Femtosecond energy- and angle-resolved photoelectron spectroscopy

We present a formulation of energy- and angle-resolved photoelectron spectr a for femtosecond pump-probe ionization of wave packets and results of its application to the (1)Sigma(u)(+) double-minimum state of aligned Na-2. The formulation is well-suited for inclusion of the underlying dynamics of mol ecular photoionization and its dependence on molecular geometry. Results ar e presented for three typical pump laser energies selected so as to investi gate qualitatively different patterns of the spatio-temporal propagation of wave packets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions are also report ed for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper d escription of the underlying photoionization amplitudes and their dependenc e on geometry for unraveling wave packet dynamics from pump-probe photoelec tron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distr ibutions on relative orientation of the molecule and polarization of the pr obe pulse are also seen to be potentially useful for real-time monitoring o f molecular rotation. (C) 2000 American Institute of Physics. [S0021-9606(0 0)00720-0].