Photodissociation of bromine on the Br2 (Πu1) state is probed with ultrafast extreme ultraviolet (53.7 nm) single-photon ionization. Time-resolved photoelectron spectra show simultaneously the depletion of ground state bromine molecules as well as the rise of Br (P 32 2) products due to 402.5 nm photolysis. A partial photoionization cross-section ratio of atomic versus molecular bromine is obtained. Transient photoelectron spectra of a dissociative wave packet on the excited state are presented in the limit of low-power-density, single-photon excitation to the dissociative state. Transient binding energy shifts of "atomic-like" photoelectron peaks are observed and interpreted as photoionization of nearly separated Br atom pairs on the Br2 (Πu1) state to repulsive dissociative ionization states.
Bibliographical noteFunding Information:
The authors gratefully acknowledge funding from the U.S. Air Force Office of Scientific Research (Grant Nos. FA9550-04-1-0083 and F49620-03-1-0212) with additional equipment and support from the Department of Energy under Contract No. DE-AC05CH11231 and the National Science Foundation Extreme Ultraviolet Center under Contract No. EEC-0310717. FIG. 1. Schematic potential curves for Br 2 and Br 2 + system. The different stages that are probed by 53.7 nm single photoionization are labeled (i) the Br 2 ( X ) ground state, (ii) fully dissociated atomic Br ( P 3 ∕ 2 2 ) atoms, (iii) the excited wave packet on the dissociative Br 2 ( Π u 1 ) state, and (iv) nearly separated Br atom pairs on the Br 2 ( Π u 1 ) . FIG. 2. Schematic of the experimental setup for ultrafast pump-probe photodissociation of Br 2 . FIG. 3. Δ PE spectrum at long time delays showing (i) the depletion of the ground state as negative peaks and (ii) the atomic Br ( P 3 ∕ 2 2 ) binding energy peaks. FIG. 4. Δ PE spectra at short time delays, shown by a black line, are compared to long time delays shown by the gray line. Statistical error bars are shown for the short time data while the much smaller error bars for the long time delays are omitted for clarity. (a) Spectral region showing the transient assigned to the photoionization of a wave packet on the dissociative Π u 1 state, labeled (iii) in Fig. 1 . (b) Spectral region showing atomic like peaks, assigned to ionization of the nearly separated Br atoms to dissociative ionization states. Note that different energy bin sizes are used for the two spectral regions. FIG. 5. Time dependence of the different spectral features: full circles show the appearance of atomic product signal, Br ( P 3 ∕ 2 2 ) ionization to Br + ( P 3 ) , x symbols show the depletion of the ground state signal X Br 2 ( Σ g 1 ) ionization, and open circles show the transient signal from the Π u 1 wavepacket ionization (multiplied by a factor of 3). Full lines correspond to Gaussian and error function fits to the data.