Ultrafast dynamics of autoionizing states in O2 probed by laser-field-assisted XUV photoionization

Dynamics of the autoionizing states in O-2 was investigated using the method of laser-field-assisted extreme-ultraviolet photoionization. Harmonics, ranging from 9th to 21st orders generated in Kr with 25 fs laser pulses at 820 nm, were employed to carry out photoionization of O-2. Distinct autoionization features in the photoelectron spectrum, stemming from the resonant excitation of superexcited states of O-2 by the 11th harmonic, were observed. The superexcited states, referred to as autoionizing states, were identified to be the. nu' = 0, n = 5 Rydberg states converging to the b(4) Sigma(-)(g) state of O-2(+). Infrared laser pulses were utilized as the probe to monitor time-varying characteristics of photoionization and autoionization of O-2. A decay time of 21 fs was obtained for the autoionizing states of O-2 by deconvoluting the pump-probe results in O-2 and Ar.