High-resolution photoelectron spectroscopic study of the first electronic states of Kr2+

Abstract

The pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of Kr2 has been recorded between 103 500 cm−1 and 118 000 cm−1. Photoelectronic transitions to four [the I(1/2u), I(3/2u), II(1/2u), and II(1/2g) states] of the first six electronic states of Kr2+ have been observed. The photoelectronic transition to the ground I(1/2u) state consists of a long progression of vibrational bands, starting at v+=0. From the resolved isotopic substructure of vibrational levels with v+⩾15, the absolute numbering of the vibrational quantum number could be determined. The analysis of the spectrum has led to improved values of the adiabatic ionization potential [IP(I(1/2u))=(103 773.6±2.0) cm−1], the dissociation energy [D0+(I(1/2u))=(9267.8±2.8) cm−1] and to the determination of an analytical potential energy curve that reproduces the experimental data from v+=0 to beyond 81% of the dissociation energy. The transitions to vibrational levels of the I(1/2u) state with v+⩽30 and v+⩾65 have vanishing Franck–Condon factors for direct ionization from the ground neutral state and gain intensity from transitions to low Rydberg states that belong to series converging on excited electronic states of Kr2+. In the region immediately below the first dissociation limit of Kr2+, a second progression was observed and assigned to a photoelectronic transition to the I(3/2u) state. The adiabatic ionization potential [IP(I(3/2u))=(112 672.4±2.0) cm−1], the dissociation energy [D0+(I(3/2u))=(369.1±2.8) cm−1] and vibrational constants could be extracted for this state. Two further progressions were observed below the second dissociation limit of Kr2+ and assigned to transitions to the II(1/2u) and II(1/2g) states. The adiabatic ionization potentials [IP(II(1/2u))=(117 339.7±2.0) cm−1, IP(II(1/2g))=(117 802.6±2.0) cm−1] and the dissociation energies [D0+(II(1/2u))=(1071.7±2.8) cm−1, D0+(II(1/2g))=(608.8±2.8) cm−1] were determined for these two ionic states. In the region just below the ionic dissociation limits, artifact lines are observed in the PFI-ZEKE photoelectron spectra at the position of transitions to Rydberg states of the krypton monomer. At the lowest threshold, collisional and associative ionization of the long lived atomic Rydberg states leads to the formation of ZEKE electrons; at the upper threshold, the rapid autoionization of the atomic Rydberg states forms high ion concentrations, and the electrons that remain trapped in the ion cloud are released by the delayed pulsed field used to produce and extract the PFI-ZEKE electrons.