Excited state photoelectron spectroscopy of anisole¤

C. G. Eisenhardt,a A. S. Gemechu,a H. R. Chelli,bc G. Cardinibc and S. CalifanobcBaumga� rtel,*a

a Institut ChemieÈPhysikalische und T heoretische Chemie, Freie Berlin,fu� r Universita� tT akustr. 3, 14195 Berlin, Germany

b Dipartimento di Chimica, University of Florence, via G. Capponi 9, 50121 Florence, Italyc L ENS, L argo E. Fermi 2, 50125 Florence, Italy

Received 11th June 2001, Accepted 16th August 2001First published as an Advance Article on the web 17th October 2001

Excited state photoelectron spectra of anisole have been measured using di†erent excitation pathways. Theyhave been chosen according to vibrational levels in the state as observed in the REMPI spectrum. TheS1combination of extended ab initio calculations with the spectroscopic results leads to the frequencies andassignment of the 42 normal modes of the anisole cation and neutral anisole. A strong inÑuence of theexcitation pathway on the appearance of the photoelectron spectra has been observed. With increasingexcitation energies the dominant signals in the spectrum are shifted to higher energy. The analysis of thespectra reveals that the major contribution to the intensity is caused by the population of vibrationalcombination states in the ion. Increasing the energy of the intermediate state changes the population ofS1these Ðnal states selectively. In addition, the population of the state is strongly supported by changes of them9bgeometry in the intermediate state.S1

1. Introduction

The understanding of the structure and electronic propertiesof van der Waals complexes between benzene derivatives andsmall molecules has witnessed in the last few years a consider-able success thanks to the synergetic combination of molecu-lar beam experiments coupled to di†erent laser spectroscopictechniques and ab initio quantum mechanical calculations.

Di†erent research groups have studied in detail aggregatescontaining small molecules associated to a phenol molecule orto its cation.1h21 Hydrogen bonding plays a very importantrole in the structure of these complexes in which phenol actsas proton donor to small molecules like water, ammonia, etc.acting as proton acceptors. Recent ab initio calculations haveactually shown that structures in which the H-bonded mol-ecule lies in the same plane as the aromatic ring are energeti-cally favored with respect to structures in which it lies abovethe benzene ring, as found in complexes of benzene with N2 ,CO, and rare gases.CS2Very little however is known about complexes in which thearomatic molecule acts as a proton acceptor22 and even lessabout their cationic species. In particular, if the OÈH group ofphenol is blocked by substitution of the hydrogen atom withan aliphatic group, the possibility of hydrogen bond couplingto an acceptor molecule is lost and the question ariseswhether the complex formation is dominated by the inter-action with the p electron system, as occurs in the benzenecomplexes, or by the interaction with the phenyl CÈH bonds.

In order to understand the structure and the dynamics ofsuch aggregates we have studied (by REMPI and photoelec-tron spectroscopy) anisole and the corresponding cations,implementing the structure determination by ab initioquantum mechanical calculations.

¤ Dedicated to Professor F. on the occasion of his 80th birthday.Do� rr

The interpretation of the spectra of the complexes is ratherdifficult and requires as a preliminary step a correct under-standing of the spectra of the parent molecules. For thisreason, in the present paper we report on the interpretation ofthe REMPI and photoelectron spectra of anisole and of theanisole cation, with particular emphasis on the importance ofdi†erent ionization pathways. In forthcoming papers we shalldiscuss the structure and spectra of their complexes with aseries of small molecules, including andCO2 , NH3 N2O.

The vibrational spectrum of anisole in the electronicgroundstate has been reported and discussed by severalauthors, but Ðnally it was Balfour23 who gave a complete andconvincing assignment of the normal modes based on the IR-spectra of anisole and di†erent speciÐc deuterated anisoles.His assignment Ðts well to the energies and characters of thesemodes as obtained by quantum chemical calculations per-formed in this work.

In the past ab initio calculations were made using the HF-level with STO-3G basis set.24 Recently Rumi and Zerbi25used the HF-level of the theory with the 3-21G and 6-31G**basis set to investigate the pi-electron conjugation and backdonation e†ect on the infrared activity of the stretchingCH3vibrations. Using the 3-21G basis set they also calculated thedependence of the vibrational pattern (Raman and IR) and theÐrst and second hyperpolarizability from the torsional anglearound the arylic carbonÈoxygen bond. Comparable highlevel calculations have been published recently by Tsuzuki etal.26 They investigated the inÑuence of intramolecular hydro-gen bonding in o-hydroxyanisole on the torsional barrier ofthe group. To our knowledge no calculations on theOÈCH3anisole cation are yet available. It should be mentioned thatquantum chemical calculations based on the density function-al theory or HF methods in the present state work well onlyfor the electronic ground state. Calculation of the Ðrst excitedstate with sufficient accuracy is not possible at the momentwith commonly available programs. Therefore the assignment

5358 Phys. Chem. Chem. Phys., 2001, 3, 5358È5368 DOI: 10.1039/b105106g

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of vibrations in the state is based on comparing their fre-S1quencies with those in the electronic ground state. Thegeometry of anisole in the state has been obtained from theS1analysis of the rotational structure in the highly resolved

(0,0) transition.27S1ÈS0The He(I) photoelectron spectrum of anisole has beenreported in the literature.28 It reveals clearly the sequence ofelectronic states up to 21.2 eV but due to the limited energyresolution and signal to noise ratio in the early days of photo-electron spectroscopy vibrational spacing is hardly recog-nized.

Excited state photoelectron spectroscopy in its simplestform is performed by using a single laser tuned so as to giverise to a speciÐc excited vibronic state. Absorption of a secondphoton then produces the ion, if the total energy of the twoabsorbed photons exceeds the Ðrst ionization potential of themolecule. Using low laser power, the ionization is a sequentialprocess. To obtain appropriate resonant intermediate statesthe measurement of the REMPI spectrum prior to the excitedstate photoelectron spectroscopy is unavoidable.

The combination of excited state photoelectron spectros-copy with the molecular beam technique improves the energyresolution and the signal to noise ratio considerably in com-parison to conventional He(I) photoelectron spectroscopy. Itopens the possibility to observe the vibrational states in largercations directly or indirectly via overtones and combinations.

2. ExperimentalFig. 1 shows a schematic drawing of the experimental set-up.It consists of a supersonic beam (expansion chamber) coupledto a time of Ñight spectrometer, a tunable dye laser and a dataacquisition system. The set-up has brieÑy been described inprevious works.29,30 In the following some important detailsof the instrument will be outlined.

The photoelectron spectrometer (““magnetic bottle ÏÏtype31,32) is made up of a permanent magnet, anSm2Co17adjustable solenoid magnet, a drift tube and a detector. Thephotoionization occurs 4 mm above the permanent magnet.In the ionization region the magnetic Ðeld has a value ofabout 0.1 T. In the magnetic Ðeld the electrons are tightlyconstrained to follow the local Ðeld lines. The weak Ðeld (0.25mT) of the long coaxial solenoid magnet directs the producedphotoelectrons through a small aperture down the full length(100 cm) of the time-of-Ñight tube. The Ñight tube is shieldedfrom stray magnetic Ðelds by a layer of ““l ÏÏ-metal(Vacuumschmelze). At the end of the tube the electrons areaccelerated towards a double microchannel plate detector. Byusing two additional electrodes (cf. Fig. 1) the photoelectronspectrometer can be operated as a two-step TOF massspectrometer33 for REMPI spectroscopy. The third aperture

Fig. 1 The essential parts of the experimental set-up.

of the mass spectrometer acts as the Ñight tube entrance. Themain vacuum chamber and the tube are manufactured of alu-minium to avoid magnetic inÑuence. The surfaces are coatedwith copper to minimize surface charging and to improve thevacuum efficiency. All parts of the spectrometer are carefullygrounded.

The detector output is coupled to a fast transimpedanceampliÐer via a high-voltage capacitor.34 The signals arerecorded with 1 ns resolution on a digital storage oscilloscope(Le Croy 9370) and read out via a GPIB interface by a com-puter.

We used a 50 lm nozzle and a 300 lm skimmer to admitthe molecules from the molecular beam source to the ioniza-tion chamber. Ionization is achieved by the frequency-doubledoutput of a Nd-YAG (10 Hz repetition rate) pumped dye laser(Estla Ltd. DL-MIDI), calibrated with a neon OG lamp. Cou-marin 153 dissolved in methanol is used as dye. Theresolution of the REMPI spectra is limited by the spectrallaser width to 0.25 cm~1. The energy resolution of the photo-electron spectrometer depends strongly on the electron kineticenergy. The 1 ns sampling rate of the oscilloscope corre-sponds, for example, to an energy resolution of 0.34 meV forelectrons of 0.44 eV kinetic energy and to 3.9 meV for elec-trons of 2.2 eV kinetic energy. Since the spectrometer collectsalmost every emitted photoelectron, the halfwidth of thesignal depends mainly on the ““ turning timeÏÏ of those elec-trons which start moving in the direction opposite to that ofthe detector. This time is also a function of the electronenergy. For an electron kinetic energy of 0.44 eV the FWHMof the resulting signal is equal to 18 meV. The energy scale hasbeen calibrated using KimuraÏs value of 8.2320^ 0.0004 eV35for the adiabatic ionization potential of anisole. The onset ofour electron yield refers to this energy. In the literature28 avalue of 8.21 ^ 0.02 eV based on He(I) spectra has beenreported. With our calibration procedure we are able tomeasure the energies of all other states with an absolute accu-racy of 5 meV.

Each PES is an average of 5000 time of Ñight spectra. Thesignal to noise ratio is excellent ; even at a photoelectronenergy of 0.2 eV it exceeds a value of Ðve.

3. Results and discussion

3.1. Ab initio calculations

The calculations were carried out by the density functionaltheory36 (DFT) as implemented in the GAUSSIAN 98package.37 In DFT good results are achieved by using asmaller basis set than required in other correlated methods.However, it is important to choose an appropriate com-bination of basis set and exchange-correlation functionals, asshown by Rauhut and Pulay38 and by Scott and Radom.39The combination of the 6-31G(d) basis set40 with the B3-LYPexchange-correlation functional represents a good compro-mise between accuracy and computer time cost. Of course, theagreement of the calculation with experimental data can beimproved using a larger basis set. Therefore we used theB3-LYP functional and the 6-311G&&(d,p) basis set.

The B3-LYP functional is deÐned in terms of the DiracÈSlater (DS), HartreeÈFock (HF), Becke (B88),41 LeeÈYangÈParr (LYP)42 and VoskoÈWilkÈNusair (VWN)43 functionalsaccording to the expression :

FB3vLYP\ 0.8Fx(DS)] 0.2Fx(HF)] 0.72Fx(B88)

] 0.81Fc(LYP)] 0.19Fc(VWN) (1)

The minimum energy geometry of both anisole and anisolecation were obtained using ““very tight ÏÏ convergence criteria.The maximum force on the atoms and the root mean squareof the forces were less than 10~6 a.u. The same criterion (10~6a.u.) was used for the maximum atomic displacement (between

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consecutive self-consistent iterations) and the root meansquare of the atomic displacements. During the calculation ofthe second derivatives with respect to the atomic displace-ments, a very Ðne grid37 was adopted for the numerical com-putation of the integrals to increase the accuracy. In theliterature several scaling frequency factors are normally38 usedfor di†erent types of internal coordinates. We have found thata satisfactory Ðt of the experimental frequencies is obtained byscaling the calculated frequencies in the range up to 2000cm~1 by the factor 0.973 and those in the higher frequencyrange by the factor 0.963.

The calculated bond lengths and angles for neutral anisoleand for the anisole cation are shown in Fig. 2. Comparison ofthe two structures shows some signiÐcant di†erences. In thecation the angle increases by more than 4¡ withCÈOÈCH3respect to the neutral molecule. In addition the length of thetwo CÈC bonds adjacent to the CÈO bond increases by about0.04 from anisole to the anisole cation, whereas that of theA�two next CÈC bonds decreases by about 0.03È0.02 TheA� .other two CÈC bonds increase, both by more than 0.02 A� .These features show the increase of quinoidal character for theanisole cation as found for the Ðrst excited state from the rota-tional structure of the transition.27S1 ^ S0The calculated ab initio normal mode frequencies of neutralanisole and of the anisole cation are compared to the experi-mental frequencies in Table 1.

The calculated frequencies agree very well with the experi-mental ones, di†erences being on average of the order of fewwavenumbers. The assignment of the normal modes of anisolehas been previously discussed by Owen and Hester44 and,more recently, by Balfour.23 Our assignment, based on thecomparison with the ab initio calculations, agrees well with

Fig. 2 Calculated ab initio structures of anisole and anisole cation.Bond lengths in and angles in degrees.A�

BalfourÏs assignment. The only di†erence concerns the CH3symmetric stretching mode. The frequency of this mode,assigned by Balfour to a band at 2834 cm~1, is too low withrespect to known stretching frequencies of methoxyCH3groups in substituted benzene compounds. For this reason,Owen and Hester44 discussed the possibility that theoccurrence of this band is due to a Fermi resonance betweenthe symmetric stretching mode and the overtone of theCH3symmetric bending mode at 1442 cm~1 and suggested asCH3an alternative assignment for the stretching mode a band inthe 2950 cm~1 region. Recently, Rumi and Zerbi25 have pre-sented strong evidence against the assignment of the band at2834 cm~1 to a fundamental mode. Our calculation showsthat there is no calculated frequency in the 2850 cm~1 regionand therefore we assign the assymmetric stretching modeCH3to the band at 2942 cm~1 as proposed in ref. 44.

Since in anisole the group can undergo a large scaleOÈCH3rotation about the CÈO bond, its molecular symmetry shouldin principle be classiÐed according to Longuet-Higgins theoryas belonging to the group. The group is isomorphousG4 G4with the group and has the same character table. ThisC2vpresents no difficulty for the classiÐcation of the skeletonmodes but requires a di†erent classiÐcation of the rotationalmodes. Since, however, in previous papers the skeleton modeswere classiÐed according to the symmetry species A1, A2 , B1and of the group and those of the methoxy groupB2 C2vaccording to the symmetry species a@ and aA of the group,Csfor an easier comparison with the literature, we shall adoptthis second classiÐcation. For the vibrations of the aromaticring we adopt the well-known notation of Wilson.46

For the anisole cation the frequencies of the normal modesare not available in the literature. The experimental fre-quencies listed in Table 1 up to about 1100 cm~1 are takenfrom an unpublished ZEKE spectrum measured by Kimura.35The other frequencies were assigned from an analysis of theÐne structure of the 0 ^ 0 photoelectron spectrum discussedin section 3.3.1, using as a guide the calculated frequencies.The procedure allowed us not only to identify with high accu-racy the normal modes, but also a large number of binary andternary combinations in the spectrum and this makes us con-Ðdent in the reliability of the proposed assignment.

3.2. The REMPI spectrum

In order to understand the features of the photoelectronspectra it is meaningful to assign the resonant intermediatevibrational states used in the photoionization process. Themeasurement of a REMPI spectrum is a necessary step inidentifying the energy of the resonant intermediate states usedin the di†erent excited state photoelectron spectra.

The anisole REMPI spectrum shown in Fig. 3 has beenmeasured in the range 276È267 nm (36 232È37 453 cm~1), i.e.in a range extending for about 1200 cm~1 above the origin ofthe transition at 36 382 cm~1, in reasonable agree-S1 ^ S0ment with the value of 36 386 cm~1 reported in the liter-ature.47 Beside the vibrationless state, several vibrationalS1states of the Ðrst excited electronic level are clearly identiÐed.These states are listed in Table 2. They were assigned by com-parison to the experimental and calculated vibrational fre-quencies of the state. Our assignment Ðts well thatS0obtained by Balfour47 from the high resolution spectrum ofthe transition. There is only one discrepancy betweenS1 ^ S0ours and BalfourÏs assignment. By comparison of the corre-sponding frequencies in anisole and anisole` we assign theband at 427 cm~1 to and not to an overtone as done bym16aBalfour.

3.3. The excited state photoelectron spectra

Excited state photoelectron spectra of aromatic moleculeshave been reported in the literature for benzene,48 toluene,49

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Table 1 Assignment of the normal modes of anisole and of the anisole cation. The calculated values are scaled by a factor of 0.973 in the range80È2000 cm~1 and by a factor 0.963 in the range above 2000 cm~1. The experimental data given in column 1 are taken from the assignmentgiven by Balfour.23 Assignment marked with an asterisk are tentative

Anisole Anisole cation

N Sym. Assignment lexp/cm~1 lcalc/cm~1 lexp/cm~1 lcalc/cm~1

1 aA COC torsion 81.5 90 110 1142 B1 m10b CH3 torsion 209 203 165 1473 B2 m9b X sensitive 260 250 239 2324 aA OÈCH3 torsion 263 266 220 2325 A2 m16a CC twisting 415 412 365 3676 a@ CÈOÈC bending È 433 432 4277 B1 m16b CÈC twisting 511 502 437 4328 A1 m6a X sensitive 553 543 540 5329 B2 m6b ring 618 610 568 560

10 B1 m4 X sensitive 690 669 616 60311 B1 m11 CÈH bending 752 738 771 76312 A1 CÈOCH3 stretch 788 777 727 76213 A2 m10a CÈH bending 819 806 788 77614 B1 m17b CÈH bending 880 867 925 91915 A2 m17a CÈH bending 956 941 996 97716 B1 m5 CÈH bending 975 956 983 97417 A1 m12 ring bending 997 981 978 97118 A1 m1 ring breathing 1022 1015 983 97319 a@ m18a OÈCH3 stretch. 1039 1041 946 93720 B2 m18b CÈH bending 1073 1075 1077 108321 a@ CH3 rocking 1143 1138 1107 111422 B2 m15 CH bending 1151 1147 1122 113723 A1 m9a CÈH bending 1169 1165 1175 117524 aA CH3 rocking 1180 1173 1139 116325 A1 m7a CÈOCH3 stretch. 1253 1248 1328 133726 B2 m3 CÈH bending 1292 1305 1301 129327 B2 m14 CÈC stretching 1332 1330 1345 136028 a@ CH3 sym. def. 1442 1437 1412 141429 B2 m19b CÈC stretch def 1455 1449 1491 150330 a@ CH3 antisym. def. 1452 1456 1454 145531 aA CH3 antisym. def. 1464 1469 1435 144532 A1 m19a CÈC stretching 1497 1492 1473 148233 B2 m8b CÈC stretching 1588 1542 1482 150334 A1 m8a CÈC stretching 1599 1606 1574 159735 A1 CH3 sym. stretch. 2900 2903 2957 296036 aA CH3 asym. stretch. 2942 2964 3049 305237 a@ CH3 asym. stretch. 3004 3034 3095* 309538 A1 m7b CÈH stretching 3026 3063 3098* 309639 B2 m13 CÈH stretching 3037 3070 3103* 310340 A1 m2 CÈH stretching 3062 3089 3105 310741 B2 m20b CÈH stretching 3092 3093 3113 311642 A1 m20a CÈH stretching 3105 3101 3121 3122

halogenated benzenes,50 benzaldehyde51 and naphthalene.24The structure of the spectra of these compounds has been dis-cussed using a few speciÐc normal modes which Ðt the mostintense observed peaks. Using di†erent intermediate states the

Fig. 3 REMPI spectrum of anisole. The vibrational states labelled inthe spectrum (except 9a) have been used as intermediate states for theexcitation of the photoelectron spectra.

shape of the spectra changes considerably. In this section wereport on the excited state photoelectron spectra of anisole,using the vibrationless state and its vibrationally excitedS1states the stretch, and asm9b , m16a , m6a , m6b , CÈOCH3 m12 m1

Table 2 Vibrational frequenciesa in the state of anisole asS1obtained from the REMPI spectrum

0 ^ 0 Symm. l(S1)b l(S1)c l(S1)d l(S0)e

9b B2 260 259 260 25016a A2 427 È 415 4126a A1 501 495 553 5436b B2 526 526 618 610

CÈOCH3 stretch A1 756 759 788 77712 A1 942 938 997 9811 A1 950 953 1022 10159a A1 992 993 1169 11653 B2 È 1127 1292 1305

14 B2 È 1177 1332 13307a A1 È 1271 1253 1248

19a A1 È 1451 1497 14928b A1 È 1567 È 1542

13 A1 È 3097 È 3101

a In cm~1. b This work. c Ref. 47. d Ref. 35. e Calc.

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intermediate states. The 2-photon absorption at the corre-sponding frequencies exceeds the ionization potential ofanisole.

The resulting di†erent photoelectron spectra, shown in Fig.4, exhibit a characteristic coarse structure, depending uponthe energy of the vibrationally excited state used in the di†er-ent ionization pathways. In the ““0È0ÏÏ spectrum there arefourteen discernible peaks, but only the Ðrst six peaks from Ito VI are labelled in Fig. 4.

If the spectra are plotted on an extended scale, as shown inFig. 5, one recognizes a reproducible Ðne structure in the elec-tron yield curve. At the origin the spectra are blurred to a fewstructural features, mainly inÑections, but peaks become moreand more pronounced on going to lower photoelectron ener-gies.

3.3.1. The ““0^ 0ÏÏ photoelectron spectrum. The moleculargeometry of anisole in the state has been recently deter-S1mined from the rotational structure of a high resolution LIFspectrum of the transition.27 Anisole in the state isS1 ^ S0 S1still planar, as in the state, but its geometry, as expectedS0

from the lowering of the electron density in the ring, is anintermediate between that of anisole and of anisole`. In par-ticular, the angle is increased by 2.1 degrees withCÈOÈCH3respect to neutral anisole. The four CÈC bonds not parallel tothe CÈO bond are increased in length.

The photoelectron spectrum of Fig. 4(a), obtained via thevibrationless state, will be now discussed below in someS1details. Four peaks (I, IV, V and VI) dominate the coarsestructure. In the spectrum of anisole cation, as can be seenfrom Table 1, there is a strong accumulation of normal modesin Ðve spectral regions, that can be arranged in Ðve bunches ofmodes. It is therefore reasonable to expect that the populationof these Ðve bunches will be imaged in the coarse structure ofthe spectrum.

The most intense spectral feature is signal I. The signalintensity rises steeply from the onset at 8.232 eV to amaximum at 8.26 eV. The shape of the signal is very smoothand no structural features are recognizable except two inÑec-tions close to the maximum. The energy di†erence betweenthe onset and the maximum of the signal correlates well withthe mode of the cation. Obviously the propensity rulem9b(*l\ 0) is not valid in this case, *l\ 1 being favored in the

Fig. 4 Photoelectron spectra of anisole via several vibrational levels in the state. The arrow shows the adiabatic ionization potential. TheS1range of the signals IÈVI represents the low resolution part of the spectra. Above signal VI on an extended scale a rich and reproducible Ðnestructure is observed. A detailed analysis of this Ðne structure will be published in a forthcoming paper.

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Fig. 5 Assignment of the CÈH stretching vibrations and of some combinations of the anisole cation as an example of the 0È0 photoelectronspectrum in the high resolution range.

ionization from the vibrationless state This and the highS1.intensity observed is supported by the variation in thegeometry of the state which favors the excitation of theS1 m9bmode.

In previous papers49,50 the maximum of the Ðrst strongsignal in the 0 ^ 0 spectrum was assigned to the ionizationpotential without comment. We assign the onset of the elec-tron yield curve to the adiabatic ionization potential and themaximum of the Ðrst strong signal to the vertical ionizationpotential. The adiabatic ionization potential correlates to thevibrationless electronic ground state of the ion whereas thevertical ionization potential correlates with the vibrational

state of the ion reached in a vertical transition from the initialstate according to the FranckÈCondon principle. The maincontribution to signal I arises from one single normal mode,but from the width of the peak, contributions of other normalmodes and of their overtones or combinations are very likelyto occur, as dicussed below.

The three inÑections close to the maximum are correlatedwith the torsional mode and with combinations of theOÈCH3torsional mode with the CÈOÈC and with theCH3 OÈCH3torsional modes. All other fundamental modes of the cationfalling in the range of peak I up to about 665 cm~1 as well asother possible overtones and combinations are not discernible

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in the spectrum. This includes the mode around 4782m9bcm~1 which may have considerable intensity.Signal II is weak and appears as a shoulder in the electronic

yield curve. It reveals some structure on an extended scalerepresentation. Using the frequencies of Table 1, the observedpeaks are assigned to the normal modes stretching,CÈOCH3and In addition, several symmetry allowed com-m11 m10a .binations Ðt well the observed peaks. Some of them areenhanced by superposition of di†erent binary combinations.The contribution of the mode to these combinations ism9bpreferred due to the high population of this state. Com-binations of with the in-plane CÈOÈC bending and withm9bmodes and are separately identiÐed.m6a m6bSignal III is also weak and smooth in shape. It shows inÑec-tions on the low energy side close to the maximum at 989cm~1 and two noticeable features at 1007 and 1029 cm~1.The normal modes and and them17b , m12 , m5 , m1 m17a OÈCH3stretching mode contribute to peak III. The maximum is cor-related with mode whereas the other modes contributem17a ,to the low energy wing of the signal. Binary combinations arealso expected in this range. A detailed inspection of the spec-trum reveals the position of some of them. At the maximum ofpeak III the combinations of the torsion with andOÈCH3 m11of the CÈOÈC bending mode with are superimposed onm6bthe mode. The signal observed at 1007 cm~1 may bem17aexplained mainly by the combination of the CÈOÈC bendingmode with The second peak in this range at 1029 cm~1 ism6b .assigned to the combinations torsion, CÈOÈCm10a ] CH3and to some ternary combinations of withtorsion] m17b m9bother modes like and The2m9b] m6b m9b] CH3 torsion] m4 .low intensity of the signal points to low FC factors for thetransition leading to these states. The normal mode m3expected close to the minimum between peaks III and IV ishardly observable.

Signal IV is very smooth on its low energy side. Around themaximum at 1212 cm~1 one Ðnds structures at 1192 and 1229cm~1. The low energy side is obviously dominated by thenormal modes and by the rocking modesm18b , m15 , m9a CH3a@ and aA which cannot be separated in the spectrum. The highenergy side shows several steplike inÑections. It is not mean-ingful to discuss all di†erent combinations which may contrib-ute to the signal. It is sufficient to point out that it gains mostof its intensity from binary combinations of withm9b m5 , m12 ,

and and from ternary combinations of with otherm1 m18a m9bmodes. Around the maximum the combinations m16b ] m10b ,and the combinations of the CÈOÈC torsionm16b] m10a ,mode with as well as withm17a m17b , (m9b] m18b), (m9b] m5)and are superimposed.(m9b] m18a)For signal V one Ðnds a situation similar to that of signal

IV. The signal appears smooth, but shows a signiÐcantnumber of inÑections. The number of normal modes and com-binations, in particular those including or the CÈOÈC in-m9bplane bending mode is very high. The normal modes andm7aare expected in the onset region of this signal and arem14observed as inÑections. The three deformation modes of themethyl group and the normal modes andm19a , m19b , m8a m8bgive rise to clear inÑections and signals around the maximum.The expected position of the antisymmetric deformationCH3mode coincides with the maximum of signal V at 1436 cm~1.The main contribution of combinations arises from couplingof with and with the rockingm9b m9a , m7a , m15 , m14 , m19b CH3mode. Combinations of the CÈOÈC in-plane bending mode aswell as of and with and with them6a m6b m12 , m1, m18b CH3rocking mode can be localized as inÑections.

Signal VI covers the range 1671È2163 cm~1 above theonset. In this range no fundamental modes are expected andthus the signal is due only to the high density of combinationsin this energy range. Again the symmetry allowed com-binations of and of of the CÈOÈC in-plane bending modem9bwith other normal modes dominate, beside those of andm6a

Actually, in the same frequency range several ternarym6b .combinations including and may contribute signiÐ-m9b 2m9bcantly to the signal and prevent the appearance of a detectableÐne structure.

At this point of the analysis we conclude that bunches offundamental vibrations are not responsible for the strongsignals observed in the spectrum. Fundamental modes con-tribute to the electron yield curve, but the coarse structure isstrongly inÑuenced by the manifold of binary and ternarycombinations as convincingly demonstrated by the high inten-sity of signal VI.

In the following region above signal VI some small signals(VIIÈXIV) are observed and the electron yield curve seems tobecome more noisy. But the detailed inspection of this rangereveals a rich and reproducible Ðne structure of the spectrum.From the analysis of this Ðne structure the coupling of normalmodes to binary and ternary combinations can be observed..In this paper we present only some related examples demon-strating the development of Ðne structure with decreasingphotoelectron energy. More details will be discussed in aforthcoming paper.

According to the assignment of the anisole cation modes(Table 1), most of the CÈH stretching fundamentals occurbetween 3095 and 3121 cm~1. As shown in Fig. 5, com-binations of these modes with one or two lower frequencymodes lead to the Ðne structure of the spectrum.

Fig. 5(a) shows the 0 ^ 0 spectrum in the narrow range3090È3125 cm~1 with a point to point distance of about 1.8cm~1. There are four signals which Ðt the positions calculatedfrom the frequencies of Table 1. The six CÈH stretching vibra-tions are not completely resolved, but can be easily identiÐed.

Fig. 5(b) and (d) show the combinations of the same CÈHstretching modes with and with the CÈOÈC in-planem9bbending mode. The signals are shifted in the spectrum by 240and 432 cm~1, respectively. The spacing between the signalsof the CÈH stretching modes remains constant. There aresome additional signals in this part of the spectrum which canbe assigned to combinations of other fundamental modes.

Fig. 5(c) shows the spectral region where the combinationsof the stretching modes with the overtone are expected.2m9bOne Ðnds a reasonable agreement with the spectrum,although the superposition with the combinations m1] 2m19b ,

and leads to a considerable modiÐcationm12] 2m3 m1] 2m3of the signals.Fig. 5(e)È(g) show the parts of the spectrum where the

spacing between states becomes very narrow and their super-position leads to a complicated signal pattern. The point topoint distance in these Ðgures is about 0.5 cm~1 and thesignal to noise ratio in Fig. 5(g) is close to 5.

In the spectrum 5(e) one expects the combinations ofnormal mode and with the CÈH stretching modes. Them12 m1spectrum in this region is not well resolved but still one canrecognize the expected sequences of combinations beside othersignals. An analogous situation is encountered in Fig. 5(f ).Most of the signal in this spectral region can be assigned tocombinations of and with the aforementionedm19a , m19b m8bgroup of CÈH modes. In Fig. 5(g) the ternary combinations of

with and with the CÈH stretching modesm9b m19a , m19bproduce most of the signals.The complete analysis of the Ðne structure of the spectrum

shows that the calculated combination modes Ðt the structureof the experimental spectrum very well. This conÐrms ourassignment presented in Table 1 for the cation.

3.3.2. Photoionization via di†erent vibrational states of S1.

The excited state photoelectron spectra obtained using asresonant intermediate states of the normal modesS1 m9b ,

stretch, and are shown in Fig.m16a , m6a , m6b , CÈOCH3 m12 m14(b)È(f ). Table 3 summarizes the position of the maxima withrespect to the onset in the di†erent spectra. Only the coarse

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Table 3 Positions of the maxima IÈVI in the photoelectron spectra (Fig. 4). The frequencies are given relative to the onset of the spectrum

0 ^ 0 9b 16a 6a 6b CÈOCH3 str. 12 1Symm. È B2 A2 A1 B2 A1 A1 A1l1(S1) È 260 427 501 526 756 942 950Signal P È È 239 228 240 323 242 268Signal I 241 481 697 882 823 1259 1522 1449Signal II 792 È 1141 È È È È ÈSignal III 990 È 1140 È 1359 È È ÈSignal IV 1212 1461 1661 È 1820 2039 2208 2245Signal V 1436 1648 1867 1950 2028 2319 2399 2440Signal VI 1889 2125 2349 2421 2458 2735 2890 2931

structure of the spectra will be discussed here, using thesignals IÈVI of the 0 ^ 0 spectrum as reference.

As shown in Fig. 4(a)È(f ), the spectral pattern changes con-tinuously. The strongest signals are shifted to higher energy. Asmall pre-signal P appears in the spectrum just before signal I.The position of the maximum of this signal remains constantat higher excitation energies, whereas the positions of theother signals are shifted. In the following we discuss the struc-ture of these spectra and analyze the di†erences between them.

The intensity of the electron yield curve represents thepopulation of vibrational states in the ion.

Generally, the population of fundamental modes is higherthan that of combinations but the number of combinationsstrongly exceeds that of the fundamentals. The populationprobabilities and the position of the combination states aremore inÑuenced by the ionization pathways than the normalmodes. By choosing a speciÐc vibrational mode as a resonantintermediate state, the population of this mode and of its com-binations with other modes is preferred after ionization.Therefore, with increasing energy of the resonant intermediatestate, the position of the signals caused by the population ofcombination states is shifted to higher energies and the inten-sity of these signals changes.

3.3.3. The photoelectron spectrum. Photoionization““m9b

ÏÏvia the state of is of special interest because of the elec-m9b S1tronic motion coupling with this vibration. The onset of theelectron yield curve is, within error limits, identical to thatobtained in the 0 ^ 0 spectrum but the shape of the coarsespectral pattern changes both in position and intensity of thesignals.

In comparison to the 0 ^ 0 spectrum the strongest signal Iin the spectrum is shifted by 240 cm~1 to higher energy,m9bthe intensity and the width being increased. It shows a kinknear the onset which correlates with the state of the ion.m9bThe maximum of the signal has to be assigned to the 2m9bstate and at the position of the state the electron yield3m9bcurve has still a remarkable intensity. The position of themaxima conÐrms again that the *l\ 1 transition is preferredupon ionization from the state. The electron yield curve isS1very smooth except for some inÑections in the high energywing which can be tentatively assigned by using the fre-quencies of Table 1. The main contribution to the high inten-sity and to the width of signal I may be due to transitions tothe states and to other vibrational states. The positions of2m9bthe CÈOÈC bending mode and of and arem16a , m6a , m6b m4expected to be close to the maximum. At the end of the highenergy wing of signal I there is an accumulation of fundamen-tal states rocking and(m17b , m18a , m12 , m5 , m1, m17a , m18b , CH3giving rise to inÑections in the spectrum. The high inten-m15)sity of the signal points also to considerable contribution ofthe transitions to binary and ternary combination statesincluding preferentially and probablym9b , 2m9b 3m9b .

The weak signals II and III in the 0 ^ 0 spectrum are com-pletely integrated in the strong signal I and in signal IV.

Signals IV and V are shifted to higher energy and theirintensity ratio is reversed in comparison to that of the 0 ^ 0spectrum.

The main reason for the enhanced intensity of signal IV isthe accumulation of fundamental vibrational states on the lowenergy side rocking, close to the(CH3 m9a , m3 , m7a , m14),maximum (symm. deformation a@, asymm. deforma-CH3 CH3tions aA and and on the high energy side andm19a) (m8b , m19bof the signal. In addition, the population of combinationm8a)states including preferentially the and the CÈOÈCm9b , 2m9bbending modes is expected to contribute to this signal. Thesignal shape is very smooth.

The intensity and the Ðne structure of signal V has to beaccounted for only in terms of combinations. At the signalmaximum the states symm. deforma-m9b] m9a , m9b] CH3tion, bendingm9b ] CÈOÈC bending] m12 , m9b] CÈOÈC

are superimposed on other combinations including] m1essentially and its overtones. Signal VI corresponds tom9bsignal VI in the 0^ 0 spectrum, but the maximum is shifted tohigher energies by 236 cm~1 and the intensity is increased.The signal is very smooth, its intensity can be explained onlyby the superposition of several combination states. A detaileddiscussion of the long list of combination states expected inthis energy range would be meaningless. It is sufficient topoint out that near the maximum several combinationsincluding and are accumulated. The FrankÈCondonm9b 2m9bfactors for transitions into these states seem to be rather high.This can be easily understood considering that the atomic dis-placements in the vibration prepare the structural changesm9bfrom the to the state.S0 S1

3.3.4. The photoelectron spectrum. At a Ðrst glance““m16a

ÏÏthe coarse shape of this spectrum and the number of signalsseem to be similar to those of the 0 ^ 0 spectrum. A closerinspection reveals, however, very remarkable di†erences.

The intensity of all signals in the spectrum is reduced incomparison to the 0 ^ 0 and to the spectrum. A smallm9bpre-signal P appears in front of the strongest signal I. Themaximum of signal P is located at 240 cm~1 above the onsetand therefore we assigned it to the vibration of the ion.m9bThe low population of the state by ionization reÑects a lowFC factor. This is reasonable since the coupling of andm16ais symmetry forbidden and because there is no support bym9bthe electronic motion in S1.The origin of the strong signal I coincides with the positionof the state of the ion. The maximum, above the onset,m16acorrelates very well with the (*l\ 1) state of the ion.2m16aThe width of signal I is only 300 cm~1 and the shape is verysmooth. This prevents a detailed analysis of the states whichcontribute to it. In the range of signal I one expects funda-mental states in the frequency range between andm16b m17a .Especially the modes and the stretchingm11, m10a C-OCH3may contribute to the intensity close to the maximum. Thehigh energy wing of the signal presumably contains com-binations of with fundamental modes. The contribution2m16a

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of combination states including the mode to the intensitym9bcannot be excluded but does not seem to be signiÐcant in thiscase.

Signal II is observed between 1050 and 1350 cm~1. It isweak but shows some remarkable structural features whichcan be clearly assigned to the vibrations and of them18b m14ion. In addition a few combinations of with other funda-2m16amentals can be correlated with details in the Ðne structure ofthe signal. The energy range of signal II in the spectrumm16acovers nearly the range of signal IV in the 0 ^ 0 spectrum inwhich also the contribution of fundamental modes dominates.This kind of signal is stable in position relative to the onset, incontrast to signals predominantly due to combination modeswhich migrate on the energy scale depending on the modespreferably populated in the di†erent excitation pathways.

The very weak signal III is observed in the range 1410È1580cm~1 above the onset. As can be seen from Table 1, it consistsof an accumulation of normal mode states. This is conÐrmedin detail by the considerable Ðne structure of the signal whichallows the assignment of single states between the symmetric

deformation and mode This small signal becomesCH3 m8b .visible due to the shift of the combination states in the highenergy wing of signal V in the 0 ^ 0 spectrum to higherenergy. States of combination modes hardly contribute to thissignal.

The signals IV and V in the range 1580È2110 cm~1 overlapeach other. Their intensity is completely due to combinationstates. The main contribution is due to combinations of 2m16awith fundamental modes up to the symmetric deforma-CH3tion at 1412 cm~1.

Signal VI (2110È2600 cm~1) is very intense and smooth inshape. The Ñat maximum extends from 2325 to 2345 cm~1.Close to the onset of the signal one expects combinations of

with other normal modes. Ternary combinations of the2m16atype are accumulated in the range of this2m16a ] mi] mjsignal together with a manifold of other ternary combinations.The shift of the maxima of signals I, IV, V and VI in the

spectrum vs. the corresponding signals of the 0 ^ 0 spectrumis of 447 cm~1. This value is the di†erence betweeen theenergy of the and states in the ion. It indicates that2m16a m9bthe signal shift is dominated by the contribution of com-bination states.

3.3.5. The photoelectron spectrum. The spectrum““m6a

ÏÏshows a sharp onset followed by a small pre-signal P. Themaximum of this small signal coincides with the mode inm9bthe ion. In the energy range of the signal one expects funda-mental modes up to and in fact inÑections of the electronm16byield curve can be assigned to these modes.

The maximum of the strongest signal I in the spectrum isshifted of 882 cm~1 from the onset. It is very smooth and onlya few weak inÑections are recognized in the high energy range.The signal width is signiÐcantly enhanced in comparison tothe 0 ^ 0 spectrum and covers 1100 cm~1. The relative inten-sity of the maximum is lower than in the spectrum andm9bhigher than in the spectrum. In the energy range of signalm16aI one expects contributions from all normal modes between

and plus contributions from a manifold of com-m6a m8bbination states. The analysis of the weak inÑections observedin the high energy wing of the signal indicates that ternarycombinations of the modes CÈOÈC in-plane bending andm9b ,

with two other normal modes give obviously important2m16acontributions to the signal intensity.The signals II and III are not separated and signal IV may

be recognized as a shoulder in the low energy wing of signalV.

Signal V is again very broad but has a considerable amountof Ðne structure, completely due to overtones and com-binations. The main contributions originate from ternary

combinations including modes the CÈOÈC in-planem9b ,bending, and the stretching mode.m6a , m6b CÈOCH3The third prominent signal (VI) in this spectrum also resultscompletely from combinations. The high density of statesblurs the structure and only few weak inÑections arise fromthe accumulation of di†erent states.

The CÈH stretching modes are expected in the decreasingpart of the spectrum above 8.55 eV but their position cannotbe clearly identiÐed.

3.3.6. The photoelectron spectrum. The spectrum““m6b

ÏÏshows again the characteristic small pre-signal mainly due tothe mode.m9bThe state of the ion is located at the onset of the Ðrstm6bstrong signal. The maximum of this signal, shifted by 823cm~1 vs. the onset of the electron yield curve, may be assignedto the combination A considerable contribution tom6b] m9b .this signal is expected from the fundamental modes between

and as well as from combinations. Among these, thosem6b m9bincluding the CÈOÈC in-plane bending, and arem9b , m6a m6bpreferred. Taking into account possible ternary combinationsit can be understood that the shape of the signal is verysmooth and that only minor inÑections can be recognized.

Signal II is not observed. The neighbouring small signal IIIis mainly based on the states of normal vibrations between m3and The maximum of the signal correlates with modem8a . m7a .Some structural features in the electron yield curve Ðt com-binations of with modes between the stretchingm6b CÈOCH3and m9b .

Going forward in the spectrum a signal with a doublemaximum IV and V is observed. This signal is completely dueto combinations. Combinations of mode with the modesm6bbetween and may be the most important contribu-m18b m8ations to the signals. However, there are more than 70 ternarycombinations located in this energy range. Therefore it is notsurprising that there are no structural features which can beexplained by simple combinations.

A comparable situation is found in the next strong signalVI. It covers a broad range of more than 700 cm~1 in whichall combinations between up to and am12 ] m7a m8a ] m14series of ternary combinations of of the CÈOÈC in-planem9b ,bending mode, of and of with two other normal modesm6a m6bare expected.

3.3.7. The stretchingÏÏ photoelectron spectrum.““C–OCH3This spectrum again shows a pre-signal with its maximum at

the position of the state of the ion.m9bThe strongest signal I is shifted to higher energies and theposition of the maximum may be assigned to the combinationof the Other combinations of theCÈOCH3 stretching] m6a .

stretching mode with with the CÈOÈC in-planeCÈOCH3 m9b ,bending mode and with mode also contribute to thism6asignal, although the main contribution to the high intensityarises from fundamental vibrations. At the low energy side ofthe signal one expects the fundamental modes CÈOCH3stretching (727 cm~1) and (1175 cm~1). The shape of them9aelectron yield curve in this energy range is very smooth sothat the position of these vibrations cannot be located. Thehigh energy wing of the signal exibits clear inÑections, many ofthem assigned to fundamentals between mode (1301 cm~1)m3and mode (1574 cm~1). In addition there are overtonesm8aand combinations falling in this energy range. The super-position of states leads to the observed Ðne structure. Signalscorresponding to the signals II and III of the 0 ^ 0 spectrumare not observed.

The next coarse structures in the spectrum given by themaxima at 2039 cm~1 (signal IV), 2319 cm~1 (signal V) and2795 cm~1 (signal VI) are completely due to the accumulationof overtones and combinations. Beyond about 3000 cm~1 the

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spectrum shows a more pronounced Ðne structure but theCÈH stretching modes are hardly identiÐed in some weakinÑections in the range 2950È3150 cm~1. It must be men-tioned that beginning with this spectrum the Ðne structure isless pronounced even under improved resolution. This may bedue to the increasing IVR which reduces the lifetime of theÐnal states obtained after ionization.

3.3.8. The and the spectra. As can be seen from Fig.m12

m14 the loss of coarse structure which begins with the CÈOCH3stretching spectrum continues here. The coarse structure of

the and of the spectra is very similar. This is very rea-m12 m1sonable since the intermediate vibrational states are very closein energy and belong to the same symmetry species.

The spectrum also shows the pre-signal due to them12 m9bmode but it is now weaker and very Ñat.Signal I is the most intense and is followed by a broad

signal, in which the three maxima III, IV and V and the broadsignal VI are identiÐed. Behind this signal the intensity strong-ly decreases. In this range no distinct signals are observed buton an extended scale a Ðne structure is still observed. Like inthe stretching and in the spectra the structuralCÈOCH3 m1features seem broader in energy. This e†ect again may be dueto the increasing IVR as mentioned above.

The origins of the speciÐc shape of the and of them12 m1spectra are very similar. All vibrational states of the ionbetween and contribute to the intensity of signal I. Inm6a m8aaddition there are several combinations in the range of signalI which blur the Ðne structure. Near the maximum of thissignal the combinations of mode and of mode withm12 m1modes and with the overtone are expected. Am6a , m6b , 2m9bdetailed inspection of the spectrum reveals that in the rangebetween 800 and about 2900 cm~1 binary and ternary com-binations dominate. Among the ternary combinations obvi-ously those of fundamental modes with the combinations m12stretching and] m9b , m12 ] m16a , m12] m6b , m12] CÈOCH3the overtone are the most important ones. The strong2m12signal V in the spectrum is due to the accumulation of thism12kind of vibrations.

The shape of the spectrum develops according to them1same principles outlined for the spectrum, i.e. the ternarym12combinations become more important for the general appear-ance of the spectrum mode taking the role of modem1 m12 .There are only minor changes in the Ðne structure of the inm1comparison to the spectrum. They result from the di†erentm12population of modes and in the state as can be seenm12 m1 S1from the REMPI spectrum.

4. ConclusionIn this paper we report on the REMPI spectrum and thephotoelectron spectra of anisole. The interpretation of thespectra has been made on the basis of ab initio calculations inthe framework of the density functional theory. The ab initiocalculated frequencies of the 42 normal modes of neutralanisole are in very good agreement with the experimentalvalues reported by Balfour.23 The calculated normal mode fre-quencies of the cation are conÐrmed by the experimentalvalues obtained from the ““0È0ÏÏ photoelectron spectrum. Thecalculated variation of the geometry of the anisole cation withrespect to that of neutral anisole is supported by highresolution investigations of the transition. The neutralS1ÈS0and the ionic molecule are both planar but the CÈOÈC anglein the cation is increased by almost 4 degrees in comparisonto neutral anisole.

The REMPI spectrum reveals several vibrations in the S1state. The frequencies obtained for these vibrations are inexcellent agreement with the data reported by Balfour. Severalexcited state photoelectron spectra have been measured. The

coarse structure of these spectra is characterized by a series ofprominent signals depending on the energy of the stateS1vibration used as an intermediate state in the 2-photon ioniza-tion. The observed position and intensity of these signalschanges considerably on going from the ““0È0ÏÏ to the ““m1 ÏÏspectrum. With increasing excitation energy the signals areshifted to higher energies, except for a small pre-signal theposition of which remains unchanged. The population ofvibrational states in the ion obviously depends on the vibra-tion in the intermediate state and therefore the ionizationS1pathway inÑuences the spectral pattern.

The selected population of vibrational states and their com-binations is clearly detected in comparing the ““0È0ÏÏ spectrumto the and the spectra, where the the 2““m9b ÏÏ ““m16a ÏÏ m9b , m9band the 2 states and their combinations are preferred. Inm16aaddition, the geometry change in the state supports theS1population of the and of the CÈOÈC in-plane bendingm9bmodes in the ion. In this way the positions of highly popu-lated levels are continuously shifted to higher energies.

The normal modes as well as the combinations contributeto the intensity of the signals. The position of the Ðnal states isnot inÑuenced by the di†erent excitation pathway but theirpopulation changes. Owing to the large number of binary andternary combinations, their contribution to the intensity of thesignals obviously exceeds that of the normal modes. There-fore, the observed shift of the prominent signals in the di†er-ent photoelectron spectra results from changes in thepopulation of selected combinations.

Besides the coarse structure at lower photoelectron energiesa pronounced Ðne structure is observed, mainly due to binaryand ternary combinations of the CÈH stretching modes withother normal modes. The analysis conÐrms the results of theab initio calculations.

AcknowledgementOne of us (S.C.) expressess his deepest thanks to the Alex-ander von Humboldt Stiftung for the award that has o†eredhim the possibility of working in Germany. This research wassupported by the E.C. (contract No HPRI-CT-1999-00111), bythe Deutsche Forschungsgemeinschaft, the Fonds der Chemis-chen Industrie and by the Italian MURST.

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