Mass analyzed threshold ionization of chlorobenzene and chlorobenzene · Ar1

25 July 1997

ELSEVIER

CHEMICAL PHYSICS LETTERS Chemical Physics Letters 273 (1997) 421-428

Mass analyzed threshold ionization of chlorobenzene and chlorobenzene • Ar 1 Gerhard Lembach, Bernhard Brutschy lnstitut fiir Physikalische und Theoretische Chemie, J. W. Goethe Universiti~t Frankfurt, Marie-Curie-Strafle 11, 60439 Frankfurt / Main, Germany

Received 9 April 1997; in final form 15 May 1997

Abstract Mass analyzed threshold ionization (MATI) spectra of the 35C1 and 37C1 isotopomers of chlorobenzene have been recorded via different vibrational S~ states. Most cationic states exhibit only a small isotopic shift of 2-4 cm-1. The strongest isotope effect is observed for the 6a vibrational mode, which differs by about 6 cm-1 for the two isotopomers. MATI spectra have also been recorded for the Van der Waals complex chlorobenzene - Ar I. Upper limits for the dissociation energies of the complex in its cationic state, as well as in its neutral states S O and S~ could be determined. © 1997 Elsevier Science B.V.

I. Introduction The cationic ground state of chlorobenzene has been the subject of numerous studies [1-4]. Up to now, the most detailed spectroscopic investigation of chlorobenzene in its ionic state has been performed by Wright et al. using zero kinetic energy photoelectron spectroscopy (ZEKE-PES) [5]. By using different vibrational levels in the S~ state as an intermediate state in the resonant two-photon ionization process, they could determine the adiabatic ionization energy and the frequencies of different vibrational modes in the ion with high accuracy. Even though the main topic of our investigations was the fragmentation dynamics of the chlorobenzene. Ar 1Van der Waals (VdW) complex, the MATI spectra of the chlorobenzene monomer also reveal some interesting new results. Due to the small iso-

topic shift of the S l intermediate states, the two isotopomers 35C1 and 3VC1 of chlorobenzene are excited simultaneously in the resonant ionization process. Despite the high spectral resolution of the ZEKE-PES method, Wright et al. did not find any indication of an isotopic shift in the ionization energy or any vibrational frequency. Due to missing mass information, the analysis of the ZEKE spectra is complicated. Since in MATI spectroscopy ions instead of photoelectrons are detected, this technique is particularly suitable for the measurement of isotope effects. The combination of state and mass selectivity also offers the possibility of monitoring the fragmentation dynamics of molecules and clusters. In this Letter, we present threshold ion spectra of the (chlorobenzene. Arl) + VdW complex and its fragmentation product, chlorobenzene +, as well as

0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI1 S0009-261 4(97)00612-X

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G. Lembach, B. Brutschy/ Chemical Physics Letters 273 (1997) 421-428

threshold ion spectra of the free chlorobenzene + monomer. The first appearance of a band in the fragment spectrum gives an upper limit for the dissociation energy of chlorobenzene - Ar 1 in the cationic state. The spectra were obtained by exciting different vibrational states in the S~ intermediate state.

2. E x p e r i m e n t

The experimental apparatus has been described in detail elsewhere [6]. Briefly, a pulsed molecular beam is produced by expanding the sample seeded in 5 bar of helium or argon through a 300 I~m nozzle (General Valve VAC-1250). The skimmed supersonic beam is intersected by the beams of two frequency-doubled dye lasers (Lambda Physik FL 2002), pumped synchronously by an excimer laser (Lambda Physik LPX 200). The calibration of the two dye lasers was carried out by recording iodine absorption spectra and later verified by means of a wavemeter (ATOS, Lambdameter LM 007). The reflectron time of flight (RETOF) mass spectrometer is mounted perpendicular to the axis of the molecular beam. The neutral Rydberg molecules are spatially separated from the simultaneously produced direct ions by a weak electric separation field (0.2-0.8 V / c m ) , applied some 100 ns after the laser excitation pulse. After a separation time of 10-30 Ixs an extraction pulse of 2000 V / c m induces the field ionization of the remaining high-lying, long-lived Rydberg states. Since the directly produced ions and those originating from Rydberg molecules are spatially separated, they are on different potentials and thus are accelerated to different kinetic energies. The directly produced ions can be easily suppressed by a pulsed reflector field in the usually field-free drift region or by an appropriate reflecting voltage at the ion mirror such that only threshold ions are detected. The ion-signal, detected by a set of two multichannel plates, is amplified and passed into a CAMAC based transient recorder (LeCroy TR8828C). The recorded mass spectra are averaged and analyzed by a personal computer. The quoted absolute energies of the adiabatic ionization energies are accurate to + 5 cm -~ while the relative energies (related to the adiabatic ionization energy) are accurate to _+2 cm 1

3. Results

3.1. One color resonant two-photon ionization (1CR2PI) spectra

Fig. 1 shows the (1 + 1) R2PI spectra of chlorobenzene (upper trace) and the chlorobenzene • Ar~ VdW complex (lower trace). Both ion-yield spectra were recorded simultaneously. Bands marked by an asterisk originate from the dissociation of the complex, which is induced by the high excess energy in the ion after one-color R2PI. The S~ origin of chlorobenzene is observed at 37049.4 cm -~. The following MATI spectra of the monomer were recorded via the band origin 0 ° in the S 1 intermediate state and the two narrow spaced transitions at 520.5 and 524.0 c m - 1. The latter have been assigned

CIB+

0o

16a116b 1 6b ~

! 18b I 6a 1 ~

h

•

h,

*

h

1.

(CIB'Ar)+

0o

6b 1 18b I 6a 1 '

I

37000

'

I

r

I

37200

37400

37600

Photon Energy [cml] Fig. 1. 1C-R2PI spectra of chlorobenzene (upper spectrum) and chlorobenzene-ArI (lowerspectrum).Bands originatingfromVdW fragmentation are markedby an asterisk. Hot bands are markedby h.

423

G. Lembach, B. Brutschy / Chemical Physics Letters 273 (1997) 421-428

by Cvita~ and Hollas to a Fermi resonance between the 6b I and 11116a ! combination mode [7]. The combination mode was later reassigned to 16a ~16b 1 by Jain and Bist [8]. The S 1 ~ S O 0 ° transition of the chlorobenzene • Ar I complex is red-shifted by 26 cm-1 relative to that of the isolated monomer and is observed at 37023.3 cm -1 Out of the three intermolecular modes, only the stretching vibration s z (42.6 cm -~) can be identified clearly. The MATI spectra of the complex were recorded by excitation of the S 100 and S16b 1 intermediate states.

3.2. Vibrational spectra of the chlorobenzene cation The MATI spectra of the 35C1 and 37C1 isotopomers of chlorobenzene recorded via the vibrationless $10 ° state are shown in Fig. 2. Also shown in Fig. 2 is the threshold ion spectrum of the phenyl ion (C6H~') , which arises from the unimolecular fragmentation reaction of the chlorobenzene cation due to multiphoton absorption [9,10]. The fact that even in the case of intramolecular fragmentation of the ion core no autoionization takes place and the Rydberg electron remains with the ionic fragment was first demonstrated by Alt et al. [11]. The assignment of the spectra is based on the work of Wright et al. [5] and Asselin et al. [4]. A summary of the observed transitions is given in Table 1. The bands have a width of about 9 cm-1 (FWHM -- full width at half maximum). The band of the vibrationless cationic ground state appears at a two-photon energy of 73170 cm -1. Since the separation of the directly produced ions from the neutral Rydberg molecules is achieved by a weak electric field, the highest Rydberg states are field-ionized during the separation time. The recorded signal is due to the remaining long-lived Rydberg states, located some wavenumbers below the ionization potential. The lowering of the adiabatic ionization potential can be estimated by A E = 4 . 0 f f cm -j [12]. Thus, the measured value has to be corrected by about 3 cm-1 and the field corrected adiabatic ionization energy (AIE) amounts to 73173 + 5 cm -~. This value is within the quoted errors of Wright et al. (73170 _+ 5 cm -1) [5], determined by ZEKE-PES, and Ripoche et al. (73176 _+ 10 cm -1 ) [3], determined from photoionization efficiency spectra. The band at 420 cm ~ (414 cm -1,

Internal Energy [cml ] 0 I

200 '

'

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I

.

400 .

.

.

.

600

.

I

'

800 '

'

I

'

1000 '

'

I

'

1

I

'

'

35CIB+

00

6a ~

i I ~

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6a 2 i 11 1 9al 121 t 18a

i I

•~-, ~ 37CIB+ "~

0o

6a ~ I

~[...~ 'rZ~

6a2

~

1'!-^

I 9al

12l~d~Ta7al.16La?

C6H5 +

0o ] 3 0'0

6a1 ~ '

'

~,,1.~ . . . . t~ AllJ,.t '

'

4 0'0

'

4]0'0

'

Two-Photon Energy [crn-1] Fig. 2. Threshold ion spectra of the 35C1and 37C1isotopomersof chlorobenzene and the phenyl+ fragment,recorded via the S~0° intermediate state.

37C1 isotopomer) above the vibrationless ground state of the cation is assigned to the 6a I vibrational mode. After the excitation of an intermediate state with a 1 vibrational symmetry, it is expected that only totally symmetric vibrations are observable. Furthermore, the propensity rule (A v = 0) favors the transition into a state with the same vibrational mode. Therefore, by excitation via the vibrationless $10 ° intermediate state, the strongest band should appear at the vibrationless cationic ground state. This was also observed by Wright et al. in their ZEKE-PES measurements. The reason for the almost equal intensities of the 0 ° and 6a ~ bands in the MATI spectrum could not be explained. Most of the intense bands above the 6a 1 level are due to vibrational modes of a I symmetry. These are 121 (714 c m - 1 / 7 1 0 cm-1), 11 (971 c m - 1 / 9 6 9 c m - l ) , 18a I (992 c m - I / 9 9 2 c m - l ) , 7a 1 (1115 c m - l / l l l l cm -1) and 9a I (1194

G. Lembach, B. Brutschy / Chemical Physics Letters 273 (1997) 421-428

424

Table 1 Summary of the main bands (in cm - ~ ) in the cationic state of chlorobenzene Mode (Wilson)

Symmetry

So

6a I

a]

121

aI

417 706

6a 2 11 18a t 7a t

at a] as aI

1003 1026 1093

9a I 6a 3 16a i 16b I 18b I 6b s 16al16b I 6aJ6b ] 16aJl6bJ6al

at as a2 bI b2 b2 b~ b2 b2

a

Cation

S i

CI 378.5 671 a

931 a 966 a 1065 a

1153

981 a

403 467 295 615

203 ~ 320 a 287.3 520.5 524

420 714 764 841 971 992 1115 1128 1139 1194 1261 343 393

C1 414 710 830 969 992 1111 1139 1191 1242

35

37

526 736 943 1155 1238

526 734 937 1149

a From Ref. [16].

c m - ] / 1191 c m - l). The first value in brackets corresponds to the vibrational frequency of the 35C1 and the second to that of the 37C1 isotopomer. In addition, also two overtones of the 6a vibration, the 6a 2 (841 c m - 1 / 8 3 0 c m - 1 ) and 6a 3 (1261 c m - I / 1 2 4 2 c m - i ) are observed. At 343 c m - t (not observed in the 37C1 spectrum) appears a relatively weak band which was assigned by Wright et al. (347 cm - l ) to the 16a 1 mode. The appearance of a mode with a 2 symmetry was rationalized by the assumption of a vibronic interaction or a Herzberg-Teller interaction. The MATI spectrum of the phenyl + fragment (lowest trace in Fig. 2) mainly corresponds to the spectrum of the 35C1 isotopomer, due to its greater abundance. A band splitting can only be resolved for the 6a I mode. The relatively sharp peak at 927 cm-~ (marked by an asterisk) in the phenyl + spectrum is ascribed to the Sz0 ° resonance of chlorobenzene. The appearance of an S 1 band in the MATI spectrum is attributed to the incomplete separation of the great number of direct ions, produced while the ionizing laser passes the strong S] ~ S O resonance.

Fig. 3a shows the threshold ion spectra after excitation of the S~6b ~ intermediate state. No ion signal is observed in the region of the AlE. This is due to the different vibrational symmetries of the excited Sl6b ~ intermediate state (b e symmetry) and the vibrationless cationic ground state (a~ symmetry). The first intense band at 526 cm-~ (526 cm -~) is assigned to the 6b ~ mode. Thus, the band at 943 cm ~ (937 cm -1) may be assigned to the 6b~6a ~ combination mode. The band at 736 cm -1 (734 c m - ~) was assigned by Wright et al. to the combination mode 16a 116b ~. By subtracting the frequency of the 16a I mode, a value of 393 cm -~ is obtained for the 16b I vibration. The last two intense bands lie beyond the region investigated by Wright et al. Due to the observed isotopic shift in the band at 1155 c m - ] (1149 c m - l ) of 6 c m - t, a participation of 6a l mode is expected. The difference of 735 cm-~ perfectly corresponds to the frequency of the 16a ~16b ], a mode favored due to Fermi resonance. Thus, this a)

via $16b]

6b 1 16al16b I 6b16a1

o

73200

73400

73600

73800

74000

74200

74400

b)

via Sl16al16b I ~~

6b I

16al16b 1 }

I

}

~

16al16b16al 6b16al '

37CIB+ '

I

'

73200

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73400

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73600

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73800

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I

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74000

'

'

I

'

74200

'

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I

'

74400

T w o - P h o t o n E n e r g y [cm "I] Fig. 3. Threshold ion spectra of the two chlorobenzene lsotopomers with the first laser tuned to (a) the St6b ~ and (b) the 16a s 16b j intermediate state.

G. Lembach, B. Brutschy / Chemical Physics Letters 273 (1997) 421-428

band is assigned to the 16al16b~6a 1 combination mode. The weak band at 1238 cm -~ could not be assigned. Fig. 3b shows the threshold ion spectra recorded via the second component of the Fermi resonance in the intermediate state (Fig. 1), which was assigned to the 16a 116b I combination mode. The strongest band at 736 cm -~ (736 cm -~) is attributed to 16a~16b 1 in the cation. The relative increase in the intensity of the band at 1155 cm -~ (1149 cm -1) supports the preceding assignment to the 16a ~16b ~6a ~ combination mode. This assignment is also supported by the photoelectron spectra of Asselin et al. [4].

Internal Energy [cm "1] 0 I

200 '

,

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400 '

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600 '

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•

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800

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•

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1000 '

•

'

I° ~ bxn

I

121)0

'

•

•

I

'

(35CIB'Ar) +

6a 1

\l~ b n

IJi

I, 9a 1

3 5 C l B

3.3. Threshold ion spectra o f (chlorobenzene . Ar]) ÷

The threshold ion spectra of (chlorobenzene. Ar~) + recorded via the vibrationless S~0 ° level are shown in Fig. 4. A summary of the identified vibrations is given in Table 2. The threshold ion spectra of this complex are characterized by low frequency vibrational progressions superposed on all intramolecular modes of chlorobenzene. Analogous to the fluorobenzene- Ar~ complex, these progressions are due to the intermolecular bending vibration b x in the symmetry plane crh of the complex. It is excited due to a change in the binding potential of the ionized complex. Up to 3 quanta of the bending mode appear at the AlE, exhibiting a vibrational spacing of about 15 cm- 1, with an average width of 5 c m - l (FWHM). In contrast to the Franck-Condon pattern observed for cationic states of fluorobenzene • Ar~, the maximum in each progression lies at the first band. The most striking characteristic of the threshold ion spectrum of the complex is that only the vibrationless ground state and the 6a 2 intramolecular vibration appear in the mass channel of the complex. The bands above 6a I (420.6 cm -1) only appear in the mass channel of the chlorobenzene ÷ fragment. The 0 ° transition into the vibrationless cationic ground state is observed at 35958 cm - l , giving a field-corrected AlE of 72984 + 5 cm-~. Thus, the AIE of the complex is lowered by 189 c m - 1 relative to the AIE of the monomer. The observed bands in the threshold ion spectrum of the fragment chlorobenzene ÷ are those missing in the spectrum of the complex. They are assigned to the intramolecular modes 121 (714 cm l), 11 (971 cm-1), 9a I (1192

425

+

1

11

12

6a 2 /

.

I

t

!6a

C6H5 +

'

I

*

'

'

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,

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,

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,

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'

73000 73200 73400 73600 73800 74000 74200 Two-Photon Energy [cm 1] Fig. 4. Threshold ion spectra of the chlorobenzene. Ar I complex recorded via the S~0 ° state. The upper spectrum was recorded at the mass of the complex while the spectra in the middle and the lower part were recorded in the mass channels of the chlorobenzene + and phenyl + fragments, respectively.

Table 2 Summary of the main bands (in cm 1) in the cationic states of (chlorobenzene. Arj)+ and its chlorobenzene + fragment Mode (Wilson) Symmetry S l

Cation

C6H35CI.Ar I C6H35CI.Arl C6H35C1 6a 1 121 6a 2 11 * 9a I 6a 3 6b I 6bl6a I bx s~

aI aI a1 a1 aI aj b2 b2

378

522

420.6

526 15

42.6

714 840 971 1091 1192 1261 526 942

G. Lembach, B. Brutschy / Chemical Physics Letters 273 (1997) 421-428

426

c m - 1), and two overtones of the 6a mode at 840 and 1261 cm -1, respectively. Due to the increased number of vibrational states resulting from the low vibrational progressions of the intermolecular bending mode b x, the spectrum looks congested between the 11 and 9a ~ vibrations. The MATI spectrum of the phenyl + fragment exhibits in addition to the 0 ° and 6a~ transitions, which are due to multiphoton processes, a single band (marked by an asterisk) at 1091 cm - I , which is also found in the spectrum of chlorobenzene ÷ fragment. By comparing its one-color band position of 37049 cm-1 (ionizing laser) with bands observed in the R2PI spectra (S 1 ~ So), this band may be ascribed to the strong S 1 ~--S 0 0 0 transition in the chlorobenzene monomer followed by direct ionization, as mentioned in the previous section. Internal Energy [cmI] 0 '

I

200 '

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400 '

'

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600 .

.

.

.

.

800

.

I

1000 '

'

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6b 1'

I

'

1200 '

'

I

'

'

(35CIB.Ar)+

Fig. 5 shows the threshold ion spectra of the (chlorobenzene .Arl) + complex and its fragments after excitation of the $16b 1 intermediate state. Only a weak band is observed at the energy of the vibrationless ground state of the cationic complex. The strongest band in the (chlorobenzene. Arl) ÷ spectrum is observed at 526 cm-~ and is assigned to the 6b ~ vibration. The next band above 6b 1 appears in the fragment spectrum and is due to the combination 6b16a I mode (942 cm-~). A very weak feature is observed in the region of the 6b 1 vibration which is likely due to multiphoton absorption. However, a field-induced fragmentation of the clusters, as discussed recently by Grebner et al., cannot be excluded [ 13]. It should be pointed out that the spectra recorded in the phenyl ÷ mass channel in Fig. 4 and 5 correspond to the band structure of the (chlorobenzene. Ar~)+ complex. This is only possible if the Rydberg electron both survives the inter- and intramolecular fragmentation without being autodetached. This finding gives further evidence for the quasi complete decoupling of the Rydberg electron and the ion core.

4. Discussion

00

4.1. Chlorobenzene

.,J± .......... ,h.L-Jl ,ll~=i Jatl...,J.,,~.Alfl~.~.~L~,. ~k~ 6b16a I

'

I

'

73000

'

'

I

'

73200

'

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I

'

73400

'

'

I

'

73600

'

'

I

'

73800

'

'

I

'

74000

'

'

I

'

74200

T w o - P h o t o n Energy [cm 1] Fig. 5. Threshold ion spectra of the chlorobenzene.Ar I complex recorded via the Si6b I state. The upper spectrum was recorded at the mass of the complex while the spectra in the middle and lower parts were recorded in the mass channels of the chlorobenzene ÷ and phenyl + fragments, respectively.

As shown by Morlet-Savary et al., the S I band origins of both isotopomers of chlorobenzene lie within 0.2 cm -j [14]. To maximize the signal-tonoise ratio, the intermediate states were excited at the maximum of the S 1 bands of the 35C1 isotopomer. Due to the small isotopic shift, also a large portion of the 37C1 isotopomer was excited. Since MATI spectroscopy is mass-selective, non-isotope selective excitation has no effect on isotope selectivity in cationic states. Thus, in contrast to ZEKE-PES no spectral broadening due to unresolved isotope effects are expected. In their extensive ZEKE-PES investigations on cationic chlorobenzene, Wright et al. could not find any indication of an isotopic shift in the ionization energy or any vibrational frequency [5]. Also in this work, no isotope induced shift in the adiabatic ionization energy could be observed within experimental resolution. On the other hand, the MATI spectra clearly exhibit a small shift of 2 - 4 cm-1 for most of the vibrational modes. The red-shift of the

G. Lembach, B. Brutschy / Chemical Physics Letters 273 (1997) 421-428

bands in the threshold ion spectrum of the 37C1 isotopomer is due to the increased mass of the chlorine atom. Since the participation of the chlorine atom is different in different vibrational modes, the isotopic shift is mode specific. The isotope effect is particularly strong for the 6a vibration, which is shifted by about 6 cm - I . The width of the ZEKE peaks observed by Wright et al. had been about 5 cm-~ (FWHM) at best (low extraction fields). Since in ZEKE-PES the spectroscopy of cationic states is performed by the detection of (threshold) photoelectrons, both isotopomers contribute to the ZEKE spectra and thus cause a broadening of the observed ZEKE bands. However, in the case of the 6a mode, the isotope effect is strong enough that at least the first 6a 2 overtone should be resolved in the ZEKE spectra. In the ZEKE spectrum recorded via the $10° intermediate state, Wright et al. observed a band at 833 c m - ~ just below 6a 2, which they assigned to the 6b ~18b ~ combination mode. This band does not appear in the MATI spectrum of the 35C1 isotopomer (upper trace in Fig. 2). By comparing the frequencies in the MATI spectra of the two isotopomers it becomes clear that this band arises from the shifted 6a 2 vibration of the 37C1 isotopomer. 4.2. C h l o r o b e n z e n e • A r I

The threshold ion spectra of the chlorobenzene. Ar~ complex are characterized by low frequency vibrational progressions with up to 3 quanta of the intermolecular bending mode. The appearance of these progressions indicates that a significant structural change in the complex takes place upon ionization. This is due to strengthening of the intermolecular bond, induced by the additional interaction of the positive charge localized in the chlorobenzene and the polarizable argon atom. It is interesting to compare these progressions with those observed for the fluorobenzene • Ar~ complex. While the latter cluster also exhibits at least 3 quanta of the b x VdW mode, the maximum in these progressions was observed at the second quanta of the bending mode. The fact that in the MATI spectra of chlorobenzene • Ar~ the maximum is observed at the origin of each progression indicates a smaller structural change compared to the fluorobenzene. Ar 1 complex. This is supported by the smaller red-shift of the AIE upon complexation

427

of about 189 cm -~ compared to 223 cm -~ for the fluorobenzene • Ar I complex. The first significant threshold ion signal in the fragmentation spectra marks an upper limit for the dissociation threshold of the complex in the cationic state. In the threshold ion spectrum of the chlorobenzene ÷ fragment recorded via the S~0 ° intermediate state, the first band appears at 714 cm -~ which is assigned to the 12 ~ vibration (see Fig. 4). After excitation of the 6b ~ vibration in the intermediate state, the first significant ion signal is observed at 942 cm -~ and is due to the 6b~6a ~ combination mode (see Fig. 5). A weak band appears at the band position of the 6b ~ mode. One reason for the simultaneous appearance of an ion signal in the threshold ion spectra of the complex and the fragment product could be a multiphoton process which would result in a highly excited ion core [15]. Another possible explanation for this characteristic could be the recently discussed coupling of Rydberg states which belong to Rydberg series converting to different vibrational states [13]. For the reasons stated above and the fact that the ion signal from the 6b ~ mode in the fragment channel is only weak, the dissociation threshold for the cationic complex is supposed to lie somewhere between 6b ~ and 12 ~. Thus, an upper limit for the dissociation energy D+ in the cation is 714 cm -1. The dissociation energy D O in the neutral electronic ground state may be derived from the relation D O= D+ + AIEcomplex - AIEMo. . . . . or D O= D+ - AAIE, where AAIE = AIEMonomer - AIEcnmplex represents the shift (lowering) in the ionization energy upon complexation. With a red-shift of the AIE of 189 cm-~ we find an upper limit for D O of 525 cm -1. An upper limit for the dissociation energy D~ in the electronically excited intermediate S~ state is given by the relation D l = D + - A h u 2, where A h u 2 denotes the difference in the ionization energies of the monomer and the complex relative to the corresponding S~0 ° states. With A h u 2 ( C I B . A L, CLB) = 163 cm -~ we find an upper limit for D~ of 551

c m - 1. 5. Conclusions In this work, we presented the threshold ion spectra of chlorobenzene and of the chlorobenzene • Ar~

428

G. Lembach, B. Brutschy / Chemical Physics Letters 273 (1997)421-428

complex, recorded via different vibrational levels of the S~ state. In contrast to the ZEKE-PES study of Wright et al. [5], with MATI spectroscopy we could find small isotopic shifts in the vibrational modes of the ionic state, which amount to 6 cm -~. Upper limits for the dissociation energies of the Van der Waals complex could be determined from the appearance of a threshold ion signal in the fragment channel, originating from cationic states of the complex. An upper limit for the dissociation energy D+ in the cationic ground state is given by the 12 ~ vibration with 714 cm -~. For the neutral complex, the dissociation energies, D O in the electronic ground state S O and D 1 in the electronically excited S t state are estimated to be lower than 525 and 551 c m - 1 , respectively.

Acknowledgements This work was sponsored by the DFG-Schwerpunkt "Molekulare Cluster". The authors also gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft, the Fond der Chemie and the J.W. Goethe Universit~t Frankfurt and Dr. A. Steiger for his technical assistance.

References [1] S.L. Anderson, D.M. Rider, R.N. Zare, Chem. Phys. Lett. 93 (1982) 11. [2] K. Walter, K. Scherm, U. Boesl, J. Phys. Chem. 95 (1991) 1188. [3] X. Ripoche, P. Asselin, F. Piuzzi, I. Dimicoli, Chem. Phys. 175 (1993) 379. [4] P. Asselin, A. Gouzerh, F. Piuzzi, I. Dimicoli, Chem. Phys. 175 (1993) 387. [5] T.G. Wright, S.I. Panov, T.A. Miller, J. Chem. Phys. 102 (1995) 4793. [6] G. Lembach, B. Brutschy, J. Phys. Chem. 100 (1996) 19758. [7] T. CvitaL J.M. Hollas, Mol. Phys. 18 (1970) 101. [8] Y.S. Jain, H.D. Bist, J. Mol. Spectrosc. 47 (1973) 126. [9] J.L. Durant, D.M. Rider, S.L. Anderson, F.D. Proch, R.N. Zare, J. Chem. Phys. 80 (1983) 1817. [10] X. Ripoche, I. Dimicoli, J. Le Calv6, F. Piuzzi, R. Botter, Chem. Phys. 124 (1988) 305. [ll] C.E. Alt, W.G. Scherzer, H.L. Selzle, E.W. Schlag, Chem. Phys. Lett. 224 (1994) 366. [12] W.A. Chupka, J. Chem. Phys. 98 (1993) 4520. [13] Th.L. Grebner, P.v. Unold, H.J. Neusser, J. Phys. Chem. A 101 (1997) 158. [ 14] F. Morlet-Savary, C. Cossart-Magos, I. Dimicoli, J. Le Calv6, M. Mons, F. Piuzzi, Chem. Phys. 142 (1990) 219. [15] H. Krause, H.J. Neusser, J. Chem. Phys. 99 (1993) 6278. [16] H.D. Bist, V.N. Sarin, A. Ojha, Y.S. Jain, Appl. Spectrosc. 24 (1970) 292; H.D. Bist, V.N. Satin, A. Ojha, Y.S. Jain, Spectrochim. Acta A 26 (1970) 841; Y.S. Jain, H.D. Bist, J. Mol. Spectrosc. 47 (1973) 126