Rovibrational laser jet-cooled spectroscopy of the NH3-Ar complex in the nu(2) umbrella region of NH3: comparison between new infrared data and an ab initio calculated spectrum

摘要

Five ortho and para bands of the nu(2) umbrella mode of the NH3-Ar van der Waals complex have been recorded at high resolution using jet-cooled infrared laser spectroscopy. A rovibrational analysis provides accurate band centres and upper state rotational constants for the Pi(s)(j = 1, k = 0) - Sigma(a)(j,k = 0) and Sigma(s)(j = 1, k = 0) - Sigma(a)(j,k = 0) ortho bands. The puzzling para bands observed in the region of the lower and upper components of the inversion splitting doublet have been assigned by comparison with rovibrational and tunnelling levels and transitions calculated ab initio. The latter calculations are based on the four-dimensional potential energy surface reported by Loreau et al. [J. Chem. Phys. 141, 224303 (2014)], which takes explicitly into account the umbrella motion of the ammonia molecule. The very good agreement found between Pi(s/a,lower)(j = 1, k = 1) - Sigma(a)(j = 1, k = 1) and Pi(s/a,upper)(j = 1, k = 1) - Sigma(s)(j = 1, k = 1) experimental and calculated transitions has been exploited to determine precisely two different inversion splittings in the nu(2) state (32.003(1) and 36.008(1) cm(-1)) from extrapolated Q(0) line frequencies and to obtain a qualitative picture of Coriolis couplings present in both the nu(2) = 0 and nu(2) = 1 states.