High-resolution FFT spectroscopy of strong-field dissociative photoionization of molecular oxygen by 800 nm pulses

We investigate the wavepacket that remains bound in excited cationic states of oxygen after interaction iwth an intense 45-fs, 800-nm pulse. A much weaker probe pulse is used to dissociate these still-bound molecules. The momentum distribution of O⁺ is measured as a function of pump-probe delay and then Fourier transformed to obtain rotational-state-resolved quantum beat spectra. The sub-cm^-1 resolution of the Fourier transform allows unambiguous identification of the electronic, vibrational and rotational states populated by the pump and then disscociated by the probe.  Although tunnel ionization is expected to populate the lower-lying a⁴∏_g state more effectively than the b⁴∑⁻_g state, the Fourier spectrum shows no signature of a wavepacket in the former. All the identifiable lines are due to rotational states in the higher-lying b manifold. The experiment confirms the role resonant coupling between the two states by the 800 nm pulses [1] and reveals the importance of rovibrational excitation and predissociation in determining the momentum spectrum of the O⁺ fragments.

1. Xue et al., Phys. Rev. A 97, 043409 (2018).