Rotationally resolved threshold photoelectron spectroscopy of O2 using coherent XUV: formation of vibrationally excited ions in the Franck–Condon gap

Abstract

The rotationally resolved photoelectron spectrum of high vibrational levels of O2+ in the Franck–Condon gap was investigated using pulsed field ionization, zero kinetic energy photoelectron spectroscopy. By using a coherent extreme ultraviolet light source for single-photon excitation, the ν+ = 6–24 vibrational levels of O2+X2Πg were studied. This is the first time levels higher than ν+ = 14 have been observed with rotational resolution. The highest level studied in the present work had a vibrational energy corresponding to 70% of the well depth. Along with the novelty of the spectroscopic technique, the present results reveal interesting and new ionization dynamics. All levels observed are Franck–Condon forbidden, and are not observed in a conventional photoelectron spectrum. There was no direct relation between the vibrational bands and the autoionizing states observed in the photoionization efficiency spectrum in the same energy region, and the rotational line intensities observed in the various bands in the current threshold photoelectron spectra were all similar. The mechanism of this process was different from the "resonant autoionization" model, which involves coupling between a Rydberg state with a low principal quantum number, a dissociative state, arid the Rydberg series converging to the vibrationally excited ion states. Instead, the excitation process was believed to be more direct, involving mainly a dissociative state (or states) and the Rydberg states with a vibrationally excited ion core. Further investigation of this mechanism is still necessary, but the formation of these highly vibrationally excited ions opens a new horizon in state-selective reaction dynamics. With the coherent XUV light source and the PFI-ZEKE technique, a wide range of vibrational energies (up to 4 eV) can be deposited into the O2+ reactant with rovibrational selectivity.