An Experimental Investigation of Collisions of NH 3 with Para‐H 2 at the Temperatures of Cold Molecular Clouds

Abstract

Experimentally measured cross sections for collisional broadening of ammonia by J = 0, para-H2 are presented for the (J,K) = (1, 1),(2, 2), and (3, 3) NH3 inversion transitions at temperatures from 12.5 to 40 K. The cross sections were obtained in a quasi-equilibrium environment utilizing the collisional cooling technique. These are the first laboratory measurements of interactions between NH3 and J = 0, para-H2 at the temperatures of cold molecular clouds. The measured cross sections are compared to theoretical predictions using two existing ab initio NH3-H2 potential surfaces and to previously measured He broadening cross sections. In comparison to He broadening cross sections at these temperatures, the para-H2 results are up to 4 times larger. This is in contrast to 300 K experimental results where para-H2 cross sections are only 50% larger than He.