Picosecond Coherent Raman Spectroscopy of Excited Electronic States of Polyene Chromophores

Abstract

The results are presented of a detailed analysis of the vibrational structure of polyene molecules in the excited electronic states. The molecules in a solution were excited by picosecond UV pulses and then probed by means of resonant coherent anti-Stokes and Stokes Raman scattering (CARS and CSRS).

The polarization sensitive CARS spectra of trans-stilbene in the S₁ state were measured with high resolution and signal-to-noise ratio. The technique of coherent ellipsometry was applied to determine the complex components of the third-order susceptibility tensor. The extremely high value of the resonant electronic hyperpolarizability of S₁ trans-stilbene detected was the result of significant π-electron delocalization in the polyeneq chain under the electronic excitation. The observed phases of different vibrational resonances measured relative to the phase of electronic susceptibility are considered as new molecular characteristics which reflect the features of the electron-vibronic coupling in the excited molecule.

The conformational and vibrational relaxation in excited trans-stilbene was studied in the presence of optical depletion of the S₁ state. The depletion of the ground state of β-carotene under the laser excitation and subsequent recovering kinetics was detected by time-resolved CARS.