Visualizing coherent vibrational motion in the molecular iodine B3Π0+u state using ultrafast XUV transient-absorption spectroscopy

Abstract

Attosecond probing of core-level electronic transitions in molecules provides a sensitive tool for real-time observation of chemical dynamics. Here, we employ ultrafast extreme-ultraviolet (XUV) transient-absorption spectroscopy to investigate the excited state electronic and nuclear dynamics in a prototype molecule, ${\mathrm{I}}_{2}$. A few-femtosecond visible pump pulse is employed to excite the ${\mathrm{I}}_{2}$ molecule and an attosecond XUV pulse is used to probe the dynamics through iodine-$4d$ core-to-valence transitions. A highly extended vibrational wave packet $({\ensuremath{\nu}}^{\ensuremath{'}}=10--50,\phantom{\rule{0.28em}{0ex}}{\ensuremath{\nu}}_{max}^{\ensuremath{'}}=25)$ is prepared by one-photon absorption in the valence excited $B\phantom{\rule{0.28em}{0ex}}{}^{3}{\mathrm{\ensuremath{\Pi}}}_{{{0}^{+}}_{\mathrm{u}}}$ state of ${\mathrm{I}}_{2}$ and its motion is directly mapped due to the strong shift of the XUV core-level transition with internuclear separation. Through the imaging of this vibrational motion, we directly reconstruct the transition energy between the valence and the core-excited states as a function of internuclear distance. Besides single-photon dynamics, distinct direct dissociation pathways arising from two-photon pump absorption are also revealed.