High-Resolution Photoelectron Spectroscopy of the X^+ 2Σ⁺ Ground State of CaAr⁺

The structure of the X^{+ 2}Σ⁺(<i>v</i>⁺) ground state of CaAr⁺ was characterized by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy. The spectra were recorded from the CaAr metastable a ³Π_0^-(<i>v</i>″ = 0) state using a resonance-enhanced (1 + 1') two-photon excitation scheme via the intermediate Ca(4s5s)Ar ³Σ⁺ (<i>v</i>' ≤ 9) state. The adiabatic ionization energy of the metastable a³Π_0^- state of CaAr, corresponding to the CaAr⁺ X^{+ 2}Σ⁺ (<i>v</i>⁺ = 0, <i>N</i>⁺ = 0) ← CaAr a ³Π_0^- (<i>v</i>″ = 0, <i>J</i>″ = 0) ionization threshold, was determined to be <i>E</i>_I/(<i>hc</i>) = 33432.86(20) cm^-1. Vibrational levels of the X^{+ 2}Σ⁺ ground electronic state of CaAr⁺ in the range <i>v</i>⁺ = 0 - 15 have been characterized with partial resolution of the rotational structure, allowing an accurate determination of the potential-energy function which takes into account long-range interactions. The dissociation energy and equilibrium bond length of the X^{+ 2}Σ⁺ ground state of CaAr⁺ were determined to be <i>D</i>_e = 845(4) cm^-1 (<i>D</i>₀ = 810(4) cm^-1) and <i>R</i>_e = 2.990(3) Å, respectively.