UC Berkeley

Excited state photoelectron angular distributions (PADs) are measured in two-color two-photon ionization experiments on autoionizing resonances in atomic and molecular targets. Atomic krypton is excited to 4s¹4p⁶6p¹ configurations, which decay on the femtosecond timescale. By ionizing these states with a delayed 801 nm probe pulse, the population decay and photoionization dynamics are simultaneously measured directly and in real time. In the case of the 4s¹4p⁶6p¹ excitation, two slightly split J=1 autoionizing states are populated, one with a singlet multiplicity and the other a triplet. A novel approach to separate the overlapping resonance contributions to the total PAD is demonstrated using femtosecond time-resolution to isolate the individual PADs of each state by way of their different autoionization lifetimes. The measured PAD for the singlet state is decomposed into the ratio of radial dipole matrix elements and relative phase difference between the two states. The triplet PADs are predicted to result in anisotropies independent of the nature of the atom or excited state from which the detected electron originates. Measurement of the anisotropy from this state shows a significant deviation from the predictions, which is most likely a result of configuration mixing from Fano and spin-orbit interactions. In another set of experiments, transiently populated superexcited states (SESs) in molecular oxygen are prepared and probed by 805 nm pulses as the autoionization and predissociation relaxation channels compete. Three neutral product channels are detected and formed on the femtosecond time scale, assigned as predissociation products of different vibrational SESs. The decay lifetime of the v=1 SES agrees excellently with lifetimes extracted from high-resolution experiments that probe the ionic core spectral line widths of the vibrational levels. Results also reveal a long lived state that is unassigned and whose spectroscopic signature is observed as a depletion in the autoionization signal by the 805 nm probe up to several hundred picosecond time delays. Future work is needed to identify this feature.