High-resolution photoelectron spectroscopy of the ground and first excited electronic states of MgKr⁺

Abstract

We report on the characterisation of the lowest three electronic states of MgKr${}^{+}$, the X${}^{+}$ ${}^2{\mathrm{\Sigma }}^{+}$ ground state associated with the Mg${}^{+}$(3s) ${}^2{\mathrm{S}}_{1/2}$ + Kr(4p)${}^6$ ${}^1{\mathrm{S}}_0$ dissociation limit and the A${}^{+}$ ${}^2{\mathrm{\Pi }}_{\mathrm{\Omega }}(\mathrm{\Omega }=1/2,3/2)$ and B${}^{+}$ ${}^2{\mathrm{\Sigma }}^{+}$ excited states associated with the Mg${}^{+}$(3p) ${}^2{\mathrm{P}}_{1/2,3/2}$ + Kr(4p)${}^6$ ${}^1{\mathrm{S}}_0$ dissociation limits. The vibrational structure of the X${}^{+}$ ground state was measured by pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy starting from the a ${}^3{\mathrm{\Pi }}_0$ metastable state of MgKr generated by laser ablation of a Mg rod in a Kr supersonic beam. The vibrational assignment was derived from the isotopic structure of the spectra and the adiabatic ionisation energy of the $\mathrm{a}\to {\mathrm{X}}^{+}$ ionising transition was determined to be 38182.4(2.0) cm${}^{-1}$. Partially rotationally resolved spectra of the A${}^{+}\leftarrow$ X${}^{+}$ and B${}^{+}\leftarrow$ X${}^{+}$ transitions were recorded by monitoring the yield of Mg${}^{+}$ ions produced by dissociation of MgKr${}^{+}$ using the technique of isolated-core Rydberg-dissociation spectroscopy. Analytical potential-energy functions of the X${}^{+}$, A${}^{+}$, and B${}^{+}$ states were derived using a global model of these states that includes the spin-orbit interaction. The observation of the highest vibrational levels of the B${}^{+}$ state and the onset of the Kr + Mg${}^{+}$(3p) dissociation continuum enabled the precise determination of the dissociation energies of the X${}^{+}$, A${}^{+}$ and B${}^{+}$ states of MgKr${}^{+}$ and of the a state of MgKr. The X${}^{+}$, A${}^{+}$ and B${}^{+}$ states of MgKr${}^{+}$ are important stepping stones to access the Rydberg states of MgKr${}^{+}$ and the ground state of MgKr${}^{2+}$ by resonant multiphoton excitation.

GRAPHICAL ABSTRACT

Keywords:

• MgKr
• PFI-ZEKE photoelectron spectroscopy
• doubly charged ions
• Rydberg states
• photoionisation

Acknowledgments

It is a particular pleasure to dedicate this article to Professor Wim Ubachs and to thank him for being a most inspiring colleague for many years. We thank Josef A. Agner and Hansjürg Schmutz for their technical assistance and maintenance of the spectrometer and Dominik Wehrli for his early contributions to this project.

Disclosure statement

There are no conflict of interest to declare.

Additional information

Funding

This work is supported financially by the Swiss National Science Foundation (grant No. 200020B-200478).