Multiphoton infrared excitation spectrum of the SF6 molecule taking into account the fraction of the particles interacting with the field

A new method is proposed for determining the fraction of the particles interacting with the field in multiphoton excitation of molecules. The method is based on saturation of the infrared absorption due to acceleration of the rotational relaxation by a buffer gas, and was used to measure the true excitation spectrum of the SF₆ molecule in an infrared laser field. Estimates were made of the rotational and translational relaxation parameters in the SF₆–N₂ system.