Interaction of wide band gap single crystals with 248 nm excimer laser radiation. V. The role of photoelectronic processes in the formation of a fluorescent plume from MgO

We examine the formation of a fluorescent plume during pulsed laser irradiation of wide band gap materials at sub-band gap photon energies. We show that fluences near the threshold for plume fluorescence in MgO do not produce adequate electron densities for significant laser-plume interactions via inverse bremsstrahlung processes. At a wavelength of 248 nm, 30 ns pulse width, and fluences well below the onset of detected plume fluorescence, we observe intense Mg+ emissions and electron temperatures in excess of 1 eV. The onset and growth of plume fluorescence closely parallels the neutral emission intensities, suggesting that the lack of plume fluorescence below the fluence threshold is due to the lack of gas phase neutral Mg. Time-resolved measurements of the atomic line emissions show peaks well after the peak laser intensity, arguing against direct laser-plume interactions. We propose that electron acceleration involves electrostatic interactions between photoelectrons and photoelectronically emitted ions. The unusual temporal evolution of the atomic line emissions can be attributed to the time dependence of the spatial overlap between neutral particles and the accelerated electrons that collisionally excite the atomic lines.