DIRECT OBSERVATION OF ALL QUASIBOUND ENERGY LEVELS AND LIFETIMES IN THE $Na_{2} B {^{1}}\Pi_{u}$ STATE BY CW MODULATED GAIN SPECTROSCOPY
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Date
1984
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Ohio State University
Abstract
Much of the interest in the $Na_{2} B{^{1}}\Pi_{u}$ state is because of the $\sim 400 cm^{-1}$, rotationless barrier to dissociation. It arises from the repulsive resonance interaction of the sodium atoms, one in the $3 {^{2}}S_{1/2}$ and one in the $3 {^{2}}P_{3/2}$ state, as they approach each other. This barrier supports seven or eight vibrational levels, some of which are observed to exhibit lifetime shortening from tunnelling through the barrier. The last bound and all quasibound energy levels, normally inaccessible from thermally populated levels of the $Na_{2}$ ground state, have been populated via a triple resonance excitation scheme involving three single mode cw lasers, one of them being an optically pumped intramolecular $Na_{2}$ laser (OPL). The signal is detected by monitoring the change in this OPL output when the probe laser is tuned into resonance between the lower OPL level and a specific rotation-vibration energy level of the B-state. The lifetimes are measured from the absorption linewidths. The observed values cover the range from $\sim 88ps (\sim 1.8GHz)$ to as short as $\sim 8ps (\sim 20GHz)$. A semi-classical treatment of the tunnelling lifetimes yields parameters which determine the long range form of the potential such as Van der Waals coefficients $C_{3}$ and $C_{6}$ and the exchange energy parameters.
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Author Institution: Department of Chemistry, Massachusetts Institute of Technology