Balmer series emission in the afterglow of high-pressure, laser-induced hydrogen and hydrogen-argon plasmas

The afterglow of photoionised plasmas created in hydrogen and hydrogen with a small fractional addition of Ar (3.5 {\%} ) has been studied by focussing a 15 ns, 10 Hz, 1064 nm laser pulse into gas that is at a pressure of 10$^{5 }$Pa. At the focus, the laser power density is about 10$^{11}$ W/cm$^{2}$. Light emission is dispersed by a 0.6 m monochromator, with a 1220 line/mm grating blazed at 500 nm and detected using an image-intensified linear diode. In pure hydrogen, H$\alpha $, H$\beta $, and H$\gamma $ emission was observed to about 4 $\mu $s. In the mixture, the peak emission intensity is enhanced by a factor of about 2, H$\delta $ and H$\varepsilon $ lines are also observed, and the emission of the H$\alpha $ and H$\gamma $ was observable to about 6 $\mu $s. The electron density, determined from Stark-broadening, is found to have a complex temporal behaviour. From an initial value at 0.3 $\mu $s of about 3.5$\times $10$^{16}$ cm$^{-3}$ in pure hydrogen and 6.5$\times $10$^{16}$ cm$^{-3}$ in the H$_{2}$/Ar mixture, the density falls by an order of magnitude by 1.5 $\mu $s. Thereafter, in pure hydrogen, an increase in density by about 1$\times $10$^{15}$ cm$^{-3}$ over a period of about 1 $\mu $s is observed, followed by a decrease. In the mixture, a plateau occurs in the density temporal behaviour. It is also found that the line emission intensity decay rate changes at about 1.5 $\mu $s (pure). WG was a Mellon Fellow at Wesleyan University.