TRANSFER OF ROTATIONAL POPULATION IN NH(3) BY INFRARED-INFRARED DOUBLE-RESONANCE

In this paper we report experimental and theoretical results on total inelastic cross-sections for NH3 molecules colliding with H-2 at room temperature. The use of time resolved infrared-infrared double resonan ce allows one to follow the population relaxation for different energy levels of NH3. A CO2 pulsed pump laser tuned to a line in the nu2 ban d of NH3 induces a transient transfer of population and the return to equilibrium is probed with a c.w. diode laser turned to lines in the n u4 band of NH3. The time evolution of the probe absorption for differe nt pressures of the two gases is then related to the inelastic cross-s ections of the states involved. The so-determined cross-sections for t he (j = 5 k = 4 epsilon = + 1) and (j = 1, k = 1, epsilon = - 1) state s of NH3 are respectively 27 angstrom2 and 31 angstrom. We have also c alculated inelastic cross-sections using a semi-classical model and an ab initio potential energy surface for different values of ( j, k, ep silon) (i.e. j = 1,2,3, k = 1,2,3 and epsilon = +/- 1) and of j2 (tota l angular momentum of H-2). These calculated values are in agreement w ith the experimental results.