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 measuredand used to characterise the rovibrational structure of the X⁺ ²Σ⁺ ground electronic state of MgAr⁺ and the photoionisation dynamics of MgAr. The Rydberg series and ionisation continua associatedwith the MgAr⁺ X⁺ (v⁺ = 0 − 7, N⁺ = 0 − 6) states were accessed from the a³Π₀ metastable stateof MgAr via selected -doublet components of the rotational levels of the Mg(3s3d)Ar ³Π₀ excitedelectronic state in a resonant two-photon excitation sequence. The parity and rotational-state selec-tion was exploited to identify the dominant autoionising Rydberg series in the photoionisationspectra and to characterise the partial-wave composition of the photoelectron in the PFI-ZEKE photo-electron spectra. Numerical evaluation of the cross-correlation of the observed photoionisation spec-tra with calculated spectra of unperturbed Rydberg series were used to determine cross-correlationionisation-energy (CRIE) spectra with full resolution of the rotational structure, following a procedureintroduced by Neuhauser, Siglow and Neusser [J. Chem. Phys. 106, 896 (1997)]. Combining the infor-mation contained in the PFI-ZEKE photoelectron spectra, the photoionisation spectra and the CRIEspectra provided an improved description of the rovibrational photoionisation dynamics of MgArand of the level structure of the ground electronic state of MgAr+ as well as accurate values of thecorresponding ionisation energies.