Photodissociation of gaseous olefinic cationsJerry M. Kramer and Robert C. Dunbar Citation: The Journal of Chemical Physics 59, 3092 (1973); doi: 10.1063/1.1680447 View online: http://dx.doi.org/10.1063/1.1680447 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/59/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Conformationally selective photodissociation dynamics of propanal cation J. Chem. Phys. 134, 054313 (2011); 10.1063/1.3540659 Photodissociation of carbon cluster cations J. Chem. Phys. 86, 3862 (1987); 10.1063/1.451946 The photodissociation of some alkyl iodide cations J. Chem. Phys. 75, 1820 (1981); 10.1063/1.442261 Cation transport in gaseous, critical, and liquid benzene and toluene J. Chem. Phys. 72, 1989 (1980); 10.1063/1.439346 Photodissociation of nitrous oxide cation J. Chem. Phys. 66, 1616 (1977); 10.1063/1.434083

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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 59, NUMBER 6 15 SEPTEMBER 1973

Photodissociation of gaseous olefinic cations

Jerry M. Kramer and Robert C. Dunbar Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106

(Received 27 March 1972)

An ion cyclotron resonance spectrometer was used to study the wavelength dependence of the photodissociation of cations in propene and several isomeric butenes. Photodissociation onsets were determined and compared with electron impact and thermodynamic values. In all cases the photodissociation onsets were found to be above both the thermodynamic onset and the first excited state of the cation as determined by photoelectron spectroscopy. The onset results were compatible with a photodissociation mechanism that proceeds by initial excitation into the first excited state, followed by

dissociation. A large increase in the photodissociation cross section for all the cations studied was observed at '" 3.7 eV. This increase correlated with the second excited states of all the cations except I-butene. Internal excitation of the parent cations was investigated by the addition of inert gas and by changing the electron beam energy. Photodissociation onsets were also found to be valuable in choosing between alternative structures for gas-phase cations. At least two noninterconverting isomers of C4H~ were shown to exist.

INTRODUCTION

The photodissociation of polyatomic cations using the ion cyclotron resonance (ICR) technique was recently reported,1 and the potential utility of this spectroscopic method for obtaining informa­tion about the energetics of ion fragmentation and the optical absorption properties of ions was point­ed out. As a necessary foundation for the inter­pretation of photodissociation spectra in these terms, we conSider it important to investigate the roles played by the thermochemistry of the photo­dissociation process and by the presence and en­ergies of excited states of the irradiated cation in determining the wavelength dependence of photo­dissociation, and particularly the energy of photodissociation onset. Simple olefinic molecules were chosen as providing a promiSing system for which a large body of data from other techniques is available to aid the interpretation, and it was found that photodissociation of olefinic parent cat­ions proceeded readily via loss of hydrogen rad­ical and (for butenes) loss of methyl radical. We have examined the photodissociation spectra of the propene cation and several isomeric butene cations with particular attention to determining the energy of the dissociation onset, and it was found possible to make a reasonably consistent interpretation of the results in terms of the thermochemistry of the processes and the location of the first excited state of the parent cation.

The photodissociation spectrum of the methyl chloride parent cation has been reported l from threshold to 3000 'A. The observed onset was found to lie about 0.6 eV above the thermochemical threshold for the dissociation, and it was consid­ered likely that dissociation involved the first ex-

3092

cited state of the cation. This spectrum was also taken to suggest that a higher excited state of the cation was involved in dissociation at shorter wave­lengths.

Ion photodissociation spectra have potential value in obtaining structural information about cations. This is illustrated by the spectra for isomeric bu­tene cations, which provide evidence against a slow isomerization of a I-butene cation to 2-butene cat­ion. We expect that this approach can provide a useful new approach to the difficult problem of gas-phase ionic structure investigation, comple­menting older methods. 2

EXPERIMENTAL

The photodissociation of isomeric butene and propene cations was observed with a Varian ICR-9 ion cyclotron resonance spectrometer and standard square cell, having dimensions 2. 54 cm x 2.54 cm x 13 cm. High trapping voltages of 1. 5-2.0 V and low analyzer and source drift voltages of less than O. 3 V provided electric field conditions that were efficient for the trapping of ions. Although it is not understood why these electric field conditions lead to efficient trapping of ions in an ICR cell, this phenomenon, originally observed by Henis,3 has subsequently been used to study the photodis­sociation of cations, 1,4 photodetatchment of anions, 5

as well as photon-induced ion-molecule reactions. 6

The trapped-ion literature is quite extensive and we can add no new interpretation of this effect.

Parent ions could be trapped for periods rang­ing from about 3 to 30 sec depending upon the particular ion. The trapping time for a given par­ent cation was found to correlate with its reaction cross section. Isobutene cation with a reaction

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PHOTODISSOCIATION OF GASEOUS OLEFINIC CATIONS 3093

cross section 7 of 36 x 1O-16cm2 had the shortest lifetime, 1-butene cation7 (a = 17 x 1O-16cm2) was intermediate, and 2-butene cation7 (a= 7x 1O-16cm2

),

the least reactive of the butene cations, had the longest lifetime. Recent total reaction rate data8

for thermal C4HS ions put the reaction rate con­stant of 1-butene slightly higher (6. Ox 10- 10 ccl mole· sec) than that for isobutene (5.4 x 10-10

),

while keeping that of cis-2-butene much lower (0. 37x 10-10

). The ion lifetimes found in our ex­periments do not correlate fully with these rates, but whether or not an exact correlation exists ap­pears to us a matter of small concern: It seems probable that one factor affecting the ion lifetime is the reaction rate. A second effect on the ion lifetime is collisional loss of ions from the cell.9

Trapping times were estimated by abruptly ter­minating the filament current and observing the parent ion signal decay. Under normal ICR con­ditions (millisecond lifetimes) the decay in the parent ion signal was too fast to observe by eye, while trapped ion conditions produced parent ion signals that lasted for many seconds.

Parent cations were produced by electron im­pact at a nominal electron energy of 11. 2 eV (fila­ment to trapping plate) and 1 pA emission current.10

To investigate the effect of electron energy on the photodissociation curves, spectra were also taken at 12 eV. The pressure in the cell, measured by the current monitor of the ion pump, was kept low at - 8 x 10-8 torr to minimize ion - molecule reac­tions and maximize ion lifetimes. The spectrom­eter was operated at room temperature and no pressure increase was observed when wavelength­selected light illuminated the cell.

Photodissociation spectra were obtained in the following way. The photodissociation product cat­ion was continuously monitored by the marginal oscillator detector and magnetic field modulation was used to detect the cation signal. Since the photodissociation product ions were not present without light, the resonance condition for product ions was obtained by maximizing the photodissocia­tion signal at a convenient wavelength. An alter­native tuning procedure of forming the fragment ion by electron impact at higher electron energies was considered inferior, because a change in elec­tron energy led to a small shift in the resonance condition.

After the fragment ion was tuned, the ion inten­sity was monitored on a recorder, alternatively with the light off and on. Typical raw data are shown in Fig. 1. The duration of the light on was long compared to the lifetime of the ions in the

cell.

To determine the reactant ion in the photodisso­ciation reaction, cyclotron resonance ejection techniques were used.! The double resonance ra­diofrequency oscillator, applied to the analyzer drift plates, was used to eject possible reactant ions as the photodissociation product ion was moni­tored at a given wavelength. The double resonance oscillator was swept over a frequency range that included the parent cation as well as all observed ion-molecule reaction product cations.

Most spectra were obtained with a Hanovia 2.5 kw xenon arc, but occasionally a 2.5 kw Hg-Xe are was used. The arc was housed in a Schoeffel LH 152 N lamp housing with a 2t-in. -diam double quartz condenser. The unmodulated light beam was passed through a 100-mm-Iong water filter with quartz windows and a 3-mm-thick Optical Industries heat-absorbing filter. Baird Atomic 2 in. x 2 in. A bandpass interference filters spaced 200 A apart from 4000 to 4400 A and from 4800 to 7000 A and 150 A bandpass filters at 7500 and 8100 A provided wavelength selection. Wavelength-select­ed light was then condensed by a quartz lens through a 11 in. diam pyrex window located at the end of the ICR cell. A stop between the lamp and water filter allowed the light to be directed into or away from the cell.

The photodissociation yields from threshold to 4000 A were corrected for the transmission and bandwidth of the interference filters at half-height as measured on a Cary 14 spectrometer. The photodissociation yields were also corrected for

..J « z (!)

en

4000 4200 4400 4800 5000 5200 5400 5600

. FILTER WAVELENGTHS (A

FIG. 1. The photodissociation of the cis-2-butene cat­ion. The C4H~ cation was monitored as light, wavelength selected by interference filters, illuminated the ICR cell. The base line corresponds to the C4H~ signal in the ab­sence of light. Experimental parameters: cis-2-butene pressure 8.6 x 10-8 torr; electron energy 11.2 eV; trap­ping voltage 2.0 v.

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3094 J. M. KRAMER AND R. C. DUNBAR

TABLE 1. C4H~ energy onsets and first excited states of the cations (electron volts).

C4H8 - C4H~ + H

hv dissoc. Electron impact Thermodynamic First excited onseta onsetb onsetC state of cati onot

1-C4H~ 2. OO± 0.07 1.42,e 1. 97,e 2. 2ge 1. 48 1. 71f

cis-2-C4H"B 2.30±0.1 1.7,e1.95e 2.00 2.16f

trans-2-C4H"B 2.38±0.1 2.11e 2.04 2.34 f

iso-C4H"B 2:2.38±0.1 2.44,e 2. 46e 2.00 2.19f

~his work. The error limits correspond to the 200 A energy resolution as well as to the observed reproducibility of the onsets. ~he electron impact onset is the appearance potential (A. P.) for the fragment ion minus

the ionization potential (I. P.). Only the studies which determined both the I. P. and the A. P. were used to determine the electron impact onset. The underlined values refer to studies that obtained an I. P. of the parent neutral within 0.15 eV of the accepted photo­ionization value. ~he thermodynamic onset is the C.Hf of the dissociation reaction. All C.H/s from

Ref. 11. dSecond I. P. minus first I. P. eReference 11 and 12. fReference 14.

the spectral energy output of the xenon lamp.

In the region below 4000 A the heat filter and interference filters were replaced with a Schoeffel GM 250 Ebert-type grating monochromator. A grating with 1180 grooves/min., blazed at 3000 A, and with matched 4 mm slits gave a 132 A band­pass. Slits of 2 mm (66 A bandpass) and 1 mm (33 A bandpass) were used where intensity was sufficient. For wavelengths below 3000 A a 1 in. sapphire window was used.

Below 4000 A no corrections were made for the light source and monochromator efficiency as a function of wavelength. Qualitatively, the com­bined efficiency of the xenon arc and monochroma­tor decreases monotonically from 4000 to 2000 A.

The optical train was aligned coarsely by eye. The fine alignment utilized the photodissociation process itself, as described above.

RESULTS

The thresholds for the photodissociation pro­cesses,

(1a)

(lb)

and

(2)

are shown for Reaction (la) in Table I, and for Re­actions (lb) and (2) in Table n, for the 1-butene,­cis- and trans-2-butene, isobutene, and propene cations. Since our results are the only experi-

mental data available using the photodissociation techniques, we have included electron impactl!-13 and thermodynamic datal! from the literature for comparison. In addition, the first excited states of the cations, determined by photoelectron spec­troscopy, have been included. 14

The photodissociation curves for loss of H [Re­action (1a)1 from 1-butene, cis-2-butene, and iso­butene cations from threshold to 4000 A are shown in Fig. 2. Curves similar in shape to the cis-2-butene cation and isobutene cation were observed for loss of CH3 [(Reaction (lb)] from isomeric bu­tene cations as well as loss of H from the propene cation [(Reaction (2)] with the thresholds as given in Table n.

The photodissociation curve for loss of H from the trans-2-butene cation from 4000 to 2600 A is shown in Fig. 3. All photodissociation curves, for all olefinic cations studied, for loss of H as well as loss of CH3, show the same large increase in photodissociated products at - 3300 A. No vi­brational structure, even with a 33 A bandpass, was observed in these curves.

The falloff in the trans-2-butene photodissocia­tion curve at - 2800 A is almost surely due to the decrease in the energy output of the lamp as one goes to shorter wavelength. The true shape of the photodissociation curve below - 2900 A is un­certain. The photodissociation curves of all the cations show a falloff at - 2800 A.

The photodissociation of C6Ht2'

(3)

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PHOTODISSOCIATION OF GASEOUS OLEFINIC CATIONS 3095

TABLE II. esH; energy onsets and first excited states of the cations (electron volts).

hv dissoc. onset"-

Electron impact Thermodynamic First excited onsetb onsetC state of cationd

C3H~ 2.38±O.l 2.30±O.1 2.38±O.l 2.48±O.1 2.38±O.l

1.8,"1.95,e2.20,e2.37e 2.26 2.07g

1-C4H; cis-2-C4H; trans-2-e4H; iso-e4Wa

.!.,J1.,f1.95,eb .. Q/2.Ue 1. 79 1. 71g

2.25," 2.35,6 U f 2.31 2.16g

~ .. !i/ 2. 63e 2.35 2.34g

2.58," 2. 6/2. 65e 2.31 2.19g

"See Footnote a, Table I. J>see Footnote b, Table I. cSee Footnote c, Table I, t.Hf of e3H5+ taken as 229 kcal/mole (Ref. 13). dSee Footnote d, Table I. 6References 11 and 12. fReference 13. gReference 14.

is also observed in the 1- and 2-butene systems. The CsHt2 cation is formed in a condensation re­action between C4Wa and C4Hs, followed by loss of C2H4•

7 The photodissociation onset for CsH9 in the 1- and 2-butene systems was found to be 2.07 eV (6000 A). The photodissociation curve for

o ......J W

>-Z o I­« u o (/)

(/)

o o I­o I Q..

I

I

1: I

/ I

/ /

A' I

I

8' /

/ I

. , I

6000 5600 5200 4800 4400 4000

WAVELENGTH (Al

FIG. 2. The photodissociation spectra for loss of H radical for (A), cis-2-butene cation; (B), I-butene cat­ion; and (e), isobutene cation from threshold to 4000 A. The error bars for each cation are the same at all wave­lengths and are only shown at 4400 A.

formation of CsHg is complicated by the following reaction sequence 7:

C4H~ ~ C4H; + H, (la)

(4)

However, complete ejection of the C6H:2 cation permits the contribution of C4H; to the CsH9 prod­uct intensity to be subtracted out. The resulting photodissociation curve for formation of CsHg from only C 6Ht2 shows the same general shape as

o ......J W

>-Z o l­e(

U o (/) (/)

o o t­o ::c Q..

C H+ ~ C H+ H 4 8 4 7 +

.' -.-/

, I

I

I

I

I

I I

, ' I '.

I \

I \

\

4000 3800 3600 3400 3200 3000 2800 2600

WAVELENGTH (Ao)

FIG. 3. The photodissociation spectrum for loss of H radical for trans-2-butene cation from 4000 to 2600 A.

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3096 J. M. KRAMER AND R. C. DUNBAR

shown in Fig. 2 for the cis-2-butene cation, shifted to longer wavelength.

The C3H5 cation, formed by photodissociation of C4H;, also undergoes a subsequent ion-mole­cule reaction,7

C TT+ ~ C H+ C 4fiS 3 5+ H3, (1)

(5)

The threshold curves for the cis-2-butene cation for loss of H at 2.30 eV and for loss of CH3 at 2.38 eV are very similar. Similar thresholds for the competing photodissociation channels [(Re­actions (la) and (lb)l are observed in trans-2-bu­tene and isobutene cations. The possibility that these similar thresholds are the result of C4H;; formed in Reaction (5) and that the C4H;; energy onset for direct formation lies at a higher energy was investigated by ejection techniques for cis­and trans-2-butene cations: The photoproduction of C4H; was monitored at different wavelengths and C3H; continuously ejected. Although Reaction (5) did contribute to the C4H; intensity, the C4H; energy onset was not determined by the C4H7 formed in Reaction (5). (The C4H; energy onset for the I-butene cation is 0.3 eV below the C3H; onset and no ambiguity results. )

Gradual thresholds were observed for all the photodissociation curves with no apparent linear region above threshold. As reported here, the thresholds are simply the longest wavelength at which a photodissociation signal could be distin­guished above the noise.

DISCUSSION

Significance of Thresholds

As discussed below, the thresholds for all the observed photodissociation processes lie above the thermochemical thresholds (even assuming no in­ternal energy of the dissociating cation), so that there must be a nonzero probability of dissocia-tion for wavelengths greater than the threshold wavelength reported here. Accordingly, we recog­nize that our thresholds must be interpreted with caution, and we wish to use our thresholds to make two different types of argument. First, the thresh­olds taken under comparable instrumental condi­tions should be characteristic spectral parameters of cations, and substantial differences in threshold behavior between cations prepared in different ways may be interpreted as reflecting differences be­tween the cations. Second, we will argue that our thresholds have some quantitative value as showing a well-defined wavelength region where the proba­bility of photodissociation rises fairly rapidly from the low value characteristic of an unfavorable direct excitation into the dissociative state to the higher

value characteristic of a favorable dissociation through the first excited state. That our thresh­olds are not simply accidental products of instru­mental sensitivity is suggested by the fact that all the thresholds are reproducible despite variations in instrumental conditions and sensitivity, and also by the success of the interpretation given be­low.

Reactive Depletion of Parent Cations

If the rate at which a cation undergoes reactive collision with neutrals is very fast compared with the photodissociation rate, then the population of ions undergoing photodissociation may be radically different in character from the population initially formed at the electron beam, due to possible se­lective depletion of reactive components of the initial population. 15 Available data for the rates of reaction of olefin parent ions with parent neutrals suggest that this effect could be severe in these systems under the conditions used in our experi­ments. s The possible severity of the problem can be assessed from the ICR single resonance spectra, calculating the fraction of parent ions which are converted to products by comparing the observed parent ion peak intensity with the sum of all the plOduct ion peak intensities. For the butenes under the conditions used, typical numbers for the per­cent depletion of the initial parent ions are 35% (2-butene), 60% (isobutylene), and 85% (I-butene). (These figures are typical, but of course depend strongly on the operating parameters of the spec­trometer.) Thus reactive depletion is typically substantial, but does not approach 100% for any conditions used. We are unable to assess further the possible effect on our results of reactive de­pletion.

Onset Energies

Dissociation of a gaseous cation can be repre­sented by the following reaction:

(6)

The energy dependence of this endothermic reaction may be studied directly by photon impact, as in the present study, or by the indirect methods of electron impact.16 The energy onset for dissocia­tion, determined by photodissociation or electron impact, is a measure of the activation energy for the dissociation reaction. A third value for the energy onset of the dissociation reaction is the thermodynamic onset [(.e.H, of Reaction (6)] where the best available !:Jl's for reactants and products are used. Hence, two sources for comparison of our photodissociation data are electron impact on­sets and thermodynamic onsets.

The electron impact activation energy for dis so-

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PHOTODISSOCIATION OF GASEOUS OLEFINIC CATIONS 3097

ciation is equal to the appearance potential (A. P. ) for the fragment ion minus the ionization potential (1. P. ) of the parent cation.

A.P. (7)

1. P. (8)

Ea=A.P.-I.P. (6)

If the experimentally determined A. P. and 1. P. correspond to reactants and products in their ground states and no excess kinetic energy or ki­netic shifts are involved, then the electron impact onset (A. P. - 1. P.) is a good measure of the bond dissociation energy of the AB+ cation.17 The same general conclusions apply in relating the photo­dissociation onset to the bond dissociation energy. The thermodynamic onset is probably the best available measure of the true bond dissociation energy.

A number of differences exist between the photo­dissociation and electron impact methods. Electron impact involves the excitation of neutral molecules and, therefore, excited neutral states may be in­volved in the ionization process. The manifold of excited neutral states is presumably unavailable in the photodissociation process. In addition, the dissociation of cations by photon impact involves the absorption of photons and hence selection rules and transition probabilities governing that absorp­tion. Therefore, it is of interest to determine whether any correlation exists between the photo­dissociation onsets and the electron impact or thermodynamic onsets.

All the photodissociation curves were observed to have gradual thresholds. Even though the photo­dissociation onsets determined from these curves may be upper limits determined by sensitivity, the photodissociation onset values are in the right range as compared with the electron impact and thermo­dynamic onsets.

The photodissociation of the cis-2-butene cation (and trans- 2- butene cation also) shows the strongest photodissociation signals of the five olefins studied at shorter wavelengths (e.g., 4000 A) and has the steepest threshold region. Therefore, of all our photodissociation onset results, the values for the 2-butene cations should be closest to the true threshold. The C3lf5 onset of the cis-2-butene cat­ion is in good agreement with the value expected from both electron impact and thermodynamic val­ues, but the C4H; onset unexpectedly lies signifi­cantly above both the electron impact and thermo­dynamic values. This suggests the necessity of considering an additional factor affecting the photo-

dissociation results which is not operative in elec­tron impact experiments; apparently the optical absorption properties of the cation cannot be ne­glected.

Tables I and II show that all the photodissocia­tion onsets are above the first excited state of the cation. Although our results do not prove that photodissociation proceeds through the first excited state of the cation, our data are consistent with this interpretation and we regard this as the most attrac­tive explanation of the differences between photo­dissociation results and those from other techniques. Presumably, the photodissociation process proceeds by an initial absorption of the photon. For a cation to absorb energy, it seems reasonable that the pho­ton energy correspond to the energy of the excita­tion to some excited state of the cation. Hence, if photodissociation proceeds through the first excited state of the cation then the excitation energy to the first excited state will be one factor in determining the wavelength dependence.

The C3H; photodissociation onsets for propene, cis- and trans-2-butene, and isobutene cations show reasonable agreement with the thermodynamic onsets (within 0.13 eV) and are also above the first excited state of the cation. The C4H; photodissocia­tion onsets for cis- and trans-2-butene and isobu­tene cations show poor agreement with the thermo­dynamic onsets, but reasonable correlation between the first excited states of these cations and their photodissociation onsets. The C4H; onset for the 1-butene cation shows poor agreement with the thermodynamic onset and only fair correlation with the first excited state, while the C3H5 onset for the 1-butene cation correlates poorly with both the thermodynamic onset and the first excited state.

The comparison between the electron impact onsets and the photodissociation onsets is compli­cated by the spread of electron impact values. The one electron impact value for the propene cation (2.37 eV) and the one for C3H; from cis-2-butene cation (2.35 eV) agree with the photodissociation onsets, while other electron impact values for these cations are in poor agreement. For trans-2-butene and isobutene cations, the C3H; photo­dissociation onsets are below the electron im­pact onsets. The C3H; photodissociation onset for the 1-butene cation correlates poorly with the electron impact onsets. Poor agreement with the electron impact onsets is also found for C4H; pro­duction from at least the 2-butene cations.18

In general, all the photodissociation onset values are above both the first excited state and the ther­modynamic onset. The agreement between electron

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3098 J. M. KRAMER AND R. C. DUNBAR

impact onsets and photodissociation onsets ranged from good to poor. Although sensitivity may in­fluence our thresholds, the C3H; onsets for four of the five olefin cations show good agreement with the thermodynamic onsets. The anomalous behav­ior of the C3H; onset for the I-butene cation is not understood.

Behavior at Short Wavelength

The photodissociation spectra for loss of H, and for loss of CH3 , for all the olefin cations studied show a sharp rise in photodissociation at approxi­mately 3300 A. This is illustrated for the trans-2-butene cation in Fig. 3. This sharp rise in pho­todissociation at about 3.7 eV correlates with the second excited state of all the isomeric butene cations, except the I-butene cation, and of the pro­pene cation, as seen in Table III. Such a sharp rise in dissociation cross section could well arise from photon excitation to the second excited state of the cation. If the I-butene cation retains the I-butene structure, then our results suggest that the I-butene cation has a second excited state about 3.7 eV above its ground state.

Effect of Inert Gas

Since parent cations were produced by electrons having energies 1 or 2 eV above the ionization po­tentials of the neutrals, the possibility of vibra­tional excitation of the parent cations existed. Vi­brationally excited parent cations will have photo­dissociation thresholds at lower energy than ground state cations, and hence the desired threshold for the ground vibrational state of the parent cation will be obscured. To test for possible vibrational excitation the photodissociation experiments were reinvestigated in the presence of an inert gas (CH4).

The electron energy was 11. 2 eV and no CH; was formed by electron impact. No ion-molecule re­actions of butene cations and CH4 neutrals were observed.

The trans-2-butene cation plus CH4 system illus­trates the role played by CH4• The peak heights of the parent cation and an ion-molecule reaction product cation were followed as a function of CH4 pressure as shown in Fig. 4. Both the parent cation and the parel,t-plus-one cation peak heights decreased with increasing CH4 pressure, presumably due at least

--.. ~--""

--l 17 (0)

« z (!)

(f) 15 .~

+00 13 I

OV

--l 7 (b) « z (!)

(f) 5

+0'> 3 I

OV

r--+-+-+-+--t-+-+--+--+-+I" -+--1-1--1--(c)

0-4 •

'+ ~+ 00 02 I I " oVov

0 5 10 15 20 25 30 35

c~ PRESSURE X 108

TORR

FIG. 4. The effect of added CH4 on the trans-2-butene mass spectrum. Trans-2-butene pressure 3.4 x 10-8 torr; electron energy 11.2 eV; trapping voltage 2.0 V. Peak heights are in arbitrary units, and spectra were taken with an Hg-Xe arc. (a) C4Ifs parent cation peak height vs CH4 pressure. (b) C4H; parent-plus-one cation peak height vs CH4 pressure. (c) Ratio of C4Ws to C4H; as a function of CH4 pressure. '

in part to collisional loss of ions from the cell. At the highest methane pressure, 3. 6 x 10-7 torr, the ratio of parent-pIus-one cation to parent cation was about half its low-pressure value; a relative decrease in reaction product peaks with increasing methane pressure was generally observed, and presumably reflects both a decrease in parent-ion trapping time and a collisional deactivation of re­active excited parent ions.

The calculated collision rate of CH4 with the trans-2-butene cation at 3.6 X 10-7 torr, assuming a momentum rate transfer constant of 1 x 10-9

cc/molecule . sec, is 12 collisions/sec. The trans-2-butene cation lifetime in the cell is greater than 15 sec, and therefore the parent cation col-

TABLE III. The second excited states of the cations (electron volts).

Cation C3H; 1-C4Ha cis-2-C4R8 trans-2-C4Ha iso-C4Ha Second excited statea 3.32b 4. S7b 3.2Sb 3.46b 3.4Sb

~hird 1. P. minus first 1. P. bReference 14.

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PHOTODISSOCIATION OF GASEOUS OLEFINIC CATIONS 3099

lides with CH4 more than 180 times in its lifetime.

Over the range of methane pressure from 0 to 3. 6 x 10-7 torr, the ratio of photoproduced C4H; ions to C4H~ did not change significantly for any wave­length from 5000 to 4000 Ao However, the C4H; photodissociation peak for 5200 A light (the thresh­old wavelength in the absence of methane) was en­tirely suppressed by even 1 x 10-8 torr of methane. This dis sappe arance of the onset peak at 5200 A in­dicates that trans -2-butene cation had approximately 200 A (1000 cm- I

) of quenchable excitation or about one vibrational mode. (A high-resolution photo­electron spectrum of trans-2-butene shows the vi­brational spacing in the ground state of the cation to be -1400 cm-I • )19

The high-resolution photoelectron spectrum of trans-2-buteneI9 supports our observations regard­ing vibrational excitation of trans-2-butene by elec­tron impact. High resolution photoelectron spectra containing vibrational structure provide a good measure of the transition probabilities for excita­tion of the neutral molecule to particular vibrational levels of an electronic state of the cation. The transition probability for excitation to the ground electronic state of the cation should determine the vibrational states accessible by electron impact. The trans- 2-butene high- resolution photoelectron spectrum shows that the transition probability for excitation to the v = 0 and v = 1 vibrational states of the ground state of the cation are approximately equal and that excitation to the v = 0 and v = 1 states accounts for approximately 70% of the excitation probability. (Excitation to v = 2 has - 20% proba­bility.) Hence, the photodissociation result ob­tained with CH4 , that trans-2-butene cation has about one vibrational mode excited, is compatible with the observation about vibrational excitation deduced from the photoelectron spectrum.

The photodissociation spectra of cis-2-butene and 1-butene cations were also investigated as a function of CH4 pressure. The photodissociation onsets for loss of H in both the 1-butene cation and cis-2-butene cation as well as the photodissociation onset for loss of CH3 in the cis-2-butene cation were found to be within 200 A of the values given in Tables I and II.

Role of Electronic Excitation

cations created by electron impact can be formed in excited electronic as well as excited vibrational states. Raising the electron beam energy from 11. 2 to 12 eV had no effect on the threshold, nor on the photodissociation band contours. As an illustration of the effect on these systems of in­creasing electron energy, an increase in the elec-

tron energy from 11. 2 to 12.0 eV for 1-butene: (a) kept the ratio of parent cation to parent-plus­one cation constant at 2.9; (b) increased the parent ion peak height by 2.5; and (c) increased the photo­dissociation yield of C4H; at 4000 A by 1.8, by 1.7 at 5000 A, and by 1.7 at 5800 A.

A change in the electron impact energy from 11. 2 to 12 eV should not alter the vibrational level population of the ground electronic state of the trans-2-butene cation. However, above an electron energy of 11. 4 eV, the first excited electronic state of the trans-2-butene cation is accessible. The absence of an effect on the thresholds from a change in electron energy suggests that the cations in the first excited state are not observed in the photodissociation experiment. This is not sur­prising-the population of ions in this state might be expected to be depleted rapidly by radiative de­cay, by reactive depletion, or by spontaneous dis­sociation. That the first excited states of these cations may be dissociative is suggested both by our results and by the lack of vibrational structure in the high-resolution photoelectron spectrum of trans-2-butene. 19

Isomeric Butene Cations

One of the most potentially useful features of the photodissociation spectra of cations is that the photodissociation curves for isomeric cations, as well as their photodissociation onsets, can give information about the structure of the parent cat­ions.

The photodissociation curves for production of C4H; from 1-butene, cis-2-butene, and isobutene cations are shown in Fig. 2. The photodissociation curve for the 1-butene cation is clearly different from the cis-2-butene cation curve, both in its onset and its shape. (The cis- and trans-2-butene cation curves are similar.) The addition of 5.0 x 10-7 torr of CH4 to cis- 2-butene and 9.3 X 10-7

torr of CH4 to 1-butene resulted in photodissocia­tion curves that still crossed and had onsets that were within 200 A of the values given in Table I. Therefore, except for the possibility that one of the cations had internal energy that was not quenched by C~, and that this internal energy re­sulted in a marked shift in the threshold and dis­tortion of the curve,20 our results show that there is at least a substantial component of the cations formed from 1-butene which differ in structure from the ions formed from cis- and trans-2-bu­tene. (We cannot, of course, rule out the possi­bility that some of the 1-butene cations rearrange to the 2-butene structure.) Because of the weak intensity of the C4H; photodissociation product from an isobutene cation, a comparison of the 1-

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3100 J. M. KRAMER AND R. C. DUNBAR

butene and isobutene cations cannot be considered as conclusive.

Recent convincing results obtained by Ausloos et al.21 using photo ionization mass spectrometry provide an important point of comparison with our results. They found that I-butene cations formed at 10 eV photon energy showed little rearrangement to the more stable 2-butene structure on the time scale of their experiments, but that 1-butene cat­ions formed at 11. 6 -11.8 eV showed preponderant rearrangement to 2-butene structure. Direct com­parison with our experiments is difficult, because our electron energy (11. 2 eV) lies between the two photoionization energies, and because of the spread of cation internal energies resulting from electron­bombardment ionization. The photoionization re­sults make it appear reasonable to assume that the conditions used in our experiments would produce a significant but not preponderant fraction of I-bu­tene cations capable of rearranging to the 2-butene structure, and this conclusion is entirely consis­tent with our results. We believe the significance of our results to lie in the demonstration that cat­ions can exist for times of the order of seconds (the time scale of our experiments) without re­arranging to the more stable 2-butene structure. Our results thus support and strengthen the con­clusion of Ausloos et al. that I-butene cations are stable against rearrangement unless they possess internal excitation.

lon-Molecule Reaction Product Cations

A suggestion of how photodissociation curves can provide potentially useful information about product ions in ion-molecule reactions is illus­trated by the photodissociation of CsHii. The CsHt2 cation is formed in a condensation reaction between C4:trs and C4Ha, followed by loss of C2H4, in the 1- and 2-butene systems7 :

(9 )

The photodissociation reaction observed was

hv (3 )

The photodissociation curve for production of CsH; in the 1- and 2-butene systems showed an onset at 2. 07 ± O. 07 e V .

While a much more extensive investigation is clearly needed to extract any structural Signifi­cance from this threshold, this observation points the way towards a potentially powerful new approach towards characterizing ion-molecule reaction prod­uct cations.

ACKNOWLEDGMENTS

Acknowledgment is made to the donors of the Petroleum Research Fund, adminstered by the American Chemical Society, to the Research Corporation, and to the National Science Founda­tion (Grant No. GP-33521X) for support of this research.

1R. C. Dunbar, J. Am. Chern. Soc. 93, 4354 (1971). 2See, for example, (a) M. L. Gross and F. W. McLafferty, J.

Am. Chern. Soc. 93, 1267 (1971); (b) J. L. Beauchamp and R. C. Dunbar, J. Am. Chern. Soc. 92, 1447 (1970); (c) L. W. Sieck, S. K. Searles, and P. Ausloos, J. Am. Chern. Soc. 91, 7627 (1969) and reference contained in these papers.

3J. M. S. Henis, J. Am. Chern. Soc. 90, 844 (1968). '4R. C. Dunbar, 1. Am. Chern. Soc. 95, 472 (1973). 5K. C. Smyth and J. I. Brauman, J. Chern. Phys. 56, 1132

(1972). 6J. M. Kramer and R. C. Dunbar, J. Am. Chern. Soc.

94, 4346 (1972). 7F. P. Abramson and J. H. Futrell, J. Phys. Chern. 72, 1994

(1968). 8L. W. Sieck, S. G. Lias, L. Hellner, and P. Ausloos, 1. Res.

Nat!. Bur. Stand. (U.S.) A 76, 115 (1972). We believe these rate constants are still sufficiently uncertain that it would not be useful to atternpt quantitative calculations of reactive depletion effects.

9We find that ion lifetimes are always of the order of (or less than) the time calculated for an ion to undergo a few hundred collisions. See also Ref. 5.

lOAlthough the absolute electron energy was not determined, similar ICR instruments have absolute electron energies that have been estimated to differ from the nominal value by no more than 0.3 eV. See S. E. Buttrill, Jr., J. Am. Chern. Soc. 92, 3560 (1970).

11J. L. Franklin et al., Nat!. Stand. Ref. Data Ser. 26 (1969). 12F. H. Field and 1. L. Franklin, Electron Impact Phenomena

and the Properties of Gaseous Ions (Academic, New York, 1957).

13G. G. Meisels, J. Y. Park, and B. G. Giessner, 1. Am. Chern. Soc. 92, 254 (1970).

14M. J. S. Dewar and S. D. Worley, J. Chern. Phys. 50, 654 (1969).

15We are grateful to a referee for bringing the importance of this point to our attention.

l"fhe energy dependence of reaction (6) may also be studied indirectly by photoionization. Insufficient data were available for comparison with the photodissociation results.

17For a general discussion see M. Vestal, Fundamental Processes in Radiation Chemistry, edited by P. Ausloos (Interscience, New York, 1968), Chap. 3.

18The electron impact for C4H/ from the isobutene cation must be viewed with suspicion. The two studies found an ionization potential 0.36 eV below and 0.60 eV above the accepted photoionization value.

19E. Haselbach and E. Heilbronner, Helv. Chim. Acta 53, 684 (1970)

20Littie is currently known about the efficiency of vibrational quenching by ion-molecule collisions. See, for example, L. Friedman and B. G. Reuben, Adv. Chern. Phys. 19, 80 (1971) for a disscussion.

21See Ref. 8. An earlier, less conclusive study of this question is reported in S. C. Lias and P. Ausloos, J. Am. Chern. Soc. 92, 1840 (1970).

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