Two-color threshold photoionization spectroscopy of jet-cooled indole clusters

Chemical Physics 105 (1986) 397-416 North-Holland, Amsterdam

397

TWO-COLOR THRESHOLD PHOTOIONIZATION OF JET-COOLED INDOLE CLUSTERS James HAGER, Department

Michael IVANCO

of Chemistry,

SPECIROSCOPY

‘, Mark A. SMITH ’ and Stephen C. WALLACE

University of Toronto, Toronto, Canada MS.9 IA1

Received 4 October 1985; in final form 7 February 1986

We report the results of a comprehensive investigation of the two-color threshold photoionixation of jet-cooled indole clusters. Using two-color photoioniaation spectroscopy, we have probed both the neutral excited levels and the ground ionic states of indole clusters containing non-polar (Ar, CH,, CF,, CsH,) and polar (H,O, MeOH, EtOH, NH,, N(CH,),) solvent species These. studies have allowed the determination of accurate cluster ionization energies (If%) as well as the assignment of electronic absorption featurea to clusters of known composition. The determination of the cluster IF, which is typically lower than that of bare indole, has allowed us to investigate the importance of charge-induced dipole and charge-dipole attractive forces in the binding of the ion-neutral clusters. In addition, we have found that the shape of the photoionixation efficiency (PIE) spectra gives valuable information regarding the relative shape and/or position of the potential energy surfaces of the neutral excited and ground ionic states of the clusters. We have also identified two distinct conformational isomers of the indole-(H,O), hydrogen bonded cluster using the techniques of electronic spectroscopy, two-color threshold ionization spectroscopy and maas analysis.

1. Introduction The study of neutral organic van der Waals clusters prepared in a supersonic expansion has been of significant interest for several years. By adjusting the expansion conditions and the constituents in the beam, one can prepare a substantial fraction of the parent organic molecule as van der Waals complexes. Thus, one has the opportunity of studying isolated clusters of a particular parent molecule associated with almost any number of solvent species that range from helium atoms to chemically relevant molecules such as

water, alcohols, and amines. Spectroscopic and dynamic studies of these unique species have provided a wealth of information regarding the nature of pairwise intermolecular forces and the early stages of solvation and nucleation [l]. Neutral van der Waals molecules have also been shown [2] to be excellent precursors for the ’ Present address: Atomic Energy Canada Ltd., Chalk River Nuclear Laboratories, Canada. * Permanent address: Department of Chemistry, University of Arizona, USA.

preparation of solvated positively charged ions. If, after the formation of the weakly bound neutral cluster, the parent molecule is photoionized, one has the opportunity to investigate the interactions between a cation and a neutral solvent molecule in the same isolated environment. Since the solvent structure around a molecular ion often determines the chemical reactivity of the charged species, it is of considerable interest to investigate the strength of such interactions in systems of well-characterized composition and energy as well as being chemically relevant. Such a study would involve the preparation of a molecular ion in a well-defined energy state and the measurement of the state and the measurement of the stabilization energy as a function of the number and chemical nature of the solvent molecules. We have used neutral van der Waals molecules produced in a supersonic jet as precursors for the production of positively charged van der Waals and molecular cluster ions. The presence of a charged center within the cluster allows for much stronger intermolecular interactions than observed for the neutral counterparts. The ionic clusters are characterized [3] by charge-induced dipole and

0301-0104/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

398

J. Hager et al. / Spectroscopy ofjet-cooled

charge-dipole interactions in addition to the dipole-induced dipole, dipole-dipole, and dispersion prevalent in the neutral precursors. Consequently, by studying both neutral and ionic clusters for the same molecular systems, one can begin to identify the magnitude of these additional ionic interaction terms [4,5]. The experimental technique we have employed in these investigations is two-color threshold photoionization spectroscopy [4-81. By choosing individual electronic resonances with the pump laser, one is able to ionize selected clusters or monomers in the supersonic expansion. Mass analysis of the resultant ions allows the unambiguous identification of the composition of the species responsible for each of the electronic absorption features. This is very important when one is investigating clusters of organic molecules with species such as water and alcohols where indirect methods of cluster composition determination can be ambiguous. This technique also provides valuable information regarding the intermolecular interactions between complicated complexing partners, such as alcohols and amines, and aromatic chromophores. This allows one to begin to develope a picture of the cluster geometries that, in these instances, have not been accessible by other means. Our approach [4-71 has been to fix the frequency of the pump laser at specific electronic absorption features and tune the ionizing laser from below the ionization onset to deeper into the ionization continuum while monitoring the ion signal. The resulting photoionization efficiency (PIE) spectrum provides a measure of the cluster or monomer ionization threshold. Typically, the cluster ionization onset is at lower energy than that of the bare probe molecule and provides a measure of the additional stabilization energy of the cluster ion with respect to the neutral precursor [3]. The additional stabilization energy of the cluster ion can be substantial, in some cases amounting to as much as several times the binding energy of the neutral complex [S]. Therefore, the Franck-Condon factors for ionization will not necessarily favor the lowest energy transition [9], i.e. to the adiabatic ionization potential. This is in contrast to many isolated molecules that we [5-71

indoie clusters

and others [8] have investigated in which Au = 0 ionizing transitions from electronically excited states are favored. One can, however, use this deviation from monomer-like behavior as an indication of the degree to which the intermolecular interactions, and thus the potential surfaces, change upon photoionization [lo]. For small changes there should be a sharp step-like rise in the ion signal at threshold similar to that seen for isolated molecules. Larger changes will be manifested by broader and more diffuse onsets. This is expected to occur for complexes containing polar solvent molecules which are strongly stabilized by charge-dipole and charge-induced dipole forces ]4,5]. In this paper, we describe the application of these techniques to indole molecular clusters. The importance of indole can be found in its relationship to protein photochemistry and its unusual condensed phase solvation and photoionization dynamics [ll]. In addition, there now exists a substantial body of information regarding neutral indole clusters prepared in a supersonic expansion. Work in our laboratory [12,13,16] and others [14,15] has served to characterize many of these neutral clusters and the relevant intermolecular interactions ranging from dispersion interactions to hydrogen bonding. Our preliminary threshold photoionization studies [4] have shown that neutral indole complexes are excellent precursors for the formation of ionic clusters. In what follows, we describe the results of a comprehensive study of the two-color photoionization of complexes of indole with nine solvent species and examine the differences in the intermolecular interactions between the neutral and positively charged clusters.

2. Experimental The experiments described here involve the use of fluorescence excitation, one-color multiphoton ionization (MPI), two-color threshold photoionization spectroscopy, and mass analysis techniques. The supersonic jet chamber and fluorescence detection/signal processing apparatus have been described previously (12,131. For the laser-induced fluorescence studies, a continuous helium free jet,

J. Hager et al. / Spectroscopy of jet-cooled inable clusters

seeded with indole vapor and the appropriate clustering molecule, was crossed with the frequency doubled (Quanta Ray WEX) light of a Nd : YAG (Quanta Ray DCR-1A) pumped dye laser (Quanta Ray PDL-1). The linewidth in the UV was = 0.2 cm-’ and pulse energies were measured to be greater than 5 mJ which could be reduced using neutral density filters. The apparatus for the two-color photoionization studies has also been described in detail in previous publications [6,7]. In the present study, the intermediate electronic level was prepared by light from the same Nd : YAG pumped dye laser system described above. The laser power of this system was maintained at low levels (in order to reduce one-color ionization signal) by inserting neutral density filters in the beam or by turning down the flash lamp power on the amplifier of the Nd : YAG laser. For the ionization step, we have used light from a XeCl excimer pumped dye laser (Lumonics 860-T with an EPD-330) providing 7 ns pulses of 5-10 mJ per pulse. Temporal overlap of the lasers and power normalization was done as described earlier [6,7]. Ions produced in the twolaser overlap region are accelerated into the quadrupole mass spectrometer housing with a very small accelerating voltage ( c 1 V/cm) where they were focused and analyzed. This small accelerating voltage had no effect on any of the threshold positions or breadths reported here. Experimental uncertainties for the ionization thresholds are obtained from the deviation from the average value for at least three spectra. For the one-color MPI experiments, only the Nd : YAG pumped dye laser system was employed. Operating at higher pulse energies than those used in the two-color experiments, we have been able to observe intense one-color ionization signal when desired. Indole (99 + 46) was obtained from Aldrich and used without further purification. The samples of the various complexing partners were obtained as previously reported [12,13]. 3. Results The results presented in this section are concerned with the electronic spectroscopy and

399

threshold photoionization spectroscopy of a number of indole clusters. The electronic spectra of interest here are the lowest energy cluster transitions to the first excited electronic state near the isolated indole band origin at 35232 cm-‘. The two-color threshold photoionization spectra have been obtained by fixing the first laser on a cluster resonance in the excited electronic state and tuning the second laser near the bare molecule ionization potential while collecting the resulting ions. Mass analysis of the cluster ions produced by two-color photoionization was also carried out in order to determine the composition of the clusters responsible for individual absorption features. First to be presented are the results of the clusters involving indole and the non-polar species argon, methane, CF,, and benzene. Following this, the data obtained for the more strongly bound indole-polar molecule clusters are described. 3.1. Indole-non-polar solvents

3.1.1. Argon, methane, perfuoromethane The electronic spectra of the In-Ar,

In-CH,, and In-CF, clusters are shown in fig. 1. For the argon and methane species, the cluster spectral shifts are to the red of the associated bare molecule transition as is commonly observed for van der Waals molecules involving an organic chromophore and a non-polar complexing partner. The shifts are - 26 and - 53 cm-’ for In-(Ar), and In-@), respectively, while the methane shifts are slightly larger being - 36 cm-’ (In-(CH,),) and -74 cm-’ (In-(CH,),). These energetics are consistent with dispersion forces being the dominant contribution to the intermolecular interactions as discussed elsewhere [12]. In contrast to this rather simple picture for the argon and methane van der Waals molecules are the results for In-CF,. As shown in fig. lc, the cluster shift is to higher energies for this complex, in contrast to expectations based on dispersion interactions alone, being 10 cm-’ per added CF., molecule. This fact has been discussed earlier [16] and is apparently due to an excited state repulsive effect between the fluorine lone pair electrons and the aromatic a-electrons of the indole molecule. Despite the differences in the magnitude and

400

J. Hager et al. / Spectroscopy ofjet-cooled

o-o

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RELATIVE

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Fig. 1. Fluorescence excitation spectra of (a) indole-argon, (b) indole-methane, and (c) indole-perfluoromethane near the indole S, origin. The asterisks in the indole-perfluoromethane spectrum mark bands assigned to intermolecular vibrations of the 1: 1 cluster. In (a) there is a small amount of indole-helium present due to the high backing pressure of the argon/helium mixture used to obtain this spectrum.

direction of these cluster shifts, it should be noted that they are approximately additive for the first two complexing species. This suggests [17] that there are two geometrically equivalent binding sites which are most probably above and below

I

indole clusters

the plane defined by the indole ring system. Recent potential surface calculations based on atom-atom interactions confirm [18] these expectations with the binding sites being associated with the benzene ring. Preliminary photoionization results for the In-Ar,,, and the In-(CH,),,, clusters have been reported in an earlier paper [4]. A brief summary of the results is included here for comparison purposes. Fig. 2 displays the two-color photoionization efficiency (PIE) spectra of these four van der Waals species obtained with the first laser tuned to the band origin of each of these systems. These spectra all exhibit very well-defined ionization thresholds red shifted from the bare indole adiabatic ionization potential [4] (62598 f 5 cm-‘). For the argon clusters, these shifts are -209 cm-’ for In-Ar, and -399 cm-’ for In-Ar,, both with respect to the ionization energy of the bare molecule. The methane van der Waals systems exhibit a larger change in IE than observed for the argon clusters. Here, the ionization threshold for In-(CH,), is shifted by -487 cm-’ and that of In-(CH,), by -900 cm-‘. This behavior is consistent with a picture in which the change in ionization energies is a function of the polarizability of the complexing species [3]. Deviations from this simple behavior, in which the change in the clustering ionization energy follows the polarizability of the clustering partner, have been observed in the In-CF, photoionization results. Since CF, is considerably more polarizable than either argon or methane, one might expect to observe a greater decrease of the indole IE for this system. The PIE spectrum displayed in fig. 3 was measured under conditions in which the electronic origin of the In-(CF,), cluster was selected by the first laser. For this cluster, the ionization threshold was found to be 702 cm-’ to higher energy than that of bare indole. In addition, the ionization onset is substantially broader than those presented to this point. The ionization thresholds for the indole monomer, In-@),, and In-(CH,), are = lo-13 cm-’ in width while those of In-(Ar), and In-(CH,), are = 15-25 cm-‘. One should note from fig. 3, however, that the ionization threshold for the CF, cluster is = 300 cm-’ in width.

AIE=-X)9

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Fig. 2. Two-color PIE spectra of (a) In-(Ar),, (b) In+&, (c) In-(CH,), and (d) In-(CH,),. These have been obtained by fixing X1 at the wavelength corresponding to the electronic origin of the respective cluster. The changes of the ionization energy of the indole monomer due to complex formation are indicated for each cluster.

J. Hager et al. / Spectroscopy ojjet-cooled

402

indole clusters

AI E = ~702 cm-’ In -CF,

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Fig. 3. Two-color PIE spectrum of In-(CF,), obtained with A, at the cluster electronic origin and A, tuned near the adiabatic IP of the indole monomer. Notice the breadth of the cluster ionization onset compared to the previous spectra in fig. 2.

We have also obtained a series of mass spectra of the In-(CF,), cluster at several different ionizing laser frequencies. Fig. 4 presents three of these mass spectra taken under different two-color conditions. When the ionizing laser is tuned z.1600 cm-’ above the In-CF, IE, the mass spectrum shown in fig. 4a was obtained. As one can see, there is no peak corresponding to the production I”’

(c)

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175 MASS

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Fig. 4. Mass spectra of the In-(CF,), cluster obtained under different two-color ionization conditions. Here, AI was fixed at the cluster electronic origin and A, at (a) 337 nm, (b) 345 nm, and (c) 353 mn.

of In+--CF,. Fig. 4b displays a similar spectrum, measured when the two-color energy was fixed 5: 900 cm-’ above the cluster threshold IE. Again, there is no detectable amount of the cluster ion under these conditions. Finally, as shown in fig. 4c, we have observed a small amount of In+-CF, when the ionizing laser is fixed at = 275 cm-’ above threshold, although fragmentation to In+ is still the dominant pathway. It should be emphasized that for these experiments the ionizing laser light was only weakly focused and the ion signal depended linearly on the intensities of both lasers. In addition, we observe no evidence of fragmentation in the two-color photoionization mass spectra of the other clusters discussed to this point. These mass spectroscopic results clearly demonstrate how two-color photoionization of monomers and clusters can differ. For most monomers studied to date [5-Q, there is a strong propensity for Au = 0 ionizing transitions because the geometry of the parent ion tends to be very similar to the neutral excited state. Such a photoionization process produces a vibrationally cold ion which cannot fragment, with the excess energy of ionization going into electron kinetic energy. The fact that we observe only small amounts of cluster ion compared with fragmentation products at ionization energies so close to the measured ionization onset means that ionizing transitions from the electronically excited levels favor large changes in

J. Hager et al / Spectrmcopy

vibrational excitation of the cluster. This makes fragmentation subsequent to ionization a much more likely phenomenon. In this section, we have presented three important results that are extremely useful in characterizing the In-(CF,), cluster: (a) the energy of the two-color PIE threshold, (b) the breadth of the two-color PIE threshold, and (c) the fragmentation pattern and characteristics of the cluster ion near the two-color ionization onset. The implications of these results are discussed below. 3. I. 2. Benzene Cluster formation of an aromatic molecule with benzene is expected to give rise to much stronger interactions than with argon or methane based on polarizability arguments. In fact, it has been shown that species containing an acidic hydrogen can form a hydrogen bond with the benzene a-electron system [13,19,20]. Such a-hydrogen bonding has been documented to give much weaker interactions than those of the same proton donors interacting with, for example, amines, being characterized by roughly less than half the bond dissociation energy [20]. Jet-cooled indole [13-151

403

of jet-cooled inahle clusters

and phenol [19] have been found to form this type of hydrogen bond in both the neutral ground and first excited electronic states. This section describes the results of a detailed investigation of indole-benzene in the neutral A and ionic 3 electronic states. Fig. 5 displays the one-color MPI spectrum of the indole-benzene system displaced to lower energies from the indole band origin. The electronic origin of In-(benzene), has been located at - 166 cm-’ with respect to the bare indole O-O transition [13]. There are several smaller bands at higher energies which can be assigned to intermolecular vibrations of the 1: 1 complex. The In-(benzene), vibrational frequencies have been measured to be 12, 30, and 72 cm-‘. The 72 cm-’ interval is roughly that expected for a weak hydrogen bond stretching frequency, while the two lower energy vibrations are probably due to intermolecular bending and/or rocking motions within the cluster. We have also identified the presence of In-(benzene), in this spectrum by mass analysis of the cluster ions produced by two-color photoionization. The band origin of this system was

In -Benzene

*

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0

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1

-100 RELATIVE

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-200 ENERGY

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Fig. 5. Onacolor MPI spectrum of In-(benzene), clusters near the indole origin. The band origin of In-(benzene), peak at - 166 cm-‘. The features marked with asterisks are due to In-(benzene),.

is assigned to the

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J. Hager et al. / Spectroscopy of jet-cooled indole clusters

identified at 183 cm-’ to the red of the indole O-O transition and 17 cm-’ to the red of the In-(benzene)t electronic origin. Thus, the addition of the second benzene complexing partner leads to only a 10% increase in the electronic spectral shift of the singly solvated species. In addition, vibrational frequencies of 13 and 71 cm-’ have been observed for the In-(benzene), cluster. These intervals are quite similar to two of those obtained for the 1 : 1 cluster. Thus, the attachment of a second benzene molecule to the 1: 1 complex does not affect the intermolecular vibrational frequencies of the singly solvated species. These results are consistent with a picture in which the first benzene molecule interacts strongly with the indole molecule through the formation of a n-hydrogen bond, and the second interacts much more weakly, probably with the indole ring system. Table 1 provides a listing of the band positions and assignments for the indole-benzene clusters. The two-color PIE spectra of In-(benzene), and In-(benzene), have been measured and show much larger changes in ionization potential than those observed for the argon and methane species. By fixing A, at the In-(benzene), electronic origin and tuning A, in the region near the IP, the PIE spectrum shown in fig. 6a was obtained. There are two major differences between this spectrum and those obtained for simpler clusters of indole asso-

Table 1 Band positions and assignments for indole-benzene clusters Electronic spectral shift (cm-‘)

Assignment

-166 -154 -136 -94 - 183 -170 - 113

In-(benzene),; Y, (12 cm-‘) v2 (30 cm-‘) vj (72 cm-‘) In-(benzene),; vt (13 cm-‘) Y* (70 cm-‘)

O-O

O-O

ciated with argon and methane. Firstly, the ionization threshold shows a substantially greater red shift, with the IE being 2632 cm-’ to lower energies than that of the indole bare molecule. This result is consistent with the much greater polarizability of benzene, compared with methane or argon, and illustrates the magnitude of charge-induced dipole interactions. This stabilization energy reflects an ionic cluster that is = 2500 cm-’ more tightly bound than the excited state neutral species. Secondly, this PIE spectrum is characterized by an ionization onset which is somewhat broader than those associated with the argon and methane clusters, being 2: 50 cm-‘. This is, however, substantially sharper than the = 300 cm-‘observed in the In-CF, PIE spectrum.

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Fig. 6. Two-color PIE spectra of (a) In-(benzene)t electronic origin of the respective cluster (table 1).

and (b) In-(benzene),.

These spectra were obtained with X, fixed at the

.f. Hager et al. / Specrrosco~y of jet-cooled indole clusters

Similar photoionization efficiency measurements of the In-(benzene), molecular cluster have been performed as is shown in fig. 6b. The ionization threshold associated with this 1: 2 complex is

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shifted by - 2908 cm-’ relative to the indole adiabatic IP. Thus, there is only an additional 276 cm-’ red shift from the In-(benzene), IE when a second benzene mokcule is added to the complex.

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406

J. Hager et al. / Spectroscopy ofiet-cooled

One should also note the much broader ionization onset than that observed in the PIE spectrum of In-(benzene),. 3.2. Indole-polar

solvents

3.2.1. In-(ROH), In this section, we present the experimental data obtained on jet-cooled indole-(ROH), clusters, where R = H, Me, Et, examined in both the electronically excited and ground ionic states. First, we consider the electronic spectra. Fig. 7 depicts the one-color MPI spectra of indole complexed with water, methanol, and ethanol. The most intense features in each of these three spectra are the band origins of the 1: 1 hydrogen bonded cluster previously identified by ourselves [13] and others [14,15]. We have recently shown [13] that these electronic spectral shifts of hydrogen bonded indole clusters are linearly related to the respective proton affinity of the complexing base, demonstrating the acid-base nature of these hydrogen bonded systems. In the lower energy regions of these spectra, there is further structure which exhibits the same concentration dependence of the proton accepting molecule as the more prominent features. As can be seen in fig. 7, this structure is sharp and is particularly regular in the In-(H,O) spectrum, exhibiting two long 34 cm-’ progressions. The In-(MeOH) and In-(EtOH) spectra in this energy region are comprised of many low-frequency transitions not readily assignable to a simple progression. We have chosen to concentrate more on the water and methanol species due to the relative simplicity of these two spectra. Transition energies for indole-water and indole-methanol are listed in table 2. In order to determine unambiguously the composition of the clusters associated with these spectral features, we have obtained the two-color mass spectra of the water and methanol cluster bands in this far red shifted region. As one can see from fig. 8, there is no evidence of the presence of In-(ROH), clusters in this spectral region, or any fragmentation leading to production of In+. These results confirm that the far red shifted transitions associated with the indole-water and indolemethanol spectra also arise from excitation of 1: 1

indole clusters

Table 2 Band positions indole-methanol

and assignments clusters

for

indole-water

Clustering partner

Electronic spectral shift (cm-‘)

Assignment

water

- 135 - 110 - 452 -418 -404 - 384 - 369 - 348 -333 -314 - 299 - 280 - 261 - 248 -160 - 137 - 112 - 412 - 436 - 429 -419 - 398 - 392 - 386 - 382 -69 -55 -41 -20

In-(H,O)I; O-O Y, (25 cm-‘) In-(H,O), *@; O-O Y, (34 cm-‘) v2 (48 cm-‘)

methanol

and

2v, VI + v2 3% 2v, + v2 4Vl 3v, + v2 % 4v, + v2 6~1

In-(MeOH),; O-O v, (25 cm-‘) 2Vl In-(MeOH), *; O-O v1 (36 cm-‘) v2 (43 cm-‘) vg (53 cm-‘) 2Vl _ 2v2

In-(Me-OH),; O-O v1 (14 cm-‘) 2v, v2 (49 cm- ‘)

‘) Far red shifted isomeric species. See text for details.

cluster species. Thus, there are two distinct sets of electronic spectra features associated with complexes with a 1: 1 stoichiometry. We have also investigated the electronic spectroscopy of water clusters of several methyl-substituted indoles. In general, we have observed the same pattern of an intense transition in the - 100 to -200 cm-’ range and weaker bands in the - 300 to -500 cm-’ spectra region. A notable exception to this behavior has been found for N-methylindole. In this case, there are no cluster electronic features red shifted from the bare molecule origin. These results lead us to suggest that all of the electronic transitions reported above for indole-(water), and indole-(methanol), involve a

J. Hager et al. / Spectrmc~y

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140

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I

407

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NUMBER

Fig. 8. Two-color photoionization mass spectra of the. In-H,0 and In-MeOH clusters obtained with A, fixed at the far red shifted electronic bands at - 452 and - 472 cm-’ respectively. The ionization laser was tuned to 3: 100 cm-’ above threshold. Note the lack of any detectable fragmentation.

hydrogen bond with the water or methanol acting as the proton acceptor. Experiments have also been carried out in order to determine the threshold ionization energies of these distinct groups of features assigned to 1: 1 clusters. By measuring the PIE spectra of individual electronic absorption features, we can accurately determine cluster ionization energies. If this value is the same for the two sets of cluster absorptions, the relevant electronic transitions are due to the same cluster. If the IEs are different and the stoichiometry of the clusters is the same, then different cluster configurations are probably responsible for the distinct intermediate state absorptions [21]. Fig. 9a displays the PIE spectra of In-(H,O), when the dominant cluster feature at - 135 cm-’ is excited in this two-step process (upper trace) and when the farthest red-shifted band at -452 cm-’ serves as the intermediate state (lower trace). The PIE spectrum in fig. 9a shows an ionization threshold red shifted by 3027 f 10 cm-’ from the bare indole IE. In addition, there is a second threshold 179 cm- ’ higher in energy which we have previously assigned [4] to the excitation of the hydrogen bond stretching frequency in the ground state of the ionic cluster. The PIE spectrum in fig. 9b was obtained with the first laser tuned to the -452 cm-’ cluster

band. Here, the cluster ionization energy is red shifted by only 978 f 20 cm-’ relative to the bare indole IE. This value is more than 2000 cm-l higher than the IE for the cluster with an electronic origin at -135 cm-‘. Thus there are two distinct In-(H,O), clusters: one with an electronic origin at -135 cm-l and the other with a band origin at -452 cm-’ which will be designated as In-(H,O):. Similar results have been obtained for the two sets of features in the In-(MeOH), electronic spectrum. As with the water clusters, the IE of the dominant electronic band is at substantially lower energy than the IE of those electronic features at lower energies. The AIE of the - 160 cm-’ band is - 3164 f 25 cm-’ from the indole IP compared with -940 f 20 cm-’ for the -492 cm-’ band. Here, however, we observe quite broad thresholds (300-800 cm-‘) associated with the PIE spectra of both conformations of In-(MeOH),. 3.2.2. In-(ROH), Under expansion conditions in which a much greater concentration of methanol vapor is present, bands located at - 20, -40, - 55, and -69 cm-’ with respect to the indole electronic origin are observed to grow in rapidly. One of these spectra is shown in fig. 10 where the new bands are marked with asterisks. The blue shift of the

J. Hager et al. / Spectroscopy of jet-cooled inable clusters

408

AI E = -3027

In-(

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WAVENUMBERS)

Fig. 9. Two-color PIE spectra of In-(H,O), obtained with k, fiied at (a) the - 135 cm-’ band. Note the large change in the indole monomer ionization energy.

1 : 2 species relative to the 1 : 1 cluster is characteristic of a bonding arrangement in which the second solvent molecule acts as a proton donor in a hydrogen bond [22]. Assignment of these features was facilitated by the two-color mass analysis, as displayed in fig. 11, showing the presence of In-(MeOH) *. The experimental conditions used in this photoionization experiment were such that the ionizing laser was tuned to = 2800 cm-’ above the IE of the In-(MeOH), cluster in order to maximize the two-color signal. This is necessary since the ioniza-

band and (b) the -452

cm-’

electronic

tion efficiency near threshold for this complex only slowly increases as a function of energy, as is shown below. However, as seen in fig. 11, this results in some fragmentation and a peak at the mass corresponding to In-(MeOH), . The low ionization efficiency of the 1 : 2 clusters is demonstrated in the PIE spectrum of the -69 cm-’ electronic band as shown in fig. 12. The ionization onset is found to be 1795 f 35 cm-’ lower in energy than that of the bare molecule. This value, however, is almost 1400 cm-’ higher in energy than the IE associated with the

409

J. Hager et al. / Spectroscopy of jet-cooled indole clusters

In-

MeOH

x x

I

I -50

-100 RELATIVE

I

I

I

-150

-200

-250

ENERGY

Fig. 10, One-color MPI spectrum of In-(McOH),. with asterisks are due to In-(M&H),.

(cd

)

The band origin of In-(&OH),

is located at -160 cm-‘. The bands marked

[In-M?OH),]+

1

I

I

I

I

I

I

I

200

I

I

150 MASS

100

NUMBER

Fig. 11. Two-color photoionization mass spectrum of In-(MeOH), obtained with h, fiied at the -69 cm-’ electronic band and A, at = 2800 cm-’ above threshold. At this two-color energy there is some fragmentation evident.

1

I

I

I

62poO

I

IONIZATION.

Fig. 12. Two-color PIE spectrum of In-(MeOH), ionization threshold.

ENERGY

(VACUUM

I

I

61.000

6L500 WAVENUMBERS)

obtained with X1 fixed at the - 69 cm-’ electronic band. Note the extremely broad

410

J. Hager et al. / Spectroscopy ojjet-cooled

strongest In-(MeOH), electronic feature. Thus the addition of a second methanol molecule to the hydrogen bonded cluster leads to a blue shift in the electronically excited state and an apparent increase in the cluster IE with respect to the 1: 1 cluster. 3.2.3. In-(NR,),,2 We have also investigated In-(NR,),,,, where R = H or CH,. The electronic spectra are shown in fig. 13. The electronic origins of the 1: 1 hydrogen bonded clusters are -227 cm-’ (ammonia) and - 323 cm-’ (trimethylamine) as reported previously [13]. At lower energies in both spectra is an additional sharp structure which appears at higher concentrations of the complexing partner. Using mass analysis with two-color photoionization, these bands have been assigned to the In-(NR,), clusters. Table 3 lists the band positions and assignments of the electronic features

indole clusters

observed when these amines form clusters with indole. The spectral shifts of the In-(NR,), species show that the addition of a second amine molecule leads to a further red electronic shift. This should be contrasted with the observations for In-(MeOH), where the second methanol molecule added to the cluster results in a blue shift from the electronic origin of the 1: 1 complex. For ammonia and trimethyl amine, the 1: 2 cluster is red shifted with respect to the 1: 1 cluster by 138 cm-’ (ammonia) and 49 cm-’ (trimethylamine). The magnitude of this incremental red shift for the second solvent molecule appears to scale with the dipole moment of the complexing partner. Table 3 Band positions and assignments dole-trimethylamine clusters

Electronic spectral shift (cm-‘)

Assignment

ammonia

- 221 -195 - 191 -95 -82 - 365 -345 - 325 -321 -306 - 301 - 286 -281 -265 - 323 -312 - 286 - 282 - 278 - 274 - 249 - 228 -219 - 208 - 205 -182 -371 - 351 - 331 - 321

In-(NH,),; O-O Y, (32 cm-‘) v1 + vt (4 cm-‘) vj (132 cm-‘) v, (145 cm-‘) In-(NH&; O-O Y, (20 cm-‘)

trimethylamine

. I 0

a

I -I(K)

I -203 RELATIVE

-400

-xx) ENERGY

-MO

(cm-‘)

Fig. 13. One-color MPI spectra of (a) In-NH, and (b) In-NMes near the indole S, origin. Band assignments are presented in table 3.

‘a and in-

Clustering partner

(a)

(b)

for indole-ammom

(NMe3)

2v1

2v, + v2 (4 cm-‘) 3Vl 3r, -I-v* 4Vl 4v, + v, 5Vl In-(NMe,),; O-O V, (11 cm-‘) v2 (36 cm-‘) vl+v) (4cm-‘) v2 +2v, v* + 3v, 2v, 2v, + v1 v4 (103 cm-‘) v4 +

v1

v4 + v, + v, v, + v2 + v, In-(NMe,),; O-O v, (2Ocm_‘) 2Vl 3Vl

411

J. Hager et aL / Spectrmcopy ojjet-cooled indole clusters

Ammonia has a greater dipole moment than trimethylamine (1.47 versus 0.612 D, respectively) and exhibits a larger incremental spectral shift for the addition of a second solvent molecule. This is a distinctly different result than that obtained for the singly solvated species. The interactions for these 1: 1 species have been determined [13] to be due to hydrogen bonding where the electronic red shift scales with the gas phase proton affinity of the amine rather than the dipole moments. Thus, the first and second amines added to the cluster interact with the indole framework through much different mechanisms. Further information regarding the way in which one and two amine molecules interact with the indole molecule has been obtained by examining the stabilization of the indole radical cation through the two-color PIE spectra. The PIE spectrum of hydrogen bonded In-(NH,), is shown in fig. 14a and exhibits an ionization threshold energy at -4152 f 20 cm-’ relative to the bare indole IE. This stabilization energy amounts to = 0.5 eV for the addition of a single complexing molecule to the indole cation. The breadth of the ionization threshold in this case is = 400 cm-‘. We have also examined the PIE spectrum of In-(NH,), ionized from the electronic origin at - 365 cm-‘, relative to that of the bare molecule, shown in fig. 14b. Here, the ionization energy of the cluster is -4529 f 40 cm-’ from the indole

(b)

species

AIP (cm-‘)

indole In-Ar, Ill-Ar,

(62598 f -209* -399* -487f -9OO* 702k -2632f -2908k -3027k -978k -3164k -940* -1795* -3433* -4152k -4529k -4450*300

In-(CH4h In-W-W2 In-(CH4h In-(bemzne)l In-(benzen+, In-(Hz% In-(H,O):@ In-(MeOH), In-(MeOH): In-(MeOH) z In-@OH), In-(NH,), In-(NH& In-(NMe,),

5) 10 10 10 10 15 15 20 10 20 25 20 30 20 20 30

‘) Far red shifted isomeric species. See text for details.

adiabatic IP, and -377 cm-’ from the IE of In-(NH,),. Thus, the second ammonia molecule leads to only a slightly increased stabilization of the ion over that shown by the first solvent molecule. Attempts to measure the two-color PIE spectra have met with only limited of In-(NMe,),,, success. Trimethylamine itself has a continuous two-photon absorption at the required frequencies of h, ( 5: 400-450 nm) and ionizes very easily

In4NH312

IONIZATION

ENERGY

(VACUUM

14. Two-color PIE spectra of (a) In-(NH,), (see table 3). Fig.

Table 4 Two-color PIE results for indole clusters

WAVENUMBERS)

and (b) In-(NH,),

obtained with A, fiied at the respective cluster electronic origin

412

J. Hager et al. / Spectroscopy ojjet-cooled

[23]. Therefore, there is a strong one-color background present in these experiments and only general observations can be made. Our data indicate that In-(NMe,), has a lower ionization energy than In-(NH,), by 2: 100-500 cm-‘. Table 4 presents a summary of the electronic spectral shifts and AIE values determined for the ten different solvent species reported here.

4. Discussion The results presented above enable us to address several important points regarding threshold photoionization of molecular clusters as well as the energetics and intermolecular interactions of neutral and positively charged indole clusters. A crucial question regarding the photoionization data is whether the ionization threshold measured within the two-color scheme accurately reflects the true adiabatic ionization potential [24] of the cluster of interest. Two-color spectra of jet-cooled monomers typically exhibit sharp (< 10 cm-‘) ionization onsets, approximately limited by the rotational cooling, which generally correspond to the adiabatic IP [5-81. This is due to the fact that the Franck-Condon factors for ionization originating from excited electronic states of isolated molecules often favor Au = 0 transitions because the potential surfaces for the electronically excited state and the ground ionic state tend to be similar. Photoionization spectroscopy of van der Waals clusters, however, is somewhat more complicated. Here, there is expected to be a significant change of the cluster potential surface upon ionization due to the much stronger intermolecular interactions of the ion compared to the neutral. Consequently, the Franck-Condon factors for ionization will not necessarily favor Au = 0 transitions and may, in fact, lead to the formation of cluster ions with substantial amounts of vibrational energy. The result will be deviations from the simple isolated molecule threshold photoionization behavior, i.e. a sharp onset which corresponds to the adiabatic IP. The broadening is thus due to the considerable range of possible vertical transitions from the electronic origin of the cluster to high energy states of

indole clusters

the cluster ion. This is the context in which we will discuss the present results of the indole clusters. 4.1. Non-polar solvents The simplest indole clusters investigated here are those with argon and methane. For both the neutral and ionic clusters, the intermolecular interactions can be described in terms of forces acting through the polar&abilities of the solvent species. For the neutrals, this would be dispersion and dipole-induced dipole interactions, and for the ions, charge-induced dipole forces. We have been able to identify the extra stabilization energy of the ionic cluster with respect to the neutral through the measurement of the change in the indole IP upon cluster formation. The AIP values obtained in this manner provide a useful way in which to estimate the binding energies of these van der Waals ions employing the relationship [3]: AIP = D,(In+-X)

- D,(In-X).

This relationship assumes that one has reliable data for the adiabatic IPs of the parent molecule and the cluster of interest. The sharp ionization thresholds we have observed for the argon and methane van der Waals molecules are very similar to those we have observed for the isolated indole molecule [4] as well as many other intermediate to large sized molecules [5-71. This indicates that these onsets correspond to the cluster adiabatic IPs. The changes in IP, therefore, can be obtained from the PIE spectra of these clusters and that of bare indole. These values are - 209 cm-’ for In-(Ar), and -399 cm-’ for In-(CH,),. One also needs an estimate of the binding energy of the ground state of the neutral cluster. Although this has not been determined experimentally, it is relatively easy to estimate by examining results from similar aromatic systems. For neutral argon systems, ground state binding energies are typically between 300 and 600 cm-’ for various aromatic probe molecules [25]. Using the measured AIP, we obtain an estimate of between = 500 and 800 cm-’ for the In+-(Ar), binding energy, or approximately, = 30% to = 60% more tightly bound than the neutral species. Methane van der Waals molecules with aniline

J. Hager et al. / Spectroscopy ofiet-cooled

[26], toluene [27], and benzene [28] have reported binding energies between 450 and 700 cm-r. Using the AIE value of -487 cm-‘, the binding energy of In+-(CH,), is determined to be in the range a900 to t. I200 cm- ‘. This represents an increase in the cluster binding energy of between 70% and 100% for the ion relative to the neutral complexes. Finally, considering the experimental results for the a-hydrogen bonded In-(benzene), cluster, we have measured a much larger AIE of - 2632 cm- ‘. Typical binding energies for such n-hydrogen bonded systems are in the 700-1200 cm-’ range [20]. Consequently, the singly solvated ionic cluster is characterized by an intermolecuhu potential well-depth of = 3300-3800 cm-‘, or roughly three times more tightly bound than is its neutral precursor. Although these binding energies are approximate they do serve to illustrate the point that photoionization of such weakly bound systems leads to a substantial strengthening of the intermolecular bond. This increased attractive interaction can be directly attributed to the charge-induced dipole f rces operative in the ionic cluster [3]. For In-( J )i and In-(CH,), this results in only a moderate change in the intermolecular interactions. Thus, one still expects to observe intense Au = 0 ionizing transitions from the neutral excited state as is commonly observed for monomers. Our threshold photoionization results show that this is the case. These PIE spectra exhibit sharp onsets with a constant ion signal for several hundred wave numbers above threshold. This supports an assignment of the ionization energies of the In-(Ar), and In-(CH,), clusters to the respective adiabatic ionization potentials. In contrast to the 1: 1 argon and methane clusters, the intermolecular bond of In-(benzene), becomes approximately three times as strong upon ionization. Consequently, there is expected to be a much larger change in the equilibrium geometry between the neutral and ionic cluster. This would be reflected by a smaller propensity for Au = 0 ionization and a more gradual onset in the PIE spectrum. Our data again support such an explanation with a broadened threshold (= 50 cm-‘) and gradually increasing ion signal as the ionizing

indole clusters

413

laser is tuned deeper into the continuum. The relatively well-defined threshold (fig. 6a) compared with several other indole clusters discussed below does, however, imply that the reported ionization onset corresponds to the adiabatic IP of the cluster. The threshold spectra of the 1: 2 indole clusters with argon and methane exhibit nearly additive changes in the indole ionization potential which allows us to assign these ionization onsets to the respective cluster adiabatic IPs. There is, however, a decidedly non-additive effect in the case of the AIEs of In-(benzene), relative to In-(benzene),. Since there is good evidence that the benzene solvent molecule in the 1: 1 cluster is associated with the indole N-H moiety through a rr-hydrogen bond, and considering the fact that there is only one of these binding sites available, these results may simply be due to the fact that two benzene molecules in the 1: 2 cluster occupy very different geometries. Thus, there would be no reason to expect nearly additive changes in threshold ionization energy. However, there is a much broader ionization onset associated with the In-(benzene), relative to that of the 1: 1 species. This indicates that there is a larger equilibrium geometry change for the 1: 2 complex upon ionization and that the ionization energy may not correspond to the adiabatic IP. The threshold photoionization behavior of the three 1: 1 cluster systems discussed above stands in sharp contrast with the results obtained for In-(CF,),. The PIE spectrum of this cluster is characterized by a broad ionization onset (300 cm-‘) and an apparent IP that is 700 cm-’ greater than that of bare indole. In addition, mass analysis of the ions produced by the two-color process shows a substantial amount of fragmentation at less than 300 cm-’ above the ionization onset. Taken together, these results strongly suggest that the measured ionization energy for this cluster does not correspond to the adiabatic IP, but rather to the Franck-Condon accessible region of the ionic potential surface which corresponds to energies higher than the true adiabatic IP. Thus, the In-CF, system presents an extreme example of potential energy surfaces for the neutral and ionic clusters with very different shapes and/or relative

414

J. Hager et al. / Spectroscopy of jet-cooled indole clusters

positions. This fact only allows us to determine an upper bound to the cluster adiabatic ionization ,potential without much more detailed information regarding Franck-Condon factors. 4.2. Polar solvents Employing the techniques of electronic spectroscopy, two-color threshold photoionization, and mass analysis, we have identified two distinct conformational isomers of 1: 1 indole clusters with water and methanol. The electronic absorption features show that the cluster associated with the lower energy set of absorption features undergoes a much greater change in equilibrium geometry upon T,V* excitation resulting in at least two long progressions in a low-frequency intermolecular vibration. Experimental evidence suggests that these two isomeric clusters are hydrogen bonded in an arrangement in which the indole N-H functional group acts as the proton donor. Consequently, we suggest that the difference between these isomers is the relative orientation or conformation of the proton acceptor relative to the indole framework within the context of such a hydrogen bond. These different geometries may involve a simple 90” rotation of the proton accepting molecule about the hydrogen bond axis or more subtle effects. Definite assignments will have to wait the results of theoretical calculations that we have recently undertaken. In the two-color threshold photoionization experiments, we have observed a much greater reduction in ionization energy associated with the polar complexing partners than those discussed above for argon and methane. This is undoubtedly due to the presence of charge-dipole intermolecular interactions in addition to the charge-induced dipole forces that dominate the interactions for non-polar solvents. For the In-(ROH), isomers, this results in an ionic cluster that is > 3000 cm-’ more tightly bound than the neutral species in one case and = 1000 cm-’ more tightly bound for the other isomer. This is reflected in qualitative differences in the appearance of the PIE spectra of these polar clusters. With the exception of the PIE spectrum of the higher energy In-(H,O)r isomer, these are characterized by onsets that are between

500 and 1000 cm-’ broad. Considering the large changes in binding energy upon photoionization for these hydrogen bonded clusters, it is reasonable to attribute these much more gradual onsets to poor Franck-Condon factors for Av = 0 ionizing transitions. The PIE spectrum of the In-(H,O), isomer with an electronic origin at - 135 cm-‘, however, exhibits two sharp ionization onsets (fig. 9a). The lower energy onset is assigned to the adiabatic IP of the cluster while the higher energy threshold corresponds to the excitation of an intermolecular vibration of In+-(HzO),. Consequently, there is a much greater propensity for Au = 0 ionizing transitions in this case than observed for indole associated with other polar molecules. The shape of this PIE spectrum has allowed us to measure directly the value of a cluster vibration that will aid us in future investigations of the intermolecular potentials of these ionic complexes. These In-(H,O), results have proven, however, to be the exception to the general rule of quite gradual ionization onsets for indole clusters involving polar solvent molecules. The PIE spectra of 1: 2 complexes with methanol and ammonia are characterized by even broader ionization onsets than the 1: 1 clusters. This is almost certainly due to dramatic changes in the intermolecular potential surfaces of the neutral and ionic species leading to very poor Franck-Condon factors for the lowest energy ionizing transitions. One cannot, therefore, determine the adiabatic ionization potentials of clusters of this type from these PIE spectra without a much more detailed understanding of the Franck-Condon factors for the ionization step and hence potential energy surface for the final ionic cluster state. However, it is interesting to note that, even for these cases, two-color photoionization near threshold produces predominantly parent cluster ion with little or no fragmentation.

5. Conclusions These investigations have explored, in a comprehensive manner, some of the characteristics of solvation of neutral and cationic indole within the

J. Hager et al. / Spectroscopy ofjet-cooled

context of isolated clusters prepared in a supersonic expansion. From the results of these studies, the following conclusions can be made. (a) In the PIE spectra of indole clusters, the widths of the ionization thresholds provide an indication of the propensity for Au = 0 ionizing transitions. Sharp thresholds and a constant ion signal deeper in the ionization continuum are characteristics of a strong propensity for Au = 0 transitions. Diffuse onsets are indicative of substantial differences in the shape and/or relative positions of the potential surfaces associated with the neutral excited and ground ionic state of the clusters. Thus, Franck-Condon factors will favor Au # 0 ionizing transitions. (b) For indole associated with the non-polar solvent species argon, methane, and benzene, the stabilization energies (relative to the neutral ground state) of the neutral excited and ground ionic states scale with the polarizability of the complexing partner. This demonstrates the importance of induction and dispersion forces in the intermolecular binding of these clusters. (c) The apparent IP of In-CF,, obtained through a two-color process, is higher than that of bare indole. This is an extreme example of poor Franck-Condon overlap between the lowest levels of the neutral excited and ground ionic states of the cluster. We have also observed significant fragmentation near threshold in the two-color photoionization mass spectrometry of In-CF, which is a direct result of these small FranckCondon factors for Au = 0 ionizing transitions producing a highly vibrationally excited van der Waals ion which can fragment. (d) We have identified two distinct structural isomers of In-(H,O), and In-(MeOH), by combining electronic spectroscopy, mass analysis, and ionization energy measurements. These observations illustrate an important use of two-color photoionization technique employing mass analysis and threshold ionization measurements, identification and characterization of chemically important neutral molecular clusters. (e) The threshold ionization energy measurements of clusters involving indole and polar solvent molecules show much greater values of AIE and broader ionization onset compared with the data

indole clusters

415

on the non-polar complexes. The large AIEs result from the much stronger association energies of the ionic cluster relative to the neutral species, and the broader onsets are due to the poorer Franck-Condon factors associated with the lowest energy ionizing transitions which accompany this large binding energy change. (f) We have measured quite non-additive AIEs for the 1: 2 indole clusters with benzene, methanol, and ammonia. This is probably a result of a substantial change in equilibrium geometry upon photoionization and true differences in the incremental stabilization energies due to the different binding sites of the two solvent molecules to the indole framework. Future studies in our laboratory will involve calculations of binding energies of both neutral and ionic clusters. This will enable us to compare experimental and theoretical values of IE and determine calculated values of cluster adiabatic IPs. In addition, we are extending our investigations to the two-color photoionization of neutral clusters with known amounts of inter- and intramolecular vibrational excitation and the characterization of the resulting ionic clusters.

Acknowledgement This work was supported in part by the Natural Sciences and Engineering Research Council of Canada.

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J. Hager et al. / Spectroscopy ofjet-cooled

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indole clusters

[22] A. Gikawa, H. Abe, N. Mikami and M. Ito, J. Phys. Chem. 87 (1983) 5083. [23] J. Hager, M. Ivanco and S.C. WaIIace, Chem. Phys. Letters 92 (1982) 112; M. Asscher and Y. Haas, Appl. Phys. 20 (1979) 291. [24] P.M. Guyon and J. Berkowitz, J. Chem. Phys. 54 (1971) 1814. [25] D.V. Brumbaugh, J.E. Kenney and D.H. Levy, J. Chem. Phys. 78 (1983) 3415; N. Gonohe, H. Abe, N. Mikami and M. Ito, J. Phys. Chem. 87 (1983) 4406; N. Gonohe, A. Shim& H. Abe, N. Mikami and M. Ito, Chem. Phys. Letters 107 (1984) 22; T.A. Stephenson and S.A. Rice, J. Chem. Phys. 81 (1984) 1083. [26] E.R. Bernstein, K.S. Law and M. Schauer, J. Chem. Phys. 80 (1984) 634. [27] M. Schauer, K.S. Law and E.R. Bernstein, J. Chem. Phys. 82 (1985) 736; M. Schauer, K.S. Law and E.R. Bernstein, J. Chem. Phys. 81 (1984) 49. [28] M. Schauer and E.R. Bernstein, J. Chem. Phys. 82 (1985) 726.