Manifestation of conical intersections in resonance Raman intensities

Abstract

This paper deals with the manifestations of conical intersections (CIs), unequivocal spectroscopic signatures of which are still elusive, in the resonance Raman intensities. In particular, the results of our calculations on the 'two state-two vibrational mode' and the 'two state-three vibrational mode' models are presented. The models comprise two excited states of different spatial symmetry, one bright and one dark, which are coupled by a nontotally symmetric mode while the energy gap between them is tuned by one/two totally symmetric modes. Time dependent theory for vibronically coupled states is employed for the calculation and analysis of Raman excitation profiles (REPs). The manifestation of intersections in REPs is studied by extensive model calculations and the results of two specific models are presented. The feasibility of using REPs to probe the role of CIs in polyatomic systems is ascertained by multimode calculations on two polyatomic systems viz., pyrazine and trans-azobenzene. The study also notes the importance of the pump excitation wavelength dependence in a femtosecond time-resolved experiment probing the intersection-induced nonadiabatic dynamics.

Nonadiabatic decay dynamics of phthalide from the light‐absorbing S3(ππ*) state‐resonance Raman spectroscopy and CASSCF study

“…This is the first time that an S 3 (1)(ππ*)/S 2 (ππ*)conical intersection has been shown to exist near the FC region of the S 3 state of phthalide. This conical intersection, identified here by resonance Raman spectroscopy in conjunction with CASSCF calculations, is markedly different from the previously discovered S 3 (ππ*)/S 1 (nπ*) conical intersections for phthalide and other molecular systems …”

“…It is found that when the ab initio calculated resonance Raman intensities deviate greatly from the experimental values, the mixing of two states in the FC region is generally involved. More specifically, when abnormally intense experimental resonance Raman intensities appear in the non‐totally symmetric fundamental modes, they generally result from overtones and combination bands with the totally symmetric fundamental modes; therefore, a curve‐crossing or conical intersection must exist between the relevant excited states . In this case, Herzberg–Teller vibronic‐coupling theory and CASSCF calculations are used to determine the exact coupling states and the associated geometric structures of the curve‐crossing or conical intersection points.…”

Probing the Interstate Coupling near a Conical Intersection by Optical Spectroscopy

“…This is in line with a previous theoretical study of the resonance Raman spectrum of pyrazine. 69,70 Here, we find that by measuring the vibrational frequencies of the reactant in the adiabatic ground and excited states as well as the electronic excitation energy, we can qualitatively estimate the value of the coupling constant λ between the electronic states using eq 3. Quantitative determination of λ requires explicit numerical simulations of the spectra as performed here.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part V

“…Chowdary et al . used RR to study conical intersections; the unequivocal spectroscopic signatures of which are still elusive in RR intensities . They noted the importance of the pump excitation wavelength in femtosecond time‐resolved experiments that probe intersection‐induced nonadiabatic dynamics.…”