Ultrafast Photodissociation of Br2: Laser-Generated High-Harmonic Soft X-Ray Probing of the Transient Photoelectron Spectra and Ionization Cross Sections

Published

October 1, 2002

Author(s)

L Nugent-Glandorf, M Scheer, D A. Samuels, V M. Bierbaum, S R. Leone

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 harmonicwavelengths (13th, 15th, 17th, 19th, and 21st of an 800 mn 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. Additional transient features are attributed to ionization of the dissociative excited state wavepacket, and new 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 mn). Analysis of the transient wavepacket 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 2greater then or equal to}3 ). At long delay fims (greater then or equal to} 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, increasing from a factor of 21 l to 56 5 for probe wavelengths of 61.5 nm to 38 nm, respectively. The intensity of the transient wavepacket 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. Thetransient wave packet ionization signal is qualitatively 10-20% of the simultaneous two-photon (400 nm + soft x-ray) ionization signal.

Citation

Journal of Chemical Physics

Volume

117

Issue

No. 13

Pub Type

Journals

Keywords

Br₂, photodissociation, pump-probe, soft x-ray, ultrafast

Citation

Nugent-Glandorf, L. , Scheer, M. , Samuels, D. , Bierbaum, V. and Leone, S. (2002), Ultrafast Photodissociation of Br₂: Laser-Generated High-Harmonic Soft X-Ray Probing of the Transient Photoelectron Spectra and Ionization Cross Sections, Journal of Chemical Physics (Accessed July 27, 2024)