Direct Measurement of Rotationally Inelastic Cross Sections at Astrophysical and Quantum Collisional Temperatures

The first measurements of rotationally inelastic collision cross sections over the 1--40 K temperature range characteristic of much of the interstellar medium are reported. Measurements of the 11,0-10,1 transition of H₂S in collision with He are based on a collisional cooling methodology, which allows the direct observation of collision processes at temperatures far below the limits ordinarily imposed by vapor pressure. These cross sections are also of fundamental interest because resonant formation of quasibound complexes mediates rotational energy transfer at low collision energy.