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An examination is done on the water-containing clusters of oxindole (OI) and 3,4-dihydro-2(IH)-quinolinone (DQ), which have been researched in the hydride stretch region of the infrared by the technique of resonant ion-dip infrared spectroscopy (RIDIRS). In conclusion, the form of the measured normal modes provides insight into the nature of the bridge fundamentals.

The electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two-photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited state potential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground state surface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°–26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The resonance-enhanced ionization spectra of the S1←S0 transitions in jet-cooled benzylidenecyclopentane and benzylidenecyclohexane are presented and analyzed to characterize the torsional conformation of these two compounds in their ground and excited electronic states. Both spectra display considerable vibronic activity in low-frequency modes which are associated with torsional motion. In the cyclopentane derivative, a single conformation is found in the ground electronic state with a torsional angle of 23° which is consistent with a symmetric double-well torsional potential in which the torsional coordinate also involves inversion of the cyclopentane ring. A planar excited-state conformation is suggested by a hindered rotor analysis of the torsional energy levels. The cyclohexane compound is found in two ground-state conformations in a He expansion but only a single conformation in an Ar expansion, suggesting that conformer interconversion can occur over a relatively low barrier. Much larger steric interactions are found in this compound which are consistent with a chair conformation of the cyclohexane ring as suggested by semiempirical molecular orbital calculations. Both electronic states of benzylidenecyclohexane appear to have nonzero torsional angles which are considerably larger in magnitude than in the cyclopentane derivative. [ABSTRACT FROM AUTHOR]

Two-photon resonant, four-photon ionization spectra of vapor phase indole and 3-methylindole have been taken using both linearly and circularly polarized excitation. From these measurements, the wavelength dependent polarization ratio Ω has been determined. The value of Ω has been used to identify the onset of absorption to the second electronically excited state La amidst overlapping vibronic transitions to the lower lying Lb excited state. This onset occurs 1000 cm-1 above the Lb origin in indole and 100 cm-1 above the Lb origin in 3-methylindole. Several resolved transitions are also seen in the indole spectrum, roughly 1400 cm-1 above the Lb origin, which are tentatively assigned to the La origin. An analysis of the indole Lb origin rotational contour suggests that the two-photon absorption tensor for this transition is traceless and is dominated by the diagonal elements. The value of Ω for the La transition implies that its two-photon absorption tensor must have a nonzero trace and hence should exhibit a very different rotational contour. [ABSTRACT FROM AUTHOR]

Electronic excitation spectra of seven tryptophan derivatives entrained in a supersonic expansion have been recorded using both resonantly enhanced two-photon ionization and laser induced fluorescence. Two derivatives, tryptophan amide and tryptophan methyl amide, were found to have substantial low frequency vibrational progressions in their excitation spectra, yet in both compounds this behavior was apparent in only one conformer. Other derivatives did not display as much vibronic activity. Conformers which had vibrational progressions were found to emit in a broad band far to the red of excitation. All other conformers were found to fluoresce most strongly in resonance with excitation. The presence of low frequency vibrational activity and red shifted fluorescence correlates well with the ability of the derivative to form an intramolecular hydrogen bond between the amine and the carboxylic acid. Backbone conformers that contain an intramolecular hydrogen bond are expected to have large dipole moments, which may strongly perturb the electronic structure of the indole chromophore. [ABSTRACT FROM AUTHOR]

The general problem of the polarization dependence of an orientationally averaged four-photon process in a rigid isotropic medium is examined. The theory is then reduced to the special case of three-photon photoselection (3PP) in which an excited state is first created by the simultaneous absorption of three identical photons and then the system is interrogated by a one-photon probe step (absorption or emission). When the probe signal is measured as a function of both excitation and probing polarizations, five unique transition tensor invariants can be determined. These five invariants provide a characteristic signature of the symmetry of the initial three-photon absorption (3PA). A symmetry analysis is illustrated within the hexagonal point groups where it is demonstrated how the measurement of a single photoselection polarization ratio formally allows, with one exception, a unique symmetry assignment to be made for any three-photon allowed transition. This measurement makes use of circularly polarized excitation with variably polarized probing. The polarized probe step in a 3PP experiment provides information concerning the three-photon transition beyond that available from a simple polarized 3PA study. [ABSTRACT FROM AUTHOR]

Although group theoretically allowed, the benzene B2u←A1g three-photon transition appears to derive intensity exclusively through a vibronic coupling route. The ν6 (e2g) vibration is dominant in providing e1u vibronic symmetry, just as it is in the one-photon transition. Several experiments are presented that provide further insight into the lack of allowed vibronic b2u activity in the three-photon spectrum. Pseudoparity, applicable to even alternate hydrocarbons in the PPP formalism, and offering selection rules beyond those based on symmetry alone, is examined as a possible explanation by looking for effects from the inductive perturbation in a fluorine substituted benzene. The possibility of nonresonant third harmonic generation in the condensed phase sample, which may lead to a viable one-photon absorption route to the excited B2u state, is also explored in both a three-photon photoselection study as well as in an experiment comparing the relative three-photon to one-photon cross sections for benzene and biphenyl. The results obtained for the benzene B2u transition and the corresponding A′2 transition in the symmetrically substituted trifluorobenzene appear to confirm the importance of pseudoparity selection rules and suggest that the third harmonic generation pathway is of minor importance. [ABSTRACT FROM AUTHOR]

A direct inverse transform procedure is presented that can efficiently determine the resonant absorption band (ABS) responsible for an experimental Raman excitation profile (REP). In this technique, an integral Hilbert transform is directly evaluated, to reveal the phase of the complex function, which, at the magnitude squared level, represents the measurable REP. Once this complex REP function is known, the complex linear electrical susceptibility is readily calculated and hence the spectrum of any property derivable from it can be determined. The inverse transform is first demonstrated on a model REP to produce the corresponding ABS. It is then applied to several experimental REPs in resonance with the S4 transition in azulene. Finally it is linked with the conventional ‘‘forward’’ transform to show how the REP of one mode may be used to predict REPs of other vibrational modes. This linked procedure is tested using published REP data for the S1 transition in azulene. [ABSTRACT FROM AUTHOR]

Raman excitation profiles (REPs) from a highly excited state in azulene are presented for several vibrational modes. These include four fundamentals, a combination, and an overtone. Experimental REPs are compared to transform theory predictions which are based on the observable absorption spectrum (ABS) of the resonant electronic state. Though both the REPs and the ABS are multimode in nature, the transform provides a single mode link between the two and allows for extraction of excited state parameters on a mode by mode basis. For several modes, excited state frequencies and Franck–Condon coupling constants are estimated. Observed discrepancies between experiment and transform for other modes raise questions as to the applicability of the ‘‘standard assumptions’’ on which the transform is based to all of the vibrational modes in this electronic state. The strongly enhanced Raman scattering also allows depolarization ratios to be determined for all the azulene symmetric ring vibrations and directly confirms previous symmetry assignments. [ABSTRACT FROM AUTHOR]