INFRARED/INFRARED DOUBLE RESONANCE: A HIGH RESOLUTION PROBE FOR THE STUDY OF IVR NEAR $9700 CM^{-1}$.

Abstract

By sequentially exciting the acetylenic C-H stretch in 1-propyne first with a $3.0 \mu m$ color center laser, followed by a $1.5 \mu m$ color center laser, we are able to access the second overtone of this C-H stretch, near 9700 $cm^{-1}$. The high resolution of the one photon experiment (10 MHz) is retained, and the S/N is limited only by the degree of fractionation of the spectrum and the noise induced from the frequency jitter of the ``pump'' laser. Locking the $3 \mu m$ color center laser to an external reference etalon resulted in an significant improvement in S/N over previous attempts. The data provide intriguing features not seen at lower energies ($v=1$ and $v=2$) in propyne or in other acetylenic molecules at similar density of states. One unique observation from the analysis of the spectrum is the presence of very strong x,y-Coriolis coupling affecting the fractionation of the lines but not the time for which the initial excitation remains localized. We plan to extend our studies at $v=3$ to other intermediate size molecules, such as cyanoacetylene, diacetylene and trifluoropropyne, in order to separate more clearly the effect on the IVR due to the amount of vibrational energy deposited in the molecule and the density of background states to which this energy can couple.