Different Rise Times of Atomic Br M$_{4,5}$ 3d$_{3/2,5/2}$ Core Level Absorptions during Br$_{2}$ C $^{1}\Pi_{u}$ $1_{u}$ State Dissociation via Extreme Ultraviolet Transient Absorption Spectroscopy

Abstract

The reported ''dissociation times'' for the Br$_{2}$ C ($^{1}\Pi_{u}$ $1_{u}$) state by various measurement methods differ widely across the literature (30 to 340 fs). We consider this issue by investigating attosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the Br M4,5 3d$_{3/2,5/2}$ edges (66 to 80 eV), tracking core-to-valence (3d to 4p) and core-to-Rydberg (3d to ns, np, n $\geq$ 5) transitions from the molecular to atomic limit. The progress of dissociation can be ascertained by the buildup of the atomic absorption in time. Notably, the measured rise times of the 3d$_{3/2,5/2}$ to 4p transitions depend on the probed core level final state, 38 $\pm$ 1 and 20 $\pm$ 5 fs for $^{2}$D$_{5/2}$ and $^{2}$D$_{3/2}$ at 64.31 and 65.34 eV, respectively. Simulations by the nuclear time-dependent Schr\"odinger equation reproduce the rise-time difference of the 3d to 4p transitions, and the theory suggests several important factors. One is the transition dipole moments of each probe transition have different molecular and atomic values for $^{2}$D$_{5/2}$ versus $^{2}$D$_{3/2}$ that depend on the bond length. The other is the merger of multiple molecular absorptions into the same atomic absorption, creating multiple timescales even for a single probe transition. Unfortunately, the core-to-Rydberg absorptions did not allow accurate atomic Br buildup times to be extracted due to spectral overlaps with ground state bleaching, otherwise an even more comprehensive picture of the role of the probe state transition would be possible. This work shows that the measured probe signals accurately contain the dissociative wavepacket dynamics but also reveal how the specific probe transition affects the apparent progress toward dissociation with bond length. Such potential probe-transition-dependent effects need to be considered when interpreting measured signals and their timescales.