Hole-Burning and Stimulated Raman−UV Double Resonance

J. Phys. Chem. 1996, 100, 10531-10535

10531

Hole-Burning and Stimulated Raman-UV Double Resonance Spectroscopies of Jet-Cooled Toluene Dimer Seiichi Ishikawa, Takayuki Ebata,* Haruki Ishikawa, Tamiko Inoue, and Naohiko Mikami Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-77, Japan ReceiVed: January 29, 1996; In Final Form: April 8, 1996X

The structure of toluene dimer generated in a supersonic jet has been investigated by hole-burning and stimulated Raman-UV double resonance spectroscopies. Hole-burning spectroscopy revealed that the broad electronic spectrum of the S1-S0 transition of the toluene dimer consists of two components due to two isomers at least. On the basis of the calculated result given by Schauer and Bernstein, the two isomers were attributed to a sandwich-shaped dimer and a T-shaped dimer, respectively. Accurate vibrational frequencies of three vibrations ν1, ν12, and ν18a for toluene-h8, toluene-d8, and the dimer were obtained by stimulated Raman-UV double resonance spectroscopy. It was found that the 121 level of bare toluene-h8 is perturbed by the vibration involving methyl rotation and its perturbation is reduced upon the dimer formation.

Introduction Clusters involving benzene and substituted benzenes are of fundamental importance as a prototype of spectroscopic investigations of many other van der Waals (vdW) clusters of aromatic molecules, and therefore, they have been studied by many workers both experimentally and theoretically. Among many problems, the determination of their structure is of special importance, since it provides a framework for investigations of intermolecular interaction and dynamics such as intramolecular vibrational redistribution (IVR) and vibrational predissociation (VP). Several methods have been developed to obtain the information of the structure, such as, high-resolution electronic spectroscopy1-40 or rotational coherence experiments,41-47 which are used to obtain rotational constants. For large size clusters or the clusters whose electronic spectra are broad, however, it is very difficult to obtain their rotationally resolved spectra. Recently, Raman spectroscopy of jet-cooled clusters with low density has been developed by the stimulated Raman-UV double resonance method or ionization-detected stimulated Raman (IDSR) technique.48-63 In this spectroscopy, the lowdensity clusters in jets are efficiently excited to a vibrational level by stimulated Raman pumping; either an increase (gain) of the vibrationally excited cluster or a depletion (loss) of the population of the vibrational ground state is monitored by using resonance-enhanced multiphoton ionization (REMPI). Felker and his co-workers measured the stimulated Raman spectra of isotope-substituted benzene clusters and proposed the T-shaped structure for the benzene dimer and a highly symmetrical structure for the trimer.60,61 Ebata et al. also proposed the T-shaped structure of the benzene dimer from the measurement of the depolarization ratio of the Raman band by using stimulated Raman-UV optical double resonance spectroscopy.62,63 Hole-burning spectroscopy is an another effective method for the investigation of cluster structure. The application of holeburning spectroscopy to jet-cooled molecules was first reported by Colson et al. who discriminated the rotational isomers of m-cresol.64 Schlag et al. measured the hole-burning spectra of the benzene dimer and succeeded in discriminating benzenes in the different sites of the T-shaped dimer.65 In contrast to extensive studies of the benzene dimer, little is known about the toluene dimer structure. Since toluene has a dipole moment (0.375 D),66 it is interesting to investigate X

Abstract published in AdVance ACS Abstracts, May 15, 1996.

S0022-3654(96)00267-5 CCC: $12.00

whether the structure of the toluene dimer is similar to that of the benzene dimer. Bernstein and his co-workers measured the 1+1′ REMPI spectra of the S1-S0 transition of the toluene dimer.67 However, it was difficult to obtain information about its structure because the electronic spectra were superimposed by the broad structureless absorption. In the present paper, we applied hole-burning spectroscopy and stimulated Raman-UV double resonance spectroscopy for toluene and its dimer. We found that the observed electronic spectrum of the toluene dimer consists of transitions of at least two isomers. Structures of isomers are discussed with respect to the similarity to that of the benzene dimer. Experimental Section Jet-cooled toluene and its dimer were generated by expanding toluene vapor seeded in He into vacuum through a pulsed nozzle having an 800 µm orifice. To avoid the formation of higher clusters, the vapor pressure of toluene was controlled by keeping the temperature of a sample container at -3 °C. The free jet was skimmed by a skimmer with a 1 mm diameter at 20 mm downstream from the nozzle to obtain a molecular beam. For the measurement of the hole-burning spectrum, a second harmonic of an optical parametric oscillator (OPO) (QuantaRay GCR-230/MOPO-730) was used for the hole-burning light source. The light beam crossed the molecular beam perpendicularly at 50 mm downstream from the skimmer. The holeburning light depopulates the ground state cluster, and the resulting depletion of the S0 population was monitored with a probe laser by 1+1 REMPI through the S1 state. The probe light was a second harmonic of a XeCl-excimer-laser-pumped dye laser (Lambda Physik Lextra 50/Scanmate). The probe laser beam crossed the molecular beam 7 mm downstream from the crossing position of the hole-burning laser beam. The delay time between the hole-burning laser pulse and the probe laser pulse was set to 4.5 µs by using a delay generator (Stanford Research DG 535). The dimer ions generated by REMPI were mass-analyzed by a time-of-flight (TOF) mass spectrometer with a 50 cm flight tube and were detected by an electron multiplier (Murata Ceratron). The hole-burning spectrum was measured by monitoring the ion signal while scanning the frequency of the hole-burning light. The experimental setup of the stimulated Raman-UV double resonance spectroscopy was described in a previous paper.63 We applied two methods described by Felker and his coworkers:52 (a) ionization gain stimulated Raman spectroscopy © 1996 American Chemical Society

10532 J. Phys. Chem., Vol. 100, No. 25, 1996

Figure 1. (a) S1-S0 1+1 REMPI spectra of toluene-h8 dimer; (b) toluene-h8 monomer; (c)-(g) hole-burning spectra of toluene dimer measured at five different monitor frequencies (νUV) at 37 278, 37 329, 37 385, 37 454, and 37 484 cm-1, respectively.

(IGSRS) and (b) ionization loss stimulated Raman spectroscopy (ILSRS). In IGSRS, we measured the ionization gain signal from the Raman pumped levels, and in ILSRS we monitored the depletion of the vibrational ground state population. A second harmonic output (532 nm) of an injection-seeded Nd: YAG laser (Quanta-Ray GCR-230) and an output of a dye laser (Quanta-Ray PDL-3) pumped by the Nd:YAG laser were used for the Raman pumping. The laser beams were focused coaxially by an f ) 250 mm lens at 10 mm downstream from the pulsed nozzle. The ground state population was also probed by 1+1 REMPI through the S1 state. The UV light for the probing was a second harmonic of a XeCl-excimer-laser-pumped dye laser (Lambda Physik Lextra 50/Scanmate). The delay time between the Raman pumping beams and the UV laser beam was controlled by the same delay generator. The UV laser beam was counterpropagated to the Raman pumping laser beams. The spectral resolution of the UV laser beam was 0.2 cm-1. The spectral resolution of the second harmonic of the injectionseeded Nd:YAG laser was 0.01 cm-1, while that of the Nd: YAG-pumped dye laser was 0.2 cm-1. The wavelength of the lasers was calibrated by a double monochromator, and the absolute accuracy was 1 cm-1. The relative accuracy, on the other hand, was 0.2 cm-1, which was confined by the spectral resolution of the dye lasers. The ion signal was amplified by an amplifier (NF BX-31) and processed by a boxcar integrator (Par Model 4400/4420). Toluene-h8 (99.5%) and toluene-d8 (99 atom %) were purchased from Wako Chemicals Co., and they were degassed before the experiment. Results and Discussion 1. Hole-Burning Spectra of Toluene Dimer. Figure 1a shows the mass-selected 1+1 REMPI spectra of the S1-S0 transition of the toluene dimer in the band origin region. In

Ishikawa et al. this measurement, only the toluene dimer ion was monitored by using a TOF mass spectrometer, and we confirmed that contribution from larger clusters due to fragmentation is negligibly small in this spectrum. As can be seen in the figure, the electronic spectrum of the toluene dimer is very broad, although two peaks are observed at 37 460 (labeled as f) and 37 485 cm-1 (labeled as g). The two peaks were also observed by Law et al.67 The bandwidths are slightly broad compared to their spectrum. Therefore, the rotational temperature of the dimers seems slightly higher in the present condition. The spectrum is so broad that it is difficult to obtain their fine structure for the analysis of the dimer structure. There may be three reasons for the broadness of the spectrum. First is that the spectrum may be composed of many transitions associated with several isomers of the dimer. Second is that the broadness may be due to the overlap of many low-frequency vibronic transitions associated with a drastic structural change and also due to a short lifetime in the S1 state, even if the transition is due to a single species. Third, it is also possible that the broadness is due to the overlap of many hot bands. However, the third possibility is thought to be unlikely, since bare toluene exhibited a well-cooled S1-S0 REMPI spectrum with almost no hot band under the same condition and in general the internal temperature of clusters is much lower than that of the bare molecule in a free jet expansion. To investigate the presence of isomers, we measured the holeburning spectra of the toluene dimer by monitoring at five different frequencies indicated as spectra c-g in Figure 1a. If the electronic transitions of several isomers are overlapped, different spectral features will be obtained in the hole-burning spectra at different monitoring frequencies. Obtained holeburning spectra are shown in parts c-g of Figure 1. Though the spectra are broad, they are clearly classified into two groups. First, spectra c-e show a similar feature with their peaks at ∼37 410 cm-1. On the other hand, spectra f and g show a rather sharp structure with their peaks at ∼37 460 cm-1. Therefore, the hole-burning spectra obtained by monitoring at frequencies lower than 37 385 cm-1 (position labeled by (e)) are different from those obtained at higher monitoring frequencies. This result indicates that the REMPI spectrum of the toluene dimer is composed of two transitions due to isomers. In Figure 2, two different spectra a and b are shown as typical representatives of these components, where spectra a and b are reproduced from spectra e and f in Figure 1, respectively. Despite the identification of two components, spectrum a extends over a wide range from 37 200 to 37 530 cm-1 so that spectra a and b are still overlapped with each other and an appropriate discrimination between them is necessary. So we subtracted the hole-burning spectrum a from spectrum b. The resulting spectrum is shown in Figure 2c. As seen in the figure, the subtracted spectrum exhibits a relatively sharp structure, with the band origin at ∼37 460 cm-1. Since the subtracted spectrum c does not extend to the region of spectrum a, both spectra represent the transitions of different species of the dimer. Thus, the hole-burning spectra reveal that two different components are overlapped in the REMPI spectrum of the toluene dimer: one has a broad structureless spectrum starting at 37 200 cm-1 with its maximum at 37 410 cm-1 as shown in Figure 2a and the other has a sharp spectrum with its origin at 37 460 cm-1 as shown in Figure 2c. The two components of the spectra are thought to be due to different isomers of the toluene dimer. According to Schauer and Bernstein, four stable isomers of the toluene dimer have been predicted by using semiempirical potential calculations.68 Three of them are sandwich-shaped dimers, and the fourth is the T-shaped dimer. Calculated

Spectroscopies of Jet-Cooled Dimer

J. Phys. Chem., Vol. 100, No. 25, 1996 10533

Figure 2. (a) Hole-burning spectrum of toluene dimer measured at monitoring frequency of νUV ) 37 385 cm-1; (b) hole-burning spectrum of toluene dimer measured at νUV ) 37 454 cm-1; (c) subtracted spectrum (Figure 2b - Figure 2a).

stabilization energies of the sandwich-shaped dimers in the S0 state were found to be much larger than that of the T-shaped dimer because dipole-dipole interactions contribute to the stability of the sandwich-shaped dimer more than to that of the T-shaped dimer. From the present experiment, it is shown that there are at least two isomers in the jet. Since the dipole moment in the S1 state is larger than in the S0 state,69 the dipoledipole interaction becomes much larger in the sandwich-shaped dimer than in the T-shaped dimer in the S1 state. Therefore, the broad and largely red-shifted spectrum (Figure 2a) can be assigned to the sandwich-shaped dimer and the sharp spectrum (Figure 2c) is assigned to the T-shaped dimer. It should be noted that the spectral red shift of the T-shaped toluene dimer (17 cm-1) is comparable with the T-shaped benzene dimer (42 cm-1). The result that both the sandwich-shaped and the T-shaped structures are stable in the toluene dimer is in good contrast with the benzene dimer, where the T-shaped structure is more stable than the sandwich-shaped structure. Different from benzene, the experimental result also suggests that the nonzero dipole moment of toluene contributes more stability for the sandwich-shaped structure than for the T-shaped structure. Another interesting point in the hole-burning spectra is the broadness of the electronic spectra of the sandwich form in Figure 2a. There may be two origins of the broadness. First is that the spectrum is still overlapped by transitions of several sandwich form isomers. As pointed out previously, three stable isomers are proposed for the sandwich form by Bernstein et al. If the isomers exhibit similar electronic spectra, their separation may be difficult even by hole-burning spectroscopy. Second is that the broadness is due to the overlap of many low-frequency vibronic bands involving intermolecular vibrations owing to a substantial structural change between the S0 and the S1 states and also due to a short lifetime of the S1 state. Hopkins et al. reported that in the benzene dimer, the lifetime of the S1 state becomes short and suggested that the dimer may form an excimer in the S1 state.70 Similar excimer formation and shortening of the S1 lifetime may occur in the toluene dimer. 2. Stimulated Raman-UV Double Resonance Spectra of Toluene and Its Dimer. (a) Toluene Monomer. Figure 3

Figure 3. Ionization loss stimulated Raman (ILSR) spectra of (a) toluene-h8 and (b) toluene-d8. The dip depths of the ILSR spectra are 20-40%. The spectral profiles were independent of the Raman pumping laser power under our experimental conditions.

TABLE 1: Frequencies of ν1, ν12, and ν18a Vibrations of Toluene-h8, Toluene-d8, and Toluene-h8 Dimer toluene-h8 ν1 ν12

784.0 1004.1 1006.2

ν18a

1032.4

toluene-d8

(toluene-h8)2 784.0

962.4

1004.1

841.8

1031.9

shows ionization loss stimulated Raman (ILSR) spectra of the ν1, ν12, and ν18a vibrations of bare toluene-h8 and -d8, where the probe UV laser frequency was tuned to the (0, 0) band of their S1-S0 transitions. As for the notation of the vibrational mode, we adopted the mode number given by Varsa´nyi;71 ν1 and ν12 are symmetric in-plane C-C stretching vibrations, and ν18a is an in-plane C-H bending vibration. The vibrational frequencies obtained are listed in Table 1. In the Raman spectrum of bare toluene-h8, there are two noticeable points: the ν12 band splits into two and their intensities are weaker than that of ν18a. It is evident that their relative intensities are quite different from those observed in the Raman spectrum of liquid toluene, where the ν12 band is much stronger than ν18a.72 As shown in Figure 3b, however, when toluene was deuterated, the doublet structure of the ν12 band disappears, showing only one strong dip at 962.4 cm-1, and its intensity is stronger than the ν18a intensity. These results suggest that the ν12 vibration of bare toluene-h8 is perturbed by other vibrations and that the perturbation is lifted by the deuteration. To investigate the perturbation, we observed the S1-S0 REMPI spectra of toluene-h8 after exciting each of the two peaks of ν12 by stimulated Raman pumping, which is shown in Figure 4. As seen in Figure 4a, the REMPI spectrum obtained by exciting the band at 1004 cm-1 shows the intense peak at 36 471 cm-1 due to the 1201 band and an additional peak at 36 527 cm-1 (+56 cm-1). On the other hand, only a single peak due to 1201

10534 J. Phys. Chem., Vol. 100, No. 25, 1996

Figure 4. (a) S1-S0 1+1 REMPI spectrum of toluene-h8 measured after Raman pumping to the lower frequency band of ν12 (1004.1 cm-1); (b) S1-S0 1+1 REMPI spectrum of toluene-h8 measured after Raman pumping to the higher frequency band of ν12 (1006.1 cm-1).

appeared when the other band at 1006 cm-1 was excited by stimulated Raman pumping, as shown in Figure 4b. The results indicate that a low-frequency vibronic band appears when the 1004 cm-1 peak of the 121 level was excited, but such a band does not appear when the 1006 cm-1 peak was excited. The frequency of the vibration (56 cm-1) is very close to the that of the 3a1 level of the methyl group rotation in the S1 state, which was reported by Bernstein et al.73 Since such a low-frequency vibration is not assigned to skeletal vibrations of toluene, the band of 56 cm-1 in Figure 4a is assigned to be the transition to the 3a1 level of the rotational motion of CH3 group combined with 1201. This result strongly suggested that the vibration that perturbs the 121 level involves the methyl rotation. Such a vibrational coupling would be removed in condensed phases because the levels due to the methyl rotor may disappear or are subjected to a drastic change. In fact, such an anomaly disappears in the dimer and the ν12 band shows a single intense peak in the dimer Raman spectrum, as will be shown later. The disappearance of the anomaly is explained by the reduction of the vibrational coupling upon cluster formation. (b) Toluene Dimer. As described above, we found that at least two stable isomers coexist in the toluene dimer. So we have to be careful to observe the Raman spectra of the dimer. Figure 5 shows the ILSR spectra of the toluene dimer measured at three different monitoring frequencies of the S1-S0 spectrum. Though their frequencies were chosen for the convenience of discriminating the isomers, the discrimination is still not enough because of the broadness of the S1-S0 spectrum of each isomer. The observed vibrational frequencies are also listed in Table 1. We first expected that the Raman band may split into two because the frequency may be different between the two isomers. Even in a single isomer having symmetrically inequivalent components, a splitting due to the two components may be observed. For example, the T-shaped benzene dimer showed two Raman bands separated by 0.6 cm-1.60-63 As seen in Figure 5, however, no prominent split or difference is observed in the Raman spectra within our spectral resolution, though all the Raman bands are broadened upon the dimer formation. The fwhm of Raman bands of toluene dimer is about 2 cm-1, which

Ishikawa et al.

Figure 5. (a) ILSR spectrum of ν12 and ν18a of toluene-h8 dimer measured at the monitoring UV laser frequency of νUV ) 37 500 cm-1; (b) ILSR spectrum of ν12 and ν18a of toluene-h8 dimer measured at the monitoring frequency of νUV ) 37 380 cm-1; (c) ILSR spectra of ν1, ν12, and ν18a of toluene-h8 dimer measured at the monitoring frequency of νUV ) 37 240 cm-1. The dip depths of the ILSR spectra are 2040%.

is 2 times as broad as that of bare toluene. Therefore, we could not resolve the vibrational band of each isomer and the broadening of the Raman bands of the toluene dimer is thought to be due to the overlap of Raman bands of the isomers that were not completely discriminated by the UV laser. The split among the Raman bands in the toluene dimer is estimated to be less than 1 cm-1, which is similar to that for the benzene dimer. The small shifts upon dimer formation and the small difference of Raman frequency between the two types of isomers indicate that the force fields of toluene are hardly affected by the intermolecular interaction of the toluene dimer, although toluene has a nonzero dipole moment. Finally, we mention the lifetime of the vibrationally excited states of the dimer. In previous papers,62,63 we obtained the decay rate of the 11 level of the benzene dimer to be 2.4 × 107 s-1, corresponding to a lifetime of 42 ns. So we prepared simultaneously the benzene dimer and the toluene dimer in the same jet expansion and tried to measure the decay lifetime of the 11 and 121 levels of the toluene dimer under the same jet condition as the benzene dimer. However, even under the condition that the ionization gain signal of the benzene dimer was observed, we could not observe any ionization gain signal of the toluene dimer. By considering the time scale of the experiment, the lifetime of the vibrationally excited toluene dimer seems to be 10 times shorter than that of the benzene dimer, which indicates that the IVR rate is much faster in the toluene dimer than in the benzene dimer. Such an enhancement cannot be simply explained by the increase of the vibrational state density originating from the methyl group. The rotational constant of the hindered rotation of the methyl group is 5.2 cm-1,73 which is comparable to the frequency of the lowest van der Waals vibration of the benzene dimer observed by Felker et al.59 Since the energy of methyl rotation is given by BJ2, the contribution of the internal rotation of methyl group to the vibrational state density may be less than that of the van der Waals vibrations. Therefore, the faster decay of the toluene dimer than the decay of the benzene dimer indicates that the

Spectroscopies of Jet-Cooled Dimer anharmonic coupling strength between the rotational motion of the methyl group and the intermolecular van der Waals modes becomes important for the fast IVR in the toluene dimer. In conclusion, we observed the hole-burning spectra of the S1-S0 transition of toluene dimer and the Raman spectra of bare toluene and toluene dimer in supersonic jets. The holeburning spectra revealed that the broad electronic spectrum of the toluene dimer consists of the electronic transitions of two isomers, and the Raman spectra of the dimer exhibit broad bandwidths associated with overlapping of bands due to the different isomers. The result indicates that there exists two types of toluene dimer, which can be a sandwich-shaped dimer and a T-shaped dimer as the theoretical calculation suggested. Acknowledgment. The authors thank Dr. Asuka Fujii for his helpful discussions. This work is supported in part by Grantin-Aid on priority-area research “Photoreaction Dynamics” and “Chemistry of Small Many Body System” from the Ministry of Education, Science, and Culture, Japan. The support from the Kurata Foundation is also gratefully acknowledged. References and Notes (1) Levy, D. H. AdV. Chem. Phys. 1981, 47, 3742. (2) Amirav, A.; Even, U.; Jortner, J. J. Chem. Phys. 1981, 75, 2489. (3) Leutwyler, S. J. Chem. Phys. 1984, 81, 5480. (4) Leutwyler, S.; Jortner, J. J. Phys. Chem. 1987, 91, 5558. (5) Troxler, T.; Knochenmuss, R.; Leutwyler, S. Chem. Phys. Lett. 1989, 159, 554. (6) Leutwyler, S.; Bosiger, J. Chem. ReV. 1990, 90, 489. (7) Troxler, T.; Leutwyler, S. J. Chem. Phys. 1991, 95, 4010. (8) Kettley, J. C.; Oram, J. W.; Palmer, T. F.; Simons, J. P.; Amos, A. T. Chem. Phys. Lett. 1987, 140, 286. (9) Hahn, M. Y.; Whetten, R. L. Phys. ReV. Lett. 1988, 61, 1190. (10) Li, X.; Hahn, M. Y.; Al-Shall, M. S.; Whetten, R. L. J. Phys. Chem. 1991, 95, 8524. (11) Bernstein, E. R. J. Phys. Chem. 1992, 96, 10105. (12) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J. Chem. Phys. 1983, 79, 1581. (13) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J. Chem. Phys. 1984, 80, 2256. (14) Beck, S. M.; Liverman, M. G.; Monts, D. L.; Smally, R. E. J. Chem. Phys. 1979, 70, 232. (15) Sugahara, Y.; Mikami, N.; Ito, M. J. Phys. Chem. 1986, 90, 5619. (16) Levy, D. H. In Structure and Dynamics of Weakly Bound Moleculer Complexes; Weber, A., Ed.; NATO ASI Series C; D. Reidel: Dordrecht, 1986; Vol. 211, p 231. (17) van Herpen, W. M.; Meerts, W. L. Chem. Phys. Lett. 1988, 147, 7. (18) Weber, Th.; von Bargen, A.; Riedle, E; Neusser, H. J. J. Chem. Phys. 1990, 92, 90. (19) Weber, Th.; Smith, A. M.; Riedle, E.; Neusser, H.; Schlag, E. W. Chem. Phys. Lett. 1990, 175, 79. (20) Langridge-Smith, P. R. R.; Carrasquillo, E.; Levy, D. H. J. Chem. Phys. 1981, 74, 6513. (21) Brumbaugh, D. V.; Kenny, J. E.; Levy, D. H. J. Chem. Phys. 1983, 78, 3415. (22) Harbersadt, N.; Soep, B. J. Chem. Phys. 1984, 80, 1340. (23) Stephenson, T. A.; Rice, S. A. J. Chem. Phys. 1984, 81, 1083. (24) Abe, H.; Ohyanagi, Y.; Ichijo, M.; Mikami, N.; Ito, M. J. Phys. Chem. 1985, 89, 3512. (25) Butz, K. W.; Catlett, D. L., Jr.; Ewing, G. E.; Krajnovich, D.; Parmenter, C. S. J. Phys. Chem. 1986, 90, 3533. (26) Kobayashi, T.; Kajimoto, O. J. Chem. Phys. 1987, 86, 1118. (27) Saigusa, H.; Itoh, M. J. Chem. Phys. 1987, 86, 2588. (28) Motyka, A. L.; Wittmeyer, S. A.; Babbitt, R. J.; Topp, M. R. J. Chem. Phys. 1988, 89, 4586. (29) Zwier, T. S. AIP Conf. Proc. 1989, 191, 692.

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