Rapid sensitization of vibrational energy levels of N2O in collisions with SF6 excited by a CO2 laser

Experimental investigations of mixtures containing predominantly N₂O and small amounts of SF₆ demonstrate that rapid interspecies pooling of vibrational energy can occur to produce a pulse of excess vibrational energy in the ν₃ mode of N₂O following excitation of SF₆ by a Q-switch CO₂ laser. This increased population in the ν₃ mode of N₂O can occur on a time scale shorter than that on which collision-induced VV processes redistribute vibrational energy among the modes of SF₆. The equilibration takes place in three discernible stages: (1) a rapid pooling of energy between a limited number of levels of the SF₆ and N₂O, then (2) a slower collision-dependent VV process that equilibrates all the vibrational modes in the system, with (3) a subsequent VT,R process that returns the system to its initial state. Argon is shown to accelerate selectively process (2) with an efficiency consistent with the previously measured ability of argon to accelerate the VV process in pure SF₆. The experimental evidence indicates that other modes in N₂O do not become involved on the time scale on which direct crossing to ν³ occurs. Additionally, on the time scale preceding the SF₆ VV equilibration, a fast collision-dependent process competes with the transfer of excitation to N₂O. The production of a pulse of excitation in N₂O is eliminated when isotopically substituted N₂O (¹⁴N¹⁵NO) is used instead under the same conditions because the crossing rate to the ν₃ mode of N₂O is decreased sufficiently when ¹⁵N is substituted for ¹⁴N that it no longer can compete with the VV equilibration among the modes in SF₆.