Spin-Orbit Coupling and Potential Energy Functions of Ar2(+) and Kr2(+) by High-Resolution Photoelectron Spectroscopy and ab Initio Quantum Chemistry.

The dependence of the spin-orbit-coupling constant of the six low-lying electronic states of Ar2(+) and Kr2(+) on the internuclear distance R has been calculated ab initio. The spin-orbit-coupling constant varies by about 10% over the range of internuclear distances relevant for the interpretation of the high-resolution photoelectron spectra of Ar2 and Kr2 and can be accurately represented by a Morse-type function for the states of ungerade electronic symmetry and by an exponentially decreasing function for the states of gerade symmetry. The spin-orbit-coupling constant is larger than the asymptotic value (at R → ∞) for the gerade states and smaller for the ungerade states. The calculated R-dependent spin-orbit-coupling constants were used to derive a new set of potential energy functions for the low-lying electronic states of Ar2(+) and Kr2(+) and to quantify the errors resulting from the widely used approach consisting of approximating the spin-orbit-coupling constant by its asymptotic value. The effects of the R dependence on the potential energy functions of the six low-lying electronic states of the homonuclear rare-gas dimer ions are found to be very small for Ar2(+) (and by inference also for Ne2(+)) but significant for Kr2(+). The shifts arising in calculations of the potential energy functions from a neglect of the R dependence of the spin-orbit-coupling constant are the result of the interplay between the differences between the binding energies of the relevant (2)Π and (2)Σ(+) states, the magnitude of the spin-orbit-coupling constant, and the magnitude and sign of the deviations between the R-dependent spin-orbit-coupling constant and its asymptotic value at large internuclear distances.

On the R-dependence of the spin-orbit coupling constant: Potential energy functions of Xe(2) (+) by high-resolution photoelectron spectroscopy and ab initio quantum chemistry.

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe(2) has been measured between 97 350 and 108 200 cm(-1), following resonant two-photon excitation via selected vibrational levels of the C 0(u) (+) Rydberg state of Xe(2). Transitions to three of the six low-lying electronic states of Xe(2) (+) could be observed. Whereas extensive vibrational progressions were observed for the transitions to the I(32g) and I(32u) states, only the lowest vibrational levels of the II(12u) state could be detected. Assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts and from the modeling of the potential energy curves. Adiabatic ionization energies, dissociation energies, and vibrational constants are reported for the I(32g) and the I(32u) states. Multireference configurational interaction and complete active space self-consistent field calculations have been performed to investigate the dependence of the spin-orbit coupling constant on the internuclear distance. The energies of vibrational levels, measured presently and in a previous investigation (Rupper et al., J. Chem. Phys. 121, 8279 (2004)), were used to determine the potential energy functions of the six low-lying electronic states of Xe(2) (+) using a global model that includes the long-range interaction and treats, for the first time, the spin-orbit interaction as dependent on the internuclear separation.

The low-lying electronic states of ArXe+ and their potential energy functions.

Photoionization and pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of ArXe have been recorded between 96 400 and 108 200 cm(-1) following resonance-enhanced two-photon excitation via selected vibrational levels of the C 1 and D 0+ Rydberg states. The PFI-ZEKE photoelectron spectra consist of three vibrational progressions corresponding to the X 1/2<--X 0+, A1 3/2<--X 0+, and A2 1/2<--X 0+ transitions. From these progressions, adiabatic ionization energies, equilibrium internuclear distances, and vibrational constants have been derived for the lowest three electronic states of ArXe+. The photoionization spectra reveal long progressions of autoionizing Rydberg states converging to the lowest vibrational levels of the A1 3/2 state. A potential model has been developed that enables a global description of the low-lying electronic states of the heteronuclear rare gas dimer ions. The model explicitly treats the effects of the spin-orbit, charge-exchange, and long-range interactions. This model was used to obtain potential energy functions for all six low-lying electronic states of ArXe+ from the experimental positions of the vibrational levels of the X 1/2, A1 3/2, and A2 1/2 states relative to the ground neutral state and existing spectroscopic data on the B 1/2, C1 3/2, and C2 1/2 states.

Potential energy curves of diatomic molecular ions from high-resolution photoelectron spectra. II. The first six electronic states of Xe2 +.

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe(2) has been measured between 90 000 and 109 000 cm(-1) following single-photon excitation from the ground neutral state. Transitions to five of the six low-lying electronic states of Xe(2) (+) could be observed. Whereas extensive vibrational progressions were observed for the X0(g) (+)-->I(1/2u), I(3/2g), and II(1/2u) photoelectron transitions, only the lowest vibrational levels of the I(3/2u) and II(1/2g) states could be detected. Unambiguous assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts of the vibrational bands and of the intensity distribution and from the modeling of the potential energy curves. Analytical potential energy curves of spectroscopic accuracy (i.e., approximately 1 meV) were determined for all six low-lying electronic states using a global model, which includes the first (charge-induced dipole, proportional to 1/R(4)) member of the long-range interaction series and treats the spin-orbit interaction explicitly. The assumption of an R-independent spin-orbit coupling constant was tested and found to be an excellent approximation.