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Intermolecular vibrations and dynamics of the anisole—benzene complex studied by multi-resonance spectroscopy
Spectrochimica Acta, Vol. 50A, No. 819, pp. 1435-1442, 1994 Copyright 0 1994 Elscvier ScienceLtd Printed in Great Britain. All rights reserved 0584-8539/94 $7.00 + 0.00 09%8539@4)EOOS3-D
Intermolecular
vibrations and dynamics of the anisole-benzene studied by multi-resonance spectroscopy MASAO
(Received
and
TAKAYANAGI
Institute for Molecular
Science,
1 December
ICHIRO
Myodaiji,
1993; accepted
complex
HANAZAKI
Okazaki 444, Japan 17 January 1994)
Abstract-The intermolecular vibrations of the anisole-benzene complex in the ground and excited electronic states have been observed by the LIF (laser-induced fluorescence) and fluorescence-dip techniques. Short progressions due to the intermolecular vibrations suggest a small structure change of the complex upon electronic excitation. The LIF excitation spectrum shows predominant progressions of 27cm-‘, which is tentatively assigned to one of the intermolecular bending modes in the excited electronic state. On the other hand, the fluorescence-dip spectrum shows only a series of bands with irregular intervals due to the intermolecular modes in the ground electronic state. The decay rates of the vibrationally excited complex in the ground electronic state have also been measured with the SEP-LIF (stimulated emission pumping-laserinduced fluorescence) technique, where the complex vibrationally excited by SEP is probed by the delayed LIF measurements. The complex excited to its purely intermolecular mode stays in the initially prepared state after a delay time of 1 ps. On the other hand, the complex excited to the intramolecular vibrational states above 5OOcm-’ does not seem to stay in the prepared states. Neither the relaxed complex nor the dissociated monomer was detected. A possible reason for this observation is discussed.
INTRODUCTION VAN DER WAALS
complexes vibrationally excited in the ground electronic state provide a unique opportunity to investigate the role of the weak intermolecular bonding and associated vibrations in the dynamics of unimolecular decomposition and IVR (intramolecular vibrational redistribution). Various techniques have been used to study their dynamics. Among them, the rates of the dynamics have often been estimated indirectly from the band widths in the IR absorption or dissociation spectra [l-5], which place only the upper limit of the rate unless the resolution of the measurements prevails over the line broadening due to the uncertainty. The rates have also been estimated indirectly from the depths of dips in the ion-dip spectra [6]. On the other hand, there have been only a few direct spectroscopic measurements of the time scales of the dynamics [7-l 11. Although the product state distribution would also give important information on the dynamics, it has been investigated so far only for the dissociation of complexes consisting of small molecules [12-151. We have applied the SEP-LIF (stimulated emission pumping-laser-induced fluorescence) technique to investigate the dynamics of vibrationally excited van der Waals complexes [lo, 16,171. A specific species excited to a specific vibrational state by SEP is monitored by the LIF technique with an appropriate delay to probe the vibrational relaxation or predissociation. The relaxation or predissociation products are probed, as well as the decay of the species in the initially excited vibrational state. We have applied this technique to some van der Waals complexes, and have found that the time scale of the relaxation and/or dissociation depends critically on the species and on the energy of the excited vibration [ 10, 16, 171. The product of the vibrational predissociation has been observed for the anisole-Ar complex [16,17]. In relation to the studies on dynamics, it is important to investigate the structure, intermolecular vibration and intermolecular potential of van der Waals complexes. LIF spectroscopy is a powerful technique to measure intermolecular vibrations in the excited electronic state. The optical pump-probe absorption depletion technique (hole-burning spectroscopy) is also a powerful tool to measure the electronic spectra of a specific species among several others produced in a molecular beam [l&19]. In this technique, the frequency of the probe laser is fixed in resonance with one of the transitions of the species of interest, and the fluorescence caused by the probe laser is monitored. The 1435
1436
M. TAKAYANAGIandI. HANAZAKI
pump (saturation) laser is applied before the probe pulse and its frequency is scanned. Fluorescence depletion is observed when the pump frequency is in resonance with the transition which has a common initial state with the transition being probed. Only poor information is available so far on the intermolecular vibrations in the ground electronic state, since it is difficult to measure the dispersed-fluorescence spectra of complexes because of their low concentration in a supersonic jet. The fluorescence-dip technique can be applied to measure the vibrational spectra of complexes in the ground electronic state with much higher sensitivity and resolution. Here we describe the results of the application of these spectroscopic techniques to the anisole-benzene complex to obtain information on the intermolecular vibrations and the dynamics of the vibrationally excited states. In the LIF-excitation and hole-burning spectra, intermolecular vibrations in the excited electronic state are observed, while intermolecular vibrations in the ground electronic state are measured with the fluorescence-dip technique. The structure of the complex and its change upon electronic excitation are discussed. We have also obtained information on the decay rate in the vibrationally excited complex by SEP-LIF measurement.
EXPERIMENTAL
Details of the experimental procedures have been described elsewhere [lo, 181. Brief descriptions of the procedure pertinent to the present study are given below. The LIF-excitation spectra were measured using the frequency-doubled output of an excimer-laser-pumped dye laser (Lumonics: HE-420~SM-B and Lambda Physik: FL3002, -0.5 mJ/pulse; line width: 0.2 cm-‘). The hole-burning measurements were performed with the above excimer-pumped dye laser system as the probe source and the frequency-doubled output of a dye laser (Quantel: TDL-50, -1 mJ/pulse; line width: 0.2-0.5 cm-‘) pumped by the second harmonics of an Nd:YAG laser (Quantel: YG571C) as the pump source. The delay between the pump and probe pulses was 100 ns. In the fluorescence-dip measurements, the output of the Nd:YAG-pumped dye laser system mentioned above was used for the dump laser, while the other frequency-doubled output of the dye laser (Quantel: TDLJO, -0.2 mJ/pulse; line width: 0.2-0.5 cm-‘) pumped simultaneously by the same Nd:YAG laser was used for the pump laser; the latter pumps the anisole-benzene complex at the O-O transition and the former dumps it from the u’ = 0 state down to a vibrational state in the ground electronic state. These lasers were also used for SEP excitation in the SEP-LIF measurements. The probe laser in the SEP-LIF measurements was the excimer-pumped dye laser system mentioned above. The delay between the SEP and LIF pulses was varied between 200 ns and 1 ys. LIF measurements with a delay shorter than 200 ns are difficult because of the interference with a tail of the fluorescence caused by the pump pulse, while the longer delay is limited by the escape of the excited species from the region of observation. Anisole (Tokyo Kasei, reagent grade) and benzene (Dojindo, spectroscopic grade) were used as received. The mixture of anisole and benzene (ca 1O:l) was vaporized at - 10 “C, diluted by helium and expanded into a vacuum chamber with a pulsed nozzle (orifice diameter: 0.8mm; pulse duration: 850~s; stagnation pressure: -2.5 atm). All measurements were performed 20 mm downstream from the nozzle. The background pressure in the chamber was held below 1O-4 Torr throughout the measurements.
RESULTS AND DISCUSSION
Figure la shows the LIF-excitation spectrum measured for the mixture of anisole and benzene seeded in He. The O-O transition of the anisole monomer is observed at 36,386cm-‘. Many LIF bands are observed in the lower energy side of the O-O band. They can be attributed to the complex of anisole and benzene, since they are not observed when either anisole or benzene is seeded in He. The bands at 36,034 and 36,265 cm-’ are likely due to different species, since their relative intensity varies by changing the concentration ratio of anisole and benzene in the jet.
Intermolecular vibrations of anisole-benzene
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Fig. 1. (a) LIF-excitation spectrum of the mixture of ankle and benzene diluted by helium in a supersonic jet. The intense band at 36,386cm-’ is the O-O band of the anisole monomer. (b) Hole-burning spectrum measured with the probe frequency set at 36,034 cm-‘. (c) Hole-burning spectrum measured with the probe frequency set at 36,265 cm-‘.
The hole-burning spectra have been measured to confirm this. A series of bands starting at 36,034cm-’ towards the higher frequency in the LIF-excitation spectrum is observed in the hole-burning spectrum measured with the probe frequency set at 36,034 cm-’ (Fig. lb), while the series starting at 36,265 cm-’ is not observed. On the other hand, only the series starting at 36,265 cm-’ is observed in the hole-burning spectrum measured with the probe frequency fixed at 36,265 cm-’ (Fig. lc). These results confirm unambiguously that the series starting at 36,034 and 36,265 cm-’ are due to different species. The series of bands starting at 36,265 cm-’ is considered to be due to the anisole-benzene, complex, since their intensity in the LIF-excitation spectrum depends on the concentration of benzene more strongly than that of the 36,034 cm-’ series. In the LIF-excitation spectrum of the anisole-benzene complex reported by LAHMANI etal. [20], the series starting at 36,265 cm-’ is not observed, presumably because they used lower concentrations of benzene than we used here. Some bands in the hole-burning spectra seem to be saturated, since relative intensities in the LIF-excitation spectrum are not reproduced in the hole-burning spectra. Positive bands in the hole-burning spectra are caused by the pump-laser-induced fluorescence, which is too intense to fade out completely within the 100 ns delay between the pump and probe pulses. The dips observed in the hole-burning spectrum of anisole-benzene, (Fig. lc) are broader than those of anisole-benzene (Fig. lb). The vibronic structure of the former becomes indistinct at - 50 cm-’ above the origin band. This is presumably due to the
M. TAKAYANAGI and I. HANAZAKI
1438
existence of larger numbers of low-lying vibrational modes in anisole-benzene,. following, only the anisole-benzene complex will be discussed.
In the
Intermolecular vibrations In the LIF-excitation spectrum, the vibronic bands due to the intermolecular vibrations in the excited electronic state of the anisole-benzene complex are observed up to - 200 cm-’ in the higher energy side from the O-O band at 36,034 cm-‘. A candidate for the low-frequency vibration is the methyl torsion in anisole (-80 cm-‘) [21]. Although the torsional vibration of the anisole monomer is not observed in the LIF-excitation spectrum, it may become optically active upon complex formation. However, a similar measurement for the anisole-&benzene complex, in which deuteriums are substituted for hydrogen atoms in the methoxy group of anisole, gives no appreciable change in the LIF-excitation spectrum except for small (l-2 cm-‘) red shifts in the higher frequency region. Hence, all the bands in this region should be attributed to intermolecular vibrations. LAHMANI et al. [20] have reported that 29 cm-’ progressions are predominant in the LIF-excitation spectrum of the anisole-benzene complex. They proposed a sandwichtype structure for the complex, where two aromatic rings are parallel, and attributed the observed progressions to one of the bending motions of two aromatic planes. They have concluded that the bending angle between the aromatic planes changes upon electronic excitation, although the change is not so large because the observed progressions due to the intermolecular modes are short. Figure 2 shows the LIF-excitation spectrum of anisole-benzene on an expanded scale. The vibronic structure has been found to consist of progressions with - 27 cm-’ intervals. The interval tends to increase slightly for higher energies. This negative anharmonicity may be consistent with the assignment of the progression to the bending mode, since the steric hindrance between two rings increases as the amplitude of the bending motion becomes larger. Sub-origins are found at 44.0, 48.5, 52.8 and 80.0 cm-‘, each of which is followed by the 27cm-’ progression. In addition, a weak band is observed at 16.8cm-‘. These frequencies are much more reliable than those measured by LAHMANI et al. [20] because of the higher signal-to-noise ratio in our measurements. Our frequencies are, however, not consistent with their calculated intermolecular frequencies based on a simple intermolecular potential [20]. We have tentatively assigned the bands at 16.8 and 80.0 cm-’ to the intermolecular torsion and stretching vibrations, respectively, because 1
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Fig. 2. LIF-excitation spectrum of the anisole-benzene complex near the O-O transition. frequency at each peak is the shift from the O-O transition.
The
Intermolecular vibrations of anisole-benzene
iz
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Fig. 3. Fluorescence-dip spectrum of the anisole-benzene complex around the O-O transition measured with the pump frequency set at 36,034 cm-‘. The frequencyat each peak is the shift
from the O-Otransition. their frequencies are expected to be the lowest and the highest, respectively, among the six intermolecular vibrations. For a further confirmation of the assignments, measurements of the dispersed-fluorescence or fluorescence-dip spectra with the pump frequency set at each vibronic band are required. Figure 3 shows the fluorescence-dip spectrum of the anisole-benzene complex measured with the pump laser set at the O-O transition of the complex, where the intermolecular vibrations in the ground electronic state are observed. In contrast to the LIF-excitation spectrum, no progression with a regular interval is observed. However, the most intense dip at 35 cm-’ appears as combination modes with the intramolecular modes of anisole, as shown in Fig. 4, where the fluorescence-dip spectra in the 500-1700 cm-’ region are shown. Figure 5 shows the SEP-LIF spectrum, where the LIF-excitation spectra of the anisole-benzene complex is measured with and without the SEP excitation to the 35 cm-’ vibrational state. Upon SEP excitation, some bands, marked with an asterisk, appear in the LIF-excitation spectrum. Among them, the band denoted as 0 can be assigned to the transition from the initially excited vibrational state to u’ = 0. The other bands with asterisks appear with frequency shifts from the “0 cm-‘” band as indicated in Fig. 5 and correspond to the 16.8,44.0,52.8,70.2 and 96.1 cm-’ vibrational states in the excited electronic states (Fig. 2). It is to be noted that the 27 cm-’ band, which exhibits progressions in the LIF-excitation spectrum (Fig. 2), is not observed in Fig. 5, suggesting that the 35 cm-’ mode in the ground electronic state has a completely different character to the 27 cm-’ mode in the excited electronic state. Dynamics of excited vibrational states The decay rates of the vibrationally excited anisole-benzene complex were measured with the SEP-LIF technique. As discussed above, the SEP-LIF spectrum of the complex measured with the SEP excitation to the 35 cm-’ vibrational state shows that the complex stays in the initially excited vibrational state. Figure 6 shows the LIF intensity of the SEP-induced band at 35,999 cm-’ plotted against the delay. The observed decay presumably corresponds to the escape of the excited complex from the point of measurement. The decay rate of the complex in this vibrational state is therefore very slow. A similar result has been obtained for the 47 cm-’ vibrational state. In contrast to the intermolecular vibrational states, the intramolecular vibrational states between 500 cm-’ and the dissociation threshold of the complex (- 1360 cm-’ [20]) prepared by the SEP excitation could not be observed in the SEP-LIF spectra. The vibrational states are observed in the fluorescence-dip spectrum as clearly resolved bands with half widths of a few cm-’ ( Fi g. 4). Since the energy is below the dissociation
M. TAKAYANAGI and I. HANAZAK~
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Fig. 4. Fluorescence-dip spectrum of the anisole-benzene complex measured with the pump frequency set at 36,034cm-‘. Since several dyes are required for the dye laser to cover the region, the spectrum has some breaks. Many combination bands of the intramolecular modes of anisole with the intermolecular mode (33-36cm-‘) are observed.
threshold, the complex cannot dissociate. Therefore, this observation may be understood by considering that the SEP excitation prepares an ensemble of a huge number of states which is formed by the coupling of a state optically accessible from u’ = 0 and many optically inactive states [22,23]. When the intramolecular vibrational states above the dissociation threshold (1360-1662 cm-‘) are excited by SEP, neither the initially prepared state nor dissociated anisole are observed in the SEP-LIF spectrum, although they are observed in the fluorescence-dip spectra. This is in contrast to the anisole-Ar complex, for which the SEP excitation to the 782 cm-’ vibrational level, which exceeds the binding energy (-4OOcm-I), produces vibrationally hot anisole monomers as detected in the LIF-excitation spectrum [16,17]. In comparing anisole-benzene with anisole-Ar, the difference in the number of intermolecular vibrational modes (six for the former and three for the latter) is important since it gives rise to several orders-of-magnitude differences in the density of states at higher energies. Non-observation of the vibrationally hot anisole monomer for the anisole-benzene complex may then be ascribed to one of the following reasons. (1) The complex does not dissociate in the time scale of observation. The slow rate is due to the high density of states in the intermolecular modes. The initially prepared states are not observed for the same reason as above. (2) The complex dissociates to give hot monomers. However, because of the high density of states in the intermolecular modes, the excess energy scatters into a much wider range of internal states than that in anisole-Ar, so that the corresponding LIF band is too broad to be detected.
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Fig. 5. LIF-excitation spectrum measured with (a) and without (b) the SEP excitation of the anisole-benzene complex to the 35 cm-’ vibrational state. In (a), bands appearing upon SEP excitation are marked with asterisks, together with the frequency shifts from the band due to the transition from the initially excited vibrational state to u’ = 0 at 35,999 cm-‘. Not only the intermolecular modes but also some intramolecular modes seem to account for the dynamics. We found that the benzonitrile-Ar complex (the dissociation threshold is < 175 cm-’ [24,25]) excited to the 1007 cm-’ vibrational state stays in the initially excited state for more than 1 ps [lo], while the anisole-Ar complex excited to the 782cm-’ vibrational state was found to predissociate within 2OOns (the dissociation threshold is - 400 cm-‘) [16,17]. The large difference in the rate of the dynamics may be due to the presence of the non-rigid methoxy group of anisole compared with benzonitrile, which should enhance the coupling between the in-plane motion of the aromatic
I
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Fig. 6. The decay of the SEP-induced band due to the transition from the 35 cm-’ intermolecular vibrational state to u’ = 0 against the delay between the SEP and LIF pulses.
1442
M. TAKAYANAGIand I. HANAZAKI
ring and the out-of-plane intermolecular motion; the coupling strength between the intramolecular and intermolecular modes controls the energy flow. Another example may be obtained by comparing anisole-benzene studied here and the benzene dimer. A slow decay rate has been reported for the 992 cm-’ vibrational state of the benzene dimer [9,11]. On the other hand, the anisole-benzene complex excited to the vibrational states in the same energy region apparently does not stay in the initially prepared state. The difference may again be ascribed to the enhanced coupling between the intramolecular and intermolecular modes of the anisole-benzene complex due to the presence of a flexible methoxy group. A further study is in progress in our laboratory to elucidate the role of the methoxy group and of the vibrational state density in the vibrational relaxation and predissociation in van der Waals complexes. Acknowledgements-We are grateful to Prof. Y. Haas of the Hebrew University of Jerusalem for his useful comments. This research was supported in part by a Grant-in-Aid for Encouragement of Young Scientists No. 04740262 from the Ministry of Education, Science and Culture, Japan.
REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [.15] [16] [17] [18] (191 [20] [21] 122) [23] [24] [25]
R. E. Miller, J. Phys. Chem. 90, 3301 (1986). F. G. Celii and K. C. Janda, Chem. Rev. 86, 507 (1986). D. J. Nesbitt, Chem. Reo. 88, 843 (1988). S. E. Novick, K. R. Leopold and W. Klemperer, Atomic and Molecular Clusters (Edited by E. R. Bernstein), p. 359. Elsevier, Amsterdam (1990). R. 0. Watts, Atomic and Molecular Clusters (Edited by E. R. Bernstein), p. 393. Elsevier, Amsterdam (1990). T. Ebata, M. Furukawa, T. Suzuki and M. Ito, J. Opt. Sot. Am. B 7, 1890 (1990). M. P. Casassa, A. M. Woodward, J. C. Stephenson and D. S. King, J. Chem. Phys. 85, 6235 (1986). M. P. Casassa, J. C. Stephenson and D. S. King, J. Chem. Phys. 89, 1966 (1988). B. F. Henson, G. V. Hartland, V. A. Venturo and P. M. Felker, J. Chem. Phys. 91, 2751 (1989). M. Takayanagi and I. Hanazaki, .I. Opt. Sot. Am. B 7, 1898 (1990). T. Ebata, M. Hamakado, S. Moriyama, Y. Morioka and M. Ito, Chem. Phys. Lett. 199,33 (1992). D. S. King and J. C. Stephenson, J. Chem. Phys. 82, 5286 (1985). M. P. Casassa, J. C. Stephenson and D. S. King, J. Chem. Phys. 85, 2333 (1986). M. P. Casassa, J. C. Stephenson and D. S. King, Faraday Discuss. Chem. Sot. 82, 251 (1986). P. Brechignac, N. Halberstadt, B. J. Whithaker and B. Coutant, Chem. Phys. Lett 142, 125 (1987). M. Takayanagi and I. Hanazaki, Chem. Phys. Lett. 190, 115 (1992). M. Takayanagi and I. Hanazaki, 1. Chem. Phys. 98, 6958 (1993). M. Takayanagi and I. Hanazaki, Chem. Phys. Lett. 208, 5 (1993) and references cited therein. W. Sherzer, 0. Kraetzschmar, H. L. Selzle and E. W. Schlag, Z. Naturforsch. 47a, 1248 (1992). F. Lahmani, C. Lardeux-Dedonder, D. Solgadi and A. Zehnacker, J. Phys. Chem. 93, 3984 (1989). W. J. Balfour, J. Molec. Spectrosc. 109, 60 (1985). W. R. Lambert, P. M. Felker and A. H. Zewail, .I. Chem. Phys. 81,2217 (1984). P. M. Felker and A. H. Zewail, J. Chem. Phys. 81, 2975 (1984). T. Kobayashi, K. Honma, 0. Kajimoto and S. Tsuchiya, J. Chem. Phys. 86, 1111 (1987). T. Kobayashi and 0. Kajimoto, J. Chem. Phys. 86, 1118 (1987).
Intermolecular
vibrations and dynamics of the anisole-benzene studied by multi-resonance spectroscopy MASAO
(Received
and
TAKAYANAGI
Institute for Molecular
Science,
1 December
ICHIRO
Myodaiji,
1993; accepted
complex
HANAZAKI
Okazaki 444, Japan 17 January 1994)
Abstract-The intermolecular vibrations of the anisole-benzene complex in the ground and excited electronic states have been observed by the LIF (laser-induced fluorescence) and fluorescence-dip techniques. Short progressions due to the intermolecular vibrations suggest a small structure change of the complex upon electronic excitation. The LIF excitation spectrum shows predominant progressions of 27cm-‘, which is tentatively assigned to one of the intermolecular bending modes in the excited electronic state. On the other hand, the fluorescence-dip spectrum shows only a series of bands with irregular intervals due to the intermolecular modes in the ground electronic state. The decay rates of the vibrationally excited complex in the ground electronic state have also been measured with the SEP-LIF (stimulated emission pumping-laserinduced fluorescence) technique, where the complex vibrationally excited by SEP is probed by the delayed LIF measurements. The complex excited to its purely intermolecular mode stays in the initially prepared state after a delay time of 1 ps. On the other hand, the complex excited to the intramolecular vibrational states above 5OOcm-’ does not seem to stay in the prepared states. Neither the relaxed complex nor the dissociated monomer was detected. A possible reason for this observation is discussed.
INTRODUCTION VAN DER WAALS
complexes vibrationally excited in the ground electronic state provide a unique opportunity to investigate the role of the weak intermolecular bonding and associated vibrations in the dynamics of unimolecular decomposition and IVR (intramolecular vibrational redistribution). Various techniques have been used to study their dynamics. Among them, the rates of the dynamics have often been estimated indirectly from the band widths in the IR absorption or dissociation spectra [l-5], which place only the upper limit of the rate unless the resolution of the measurements prevails over the line broadening due to the uncertainty. The rates have also been estimated indirectly from the depths of dips in the ion-dip spectra [6]. On the other hand, there have been only a few direct spectroscopic measurements of the time scales of the dynamics [7-l 11. Although the product state distribution would also give important information on the dynamics, it has been investigated so far only for the dissociation of complexes consisting of small molecules [12-151. We have applied the SEP-LIF (stimulated emission pumping-laser-induced fluorescence) technique to investigate the dynamics of vibrationally excited van der Waals complexes [lo, 16,171. A specific species excited to a specific vibrational state by SEP is monitored by the LIF technique with an appropriate delay to probe the vibrational relaxation or predissociation. The relaxation or predissociation products are probed, as well as the decay of the species in the initially excited vibrational state. We have applied this technique to some van der Waals complexes, and have found that the time scale of the relaxation and/or dissociation depends critically on the species and on the energy of the excited vibration [ 10, 16, 171. The product of the vibrational predissociation has been observed for the anisole-Ar complex [16,17]. In relation to the studies on dynamics, it is important to investigate the structure, intermolecular vibration and intermolecular potential of van der Waals complexes. LIF spectroscopy is a powerful technique to measure intermolecular vibrations in the excited electronic state. The optical pump-probe absorption depletion technique (hole-burning spectroscopy) is also a powerful tool to measure the electronic spectra of a specific species among several others produced in a molecular beam [l&19]. In this technique, the frequency of the probe laser is fixed in resonance with one of the transitions of the species of interest, and the fluorescence caused by the probe laser is monitored. The 1435
1436
M. TAKAYANAGIandI. HANAZAKI
pump (saturation) laser is applied before the probe pulse and its frequency is scanned. Fluorescence depletion is observed when the pump frequency is in resonance with the transition which has a common initial state with the transition being probed. Only poor information is available so far on the intermolecular vibrations in the ground electronic state, since it is difficult to measure the dispersed-fluorescence spectra of complexes because of their low concentration in a supersonic jet. The fluorescence-dip technique can be applied to measure the vibrational spectra of complexes in the ground electronic state with much higher sensitivity and resolution. Here we describe the results of the application of these spectroscopic techniques to the anisole-benzene complex to obtain information on the intermolecular vibrations and the dynamics of the vibrationally excited states. In the LIF-excitation and hole-burning spectra, intermolecular vibrations in the excited electronic state are observed, while intermolecular vibrations in the ground electronic state are measured with the fluorescence-dip technique. The structure of the complex and its change upon electronic excitation are discussed. We have also obtained information on the decay rate in the vibrationally excited complex by SEP-LIF measurement.
EXPERIMENTAL
Details of the experimental procedures have been described elsewhere [lo, 181. Brief descriptions of the procedure pertinent to the present study are given below. The LIF-excitation spectra were measured using the frequency-doubled output of an excimer-laser-pumped dye laser (Lumonics: HE-420~SM-B and Lambda Physik: FL3002, -0.5 mJ/pulse; line width: 0.2 cm-‘). The hole-burning measurements were performed with the above excimer-pumped dye laser system as the probe source and the frequency-doubled output of a dye laser (Quantel: TDL-50, -1 mJ/pulse; line width: 0.2-0.5 cm-‘) pumped by the second harmonics of an Nd:YAG laser (Quantel: YG571C) as the pump source. The delay between the pump and probe pulses was 100 ns. In the fluorescence-dip measurements, the output of the Nd:YAG-pumped dye laser system mentioned above was used for the dump laser, while the other frequency-doubled output of the dye laser (Quantel: TDLJO, -0.2 mJ/pulse; line width: 0.2-0.5 cm-‘) pumped simultaneously by the same Nd:YAG laser was used for the pump laser; the latter pumps the anisole-benzene complex at the O-O transition and the former dumps it from the u’ = 0 state down to a vibrational state in the ground electronic state. These lasers were also used for SEP excitation in the SEP-LIF measurements. The probe laser in the SEP-LIF measurements was the excimer-pumped dye laser system mentioned above. The delay between the SEP and LIF pulses was varied between 200 ns and 1 ys. LIF measurements with a delay shorter than 200 ns are difficult because of the interference with a tail of the fluorescence caused by the pump pulse, while the longer delay is limited by the escape of the excited species from the region of observation. Anisole (Tokyo Kasei, reagent grade) and benzene (Dojindo, spectroscopic grade) were used as received. The mixture of anisole and benzene (ca 1O:l) was vaporized at - 10 “C, diluted by helium and expanded into a vacuum chamber with a pulsed nozzle (orifice diameter: 0.8mm; pulse duration: 850~s; stagnation pressure: -2.5 atm). All measurements were performed 20 mm downstream from the nozzle. The background pressure in the chamber was held below 1O-4 Torr throughout the measurements.
RESULTS AND DISCUSSION
Figure la shows the LIF-excitation spectrum measured for the mixture of anisole and benzene seeded in He. The O-O transition of the anisole monomer is observed at 36,386cm-‘. Many LIF bands are observed in the lower energy side of the O-O band. They can be attributed to the complex of anisole and benzene, since they are not observed when either anisole or benzene is seeded in He. The bands at 36,034 and 36,265 cm-’ are likely due to different species, since their relative intensity varies by changing the concentration ratio of anisole and benzene in the jet.
Intermolecular vibrations of anisole-benzene
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36401
cm-1
Fig. 1. (a) LIF-excitation spectrum of the mixture of ankle and benzene diluted by helium in a supersonic jet. The intense band at 36,386cm-’ is the O-O band of the anisole monomer. (b) Hole-burning spectrum measured with the probe frequency set at 36,034 cm-‘. (c) Hole-burning spectrum measured with the probe frequency set at 36,265 cm-‘.
The hole-burning spectra have been measured to confirm this. A series of bands starting at 36,034cm-’ towards the higher frequency in the LIF-excitation spectrum is observed in the hole-burning spectrum measured with the probe frequency set at 36,034 cm-’ (Fig. lb), while the series starting at 36,265 cm-’ is not observed. On the other hand, only the series starting at 36,265 cm-’ is observed in the hole-burning spectrum measured with the probe frequency fixed at 36,265 cm-’ (Fig. lc). These results confirm unambiguously that the series starting at 36,034 and 36,265 cm-’ are due to different species. The series of bands starting at 36,265 cm-’ is considered to be due to the anisole-benzene, complex, since their intensity in the LIF-excitation spectrum depends on the concentration of benzene more strongly than that of the 36,034 cm-’ series. In the LIF-excitation spectrum of the anisole-benzene complex reported by LAHMANI etal. [20], the series starting at 36,265 cm-’ is not observed, presumably because they used lower concentrations of benzene than we used here. Some bands in the hole-burning spectra seem to be saturated, since relative intensities in the LIF-excitation spectrum are not reproduced in the hole-burning spectra. Positive bands in the hole-burning spectra are caused by the pump-laser-induced fluorescence, which is too intense to fade out completely within the 100 ns delay between the pump and probe pulses. The dips observed in the hole-burning spectrum of anisole-benzene, (Fig. lc) are broader than those of anisole-benzene (Fig. lb). The vibronic structure of the former becomes indistinct at - 50 cm-’ above the origin band. This is presumably due to the
M. TAKAYANAGI and I. HANAZAKI
1438
existence of larger numbers of low-lying vibrational modes in anisole-benzene,. following, only the anisole-benzene complex will be discussed.
In the
Intermolecular vibrations In the LIF-excitation spectrum, the vibronic bands due to the intermolecular vibrations in the excited electronic state of the anisole-benzene complex are observed up to - 200 cm-’ in the higher energy side from the O-O band at 36,034 cm-‘. A candidate for the low-frequency vibration is the methyl torsion in anisole (-80 cm-‘) [21]. Although the torsional vibration of the anisole monomer is not observed in the LIF-excitation spectrum, it may become optically active upon complex formation. However, a similar measurement for the anisole-&benzene complex, in which deuteriums are substituted for hydrogen atoms in the methoxy group of anisole, gives no appreciable change in the LIF-excitation spectrum except for small (l-2 cm-‘) red shifts in the higher frequency region. Hence, all the bands in this region should be attributed to intermolecular vibrations. LAHMANI et al. [20] have reported that 29 cm-’ progressions are predominant in the LIF-excitation spectrum of the anisole-benzene complex. They proposed a sandwichtype structure for the complex, where two aromatic rings are parallel, and attributed the observed progressions to one of the bending motions of two aromatic planes. They have concluded that the bending angle between the aromatic planes changes upon electronic excitation, although the change is not so large because the observed progressions due to the intermolecular modes are short. Figure 2 shows the LIF-excitation spectrum of anisole-benzene on an expanded scale. The vibronic structure has been found to consist of progressions with - 27 cm-’ intervals. The interval tends to increase slightly for higher energies. This negative anharmonicity may be consistent with the assignment of the progression to the bending mode, since the steric hindrance between two rings increases as the amplitude of the bending motion becomes larger. Sub-origins are found at 44.0, 48.5, 52.8 and 80.0 cm-‘, each of which is followed by the 27cm-’ progression. In addition, a weak band is observed at 16.8cm-‘. These frequencies are much more reliable than those measured by LAHMANI et al. [20] because of the higher signal-to-noise ratio in our measurements. Our frequencies are, however, not consistent with their calculated intermolecular frequencies based on a simple intermolecular potential [20]. We have tentatively assigned the bands at 16.8 and 80.0 cm-’ to the intermolecular torsion and stretching vibrations, respectively, because 1
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Fig. 2. LIF-excitation spectrum of the anisole-benzene complex near the O-O transition. frequency at each peak is the shift from the O-O transition.
The
Intermolecular vibrations of anisole-benzene
iz
I
1439
I
I
36000 35900 35950 DUMP FREQUENCY
complex
36050 / cm-’
Fig. 3. Fluorescence-dip spectrum of the anisole-benzene complex around the O-O transition measured with the pump frequency set at 36,034 cm-‘. The frequencyat each peak is the shift
from the O-Otransition. their frequencies are expected to be the lowest and the highest, respectively, among the six intermolecular vibrations. For a further confirmation of the assignments, measurements of the dispersed-fluorescence or fluorescence-dip spectra with the pump frequency set at each vibronic band are required. Figure 3 shows the fluorescence-dip spectrum of the anisole-benzene complex measured with the pump laser set at the O-O transition of the complex, where the intermolecular vibrations in the ground electronic state are observed. In contrast to the LIF-excitation spectrum, no progression with a regular interval is observed. However, the most intense dip at 35 cm-’ appears as combination modes with the intramolecular modes of anisole, as shown in Fig. 4, where the fluorescence-dip spectra in the 500-1700 cm-’ region are shown. Figure 5 shows the SEP-LIF spectrum, where the LIF-excitation spectra of the anisole-benzene complex is measured with and without the SEP excitation to the 35 cm-’ vibrational state. Upon SEP excitation, some bands, marked with an asterisk, appear in the LIF-excitation spectrum. Among them, the band denoted as 0 can be assigned to the transition from the initially excited vibrational state to u’ = 0. The other bands with asterisks appear with frequency shifts from the “0 cm-‘” band as indicated in Fig. 5 and correspond to the 16.8,44.0,52.8,70.2 and 96.1 cm-’ vibrational states in the excited electronic states (Fig. 2). It is to be noted that the 27 cm-’ band, which exhibits progressions in the LIF-excitation spectrum (Fig. 2), is not observed in Fig. 5, suggesting that the 35 cm-’ mode in the ground electronic state has a completely different character to the 27 cm-’ mode in the excited electronic state. Dynamics of excited vibrational states The decay rates of the vibrationally excited anisole-benzene complex were measured with the SEP-LIF technique. As discussed above, the SEP-LIF spectrum of the complex measured with the SEP excitation to the 35 cm-’ vibrational state shows that the complex stays in the initially excited vibrational state. Figure 6 shows the LIF intensity of the SEP-induced band at 35,999 cm-’ plotted against the delay. The observed decay presumably corresponds to the escape of the excited complex from the point of measurement. The decay rate of the complex in this vibrational state is therefore very slow. A similar result has been obtained for the 47 cm-’ vibrational state. In contrast to the intermolecular vibrational states, the intramolecular vibrational states between 500 cm-’ and the dissociation threshold of the complex (- 1360 cm-’ [20]) prepared by the SEP excitation could not be observed in the SEP-LIF spectra. The vibrational states are observed in the fluorescence-dip spectrum as clearly resolved bands with half widths of a few cm-’ ( Fi g. 4). Since the energy is below the dissociation
M. TAKAYANAGI and I. HANAZAK~
1440
J
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35200
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Fig. 4. Fluorescence-dip spectrum of the anisole-benzene complex measured with the pump frequency set at 36,034cm-‘. Since several dyes are required for the dye laser to cover the region, the spectrum has some breaks. Many combination bands of the intramolecular modes of anisole with the intermolecular mode (33-36cm-‘) are observed.
threshold, the complex cannot dissociate. Therefore, this observation may be understood by considering that the SEP excitation prepares an ensemble of a huge number of states which is formed by the coupling of a state optically accessible from u’ = 0 and many optically inactive states [22,23]. When the intramolecular vibrational states above the dissociation threshold (1360-1662 cm-‘) are excited by SEP, neither the initially prepared state nor dissociated anisole are observed in the SEP-LIF spectrum, although they are observed in the fluorescence-dip spectra. This is in contrast to the anisole-Ar complex, for which the SEP excitation to the 782 cm-’ vibrational level, which exceeds the binding energy (-4OOcm-I), produces vibrationally hot anisole monomers as detected in the LIF-excitation spectrum [16,17]. In comparing anisole-benzene with anisole-Ar, the difference in the number of intermolecular vibrational modes (six for the former and three for the latter) is important since it gives rise to several orders-of-magnitude differences in the density of states at higher energies. Non-observation of the vibrationally hot anisole monomer for the anisole-benzene complex may then be ascribed to one of the following reasons. (1) The complex does not dissociate in the time scale of observation. The slow rate is due to the high density of states in the intermolecular modes. The initially prepared states are not observed for the same reason as above. (2) The complex dissociates to give hot monomers. However, because of the high density of states in the intermolecular modes, the excess energy scatters into a much wider range of internal states than that in anisole-Ar, so that the corresponding LIF band is too broad to be detected.
Intermolecular
vibrations of anisole-benzene
1441
36100
66000
LIF
complex
EXCITATION
FREQUENCY
/
cm-t
Fig. 5. LIF-excitation spectrum measured with (a) and without (b) the SEP excitation of the anisole-benzene complex to the 35 cm-’ vibrational state. In (a), bands appearing upon SEP excitation are marked with asterisks, together with the frequency shifts from the band due to the transition from the initially excited vibrational state to u’ = 0 at 35,999 cm-‘. Not only the intermolecular modes but also some intramolecular modes seem to account for the dynamics. We found that the benzonitrile-Ar complex (the dissociation threshold is < 175 cm-’ [24,25]) excited to the 1007 cm-’ vibrational state stays in the initially excited state for more than 1 ps [lo], while the anisole-Ar complex excited to the 782cm-’ vibrational state was found to predissociate within 2OOns (the dissociation threshold is - 400 cm-‘) [16,17]. The large difference in the rate of the dynamics may be due to the presence of the non-rigid methoxy group of anisole compared with benzonitrile, which should enhance the coupling between the in-plane motion of the aromatic
I
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Fig. 6. The decay of the SEP-induced band due to the transition from the 35 cm-’ intermolecular vibrational state to u’ = 0 against the delay between the SEP and LIF pulses.
1442
M. TAKAYANAGIand I. HANAZAKI
ring and the out-of-plane intermolecular motion; the coupling strength between the intramolecular and intermolecular modes controls the energy flow. Another example may be obtained by comparing anisole-benzene studied here and the benzene dimer. A slow decay rate has been reported for the 992 cm-’ vibrational state of the benzene dimer [9,11]. On the other hand, the anisole-benzene complex excited to the vibrational states in the same energy region apparently does not stay in the initially prepared state. The difference may again be ascribed to the enhanced coupling between the intramolecular and intermolecular modes of the anisole-benzene complex due to the presence of a flexible methoxy group. A further study is in progress in our laboratory to elucidate the role of the methoxy group and of the vibrational state density in the vibrational relaxation and predissociation in van der Waals complexes. Acknowledgements-We are grateful to Prof. Y. Haas of the Hebrew University of Jerusalem for his useful comments. This research was supported in part by a Grant-in-Aid for Encouragement of Young Scientists No. 04740262 from the Ministry of Education, Science and Culture, Japan.
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