IR MULTIPHOTON ABSORPTION IN THE $A^{1}A_{2}$ STATE OF THIOPHOSGENE

$^{*}$Work performed under the auspices of U.S. Department of Energy.Author Institution:Infrared multiphoton absorption (IRMPA) in the first excited singlet state of thiophosgene has been studied under molecular beam conditions by monitoring changes in the total fluorescence yield due to predissociation. A dye laser pulse prepares specific levels of the $A^{1}A_{2}$ state which are subsequently pumped to higher levels with a $CO_{2}$ laser. Molecules which absorb IR photons to an energy level $\leq 3650 ~\mbox{cm}^{-1}$ above the vibrationless level effectively do not fluoresce, but those which absorb to an energy level $\geq 3650~\mbox{cm}^{-1}$ fluoresce with unit quantum yield. At a dye laser resolution of $0.3 cm^{-1}$, all rotational levels of given vibronic state interact with the $CO_{2}$ laser and hole burning effects are not observed. The experimental geometry constrains all electronically excited molecules to interact with the $CO_{2}$ laser, yielding absolute probabilities for nonradiative pathways (expected to be predominantly predissociation). These absolute probabilities have been used to determine relative multiphoton absorption cross sections within the vibrational manifold for levels up to $3000 cm^{-1}$ in energy ($- 0.3$ state $cm^{-1}$). Plots of absolute dissociation probabilities as a function of fluence suggest that the absorption cross section is both intensity dependent and wavelength dependent. Observations concerning the extent of intermolecular coupling of isoenergetic levels will also be discussed