Time-resolving the UV-initiated photodissociation dynamics of OCS

TL;DR

This study uses time-resolved spectroscopy and simulations to investigate the ultrafast photodissociation dynamics of OCS molecules after UV excitation, revealing unexpected vibrational wavepacket oscillations in the dissociation process.

Contribution

It provides the first detailed time-resolved analysis of OCS photodissociation, identifying vibrational wavepackets as the source of observed oscillations, which was previously unrecognized.

Findings

Observed ~100 fs oscillations in S+ channel during UV dissociation.

Wavepacket simulations suggest these oscillations are due to vibrational states in excited electronic states.

Revealed that oscillations are not related to rotational motion of CO fragments.

Abstract

We present a time-resolved study of the photodissociation dynamics of OCS after UV-photoexcitation at $\lambda=237$ nm. OCS molecules ($X\,^1\Sigma^+$) were primarily excited to the $1\,^1\!A''$ and the $2\,^1\!A'$ Renner-Teller components of the $^1\Sigma^{-}$ and $^1\!\Delta$ states. Dissociation into CO and S fragments was observed through time-delayed strong-field ionisation and imaging of the kinetic energy of the resulting CO$^+$ and S$^+$ fragments by intense $790$ nm laser pulses. Surprisingly, fast oscillations with a period of $\sim100$ fs were observed in the S$^+$ channel of the UV dissociation. Based on wavepacket-dynamics simulations coupled with a simple electrostatic-interaction model, these oscillations do not correspond to the known highly-excited rotational motion of the leaving CO$(X\,^1\Sigma^+,J\gg0)$ fragments, which has a timescale of $\sim140$ fs. Instead, we suggest to assign the observed oscillations to the excitation of vibrational wavepackets in the $2\,^3\!A''$ or $2\,^1\!A''$ states of the molecule that predissociate to form S$(^3\!P_{J})$ photoproducts.