Time-resolved Fourier transform emission spectroscopy of highly vibrationally excited molecules

Dong Qin, University of Pennsylvania


Time resolved Fourier transform emission spectroscopy has been developed to study highly vibrationally excited molecules in the visible and infrared regions. The feasibility of this technique has been demonstrated in the characterization of transient species such as methylene, and in the investigation of collisional deactivation of highly vibrationally excited molecules such as NO$\rm\sb2,\ SO\sb2$ and pyrazine. Using Fourier transform dispersed emission spectroscopy (FTDES) in the visible region, four vibrational levels in the $\tilde a$ state, i.e., (1,2,0), (1,3,0), (2,0,0) and (0,5,0), of CH$\sb2$ were characterized.for the first time. Spectroscopic analysis of these vibrational levels gave the rotational constants and vibrational term values. Renner-Teller coupling between the $\tilde a\sp1A\sb1$ and $\tilde b\sp1B\sb1$ states of CH$\sb2$ and its effect on the rotational structure of bending vibrational levels were observed from the rotational constant of the $\tilde a\sp1A\sb1$ (0,5,0) band. Using the term value of $\tilde a\sp1A\sb1$ (0,5,0) level, the barrier height to linearity in the $\tilde a\sp1A\sb1$ state was estimated. Using time-resolved FTDES (TR-FTDES), the rotational energy transfer (RET) propensity rules and the rate constants of state-to-state RET for $\tilde b\sp1B\sb1$ (0,16$\sp0$,0) state were obtained. The propensity rules observed for RET showed that CH$\sb2\ \tilde b\sp1B\sb1\ (0,16\sp0,0)$ interacted with ketene through a quadrupole-dipole interaction, with selection rules: $\rm \Delta J = \pm2,\ \Delta K\sb{a} = 0,\ and\ \Delta K\sb{c} = \pm2.$ Both RET and reaction occurred rapidly for CH$\sb2\ \tilde b\sp1B\sb1\ (0,16\sp0,0)$ in collisions with ketene. The cross section for RET ranged from one to four times the hard-sphere gas kinetic cross section, which was calculated to be $\sim$10 A$\sp2$. The reactive cross sections of $\tilde b\sp1B\sb1\ (0,16\sp0,0)$ indicated that CH$\sb2$ was more reactive in $\tilde b\sp1B\sb1$ state than in $\tilde a\sp1A\sb1$ state. Using time-resolved Fourier transform IR emission spectroscopy (TR-FTIRES), collisional deactivation of highly vibrationally excited NO$\rm\sb2,\ SO\sb2$ and pyrazine colliding with a variety of bath-gas molecules has been studied. IR emission from both laser excited donor molecules and energy receiving bath-gas molecules (such as CO$\sb2$ and N$\sb2$O) was observed and examined. By modeling the IR emission bands of excited donor molecules from TR-FTIRES spectra, the average energy content of excited donor molecules, $\langle E\rangle,$ as a function of number of collisions was extracted. Further calculation yielded the energy transfer rate, i.e., energy loss per collision, $\langle\Delta E\rangle,$ as a function of $\langle E\rangle.$ The $\langle \Delta E\rangle-\langle E\rangle$ curves for NO$\sb2$ and SO$\sb2$ showed a dramatic increase at energies that correspond to the origins of the electronic excited states of the donor molecules, indicating that an intramolecular coupling between the electronic excited state and the electronic ground state greatly enhanced the energy transfer rate. On the other hand, both $\Delta$v = 1 and $\rm\Delta v > 1$ energy transfer collisions from donor to acceptor molecules were observed by monitoring IR emission from bath-gas molecules. It was found that the $\Delta$v = 1 energy transfer collisions could be well described by a transition dipole coupling mechanism.

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Recommended Citation

Qin, Dong, "Time-resolved Fourier transform emission spectroscopy of highly vibrationally excited molecules" (1996). Dissertations available from ProQuest. AAI9627993.