Laser-induced electron emission

Abstract

Laser-induced electron emission from tungsten has been observed using $Q$-switched ruby lasers. Experimental results from tungsten indicate that processes other than thermionic emission can occur at irradiances of tens of megawatts per square centimeter and pulse durations of tens of nanoseconds. For these laser irradiances and pulse durations the emitted electron pulses have maximum currents in the range of 0.1 to 100 mA, and this maximum current is delayed approximately 5 nsec with respect to the maximum incident irradiance. The measured time delay is consistent with a thermal origin for the electron emission process. However, the relation between the emitted current and the incident laser irradiance is substantially less nonlinear than that predicted by the Richardson-Dushman equation. An effect that we call two-photon-assisted thermionic emission is proposed to explain the observed data, and the experimental results are compared to a model for this effect. In addition, results are obtained from tungsten ribbon cathodes, the initial temperature of which could be varied by changing the current in the ribbon. In this case, however, when the maximum emitted current is compared to the initial cathode temperature for a constant maximum irradiance, the results are more consistent with thermionic emission than with the two-photon-assisted thermionic emission process. This difference is as yet unexplained.