The first electronic states of Ar2+ studied by high resolution photoelectron spectroscopy

The pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectrum of Ar2 has been recorded between 116 500 cm−1 and 128 200 cm−1. The spectrum consists of a long progression of transitions to the vibrational levels of the ground A 2Σ1/2u+ state of Ar2+ with v+ up to 52, a shorter progression of four bands attributed to transitions to the first four vibrational levels of the C 2Π1/2u state and of a single sharp line assigned to the C 2Π3/2u(v+=0)←X 10g+(v=0) transition. Rotational constants of several vibrational levels of the A 2Σ1/2u+ state have been determined from high resolution PFI-ZEKE photoelectron spectra. From these measurements new information on the first electronic states of Ar2+ has been extracted. An analytical potential energy function has been derived for the A 2Σ1/2u+ state which extends to large internuclear distances (beyond 5 Å) and reproduces all measured vibrational energy levels up to v+=52. The adiabatic ionization potential for the photoelectronic transitions to the A 2Σ1/2u+, C 2Π3/2u and C 2Π1/2u states are determined to be 116 591.1±6 cm−1, 126 883.9±3 cm−1 and 128 004.1±5 cm−1, respectively, from which dissociation energies (D0+) of 10 603.7±6 cm−1, 310.8±3 cm−1, and 622.5±5 cm−1 are obtained. The vibrational levels of the C 2Π1/2u state can be described by a Morse potential with ωe=58.9±0.8 cm−1 and ωexe=1.40±0.27 cm−1, respectively. Associative ionization and collisional ionization processes involving argon atom Rydberg states induce spurious peaks in the PFI-ZEKE photoelectron spectrum. Ways to unambiguously identify these spurious peaks are discussed.