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The resonance hyper-Raman (RHR) scattering of CH3I vapor is observed as blue incident radiation is tuned through two-photon resonance with the ν2 vibronic absorption band of the predissociative B←X Rydberg transition in the UV. In analogy to linear resonance Raman spectroscopy, the RHR band shapes and polarization are found to be a sensitive function of the two-photon resonant vibronic state lifetime and detuning. The dependence of these scattering characteristics on the two-photon excited-state dephasing constant provides a technique for determining subpicosecond predissociation rates via nonlinear spontaneous Raman scattering. Theoretical fits to the observed resonant rovibrational hyper-Raman depolarization ratio dispersion curves are obtained for a vibronic B-state lifetime of 0.5±0.2 ps. This lifetime value is in agreement with previous results obtained from linear resonance Raman scattering studies. The polarization analysis of RHR scattering, in particular, provides a technique for measuring excited-state lifetimes of molecular transitions in the VUV. [ABSTRACT FROM AUTHOR]

Vibronically specific, subrotational period lifetimes of the predissociated B state (201–190 nm) of CH3I and CD3I are determined by spontaneous resonance Raman scattering measurements of depolarization ratios and rotationally resolved Raman excitation profiles (REPs). The lifetime of the electronic origin, methyl rocking (ν6), and methyl umbrella (ν2) band are the same in CH3I (0.5 ps) and CD3I (1.2 ps). However, the predissociation rate decreases by a factor of ∼3 when one quantum of ν3, the methyl–iodine stretch, is excited on the B-state surface. The only vibrational mode which is found to accelerate the B-state predissociation rate is the a1 C–H (C–D) stretch. The lifetime of the ν1 vibronic level decreases by nearly an order of magnitude for CH3I (60 fs) and by a factor of 2 for CD3I (0.6 ps) relative to the corresponding electronic origin level. In addition, K-resolved REP analysis reveals predissociation rates to be K independent. These mode-specific predissociation rates are ascribed to the shape of the multidimensional reaction coordinate in the region of the curve crossing between the bound (B state) and unbound surfaces. [ABSTRACT FROM AUTHOR]

All three scattering invariants, isotropic, antisymmetric, and anisotropic, contribute to the resonance Raman (RR) activity of randomly oriented gas phase scatterers as shown in a sum-over-all-rovibronic-states approach regardless of the symmetry of the vibrational transition. Thus, in the gas phase, totally symmetric modes may have Raman depolarization ratios (ρ)>3/4 and nontotally symmetric modes may have ρ≠3/4 for resonance excitation frequencies. Large ρ dispersion effects are predicted when the RR scattering cross section is dominated by the contribution of a single vibronic band. The extent of this purely rotational effect sensitively depends on the excitation frequency and the resonant dephasing rate. These depolarization ratio effects are demonstrated by the resonance Raman scattering of CH3I (B state) and NH3, ND3 (A state) for both a and e vibrational bands as well as for J (NH3) and K (CH3I) resolved rovibrational features. [ABSTRACT FROM AUTHOR]

The Raman scattering due to resonance with a perpendicularly polarized electronic transition of a symmetric top molecule in the gas phase is described in a sum-over-all-states approach. The derived intensity and depolarization ratio expressions in an irreducible two-photon tensor basis are applied to the analysis of the ν2 (a1 ) and ν6 (e) bands of CH3 I derived from resonance with the predissociated X→B absorption system. A lifetime of 0.5±0.1 ps is determined for the electronic origin and several K-specific rovibronic levels of the v’6 =1 band of the resonant excited state. These results are contrasted with recent dynamical interpretations of the corresponding jet-cooled CH3 I absorption spectra. [ABSTRACT FROM AUTHOR]

Resonance hyper-Raman (RHR) excitation profiles of rovibrational transitions of NH3 are observed as the incident blue radiation is tuned through two-photon resonance with the v‘2 =2 and v‘2 =3 bands of the UV X→A Rydberg transition. The excitation frequency dependence of the rotationally resolved HREPs are fit by our previously derived intensity expressions [L. D. Ziegler et al., J. Chem. Phys. 87, 4498 (1987)]. These fits to theory reveal J-dependent photodissociation rates for both vibronic bands on the A state surface that correspond to subpicosecond lifetimes. The rotationally assisted dynamical effects are consistent with the previous results of linear rotational Raman excitation profile studies and with an adiabatic centrifugal mechanism which couples rotational motion with the photodissociative reaction coordinate. [ABSTRACT FROM AUTHOR]

The intensities of vibrational resonance hyper-Raman transitions are developed in the usual framework of vibronic theory. In analogy to linear Raman scattering, the leading terms of RHR scattering, A, B, and C, are found when the electronic transitions moments, appearing in the transition hyperpolarizability, are expanded to first order in nuclear coordinate dependence. Noncentrosymmetric molecules are predicted to be the strongest nonlinear scatterers and derive activity from the RHR A term. In contrast to the vibronic analysis of linear Raman scattering, both one and two-photon allowed transitions contribute to RHR scattering cross section at the same order of vibronic theory (B term) for centrosymmetric molecules. RHR scattering spectra of gas phase CH3I, NH3, and CS2 illustrate A- and B-term RHR activity. [ABSTRACT FROM AUTHOR]

Resonance rotational hyper-Raman scattering cross sections excited by identical linearly polarized incident photons are derived for symmetric tops. In the absence of pure dephasing, the rotational structure of a RHR vibrational transition is described, in general, by five scattering invariants which are derived in an irreducible tensor framework. The pattern of relative rotational RHR intensities is a sensitive function of the two-photon resonant vibronic state dephasing constant. High quality rotational RHR spectra of NH3 are obtained using blue incident radiation at half the X→A transition energy. Fits to the observed patterns of relative rotational RHR intensities determine lifetimes of 38, 60, and 100 fs for the v′2 =1,2,3 A state vibronic bands respectively. Although seven rotational branches (N,O,P,Q,R,S,T) are possible in HR, only three (P,Q,R) are observed in these RHR spectra of ammonia due to the magnitude and relative signs of vibronic matrix elements in the scattering cross section expressions. [ABSTRACT FROM AUTHOR]

The analysis of Raman excitation profiles of rotational scattering transitions is shown to be a sensitive measure of rotationally specific intramolecular dynamics in the subpicosecond time regime. O, P, Q, R, and S Raman REPs of ammonia are reported and theoretically treated using previously derived resonance rotational scattering intensity expressions. Resonance rovibrational Raman spectra are obtained with UV Raman excitation continuously resonant with the v’2 =2 vibronic band (207–210 nm) of the NH3 X → A absorption spectrum. The resulting REP analysis reveals that the rates of photodissociation on the A state surface have a significant rotational quantum dependence in the 22 band. Rovibronic lifetimes decrease from 140 to 70 fs as J increases from 2 to 8 in this resonant A state vibrational level. These lifetime effects are largely ascribed to centrifugal effects. [ABSTRACT FROM AUTHOR]

Expressions are derived using irreducible tensor notation describing resonance rotational Raman scattering intensities for symmetric top molecules. Isotropic, antisymmetric, and anisotropic scattering contributions to the resonance O, P, Q, R, and S rotational Raman transitions are considered. Excited state lifetimes in the range of ∼0.02 to 1 ps or ∼1 to 100 vibrational periods can be determined by fitting the pattern of observed relative ΔJ=0 and ΔJ≠0 rotational or rovibrational scattering intensities. UV Raman scattering spectra resonant with photodissociative levels of the X→A Rydberg transition of NH3 and ND3 are used to illustrate this technique. Rates of ammonia photodissociation are found to have a strong dependence on the excited bending level quantum number. [ABSTRACT FROM AUTHOR]

The depolarized reduced Raman and corresponding optical Kerr effect (OKE) spectral density of ambient CS[sub 2] have been calculated by way of time correlation function (TCF) and instantaneous normal mode (INM) methods and compared with experimental OKE data. When compared in the reduced Raman spectrum form, where the INM spectrum is proportional to the squared polarizability derivative weighted density of states (DOS), the INM results agree nearly quantitatively (at all but the lowest frequencies) with the TCF results. Both are in excellent agreement with experimental measurements. The INM signal has a significant contribution from the imaginary INMs. Within our INM theory of spectroscopy the imaginary INMs contribute like the real modes, at the magnitude of their imaginary frequency. When only the real modes are allowed to contribute, and the spectrum is rescaled to account for the missing degrees of freedom, the results are much poorer, as has been observed previously. When the spectra are compared in their OKE form, the INM spectrum is found to lack the low-frequency spike which is associated with long time scale rotational diffusion, and it is not surprising that an INM theory would not capture such a feature. The results demonstrate that while the OKE and spontaneous depolarized Raman spectrum contain the same information, they clearly highlight different dynamical time scales. At higher frequencies (ω>25 cm[sup -1]) the INM OKE results are in excellent agreement with TCF and experimental results. The TCF results capture the low-frequency spike and are in agreement with experiment everywhere within the precision of the present calculations. The molecular contributions to the OKE signal are analyzed using INM methods. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]