A theoretical spectroscopic study of single ammonia molecules adsorbed on the surface of fcc-type argon clusters. Surface effects on the ν_2 vibration-inversion mode

摘要

Ammonia monomers have been adsorbed on argon clusters at low temperature T approx= 30 K by Rohmund and Huisken [1] using the pick-up technique. They measured the spectrum of the NH_3 molecules in the region of the ν_2 umbrella mode. Two broad bands centered around 970 and 1000 cm~(-1) with finer details were observed. The authors attempted to interpret the obtained spectrum on the basis of the free rotation motions of the molecules. In this paper semi-empirical atom-atom potential energy calculations are performed for the ammonia monomer adsorbed on a rigid face-centered-cubic (fcc)-type surface of the argon cluster. In the equilibrium position of the rigid molecule on the cluster surface the orientational potential energy surface exhibits two quasi-equivalent minima separated by a potential barrier of about 100 cm~(-1). The symmetry of the molecular vibration-inversion double-well potential is destroyed; the inversion motion is then forbidden in the ground state. On the basis of the two adsorption orientations, the vibrational frequency shifts are calculated and the obtained infrared bar-spectrum agrees with the experimental one.