Volume 107, number 1 CHEMICAL PHYSICS LETTERS 18 May 1984

TWO-COLOR PHOTOlONlZATlON OF VAN DER WAALS COMPLEXES OF FLUOROBENZENE

IN A SUPERSONIC FREE JET

Narishi GONOHE, Atsuo SHIMIZU, Haruo ABE, Naohiko MlKAMl and Mitsuo IT0 Department of Chemistry. Faculty of Science. Tohoku Unirersity. Sendai 980. Japan

Received 12 hkch 1984

The ionization potentials of van der MaIs compleses of fluorobenzene have been measured in a supersonic free jet by the two-color resonanceenhanced two-photon ionization technique. It is found that the ionization potential of fluoroben- zene is greatly reduced by complex&ion. The configurational change of the complex induced by the ionization is discussed.

1. Introduction

Our recent studies [ 1,2] of van der Waals com-

plexes, consisting of an aromatic molecule and a usual solvent molecule like CC14. have provided information on the stabilization of a neutral solute molecule in the electronic ground and excited states which is induced by the attachment of a solvent molecule. The stabili-

zation energy usually amount-s to several hundred

wavenumbers. It is expected that the ion of a solute molecule also

shows a considerable stabilization by the attachment of a solvent molecule. In fact, it is well known that the ionization potential of an aromatic molecule is greatly reduced in the solid or liquid phase compared

with the gas phase [3,4]. Therefore, wc extend the studies of neutral van der Waals complexes to their ions. The spectroscopic studies of the ions of van der Waals complexes will provide information not only on the ionization potential of the complex but also on the intermolecular interaction between the ion of the solute molecule and the solvent molecule. Such information is essential in understanding the chemical and physical behavior of the solute molecule in solu- tion_

Recently, it has been well demonstrated that two- color resonanceenhanced multiphoton ionization (MPI) spectroscopy is a very useful technique in the study of the ionic states and the highly excited states of a molecule [5-S]. In twocolor MPI, ionization of

22

a molecule is accomplished with two laser photons using a resonant intermediate state; the first photon

is used to excite the molecule to an electronic excited state and then the second laser is scanned across the ionization threshold. This technique leads to a direct and accurate determination of the ionization potential_

In this letter, we report the two-color MPI spectra of the fluorobenzene-(Ar), and fluorobenzene-Ccl4 complexes prepared in a supersonic free jet. The sharp rise of the ion signal was observed from which the ionization potentials of the complexes were accurate- ly determined. It was found that the ionization poten- tial of fluorobenzene is greatly reduced by complexa- tion. From the results obtained, the configurational change of the complex induced by the ionization is discussed.

2. Experimental

The experimental arrangement for the measure- ments of the two-color two-photon ionization spec- tra in combination with the pulsed supersonic jet ap- paratus has already been described elsewhere [6,7].

The dye laser system used consists of a nitrogen laser (Molectron W-24) and two tunable dye lasers (Mol- ectron DL14) pumped by the same nitrogen laser. The fit dye laser (coumarin 500) was frequency doubled by an angle-tuned SHG crystal and the second harmonic obtained (~1) was used to pump the complex

0 009-2614/84/S 03.000 Elsevier Science Publishers B.V. (North-Holland Phvsics Publinhine Divicinnl

Volume 107. number 1 CHEMICAL PHYSICS LETTERS 18 hlciy 1984

to a specific vibronic level in its SI state. The second dye laser (cournarin 540A) was also frequency doubled and the second harmonic (vz) was scanned to probe the ionic state by one-photon absorption from the S, state. After being frequency doubled, glass filters (Coming 7-54) were csed to block out the visible light from the dye lasers, which may cause the two-photon resonant multiphoton ionization process. The power of uZ from the ionizing laser was monitored continu- ously over the frequency region studied. The two W

laser beams were introduced coaxially but from op- posite directions into a vacuum chamber without de-

lay and crossed the supersonic jet IO mm downstream. The power of the frequency futcd laser (vt) was main- tained as low as possible in order to avoid direct two- photon ionization. The ionizing laser (~2) was focused by a lens of 2.5 cm focal length at the center of super- sonic free jet. The van der Waals complex was prr- pared by expandinga gaseous mixture of fluorobenzenc and Ar or CCI, seeded in 3 atm He into the vacuum chamber through a 0.4 mm diameter pulsed nozzle. The ions produced by the twocolor photoionization were directed into a detector chamber by a repeller with an appropriate voltage (1 O-25 V/cm) and were detected by a channel multiplier. The signal from the channel multiplier wasamplified by a current amplifier (Keithly 437) and then was integrated by a boxcar integrator (Brookdeal 941 S/9425).

3. Results and discussion

Figs. la and lb show the 0,O band regions of the

onecolor MPI spectra (St + So) of the fluorobenzene- Ar and fluorobenzene-CC14 compleses, respectively, in a supersonic free jet. In fig. la, two bands located at 24 and 47 cm-l to the red of the 0,O band (378 16 cm-l) of free fluorobenzene are assigned to the 0,O bands of fluorobenzene-(Ar)l and fluorobenzene- (Ar), complexes, respectively. The spectrum of tluo- robenzene-CC14 (fig. 1 b) shows a well resolved struc- ture characteristic of several low-frequency intermo- lecular vibrations [2] _ The most intense band displaced by 89 cm-l to the red of the 0,O band of free fluoro- benzene was attributed to ihe 0,O band of the com- plex. All the twocolor MPI spectra described below were measured by exciting the 0,O bands of the com- plexes by ~1.

., 1 ree

(0)

.-.a :!lJ’Ln “:L’rfJ,‘, cm -1

Fig. 1. Onecolor >lPi spectra of (a) fluorobsnzcns-iu. (b) tluorobcnzene-CC& in a supersonic free jet. The bands in- dicated by dots in spectrum (b) are relevant to the intermo-

lecular vibrations.

Fio 7a displays the two-color two-phoron ioniza-

tion s3p’ectrum of free fluorobenzene ‘n the region 01 the ionization threshold. A sharp rise of (he ion signal is clearly seen. The threshold value measured at rhe middle point of the rise shows the electric field de- pendence which is caused by field ionization of Rydberg states. Extrapolation to zero electric field

from the threshold values obtained at various fields gives the adiabatic ionization potential IP, = 74230 +5 WI-l, which is somewhat higher in ener% than

rhe value (lPO = 74217 cm-l) obtained by Smith and Raymonda [91 from the analysis of the Rydberp series observed in the vapor absorption spectrum.

Figs. 2b and 2c show the threshold spectrri for the fluorobenzene-(Ar), and the fIuorobenzene-(Ar)2 complex, respectively. which were obtained by escita- tion of their 0.0 bands with vl. A sharp rise similar to that for free fluorobenzene is observed, but the ioniza- tion threshold of the complex is red-shifted in com- parison with that of free fluorobenzene. The observed red-shift is 200 and 389 cm-t ior the fluorobenzent-

(Ar)I and the fIuorobrnzenc-(_4r)2, resprcrively. The

red-shift implies that the binding ener_q of fluoroben- zene cation in its ground state with Ar is larger than that of the ground-state neutral molecule with Ar. It

23

Volume 107, number 1 CHEhIlCAL PHYSICS LETTERS 18May 1984

(b) (a)

36iOO SCANNING LASER

WAVENUMGER / an-’ 363CO 3CLCO

TOTAL WAVENUMBER / cm-l

Fig. 2. Twocolor hlPl spectra of (a) free fluoroknzene, (b) fluorobenzene-(Ar)l, (c) fluorobenzene-(Ar)z complexes in a super- sonic free jet. YI was tuned to the 0.0 band of ezvzh molecule or complex. The electric field is 25 V/cm.

is interesting to see that the decrease of the ionization potential induced by the complexation is nearly pro- portional to the coordination number of Ar atoms. This proportionality suggests that the screening effect of the charge on the benzene ring is negligible for at least two At atoms attached.

Fig. 3 shows the threshold spectrum of the fluoro- benzene-Ccl4 complex. In contrast to the cases of free fluorobenzene and the fluorobenzene-Ar com- plex, a gradual rise of the ion signal begins at =72000 cm-l, about 2000 cm-l below the ionization poten- tial of free fluorobenzene. and continues until ==73000 cm-l. This broad threshold suggests a marked change in the geometrical structure between the S, state of the neutral complex and its ionic state. This geomet- rical change is expected, since the intermolecular in- teraction in the complex is mainly of the van der

Waals type for the S, state, but the stronger charge- induced dipole interaction is responsible for the bind- ing in the ioni.p:tate. In the case where intermolecular equilibrium distance in the complex ion is greatly dif- ferent from that in the neutral complex, one can ex- pect substantial transition probabilities from the S, level to many vibronic levels of the ionic state involv- inghigh quanta of the intermolecular stretching vibra- tion. The frequency of the intermolecular stretching

24

vibration is about 40 cm-l for the neutral complex

[2j. It would be not greatly different from this value for the complex ion, although it may be larger. In any case, the observed gradual rise of the ionization thresh- old in the fluorobenzene-CCl4 complex can be ex- plained as a superposition of many unresolved thresh- olds corresponding to the transitions to the many low-frequency vibrational levels in the ion from the S, state. The decrease of the ionization potential for the fluorobenzene-CC14 complex relative to that for free fluorobenzene is as large as 2000 cm-l, which is about 10 times larger than the decrease for the fluoro- benzeneqAr)t complex.

Very recently, Jortner et al. [ lo,1 11 carried out model calculations of the potential surfaces of van der Waals ions and discussed the reduction of the ioniza- tion potentials of benzene in its rare-gas complex rel- ative to that of the bare molecule. Following the method proposed by Jortner et al., we estimated the binding energies and equilibrium intermolecular dis- tances for the fluorobenzene-Ar and fluorobenzene- Ccl4 complexes and their ions. The intermolecular potential for the neutral complex was assumed to be a superposition of pairwise atom-atom potentials of the 6-12 type and that for the ion to be sum of the same potential as that for neutral complex and the

Volume 107. number I CHEMCAL PHYSICS LETTERS

Fig. 3. Two-color hlP1 spectrum of fluorobenzene-CC4 complex in a supersonic free jet. “1 was tuned to the 0.0 bznd oi tile com-

pies. The electric field is 75 V/cm.

electrostatic charge-induced dipole interaction poten- tial. The calculated values are given in table 1. The

results qualitatively explain the observed reduction of the ionization potential in the ion. From table 1. the following conclusions emerge:

(i) The change in the intermolecular equilibrium distance upon ionization is larger for the fluoroben- zene-Ccl4 complex than for the fluorobenzene-Ar complex. This result supports the observed gradual

Table 1

Cakulsted binding energies (D) and equilibrium intermolecu-

lar distances (ro) for neutral van der Wards complexes and their ions

Fluorobenzene-Ar Fluorobenzene-CQ

neutral ion neutral ion

10 w 3.46 3.34 4.10 4.46 D (cm-’ ) 427 289 748 1650

(382) a) (979) 3) pohuizability

of solvent 1.63 105

a) Contribution from the charge-induced dipole interaction.

rise of the ionization threshold in the fluorobenzene- CCI, c0111ple:s.

(ii) The binding criers in the neutral CC& complcs is larger than thai in the neutral _4r complex. indicat- ing the increase of the dispersive interaction will1 in- creasing the polarizability of the solvent molecule.

(iii) The calculated binding snerg oi the comples ion is much larger ihan tha1 of the corresponding nru- tral comples. The difference in the binding energ! between the neutral comples and the complex ion comes from the charge-induced dipole interaction which is characteristic of the ion. As seen from table 1. contribution from the charge-induced dipole intcr- action becomes more important for the solvent mol- ecule having grearrr polarizability.

These calculated results agree qualitatively with the observed results: the ionization potential is 100 and ;=2000 cm-I for the Ar and CC14 compleses. respec- tively. which can be compared with the calculated values of 789 - 427 = 362 and 1650 - 738 = 902 cm-l _ It is noted that ~lese reductions of the ioniza- tion potentials are much larger than the stabilization energy of the neutral comples in the excited state.

Volume 107. number 1 CHEhfiCAL PHYSICS LETTERS 18 hfay 1984

wh.itt is only 24 and 89 cm-’ for the Ar and Ccl4 comptexes, respectively [2f .

If ) N. Gonohe. N. Suzuki. H. Abe, N. Mikami and hf. ito. Chem. Phys. Letters 94 (1983) 549.

121 N. Gonohe. H. Abe, N. Mikami and hf. Ito. J. Phys. Chem. 87 (1983; 4406.

13 I J . Jortner, in: Vacuum ultmviolat radiation physics, eds. E-E. Koch. R. Haensel and C. Kunz (PergamonfVieweg. New York/B~un~bwe~, 1974) p. 291.

141 J.B. Birks, Photophysics of aromatic motecufes (Wiley- Interscience, New York, 1970) p_ 236.

15 f hf.A. Duncan, T.G. Diem and R.E. Smalley, J. Cbem. Phys. 75 (1981) 2118.

16 1 1”. Ebata, T. lmajo, N. hSkami and hf. lto, Chcm. Phys. Letters 89 (X982)45.

[ 7] ht. Fujii, T. Ebata. N. Mikami and hf. Ito, Chem. Phys. Letters 101 (1983) 578.

[S] T. Ebata, Y. Anezaki, M. Fujii, N. hlikami and hf. Ito, 3. Phys Chem. 87 (1983) 4773.

191 D.R. Smith and J.W. Raymonda.Chem. Phys. Letters 12 (1971) 269.

[IO] M.J. Ondrechen, 2. Berkovitch-Yellin and J. Jortner, J. Am. Cbem. Sot. 103 (1981) 6586.

f 111 J. Jortner, U. Even, S. Leutwyier and 2. Berkovitch- Yelling, J. Chem. Phys. 78 (1983) 309.

26