All-XUV Pump-Probe Transient Absorption Spectroscopy of the Structural Molecular Dynamics of Di-iodomethane

Author(s): Rebholz, M; Ding, T; Despre, V; Aufleger, L; Hartmann, M; Meyer, K; Stoos, V; Magunia, A; Wachs, D; Birk, P; Mi, Y; Borisova, GD; Castanheira, CDC; Rupprecht, P; Schmid, G; Schnorr, K; Schroter, CD; Moshammer, R; Loh, ZH; Attar, AR; Leone, SR; Gaumnitz, T; Worner, HJ; Roling, S; Butz, M; Zacharias, H; Dusterer, S; Treusch, R; Brenner, G; Vester, J; Kuleff, AI; Ott, C; Pfeifer, T | Abstract: In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I: 4d5/2-σ∗ transition of a gas-phase di-iodomethane (CH2I2) target. This site-specific excitation generates a 4d core hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the same I: 4d resonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger.