Application of the instanton method for analyzing tunneling splitting spectra of nonrigid molecular systems: II. Excited states

Abstract

For nonrigid molecules with two equivalent minima on their potential energy surface, expressions are obtained in terms of the instanton method for the calculation of the magnitude of the tunneling splitting of vibrational levels with the number n≥0 both in the harmonic approximation and taking into account the anharmonic corrections ∼ ℏ2. Using comparative calculations of the lowest spectral region of the tunneling splitting of the NH3 molecule as an example, it is shown that, even in the harmonic approximation, the calculation accuracy of the instanton method is as good as that of the standard Wentzel-Kramers-Brillouin method. When the anharmonic corrections to the energy levels are taken into account, the accuracy of description of the tunneling splittings of the vibration-inversion spectrum of molecules of the NH3 type in terms of the instanton method increases almost to the level of quantitative agreement with experimental data.