Quantum Monte Carlo and high-level ab initio molecular orbital investigation of dissociation channels of the positronic alkali-metal hydrides, [XH;e⁺] (X = Li, Na, and K)

Abstract

Energy thresholds for dissociation channels of positronic alkali-metal hydrides, [XH;e⁺] (X = Li, Na, and K), to XH + e⁺(positron dissociation), XH⁺ + Ps (positronium dissociation), and X⁺ + [H⁻;e⁺] (positronic hydride ion dissociation) have been calculated using quantum Monte Carlo and high-level ab initio molecular orbital methods, and including quantum zero-point vibrational energy of all of the particles. As the atomic number of X increases from Li to K, the dissociation energy to XH + e⁺ increases because the dipole moment of XH increases with the atomic number of X, while the dissociation energy to XH⁺ + Ps decreases. The energy threshold for the ionic dissociation to X⁺ + [H⁻;e⁺] is also reduced, and we obtain 0.975 (3) eV, 0.573 (12) eV, and 0.472 (19) eV for [LiH;e⁺], [NaH;e⁺], and [KH;e⁺], respectively, for this channel. Our results strongly support the conclusion that, among these three channels, the lowest energy dissociation for [XH;e⁺] is the pathway to X⁺ + [H⁻;e⁺], where X = Li, Na, and K.