Helium and hydrogen induced rotational relaxation of H2CO observed at temperatures of the interstellar medium

We have performed pressure broadening and time resolved double resonance st udies on three rotational transitions (2(12) <- 1(11), 2(11)<- 1(10), and 3 (13)<- 2(12)) of formaldehyde (H2CO) in collision with helium and hydrogen at low temperatures (T <less than or equal to>16 K). This was achieved by a pplying the collisional cooling method. The purpose of this study was to co mpare 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 H2CO- He collisions. However, we observe significantly different pressure broaden ing cross sections for H2CO-H (2)collisions, which calls into question the substitution of helium for hydrogen as a collision partner in astrophysical calculations.