Picosecond IR-UV pump–probe spectroscopic study on the vibrational energy flow in isolated molecules and clusters

Abstract

Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) from the XH stretching vibrations, where X refers to O or C atom, of aromatic molecules and their hydrogen(H)-bonded clusters are investigated by picosecond time-resolved IR-UV pump probe spectroscopy in a supersonic beam. For bare molecules, we mainly focus on IVR of the OH stretch of phenol. We describe the IVR of the OH stretch by a two-step tier model and examine the effect of the anharmonic coupling strength and the density of states on IVR rate and mechanism by using isotope substitution. In the H-bonded clusters of phenol, we show that the relaxation of the OH stretching vibration can be described by a stepwise process and then discuss which process is sensitive to the H-bonding strength. We discuss the difference/similarity of IVR/VP between the “donor” and the “acceptor” sites in phenol–ethylene cluster by exciting the CH stretch vibrations. Finally, we study the vibrational energy transfer in the isolated molecules having the alkyl chain, namely phenylalcanol (PA). In this system, we measure the rate constant of the vibrational energy transfer between the OH stretch and the vibrations of benzene ring which are connected at the both ends of the alkyl chain. This energy transfer can be called “through-bond IVR”. We investigate the three factors which are thought to control the energy transfer rate; (1) “OH ↔ next CH₂” coupling, (2) chain length and (3) conformation. We discuss the energy transfer mechanism in PAs by examining these factors.