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The linear Stark effect measured in the ν2/ν4 dyad of SnH4 by Doppler-free infrared–infrared double resonance spectroscopy has been analyzed. Fifteen double resonance signals in the vibrationally excited state and 12 previously reported signals in the ground state have been fitted simultaneously to an effective Stark Hamiltonian. The centrifugal distortion moment μ0 and the two vibration-induced moments μ44 and μ24 are determined to be μ0=1.499(11)×10-5 D, μ44=-1.317(42)×10-2 D, and μ24=3.94(23)×10-2 D. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The A˜ [sup 3]Π[sub i]←X˜ [sup 3]Σ[sup -] electronic transition of a carbon-chain molecule CCS has been observed for the first time by laser induced fluorescence spectroscopy. The species was generated with a pulsed discharge of 0.5% C[sub 2]H[sub 2] and 0.5% CS[sub 2] diluted in Ar via a pulsed supersonic jet. A number of bands in the 690-675 nm and 600-615 nm regions (14 500-14 800, 16 200-16 600 cm-1) were observed and assigned to those of CCS using ground state combination differences. Three of the bands in each region were found to belong to the three A˜ [sup 3]Π[sub i] spin-orbit components of the A˜ [sup 3]Π[sub i]←X˜ [sup 3]Σ[sup -] transition (T[sub v]=14 662.777(4) and 16 423 cm-1), and an effective set of rotational constants for the upper states were obtained. The higher energy band was heavily perturbed, preventing the determination of effective constants for this band. These bands have been tentatively assigned as two successive bands in the A˜ [sup 3]Π[sub i](n,0,0) progression with a resultant effective value of 1760 cm-1 for the A˜ [sup 3]Π[sub i] ν[sub 1] vibrational fundamental. Dispersed fluorescence spectra from the A˜ [sup 3]Π[sub i]←X˜ [sup 3]Σ[sup -] band of CCS have also been observed, yielding definitive experimental information on the vibrational structure of the X˜ [sup 3]Σ[sup -] ground state for the first time. The three harmonic frequencies were determined to be 1708.2(40) cm-1, 862.5(19) cm-1, and 269.3(13) cm-1 for ω[sub 1] (CC stretch), ω[sub 2] (CS stretch), and ω[sub 3] (CCS bend), respectively. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Excitation spectrum of the benzene-D 2 van der Waals complex in the vicinity of the S 1 ← S 0 complex [M. Hayashi and Y. Ohshima, J. Phys. Chem. A 0 1 vibronic transition of the monomer was recorded with sub-Doppler resolution by utilizing mass-selective two-color resonance-enhanced two-photon ionization. Contrary to the previous report on the benzene-H 2 complex [M. Hayashi and Y. Ohshima, J. Phys. Chem. A 117 , 9819 (2013)], both spin isomers correlating to para and ortho D 2 (with rotational angular momentum j = 1 and 0, respectively) are identified by using a gas sample of normal D 2 . Three and two vibronic bands involving vdW-mode excitation were observed for the para and ortho species, respectively, in addition to their origin bands. Comparison of the results for the two spin isomers has allowed us to make unambiguous band assignments, and vibrational frequencies of all the three vdW modes have been determined for benzene-H 2 and -D 2 . Among the three modes, the two-dimensional vdW twist is correlated to the hindered internal rotation of H 2 /D 2 and the barrier for the internal rotation has been evaluated: 72 and 66 cm -1 for benzene-H 2 and -D 2 , respectively. Vibronic-state dependence of the intermolecular distance between benzene and H 2 /D 2 is discussed on the basis of precisely determined rotational constants. Homogenous line broadening has been identified for all the observed vibronic bands, and the corresponding upper-state lifetimes are determined to be in the range of 0.3-0.7 ns.

We combine results from several different experiments to investigate the photophysics of acridine (Ac) and its hydrated clusters in the gas phase. Our findings are also compared with results from condensed phase studies. Similar to measurements of Ac dissolved in hydrocarbons, the lifetime of the first electronically excited state of isolated Ac in vacuum is too short for typical resonantly enhanced multiphoton ionization (REMPI) and laser induced fluorescence (LIF) experiments, hence no signal from REMPI and LIF can be attributed to monomeric Ac. Instead, sensitized phosphorescence emission spectroscopy is more successful in revealing the electronic states of Ac. Upon clustering with water, on the other hand, the lifetimes of the excited states are substantially increased to the nanosecond scale, and with two water molecules attached to Ac, the lifetime of the hydrated cluster is essentially the same as that of Ac in aqueous solutions. Detailed REMPI and ultraviolet-ultraviolet hole-burning experiments are then performed to reveal the structural information of the hydrated clusters. Although the formation of hydrogen bonds results in energy level reversal and energy separation between the first two excited states of Ac, its effect on the internal geometry of Ac is minimal, and all clusters with 1-3 water molecules demonstrate consistent intramolecular vibrational modes. Theoretical calculations reveal just one stable structure for each cluster under supersonic molecular beam conditions. Furthermore, different from mono- and di-water clusters, tri-water clusters consist of a linear chain of three water molecules attached to Ac. Consequently, the fragmentation pattern in the REMPI spectrum of tri-water clusters seems to be dominated by water trimer elimination, since the REMPI spectrum of Ac + ·W 3 is largely reproduced in the Ac + mass channel, but not in the Ac + ·W 1 or Ac + ·W 2 channel.

High-resolution spectra of the S1←S0 transition in jet-cooled deuterated benzenes were observed using pulse dye amplification of single-mode laser light and mass-selective resonance enhanced multiphoton ionization (REMPI) detection. The vibrational and rotational structures were accurately analyzed for the vibronic levels in the S1 state. The degenerate 6(1) levels of C6H6 or C6D6 are split into 6a(1) and 6b(1) in many of deuterated benzenes. The rigid-rotor rotational constants were assessed and found to be slightly different between 6a and 6b because of different mean molecular structures. Their rotational levels are significantly shifted by Coriolis interactions. It was found that the Coriolis parameter proportionally changed with the number of substituted D atoms.

Pure rotational transitions of the Ar-CS van der Waals complex have been observed by Fourier Transform Microwave (FTMW) and FTMW-millimeter wave double resonance spectroscopy. Rotational transitions of v(s) = 0, 1, and 2 were able to be observed for normal CS, together with those of C(34)S in v(s) = 0, where vs stands for the quantum number of the CS stretching vibration. The observed transition frequencies were analyzed by a free rotor model Hamiltonian, where rovibrational energies were calculated as dynamical motions of the three nuclei on a three-dimensional potential energy surface, expressed by analytical functions with 57 parameters. Initial values for the potential parameters were obtained by high-level ab initio calculations. Fifteen parameters were adjusted among the 57 parameters to reproduce all the observed transition frequencies with the standard deviation of the fit to be 0.028 MHz.

The Ã(3)A"-X̃(3)Σ(-) electronic transition of the HCCN∕DCCN radical was observed by laser-induced fluorescence spectroscopy. Rotationally resolved excitation spectra were observed for HCCN and DCCN, and effective molecular constants of the upper state were determined. The observed intensities of the rotational lines were anomalous, probably due to a level-dependent non-radiative decay process in the excited state. Fluorescence depletion spectroscopy was applied in order to observe non-fluorescent vibronic levels. A dispersed fluorescence spectrum was also observed to determine the vibrational level structure in the ground electronic state. The observed vibrational structures in the fluorescence depletion and dispersed fluorescence spectra were tentatively assigned based on the results of ab initio calculations.

Pure rotational spectra of the sodium chloride-water complexes, NaCl-(H(2)O)(n) (n = 1, 2, and 3), in the vibronic ground state have been observed by a Fourier- transform microwave spectrometer coupled with a laser ablation source. The (37)Cl-isotopic species and a few deuterated species have also been observed. From the analyses of the spectra, the rotational constants, the centrifugal distortion constants, and the nuclear quadrupole coupling constants of the Na and Cl nuclei were determined precisely for all the species. The molecular structures of NaCl-(H(2)O)(n) were determined using the rotational constants and the molecular symmetry. The charge distributions around Na and Cl nuclei in NaCl are dramatically changed by the complex formation with H(2)O. Prominent dependences of the bond lengths r(Na-Cl) on the number of H(2)O were also observed. By a comparison with results of theoretical studies, it is shown that the structure of NaCl-(H(2)O)(3) is approaching to that of the contact ion-pair, which is considered to be an intermediate species in the incipient solvation process.

We experimentally investigate the nonadiabatic rotational excitation process of a symmetric-top molecule, benzene, in the electronic ground state irradiated by intense nonresonant ultrafast laser fields. The initial rotational-state distribution was restricted mostly to the five lowest levels with different nuclear spin modifications by an extensive adiabatic cooling with the rotational temperature well below 1 K, and distributions after the interaction with a femtosecond double-pulse pair (3-5 TW/cm(2) each with 160 fs duration) with time delays were probed in a quantum-state resolved manner by employing resonant enhanced multiphoton ionization via the S(1) ← S(0) 6(0) (1) vibronic transition. Populations of 10 rotational levels with J ranging from 0 to 4 and K from 0 to 3 were examined to show an oscillatory dependence on the time delay between the two pulses. Fourier analysis of the beat signals provides the coupling strengths between the constituent levels of the rotational wave packets created by the nonadiabatic excitation. These data are in good agreement with the results from quantum mechanical calculations, evidencing stepwise excitation pathways in the wave packet creation with ΔJ = 2 in the K = 0 stack while ΔJ = 1 and 2 in the K > 0 stacks.

Vibrational level structure in the S(0) (1)A(g) and S(1) (1)B(3u) states of pyrene was investigated through analysis of fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation in a supersonic jet and through referring to the results of ab initio theoretical calculation. The vibrational energies are very similar in the both states. We found broad spectral feature in the dispersed fluorescence spectrum for single vibronic level excitation with an excess energy of 730 cm(-1). This indicates that intramolecular vibrational redistribution efficiently occurs at small amounts of excess energy in the S(1) (1)B(3u) state of pyrene. We have also observed a rotationally resolved ultrahigh-resolution spectrum of the 0(0) (0) band. Rotational constants have been determined and it has been shown that the pyrene molecule is planar in both the S(0) and S(1) states, and that its geometrical structure does not change significantly upon electronic excitation. Broadening of rotational lines with the magnetic field by the Zeeman splitting of M(J) levels was very small, indicating that intersystem crossing to the triplet state is minimal. The long fluorescence lifetime indicates that internal conversion to the S(0) state is also slow. We conclude that the similarity of pyrene's molecular structure and potential energy curve in its S(0) and S(1) states is the main cause of the slow radiationless transitions.

We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 0(0)(0) (a-type) and 0(0)(0)+467 cm(-1) (b-type) bands of the S(2) (1)A(1)<--S(0) (1)A(1) transition of jet-cooled azulene. The observed linewidth is 0.0017 cm(-1), which corresponds to the lifetime of 3.1 ns in the S(2) state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S(0) and S(2) states (C(2v) symmetry). Rotational constants of the S(2) state are almost identical to those of the S(0) state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S(2) (1)A(1) state of azulene was identified to be IC to the S(1) (1)B(2) state. However, this S(2)-->S(1) IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S(1) (1)B(2) state, which is also responsible for the extraordinarily fast S(1)-->S(0) IC in the isolated azulene molecule.

Pure rotational transitions in the ground state for Ar-OH and Ar-OD [Y. Ohshima et al., J. Chem. Phys. 95, 7001 (1991) and Y. Endo et al., Faraday Discuss. 97, 341 (1994)], those in the excited states of the OH vibration, nu(s)=1 and 2, observed by Fourier-transform microwave spectroscopy in the present study, rotation-vibration transitions observed by infrared-ultraviolet double-resonance spectroscopy [K. M. Beck et al., Chem. Phys. Lett. 162, 203 (1989) and R. T. Bonn et al., J. Chem. Phys. 112, 4942 (2000)], and the P-level structure observed by stimulated emission pumping spectroscopy [M. T. Berry et al., Chem. Phys. Lett. 178, 301 (1991)] have been simultaneously analyzed to determine the potential energy surface of Ar-OH in the ground state. A Schrodinger equation, considering all the freedom of motions for an atom-diatom system in the Jacobi coordinate, R, theta, and r, was numerically solved to obtain energies of the rovibrational energy levels using the discrete variable representation method. A three-dimensional potential energy surface is determined by a least-squares fitting. In the analysis the potential parameters, obtained by ab initio calculations at the RCCSD(T) level of theory with a set of basis functions of aug-cc-pVTZ and midbond functions, are used as initial values. The determined intermolecular potential energy surface and its dependence on the OH monomer bond length are compared with those of an isovalent radical complex, Ar-SH.

Fluorescence excitation spectra of the S(1)-S(0) origin band of 9-cyanoanthracene have been observed under a uniform electric field up to 200 kV/cm to explore pendular-state spectrum of an asymmetric-top molecule close to the strong field limit. The observed spectra exhibit distinct evolution of the band contour as a function of the applied electric field, which are much different from each other for different excitation configurations. An approximate method suitable for spectrum simulations of large asymmetric-top molecules in a pendular condition is developed for the analysis of the experimental results. The comparison of the observed and simulated spectra shows that the spectra are well ascribed in terms of the pendular-state selection rules, which have recently been derived from theoretical consideration of the pendular-limit representation of energy levels and spectra.