Parity- and rotational–state-selective threshold photoionisation of MgAr by field-ionisation and Rydberg-series cross-correlation methods

High-resolution photoionisation and PFI-ZEKE photoelectron spectra of MgAr have been measured and used to characterise the rovibrational structure of the 𝑋^{+ 2}⁢Σ⁺ ground electronic state of MgAr⁺ and the photoionisation dynamics of MgAr. The Rydberg series and ionisation continua associated with the MgAr⁺ 

𝑋⁺ (𝑣⁺=0−7, 𝑁⁺=0−6)  states were accessed from the  𝑎⁢³⁢Π₀ metastable state of MgAr via selected Λ-doublet components of the rotational levels of the Mg(3s3d)Ar ⁢³⁢Π₀ excited electronic state in a resonant two-photon excitation sequence. The parity and rotational-state selection was exploited to identify the dominant autoionising Rydberg series in the photoionisation spectra and to characterise the partial-wave composition of the photoelectron in the PFI-ZEKE photoelectron spectra. Numerical evaluation of the cross-correlation of the observed photoionisation spectra with calculated spectra of unperturbed Rydberg series were used to determine cross-correlation ionisation-energy (CRIE) spectra with full resolution of the rotational structure, following a procedure introduced by Neuhauser, Siglow and Neusser [J. Chem. Phys. 106, 896 (1997)]. Combining the information contained in the PFI-ZEKE photoelectron spectra, the photoionisation spectra and the CRIE spectra provided an improved description of the rovibrational photoionisation dynamics of MgAr and of the level structure of the ground electronic state of MgAr⁺ as well as accurate values of the corresponding ionisation energies.