Time-resolving the UV-initiated photodissociation dynamics of OCS

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.