Ultrafast photodissociation of Br2: Laser-generated high-harmonic soft x-ray probing of the transient photoelectron spectra and ionization cross sections

Abstract

The ultrafast dissociation of gas-phase Br2 is probed via a 400 nm pump soft-x-ray probe scheme at five different high-order harmonic wavelengths (13th, 15th, 17th, 19th, and 21st of an 800 nm Ti:sapphire laser). A series of time-resolved ultrafast photoelectron spectra reveals prompt two-photon ionization features, which allow in situ measurement of the cross correlation between the pump and probe pulses. Transient features are attributed to ionization of the dissociative excited state wave packet, and new spectral peaks are associated with the formation of atomic Br. Deconvolution of time-trace plots of the atomic signals with the cross-correlation pulse durations reveal similar dissociation times (∼40 fs) at two probe wavelengths (47 and 42 nm). Analysis of the transient wave packet photoelectron signal suggests an ionization process that occurs during dissociation, with a broad electron kinetic energy distribution at an extended Br–Br bond length (R ⩾3 Å). At long delay times (⩾500 fs), an enhancement of the ionization cross section of the Br atom compared to the Br2 molecule is observed with each of the probe wavelengths, the ratio increasing from a factor of 21±1 to 56±5 for probe wavelengths of 61.5 to 38 nm, respectively. The intensity of the transient wave packet signal on the dissociative state remains nearly constant between the 17th and 19th harmonic probes, indicating that the ionization cross section of the dissociative state has an entirely different wavelength dependence than the Br atom. The transient wave packet ionization signal is qualitatively 10%–20% of the simultaneous two-photon (400 nm+soft x-ray) ionization signal. The results are discussed in terms of the transient dynamics of dissociative state photoelectron spectroscopy, the correlation between molecular and atomic ionization probabilities, and above threshold ionization probabilities.