Absolute measurements of state-to-state rotational energy transfer between CO and H2 at interstellar temperatures

Experimental measurements and theoretical calculations of state-to-state rate coefficients for rotational energy transfer of CO in collision with H${}_2$ are reported at the very low temperatures prevailing in dense interstellar clouds (5 - 20 K). Detailed agreement between quantum state-selected experiments performed in cold supersonic flows using time-resolved infrared - vacuum-ultraviolet double resonance spectroscopy and close-coupling quantum scattering calculations confirms the validity of the calculations for collisions between the two most abundant molecules in the interstellar medium.