Ultrafast Dynamics of Neutral Superexcited Oxygen: A Direct Measurement of the Competition between Autoionization and Predissociation

Using ultrafast extreme ultraviolet pulses, we performed a direct measurement of the relaxation dynamics of neutral superexcited states corresponding to the nlσg(c⁴Σᵤ^-) Rydberg series of O₂. An extreme ultraviolet attosecond pulse train was used to create a temporally localized Rydberg wave packet and the ensuing electronic and nuclear dynamics were probed using a time delayed femtosecond near-infrared pulse. We investigated the competing predissociation and autoionization mechanisms in superexcited oxygen molecules and found that autoionization is dominant for the low n Rydberg states. We measured an autoionization lifetime of 92±6 fs and 180±10 fs for the (5s,4d)σg and (6s,5d)σg Rydberg state groups, respectively. We also determine that the disputed neutral dissociation lifetime for the ν=0 vibrational level of the Rydberg series is 1100±100 fs.