Helium and Hydrogen Induced Rotational Relaxation of H 2 CO Observed at Temperatures of the Interstellar Medium

Abstract

We have performed pressure broadening and time resolved double resonance studies on three rota-tional transitions and of formaldehyde in collision with helium (2 12 ^ 1 11 , 2 11 ^ 1 10 , 3 13 ^ 2 12) (H 2 CO) and hydrogen at low temperatures (T ¹ 16 K). This was achieved by applying the collisional cooling method. The purpose of this study was to compare laboratory measurements with the theory used to explain the anomalous absorption of interstellar formaldehyde against the 2.7 K background. Our laboratory measurements are generally consistent with this theory for collisions. However, we H 2 CO-He observe signiÐcantly di †erent pressure broadening cross sections for collisions, which calls H 2 CO-H 2 into question the substitution of helium for hydrogen as a collision partner in astrophysical calculations.