Ultrafast decay of superexcited cΣ _u-4nlσ _gv=0,1 states of O ₂ probed with femtosecond photoelectron spectroscopy

Neutral superexcited states in molecular oxygen converging to the O2+cΣ _u-4 ion state are excited and probed with femtosecond time-resolved photoelectron spectroscopy to investigate predissociation and autoionization relaxation channels as the superexcited states decay. The cΣ _u-44sσ _gv=0, cΣ _u- 44sσ _gv=1, and cΣ _u-43dσ _gv=1 superexcited states are prepared with pulsed high-harmonic radiation centered at 23.10 eV. A time-delayed 805 nm laser pulse is used to probe the excited molecular states and neutral atomic fragments by ionization; the ejected photoelectrons from these states are spectrally resolved with a velocity map imaging spectrometer. Three excited neutral O atom products are identified in the photoelectron spectrum as 4d ₁DJ°3, 4p ₁PJ°5 and 3d ₁DJ°3 fragments. Additionally, several features in the photoelectron spectrum are assigned to photoionization of the transiently populated superexcited states. Using principles of the ion core dissociation model, the atomic fragments measured are correlated with the molecular superexcited states from which they originate. The 4d ₁DJ°3 fragment is observed to be formed on a timescale of 65 ± 5 fs and is likely a photoproduct of the 4sσ _g v 1 state. The 4p ₁PJ°5 fragment is formed on a timescale of 427 ± 75 fs and correlated with the neutral predissociation of the 4sσ _g v 0 state. The timescales represent the sum of predissociation and autoionization decay rates for the respective superexcited state. The production of the 3d ₁DJ°3 fragment is not unambiguously resolved in time due to an overlapping decay of a v 1 superexcited state photoelectron signal. The observed 65 fs timescale is in good agreement with previous experiments and theory on the predissociation lifetimes of the v 1 ion state, suggesting that predissociation may dominate the decay dynamics from the v 1 superexcited states. An unidentified molecular state is inferred by the detection of a long-lived depletion signal (reduction in autoionization) associated with the BΣg-2 ion state that persists up to time delays of 105 ps.