Femtosecond energy- and angle-resolved photoelectron spectroscopy

We present a formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets and results of its application to the ^1Σ^+_u double-minimum state of aligned Na_2. The formulation is well-suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results are presented for three typical pump laser energies selected so as to investigate 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 reported for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unraveling wave packet dynamics from pump–probe photoelectron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distributions on relative orientation of the molecule and polarization of the probe pulse are also seen to be potentially useful for real-time monitoring of molecular rotation.