Helium pressure broadening of ammonia inversion transitions from 10 to 35 K

Helium pressure broadening cross sections for the (J,K)=(1,1), (2,2) and (3,3) inversion transitions of ammonia were measured at temperatures from 10 to 35 K. Measurements were taken in a quasiequilibrium cell using the collisional cooling technique. In contrast to the situation at room temperature where there is little state-to-state variation in helium pressure broadening cross sections, at low temperature the (3,3) cross sections were found to be, on average, three times larger than the (1,1) cross sections while the (2,2) cross sections were twice as large as the (1,1) values. The major factor determining the relative size of the low temperature cross sections appears to be the contribution from inelastic collisions into lower energy levels. The relative size of the cross sections thus scales with the number of rotational levels underlying a given (J,K) state. Cross sections were also calculated for comparison with the experimental data using three existing NH3-He potential surfaces. Overall, none of the three surfaces gave consistent agreement with the experimental results for all three observed transitions. The discrepancy between experiment and theory was greatest for the (3,3) data. This may be a manifestation of a previously observed tendency of NH3-He potential surfaces to underestimate the contribution from parity-conserving collisions in calculated state-to-state rates. (C) 1997 American Institute of Physics. [S0021-9606(97)03944-5].