Pulsed-laser optogalvanic spectroscopy in a high-pressure Hg discharge

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Abstract
Pulsed-laser irradiation of a high-pressure mercury discharge tuned to excite the 7 3S1←6 3P2,1,0 transitions in mercury at 546, 436, and 405 nm, respectively, decreased the voltage across the discharge and increased the current through the discharge with respect to their unperturbed values. The peak magnitude of the voltage decrease was proportional to the peak current increase, but the temporal dependence of the two signals was different. The optogalvanic signal was a maximum in the center of the discharge and decreased towards the walls. The full width at half maximum (FWHM) of the optogalvanic line profile also depended on radial position. The magnitude of the optogalvanic signals had a nonlinear dependence on laser energy, and the FWHM of the optogalvanic line profile increased with laser energy. Both of these effects were attributed to stimulated emission. Negative optogalvanic voltage signals were also observed at 408 nm (7 1S0←6 3P1), 577 nm (6 3D2←6 1P1), and 579 nm (6 1D2←6 1P1). Estimates of reaction rates support the following mechanism: Absorbed laser energy is rapidly converted to increased electron kinetic energy by superelastic collisions, which results in increased ionization in the discharge. The additional ionization decreases the voltage across the discharge and increases the current through the discharge.