High-resolution pulsed field ionization photoelectron-photoion coincidence spectroscopy using synchrotron radiation

Abstract

We have developed a sensitive and generally applicable scheme for performing pulsed field ionization (PFI) photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) spectroscopy using two-bunch and multibunch synchrotron radiation at the Advanced Light Source. We show that this technique provides an ion internal state (or energy) selection limited only by the PFI-PE measurement. Employing a shaped pulse for PFI and ion extraction, a resolution of 0.6 meV [full width at half maximum (FWHM)] is observed in the PFI-PEPICO bands for Ar+(2P3/2,1/2). As demonstrated in the PFI-PEPICO study of the process, O2+hν→O2+(b 4Σg−, v+=4, N+)+e−→O+(4S)+O(3P)+e−, the dissociation of O2+(b 4Σg−, v+=4) in specific rotational N+ levels can be examined. The simulation of the experimental breakdown diagram for this reaction supports the conclusion that the threshold for the formation of O+(4S)+O(3P) from O2+(b 4Σg−, v+=4) lies at N+=9. We have also recorded the PFI-PEPICO time-of-flight (TOF) spectra of O+ formed in the dissociation of O2+(b 4Σg−, v+=4–7). The simulation of these O+ TOF spectra indicates that the PFI-PEPICO method is applicable for the determination of kinetic energy releases. Previous PFI-PE studies on O2 suggest that a high-n O2 Rydberg state [O2*(n)] with a dissociative ion core undergoes prompt dissociation to yield a high-n′ O-atom Rydberg state [O*(n′)] [Evans et al., J. Chem. Phys. 110, 315 (1999)]. The subsequent PFI of O*(n′) accounts for the formation of a PFI-PE and O+. Since the PFI-PE intensities for O+ and O2+ depend on the lifetimes of O*(n′) and O2*(n), respectively, the PFI-PE intensity enhancement observed for rotational transitions to O2+(b 4Σg−, v+=4, N+⩾9) can be attributed to the longer lifetimes for O*(n′) than those for O2*(n). The PFI-PEPICO study of the dissociation of CH3+ from CH4 also reveals the lifetime effects and dc field effects on the observed intensities for CH3+ and CH4+. The high resolution for PFI-PEPICO measurements, along with the ability to distinguish the CH3+ fragments due to the supersonically cooled CH4 beam from those formed by the thermal CH4 sample, has allowed the determination of a highly accurate dissociation threshold for CH3+ from CH4.