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The IR spectra of benzoic acid (BA), (BA)(H2O)nand (BA)2(H2O)n(n= 1, 2) clusters, and their ring-deuterated isotopologues in the 2800–3750 cm–1region were measured with IR-vacuum ultraviolet spectroscopy under the jet-cooled condition. For (BA)(H2O) and (BA)(H2O)2, only a single isomer was observed for each species, whereas for (BA)2(H2O) and (BA)2(H2O)2, more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA)m(H2O)nand their ring-deuterated isotopologues (BA-d5)m(H2O)nfor specific values of mand n. The anharmonic analysis based on the vibrational second-order perturbation theory indicated that the complexity of the IR spectra in these clusters was due to the appearance of many bands of (i) the overtone and combination modes involving the O–H bend of H2O and the in-plane C–O–H bends and the C═O stretch of BA, and (ii) the combination modes involving the hydrogen-bonded O–H stretch and low-frequency intermolecular vibrations, with considerable intensities.

The structures of a pyrazine dimer (pyrazine)2and (pyrazine)(benzene) hetero-dimer cooled in a supersonic beam were investigated by the measurement of the infrared spectra in the C–H stretching region with infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum-chemical calculations. The stabilization energy calculation at the CCSD(T)/aug-cc-pVTZ level of theory predicted three isomers for (pyrazine)2and three for (pyrazine)(benzene) with energy within 6 kJ/mol. Among them, the cross-displaced π–π stacked structure is the most stable in both dimers. In the observed IR spectra, both dimers exhibited two intense bands near 3065 cm–1, with intervals of 8 cm–1in (pyrazine)2and 11 cm–1in (pyrazine)(benzene), while only one band appeared in the monomer. For (pyrazine)(benzene), we also measured the IR spectrum of (pyrazine)(benzene-d6), where the interval of the two bands was unchanged. The analysis of the observed IR spectra with anharmonic calculations suggested the coexistence of three isomers of (pyrazine)2and (pyrazine)(benzene) in a supersonic jet. For (pyrazine)2, the two isomers which were previously assigned to the H-bonded planar and the π–π stacked structures respectively were reassigned to the cross-displaced π–π stacked and T-shaped structures, respectively. In addition, the quantum chemical calculation and IR-VUV spectral measurement suggested the coexistence of the H-bonded planar isomer in the jet. For (pyrazine)(benzene), the IR spectrum of the (pyrazine) site showed a similar spectral pattern to that of (pyrazine)2, especially the split at ∼3065 cm–1. However, the anharmonic analysis suggested that they are assigned to the different vibrational motions of (pyrazine). The anharmonic vibrational analysis is essential to associate the observed IR spectra with the correct structures of the dimer.

This study investigated the geometric and electronic structures of dibenzo-21-crown-7 (DB21C7) and dibenzo-24-crown-8 (DB24C8) complexes with alkali metal ions, identified as M+(DB21C7) and M+(DB24C8) (M = Na, K, Rb, and Cs), respectively. We observed the ultraviolet photodissociation (UVPD) spectra of these complexes under cold (∼10 K) gas-phase conditions. The conformations of the M+(DB21C7) and M+(DB24C8) complexes were determined by comparing the UVPD spectra with the calculated electronic transitions of the local-minimum forms. The interactions between the electronic excited states of the two benzene chromophores in the M+(DB21C7) and M+(DB24C8) complexes were examined and compared with those of previously studied complexes (dibenzo-15-crown-5 (DB15C5) and dibenzo-18-crown-6 (DB18C6)). The S1–S0and S2–S0electronic excitations of the M+(DB21C7) complexes were almost localized in one of the benzene rings. In contrast, the closed conformers of the M+(DB24C8) (M = K, Rb, and Cs) complexes were delocalized over the two chromophores for electronic excitations, exhibiting strong electronic interactions between the benzene rings. For the M+(DB24C8) complexes (M = K, Rb, and Cs), the short distance between the benzene rings (∼3.9 Å) led to a strong interaction between the benzene chromophores. We conclude that this strong interaction in the M+(DB24C8) complexes correlates strongly with the broad absorption in the UVPD spectra, suggesting the presence of an intramolecular excimer for the K+(DB24C8), Rb+(DB24C8), and Cs+(DB24C8) complexes.

Vibrational spectra of the CH stretching vibration of diazine molecules, pyrimidine and pyrazine, were measured by infrared (IR)–vacuum ultraviolet (VUV) spectroscopy under the jet‐cooled gas‐phase condition. The observed IR spectra were analyzed by three anharmonic algorithms to account for the Fermi‐resonance (F‐R). The anharmonic analysis of the F‐R pattern was performed with second‐order vibrational perturbation theory (VPT2) with quartic potentials (QPs) at the DFT level of B2PLYP/6–311++G(d,p), followed by vibrational configuration interaction (VCI) method with the same QP. The VPT2 + QP method reasonably reproduced most of the bands in the observed spectra for all the species, especially for pyrimidine, a decent agreement is obtained with respect to the band positions and relative intensities. The analyses of the spectra show that all the observed spectra can be well interpreted by the F‐R between the CH stretching fundamentals and the first overtone and 1 + 1 combination bands involving the in‐plane CH bending vibrations and the contribution of the higher‐order anharmonic coupling to the observed spectra seems to lead to red‐shift of the F‐R patterns. Discrete variable representation based methods with potential energy surfaces at CCSD/aug‐cc‐pVDZ theory with 10 degrees of freedom were also carried out to assess the quality of QP at DFT level. Progress of recent anharmonic calculation enabled to analyze the Fermi‐resonance of the CHstretching vibrations of aromatic molecules

The conformation and electronic structure of dibenzo-24-crown-8 (DB24C8) complexes with K+ion were examined by ion mobility–mass spectrometry (IM–MS), ultraviolet (UV) photodissociation (UVPD) spectroscopy in the gas phase, and fluorescence spectroscopy in solution. Three structural isomers of DB24C8 (SymDB24C8, Asym1DB24C8, and Asym2DB24C8) in which the relative positions of the two benzene rings were different from each other were investigated. The IM–MS results at 86 K revealed a clear separation of two sets of conformers for the K+(SymDB24C8) and K+(Asym1DB24C8) complexes whereas the K+(Asym2DB24C8) complex revealed only one set. The two sets of conformers were attributed to the open and closed forms in which the benzene–benzene distances in the complexes were long (>6 Å) and short (<6 Å), respectively. IM–MS at 300 K could not separate the two conformer sets of the K+(SymDB24C8) complex because the interconversion between the open and closed conformations occurred at 300 K and not at 86 K. The crown cavity of DB24C8 was wrapped around the K+ion in the complex, although the IM–MS results availed direct evidence of rapid cavity deformation and the reconstruction of stable conformers at 300 K. The UVPD spectra of the K+(SymDB24C8) and K+(Asym1DB24C8) complexes at ∼10 K displayed broad features that were accompanied by a few sharp vibronic bands, which were attributable to the coexistence of multiple conformers. The fluorescence spectra obtained in a methanol solution suggested that the intramolecular excimer was formed only in K+(SymDB24C8) among the three complexes because only SymDB24C8 could possibly assume a parallel configuration between the two benzene rings upon K+encapsulation. The encapsulation methods for K+ion (the “wraparound” arrangement) are similar in the three structural isomers of DB24C8, although the difference in the relative positions of the two benzene rings affected the overall cross-section. This study demonstrated that temperature-controlled IM–MS coupled with the introduction of appropriate bulky groups, such as aromatic rings to host molecules, could reveal the dynamic aspects of encapsulation in host–guest systems.

We performed UV spectroscopy for p-coumaric acid (pCA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S1(1ππ*)–S0absorption in the 31 500–33 500 cm–1region. Both pCA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the 1ππ* state was measured by picosecond pump–probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from 1ππ* and its time profile were measured by nanosecond UV–deep UV pump–probe spectroscopy. The transient state was observed for pCA and FA and assigned to the T1state, and we concluded that the NRD process of 1ππ* is S1(1ππ*) → 1nπ* → T1(3ππ*), similar to those of methyl cinnamate and para-substituted cinnamates such as p-hydroxy and p-methoxy methyl cinnamate. On the other hand, the transient T1state was not detected in CafA, and its NRD route is suggested to be S1(1ππ*) → 1πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the 1ππ* energy and suppresses the NRD process for all the species.

The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans–cisisomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S1–S0transition; UV–UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of synand antiisomers, where the methoxy (−OCH3) groups orient in opposite directions to each other. Picosecond UV–UV pump–probe spectroscopy revealed the NRD process from the excited singlet (S1(1ππ*)) state occurs at a rate constant of ∼1010–1011s–1, attributed to internal conversion (IC) to the 1nπ* state. Nanosecond UV–deep UV (DUV) pump–probe spectroscopy identified a transient triplet (T1(3ππ*)) state, whose energy (from S0) and lifetime are 18 400 cm–1and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S1(trans, 1ππ*) → 1nπ* → T1(3ππ*) → S0(cis)".

The UV photodissociation (UVPD) spectra of Ag+complexes with benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), and dibenzo-15-crown-5 (DB15C5) [Ag+(B18C6), Ag+(DB18C6), and Ag+(DB15C5)] are observed under cold gas-phase conditions. Ag+(B18C6) and Ag+(DB18C6) show sharp vibronic bands in the 36000–37200 cm–1region, while the UVPD spectrum of Ag+(DB15C5) is very broad. These UV bands are assigned to the π–π* transition, which is localized on the B18C6, DB18C6, and DB15C5 part of the complexes. Quantum chemical calculations suggest that the broad UV feature of Ag+(DB15C5) can be attributed to the short lifetimes of optically excited ππ* states due to internal conversion (IC) to low-lying excited states that are present only for this complex. The appearance of the π–π* transition in the same UV region as that of the neutral crown ethers and their complexes with alkali metal ions indicates that the positive charge is localized on the Ag atom in these complexes. However, the fragment ions produced after UV absorption are B18C6+, DB18C6+, and DB15C5+radical ions, indicating that they are produced via charge transfer (CT) between the Ag+ion and benzo-crown ethers. The CT during fragmentation is attributed to the higher ionization energy of Ag atom when compared to the benzo-crown ethers. In the complexes, the Ag+ion is effectively encapsulated by the crown cavity of the benzo-crown ethers without transferring the positive charge from Ag+to the crown. However, UV excitation of the Ag+(B18C6), Ag+(DB18C6), and Ag+(DB15C5) complexes can reduce the Ag+ion and produce a Ag atom with high efficiency in the gas phase.

Ultraviolet photodissociation (UVPD) and IR–UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn2+(benzo-15-crown-5), Mn2+(B15C5)(H2O)n(n= 0–2), under cold (∼10 K) gas-phase conditions. Density functional theory (DFT) calculations are also carried out to derive information on the geometric and electronic structures of the complexes from the experimental results. The n= 0 complex shows broad features in the UVPD spectrum, whereas the UV spectra of the n= 1 and 2 complexes exhibit sharp vibronic bands. The IR–UV and DFT results suggest that there is only one isomer each for the n= 1 and 2 complexes in which H2O molecules are directly attached to the Mn2+ion through Mn2+···OH2bonds with no intermolecular bond between the water molecules. Time-dependent DFT calculations suggest that the π–π* transition of the B15C5 part is highly mixed with the “ligand to metal charge transfer” transition in the n= 0 complex, which can result in broad features in the UVPD spectrum. In contrast, attachment of H2O molecules to Mn2+(B15C5) suppresses the mixing, providing sharp vibronic bands assignable to the π–π* transition for the n= 1 and 2 complexes. These results indicate that the electronic structure and transition of benzo-crown ether complexes with transition metals are strongly affected by solvation.

We measure UV and IR spectra in the gas phase for EuOH+, EuCl+, and TbO+ions, which are produced by an electrospray ionization source and cooled to ∼10 K in a cold, 22-pole ion trap. The UV photodissociation (UVPD) spectra of these ions show a number of sharp, well-resolved bands in the 30000–38000 cm–1region, although a definite assignment of the spectra is difficult because of a high degree of congestion. We also measure an IR spectrum of the EuOH+ion in the 3500–3800 cm–1region by IR–UV double-resonance spectroscopy, which reveals an OH stretching band at 3732 cm–1. We perform density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these ions in order to examine the nature of the transitions. The DFT results indicate that the states of highest-spin multiplicity (octet for EuOH+and EuCl+and septet for TbO+) are substantially more stable than other states of lower-spin multiplicity. The TD-DFT calculations suggest that UV absorption of the EuOH+and EuCl+ions arises from Eu(4f) → Eu(5d,6p) transitions, whereas electronic transitions of the TbO+ion are mainly due to the electron promotion of O(2p) → Tb(4f,6s). The UVPD results of the lanthanide-containing ions in this study suggest the possibility of using lanthanide ions as “conformation reporters” for gas-phase spectroscopy for large molecules.

A new technique, surface-enhanced infrared absorption (SEIRA) spectroscopy, was used for the structural investigation of lanthanide (Ln) and actinide (An) complexes containing organic ligands. We synthesized thiol derivatives of organic ligands with coordination sites similar to those of 2-[N-methyl-N-hexanethiol-amino]-2-oxoethoxy-[N′,N′-diethyl]-acetamide [diglycolamide (DGA)], Cyanex-272, and N,N,N′,N′-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN), which have been used for separating Ln and An through solvent extraction. These ligands were attached on a gold surface deposited on an Si prism through S–Au covalent bonds; the gold surface enhanced the IR absorption intensity of the ligands. Aqueous solutions of Ln (Eu3+, Gd3+, and Tb3+) and An (Am3+) ions were loaded onto the gold surface to form ion complexes. The IR spectra of the ion complexes were obtained using Fourier transform infrared spectroscopy in the attenuated total reflection mode. In this study, we developed a new sample preparation method for SEIRA spectroscopy that enabled us to obtain the IR spectra of the complexes with a small amount of ion solution (5 μL). This is a significant advantage for the IR measurement of radiotoxic Am3+complexes. In the IR spectra of DGA, the band attributed to C═O stretching vibrations at ∼1630 cm–1shifted to a lower wavenumber by ∼20 cm–1upon complexation with Ln and An ions. Moreover, the amount of the red shift was inversely proportional to the extraction equilibrium constant reported in previous studies on solvent extraction. The coordination ability of DGA toward Ln and An ions could be assessed using the band position of the C═O band. The Cyanex-272- and TPEN-like ligands synthesized in this report also showed noticeable SEIRA signals for Ln and An complexes. This study indicates that SEIRA spectroscopy can be used for the structural investigation of ion complexes and provides a microscopic understanding of selective extraction of Ln and An.

VERy relaxing: Vibrational energy relaxation (VER) of benzene dimer isotopologues in the CH stretching vibrational level has been investigated by picosecond time‐resolved IR–UV pump–probe spectroscopy (see picture, h=C6H6, d=C6D6). A remarkable difference is found in the relaxation lifetime: the lifetime of the stem site (110 ps) is 4.5 times shorter than that of the top site (500 ps).

Ganz entspannt: Die Schwingungsenergierelaxation (VER) von Isotopologen des Benzol‐Dimers auf dem C‐H‐Streckschwingungsniveau wurde durch zeitaufgelöste Pikosekunden‐IR‐UV‐Pump‐Sonden‐Spektroskopie untersucht (siehe Bild, h=C6H6, d=C6D6). Dabei ergab sich ein bemerkenswerter Unterschied in den Relaxationslebensdauern: Die Lebenszeit des „stehenden“ Benzolmoleküls („Stem“, 110 ps) ist 4.5mal kürzer als die des „liegenden“ („Top“, 500 ps).

Structure and dynamics of gas-phase molecules and their clusters formed in a supersonic jet has been a great interest to many researchers. Studies on these simple systems provide us with the fundamental concept of many physical and chemical phenomena observed in bulk systems. These studies are also important for the verification of theoretical prediction and quantum chemical calculations. Especially molecular clusters have been thought to be a good model for molecular level understanding of intermolecular interaction and relaxation in condensed phase. However, until the beginning of 1990, studies of molecular clusters mainly concentrated on electronic spectroscopy and very few studies were reported for vibrational spectroscopy. On the other hand, vibrational spectroscopy is essential in condensed phase, and there has been a gap between cluster science and condensed phase science. In 1993 we first reported infrared (IR)–ultraviolet (UV) double resonance spectroscopy of the OH stretching vibration of phenol and its hydrated clusters in a supersonic jet. The observed vibrational spectra showed a characteristic feature of the H-bonding, that is a larger frequency shift as well as broadening of the OH stretching band, and the structures were unambiguously determined with the aid of quantum chemical calculation. It is not too much to say that the combination of double resonance vibrational spectroscopy and quantum chemical calculation has become a very popular method in cluster science. In this paper, we describe various double resonance vibrational spectroscopies in frequency domain. Typical examples of application are shown; determination of H-bonded clusters of benzonitrile, conformer selective vibrational spectroscopy for l-phenylalanine and its hydrated cluster, and study of the encapsulation structure of guest atom and molecules by functional molecules, calixarene and crown ether. Finally, time-domain studies for the dynamics of the vibrations are described. Picosecond IR–UV pump–probe spectroscopy is applied to study the vibrational energy relaxation (VER) of the OH stretching vibration of isolated aromatic molecules and the H-bonded clusters.

IR-UV double resonance spectroscopy and ab initiocalculations were employed to investigate the structures and vibrations of the aromatic amino acid, l-phenylalanine–(H2O)nclusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of l-phenylalanine (l-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for l-Phe–(H2O)1, and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In l-Phe–(H2O)2, only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of l-Phe–(H2O)3the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the π-system. Finally, in isolated l-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initiocalculation.

Two-color (1 + 1′) REMPI mass spectra of o-, m- and p-fluorophenol·ammonia (1 n) clusters were measured with a long delay time between excitation and ionization lasers. The appearance of NH4(NH3)n−1+with 100 ns delay after exciting the S1state is a strong indication of generation of long-lived species viaS1. In analogy with the phenol·ammonia clusters, we conclude that an excited state hydrogen transfer reaction occurs in o-, m- and p-fluorophenol·ammonia clusters. The S1-S0transition of o-, m- and p-fluorophenol·ammonia (1 : 1) clusters were measured by the (1 + 1′) REMPI spectra, while larger (1 n) cluster (n= 2–4) were observed by monitoring the long-lived NH4(NH3)n−1clusters action spectra. The vibronic structures of m- and p-fluorophenol·ammonia clusters are assigned based on vibrational calculations in S0. The o-fluorophenol·ammonia (1 : 1) cluster shows an anharmonic progression that is analyzed by a one-dimensional internal rotational motion of the ammonia molecule. The interaction between the ammonia molecule and the fluorine atom, and its change upon electronic excitation are suggested. The broad action spectra observed for the o-fluorophenol·ammonia (1 : n) cluster (n 2) suggest the excited state hydrogen transfer is faster than in m- and p-fluorophenol·ammonia clusters. The different reaction rates between o-, m- and p-fluorophenol·ammonia clusters are found from comparison between the REMPI and action spectra.

Infrared (IR) spectra of benzene–(water)_n cluster cations (Bz–W_n)+ (n = 1–6) in the OH and CH stretching vibrational region were observed in order to investigate their structure and reactivity. The cluster cations were prepared by two different production methods: one is due to collision between bare benzene cations and water clusters; and the other utilizes resonance enhanced multiphoton ionization (REMPI) of neutral clusters. The former method prefers the production of the most stable isomer cluster cations, while the latter would reflect the Franck-Condon restriction in the ionization process. The structures of the n = 1 and n = 2 clusters were determined on the basis of the comparison between the IR spectra and density functional theory (DFT) calculations. In the n = 1 cluster cation, the oxygen atom of the water molecule is located in the benzene ring plane and coordinates to the benzene moiety by two identical CH–O hydrogen bonds. The IR spectra of the n = 2 cluster cation showed absorption bands arising from two different types of isomers: one has a hydrogen-bonded water dimer interacting with the benzene cation; in the other isomer two water molecules are independently bound to the benzene cation. The production ratio between the isomers was found to strongly depend on the cluster ion preparation methods. Except for the case of the n = 2 cluster, the cluster cations prepared by the two different methods gave identical IR spectra. This means that quite extensive rearrangements of the cluster structure occur upon ionization of the neutral clusters, leading to the most stable form of the cluster cations. The spectral features of the n = 3 cluster cation are very similar to the n = 2 cluster, suggesting similar structures among these clusters. Higher clusters larger than the n = 3 cluster showed quite different IR spectra from those of the n ≤ 3 clusters, but their spectral features are very similar to those of hydrated clusters of protonated species, X–H+–(H₂O)_n, indicating that proton transfer reactions from the benzene cation to the water moiety occur in the larger clusters than those with n = 3.

An investigation of CH stretching vibrations in benzene and toluene in their S₁ states has been carried out using UV–IR and stimulated Raman–UV double resonance spectroscopic methods. In benzene two CH stretching vibrations, namely ν'₂₀ and ν'₂, were observed, and in the case of toluene both aromatic CH and methyl CH stretching vibrations were observed. The ν'₂₀ of benzene is strongly coupled with ν'₈ + ν'₁₄ via anharmonic resonance. The aromatic CH stretching vibrations in toluene also exhibit strong anharmonic resonance, leading to the appearance of a large number of bands in the 3000–3100 cm−1 region. The observed frequencies of CH stretching vibrations in the S₁ state in both benzene and toluene are higher than the corresponding values in the S₀ state. On the other hand, methyl CH stretching vibrations in the S₁ state of toluene occur at a lower frequency than those in the S₀ state.

Raman spectra of ring-breathing (ν12) and CN stretching vibrations (νCN) of jet-cooled benzonitrile and its clusters with Ar, H2O, CH3OH, CH3CN, CCl4, CHCl3 and CHF3 were observed by stimulated Raman–UV double resonance spectroscopy. Cluster structures were determined on the basis of the observed vibrational spectra, rotational contour analyses of the high resolution S1–S0 electronic spectra and ab initio molecular orbital calculations with the HF/6–31G(d,p) basis set. It was found that the types of the cluster structures are classified into three groups. (1) The clusters with H2O, CH3OH and CH3CN represent the ‘side’-type structure, in which the intermolecular bond is formed in the phenyl plane of benzonitrile, and the benzonitrile dimer also belongs to this structure. (2) The clusters with Ar and CCl4 form the ‘on-top’-type structure, where Ar or CCl4 is located above the phenyl plane. (3) The clusters with CHCl3 and CHF3 have the ‘linear’-type structure, in which these molecules are located at the N end of the CN group of benzonitrile. The rate constants of the vibrational relaxation for these clusters occurring after the Raman excitation to their ν12 vibrations were also observed. It was found that different rate constants are associated with the different types of cluster structures. That is, the 121 levels of the ‘side’-type clusters relax much faster than those of other types of the clusters. The results represent a clear indication that the anharmonic coupling between ν12 and intermolecular vibrations are much larger in the ‘side’-type clusters than others. Copyright © 2000 John Wiley & Sons, Ltd.

The S1-S0electronic spectrum of methyl cinnamate in a supersonic jet has been investigated to discriminate the transitions of the s-cis/s-trans conformers. Population labelling spectroscopy was applied to the conformer discrimination, and vibronic bands belonging to each conformer were identified. The relative population was estimated from the fluorescence quantum yields obtained by lifetime measurements. It was found that both conformers exist almost equally in a supersonic jet. The conformer identification of the vibronic bands was carried out based on the difference of the red-shift of their hydrogen-bonded complexes with methanol.

Stimulated-emission ion-dip spectroscopy was applied for the observation of the ground-state vibrational levels of jet-cooled phenol and its hydrogen-bonded complex with water. The observed ion-dip intensity, representing the stimulated emission from S_1 to the vibrational level in S_0 yields information about the intramolecular vibrational redistribution rate of the vibrational state. The intramolecular vibrational redistribution rate was estimated from the observed ion-dip intensities for the above species. It was found that a great enhancement of the intramolecular vibrational redistribution rate occurs with the formation of the hydrogen-bonded complex. The enhancement is ascribed to the increase of the density of the vibrational states contributed by the low-frequency hydrogen-bond modes.

The C 1132 ← X̃1'A, transition of jet-cooled SO2has been observed by degenerate four-wave mixing (DFWM) spectroscopy. Under the condition with a high input laser power, the DFWM intensity was proportional to the laser power and the DFWM spectrum represented the absorption spectrum. The relative fluorescence quantum yield was obtained by comparing the DFWM spectrum with the LIF spectrum. The vibronic dependence of the quantum yield agreed well with that of the fluorescence lifetime. It was found that the predissociation rate of the vibronic levels involving anti-symmetric stretching vibration is much faster than others.

Carbon atom generation following the predissociation of Rydberg states of CO has been investigated. The Rydberg states of CO were prepared by two-color double resonant excitation through the3sσB1Σ+state and the atomic carbon fragment was detected by resonance-enhanced multiphoton ionization. The4s–6s(v= 0), 5s(v= 1), 4p(v= 0), 3dπ(v= 0, 1), 5dσ(v= 0), 4f–5f(v= 0) Rydberg states and theW1Π(v= 2, 3) state were observed, and it was confirmed that all the states predissociate to generate the3P carbon atom. The distributions in the spin-orbit sublevels (J) of the3PJcarbon atoms were also investigated.

The S1−S0electronic spectrum of methyl cinnamate in a supersonic jet has been investigated to discriminate the transitions of the s-cis/s-transconformers. Population labelling spectroscopy was applied to the conformer discrimination, and vibronic bands belonging to each conformer were identified. The relative population was estimated from the fluorescence quantum yields obtained by lifetime measurements. It was found that both conformers exist almost equally in a supersonic jet. The conformer identification of the vibronic bands was carried out based on the difference of the red-shift of their hydrogen-bonded complexes with methanol.

Stimulated-emission ion-dip spectroscopy was applied for the observation of the ground-state vibrational levels of jet-cooled phenol and its hydrogen-bonded complex with water. The observed ion-dip intensity, representing the stimulated emission from S_I to the vibrational level in S_0, yields information about the intramolecular vibrational redistribution rate of the vibrational state. The intramolecular vibrational redistribution rate was estimated from the observed ion-dip intensities for the above species. It was found that a great enhancement of the intramolecular vibrational redistribution rate occurs with the formation of the hydrogen-bonded complex. The enhancement is ascribed to the increase of the density of the vibrational states contributed by the low-frequency hydrogen-bond modes.

Vibrational spectroscopies of small-sized hydrogen-bonded clusters of organic acids and related molecules, as well as their ions, are reviewed based on our recent results. OH stretching vibrations of the jet-cooled clusters generated by supersonic expansions are observed by the various size-selected and population-labelling spectroscopic methods; ionization detected infrared (IR) and or stimulated Raman spectroscopies for the neutral clusters in the electronical ground state (S ) and fluorescence detected IR spectroscopy for the clusters in the electronically excited state (S ). The hydrogen-bond structures of phenol-(H O) clusters are n extensively investigated on the basis of the spectral analysis combined with ab initio calculations of their stable forms and vibrations. Remarkable enhancement of the hydrogen-bond strength upon electronic excitation is demonstrated for the IR spectra of the S clusters of phenol. For tropolone-(H O) and- (CH OH) n n clusters, (phenol), and fluorobenzene-(CH OH) clusters, cluster-size-dependent n rearrangements and transformations of their hydrogen bonds are also investigated. IR dissociation spectroscopy of the cluster ions involving an ion-trapping technique is also described. The method is used to obtain vibrational spectra of (phenol), \[phenol-(H O) ] and (phenol-benzene) ; their characteristic spectra of n n the OH stretching vibrations indicate that extremely large changes of the intermolecular hydrogen bonds are induced upon ionization of the clusters. Finally, a novel method for vibrational spectroscopy of bare molecular ions, for which no dissociationspectroscopic techniqueis successful, is described and its applicationto the IR spectrum of the phenol cation is given. The method involves autoionization process following vibrational excitation of the high-Rydberg-state molecule whose core has essentially the same vibrational structure as that of the bare ion. Future applications and directions of vibrational spectroscopy of clusters are discussed.

Rotational distributions of CN(X2S+, v?=0) generated by the 266.0 nm photolysis of ICN in a supersonic free jet have been determined by the measurements of the fluorescence excitation spectrum. The distributions are expressed as the sum of three Boltzmann distributions characterized by temperatures of 12, 230, and 2000 K.

The infrared (IR) spectra of the O–H stretching vibrations of pyridine–water clusters (Pyd)m(H2O)n, with m, n= 1–4, have been investigated with infrared–vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd)m(H2O)n+by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m= 1 and 2 and stronger signals for m≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd)m(H2O)nupon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd)2+cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd)2(H2O)n, which were obtained with quantum-chemical calculations. Most of the calculated (Pyd)2(H2O)nclusters had the characteristic structure in which H2O or (H2O)2forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd)2, and an additional H2O molecule(s) extends the H-bonded network. The π-stacked (Pyd)2(H2O)nmoiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O–H stretch fundamentals and the overtones of the O–H bending vibrations in (Pyd)m(H2O)nwas found to be less pronounced in the case of (Pyd)m(NH3)nstudied previously.

The vibrational distribution of the CS(X) fragment has been studied by the UV flash photolysis and kinetic spectroscopic technique. The CS fragment was generated by the flash photolysis of CS2at ≈200 nm. The vibrational distribution is well represented by a sum of two modified Poisson distributions with different mean values. This distribution is explained by assuming the following two photodissociation processes of CS2; one is a predissociation through a triplet state and the other is a direct photodissociation.(Remark: Graphics omitted.)

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