Interplay between ultrafast electronic and librational dynamics in liquid nitrobenzene probed with two-color four-wave mixing

TL;DR

This study combines experimental and theoretical approaches to investigate how ultrafast electronic and librational motions interact in liquid nitrobenzene, revealing that near infrared pulses induce librational motion and electronic coherences detectable via four-wave mixing.

Contribution

It demonstrates the use of femtosecond pulses and quantum simulations to elucidate the coupling between electronic and librational dynamics in liquids, advancing ultrafast nonlinear optical spectroscopy.

Findings

Near infrared pulses launch librational motion.

Electronic coherences are generated during the process.

The nonlinear signal indicates molecules are excited electronically.

Abstract

We present an experimental and theoretical study of the interplay between ultrafast electron dynamics and librational dynamics in liquid nitrobenzene. A femtosecond ultraviolet pulse and two femtosecond near infrared pulses interact with nitrobenzene molecules, generating a four-wave mixing nonlinear signal that is measured in the Optical Kerr Effect geometry. The near infrared nonlinear signal is measured to be non-zero only at negative time delays, corresponding to the near infrared pulses arriving earlier than the ultraviolet pulse. We perform time-dependent Quantum Master Equation calculations, which include a classical libration model, to simulate the experiment. The simulations support the conclusion that the near infrared pulses launch librational motion, while simultaneously creating electronic coherences that result in a libration-modulated electronic nonlinear response. Furthermore, we conclude that the measured nonlinear optical signal corresponds to a non-parametric process that leaves the molecules in an excited electronic state. This work provides new insight into ultrafast nonlinear optical interactions in liquids and is an important step towards probing ultrafast electronic coherences in large molecules in the liquid phase.