Time resolved Measurements of State resolved Collisional Energy Transfer in the Electronic Ground State of $NH_{3}$ with IR-Double (Triple)-Resonance Spectroscopy

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Ohio State University

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The energy transfer of vibrationally and rotationally excited states of polyatomic molecules is of fundamental interest for molecular spectroscopy and for the kinetics of many reactive systems. In the present contribution, state resolved measurements of collisional energy transfer in $NH_{3}$ in the $v_{2}=1$ and $v_{2}=2$ regions are presented. The $v_{2}$ levels of $NH_{3}$ have been populated by $CO_{2}$ laser excitation: one photon in the $v_{2}=1$ region and two photon (identical and non-identical) in the $v_{2}=2$ region. With this technique, selected well defined states in the regions of $v_{2}=1$ and 2 have been prepared. The detection of changes in population of these and other neighboring states has been monitored by transient IR absorption spectroscopy with a diode laser. Line widths in the $3v_{2}-2v_{2}$ bands have provided some information about energy transfer in the $3v_{2}$ region. Measurements of total depopulation rates of selected states $(e. g. k(v_{2}=1, s(5,3)) = 30\mu s^{-1} torr^{-1})$, rates of collision induced symmetry change ($ a\rightarrow $s), and individual $k_{ij}(\bigtriangleup J=1)$ between different states in the energy regions $v_{2}$ and $2v_{2}$ are found to be faster than the Lennard-Jones rate ($10\mu s^{-1}-torr^{-1}$), which is in accord with theoretical expectations. The experiments show that vibrational energy loss is much slower than rotational energy transfer A detailed master equation analysis and direct state-resolved measurements provide ``state to state” rate constants and their vibrational energy / inversion splitting dependence for $NH_{3}-NH_{3}$ rotational energy trasfer. Preliminary work on foreign gas broadening will be described.


Author Institution: Department of Chemistry, Massachusetts Institute of Technology