Ultrafast Time-Resolved Soft X-Ray Photoelectron Spectroscopy of DissociatingBr2

The dissociation of excited state ${\mathrm{Br}}_2$ is probed with the novel technique of ultrafast soft x-ray photoelectron spectroscopy. Excited ${\mathrm{Br}}_2$ molecules are prepared in the dissociative ${}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state with 80 fs, 400 nm pulses, and a series of photoelectron spectra are obtained during dissociation with pulses of soft x-ray light (47 nm, 26.4 eV, 250 fs). The formation of Br atoms is readily detected and the data support an extremely fast dissociation time for ${\mathrm{Br}}_2$ on the order of 40 fs. Amplitudes of the pump-probe features reveal that the ionization cross section of atomic Br at 47 nm is $\sim 40$ times larger than that of ${\mathrm{Br}}_2$.