Investigation Of Ultrafast Molecular Dynamics Of Liquids Via Coherent, Time-Resolved Fourier Transform Spectroscopy

Abstract

ABSTRACT The molecular dynamics of liquids in the frequency range 100 GHz to tens of THz are conveniently interrogated with broadband femtosecond laser pulses through the time-resolved optical Kerr effect. The molecular dynamics are extracted from the data by a combination of the traditional semi-log plot analysis for diffusive relaxations, and a novel Fourier- transform analysis for the intra- and inter molecular vibrational dynamics. These tech­ niques are applied to the optical Kerr dynamics of liquid iodobenzene at room temperature, providing evidence for the presence of both librational motion (at short times) and diffus­ ive reorientation (at longer times) about two distinct molecular axes. 1. INTRODUCTION The molecular dynamics of polarizable liquids have been the object of intense scrutiny via time-resolved four-wave-mixing spectroscopies using nearly transform-limited 55-70 fs laser pulses at wavelengths far from any electronic resonances . In this paper we present femtosecond optical heterodyne detected optical Kerr effect (OHD-OKE) data on liquid iodo­ benzene at 298 K. At times less than 1 ps this molecular liquid exhibits extremely complex dynamics arising from intra- and intermolecular vibrational motions, with the intermolecu- lar dynamics arising primarily from a coherently driven oscillatory orientational motion (molecular libration). Using a novel Fourier transform technique for analysis of the temporal data, librational motion about two distinct molecular axes is clearly revealed. Similar results are observed in other benzenes and benzene analogs2. Additionally, evidence will be presented for diffusive reorientational motion about more than one molecular axis in this liquid on the picosecond timescale. Previous investigations on binary solutions of CS2 in various alkane solvents have revealed the influence of the local intermolecular potential on the dynamics of the intermolecular librational motion . The current Fourier transform treatment of the time domain data reveals the role of the intermolecular poten­ tial on intramolecular vibrational motions. In particular, significant depolarized intensi­ ty is induced in symmetrical vibrational modes that are rigorously polarized in the gas phase.