The X+ 2Πg, A+ 2Πu, B+ 2Δu, and $\mathrm{a}^+\ ^4\Sigma _{\mathrm{u}}^-$a+Σu−4 electronic states of ${\rm Cl}_2^+$ Cl 2+ studied by high-resolution photoelectron spectroscopy

Abstract

Partially rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the three isotopomers ((35)Cl2, (35)Cl(37)Cl, and (37)Cl2) of Cl2 have been recorded in the wavenumber ranges 92,500-96,500 cm(-1), corresponding to transitions to the low vibrational levels of the X(+) (2)Πg (Ω = 3∕2, 1∕2) ground state of Cl2 (+), and 106,750-115,500 cm(-1), where the a(+) (4)Σu (-)←X(1)Σg (+), A(+) (2)Πu←X(1)Σg (+), and B(+) (2)Δu←X(1)Σg (+) band systems overlap with transitions to high vibrational levels (v(+) > 25) of the X(+) state. The observation of Franck-Condon-forbidden transitions to vibrational levels of the X(+) state of the cation with v(+) ≥ 25 is rationalized by a mechanism involving vertical excitation of predissociative Rydberg states of mixed singlet-triplet character with an A(+) ion core which are coupled to Rydberg states converging to high-v(+) levels of the X(+) state. The same mechanism is proposed to also be responsible for the observation of Cl(+) - Cl(-) ion pairs and quartet states in the photoionization of Cl2. The potential energy function of the X(+) state of Cl2 (+) was determined in a direct fit to the experimental data. Transitions to vibrational levels of the A(+) (2)Πu, 3∕2 and B(+) (2)Δu, 3∕2 states of Cl2 (+) could be identified using the results of a recent analysis of the strong perturbation between the A(+) (2)Πu, 3∕2 and B(+) (2)Δu, 3∕2 states of Cl2 (+) observed in the A(+) - X(+) band system [Gharaibeh et al., J. Chem. Phys. 137, 194317 (2012)], and transitions to several vibrational levels of the upper spin-orbit component ((2)Πu, 1∕2) of the A(+) state were detected in the photoelectron spectrum of Cl2 (+). The a(+) (4)Σu (-)←X(1)Σg (+) photoelectron band system, which is nominally forbidden by single-photon ionization from the ground state was also observed for the first time and its vibrational and spin-orbit structures were analyzed. The (4)Σu (-) state is split into two spin-orbit components with Ω = 1∕2 and Ω = 3∕2, separated by 37.5 cm(-1). The vibrational energy level structure of both components is regular, which indicates that the splitting results from the interaction with one or more distant ungerade Ω = 1∕2 or Ω = 3∕2 electronic states.