The role of specific normal modes during non‐Born–Oppenheimer dynamics: the S1–S0 internal conversion of β‐carotene interrogated on a femtosecond time‐scale with coherent anti‐Stokes Raman scattering

Abstract

AbstractThe population flow from the first excited singlet state (S1) to the electronic ground state (S0), facilitated through the S1–S0 internal conversion, and the subsequent internal vibrational energy redistribution and vibrational cooling processes are monitored selectively with respect to the different forms of nuclear motion in all‐trans‐β‐carotene. This is realized by exciting the molecule into the second excited singlet state (S2) with an auxiliary pump laser pulse and interrogating the population recovery into the electronic ground state by means of a time‐delayed, coherent anti‐Stokes Raman (CARS) process. This spectroscopic scheme is referred to as a pump‐CARS scheme in analogy to the classical pump–probe scheme in time‐resolved spectroscopy. Here, the profound enhancement of the CARS signal intensity in the case of a Raman resonance to a specific vibrational mode is utilized as an intensity filter that amplifies the signal from the vibrational modes of interest, making the contributions from other vibrational modes negligible. This filter allows for the population flow in a specific vibrational mode to be monitored as the radiationless electronic transition between the S1 and S0 state takes place. This spectroscopic scheme opens up the possibility of identifying the vibrational motion with a large‐amplitude motion in the CC double bond symmetric stretch as the primary acceptor of population from the S1 state. A mechanism with which the other normal modes are populated is postulated. Copyright © 2002 John Wiley & Sons, Ltd.