15min:

MARKUS MENGEL AND FRANK C. DE LUCIA, Department of Physics, The Ohio State University, 174 West 18th Ave., Columbus OH 43210, USA.

We have performed pressure broadening and time resolved double resonance studies on three rotational transitions (2₁₂ 1₁₁, 2₁₁ 1₁₀, and 3₁₃ 2₁₂) of formaldehyde (H₂CO) in collision with helium and hydrogen at low temperatures (T \le 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. We can confirm this prediction as far as it constitutes a model based on H₂CO - He collisions. However, we observe significantly different pressure broadening cross sections for H₂CO - H₂ collisions which raises the question as to whether a simple transfer from helium to hydrogen as collision partner can be made.