Non-optogalvanic signal characteristic times in prebreakdown discharges

The prebreakdown regime, which exists in a gas cell during the time interval of free charge density growth from first electron-ion pair generation until the onset of breakdown, is characterized by low bias and incident field intensities. In our experiment, d. c. biased Ne and Ar gas cells at 90 torr each were illuminated with laser pulses. Both the incident laser field and d. c. bias field were smaller, sometimes much smaller, than those required for gas breakdown by each alone. Under these conditions, clear-cut electrical signals of amplitude on the order of tens of millivolts are observed as a response to laser pulse irradiation at wavelengths nonresonant to any gas atomic transitions. We called these non-breakdown responses 'prebreakdown or precursor signals'.