Abstract
By analyzing measured infrared absorption of pure $\mathrm{C}{\mathrm{H}}_{4}$ gas under both ``free'' (large sample cell) and ``confined'' (inside the pores of a silica xerogel sample) conditions we give a demonstration that molecule-molecule and molecule-surface collisions lead to very different propensity rules for rotational-state changes. Whereas the efficiency of collisions to change the rotational state (observed through the broadening of the absorption lines) decreases with increasing rotational quantum number $J$ for $\mathrm{C}{\mathrm{H}}_{4}\text{-C}{\mathrm{H}}_{4}$ interactions, $\mathrm{C}{\mathrm{H}}_{4}$-surface collisions lead to $J$-independent linewidths. In the former case, some (weak) collisions are inefficient whereas, in the latter case, a single collision is sufficient to remove the molecule from its initial rotational level. Furthermore, although some gas-phase collisions leave $J$ unchanged and only modify the angular momentum orientation and/or symmetry of the level (as observed through the spectral effects of line mixing), this is not the case for the molecule-surface collisions since they always change $J$ (in the studied $J=0--14$ range).
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