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The dynamics of infrared multiphoton absorption (IRMPA) is predicted from theory to vary with increasing density of states. Very little is known experimentally, however, about the actual dynamics of IRMPA in the low density-of-states regime. The experiments described here examine the photophysics of absorption in the molecule thiophosgene. The experiment involves infrared-visible double resonance in conjunction with molecular beam techniques. This method enables populations of the ground state, perturbed by $CO2$ laser pumping, to be determined directly in the absence of collisions. Previous studie$s1$ have shown that populations of the pumped mode $(2\nu 4)$ are sensitive to the peak power of the $CO2$ laser, and that all rotational states interact with the IR radiation field. Consequently, several important problems are examined in these studies: 1) variation of populations in pumped mode levels (up to $\nu =12$) with changes in $CO2$ laser peak power and wavelength; 2) the extent of rotational state (K structure) depletion of the vibration-less level; 3) the effects on absorption of non-driven, background modes coupled to the pumped mode ladder. Since the low-density-of-states regime plays an important role in IRMPA in thiophosgene, calculations using the model of Ackerhalt and Galbraith are in progress to understand more quantitatively the dynamics of absorption in this region. $\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$ Supported by Air Force Office of Scientific Research. $\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ Work performed under auspices of U.S. Department of Energy. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ D. M. Brenner. J. Chem. Phys. 74, 0000 (1981).