PFI-ZEKE photoelectron spectroscopy of positively charged ions: illustration with Mg+

Abstract

Pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy has been applied for the first time to a positively charged ion, Mg+. Mg+ ions have been prepared in the [Ne]3s S21/2 ground state by photoionization of Mg atoms in a supersonic expansion. The ions were then excited to very high Rydberg states located just below the ionization threshold corresponding to the formation of ground-state Mg2+ by (2 + 1′) resonance-enhanced three-photon absorption via the 3d D25/2 intermediate state. We demonstrate experimentally and in a calculation of the transition moments that only the nf Rydberg series of Mg+ is efficiently excited from the D25/2 state and extract from the Rydberg spectrum precise values for the ionization energy of Mg+ (121267.65 ± 0.06 cm−1) and the quantum defect of the nf series (0.0044(6)). The field ionization of very high Rydberg states of Mg+ was investigated using sequences of electric field pulses. Compared to the pulsed field ionization of neutral atoms or molecules, the pulsed field ionization of positively charged ions differs in three respects: (i) the shift of the ionization thresholds induced by an electric field of given strength is larger by a factor of approximately 2, (ii) the inherent presence of ions in the photoexcitation volume prevents the escape of slow electrons, and (iii) the inhomogeneous stray fields induced by these ions and their effects on the high Rydberg states reduce the selectivity of the field-ionization process and degrade the resolution of the PFI-ZEKE photoelectron spectra. By minimizing the ion density, optimizing the photoexcitation efficiency and using multipulse field-ionization sequences, we show that the resolution of the photoelectron spectra can be as high as 0.3 cm−1, which opens up the possibility to study doubly-charged cations by high-resolution photoelectron spectroscopy.