Spatially resolved spectra of resonantly pumped laser produced plasmas of lithium

Abstract

Spatially resolved visible spectra emitted from a plasma formed by focusing the output of a flashlamp pumped dye laser onto solid targets of lithium have been recorded. The effects of laser wavelength on plasma formation have been analyzed by tuning the laser to the first resonance transition in lithium at 6708 Å and comparing the results with those obtained at an off resonance wavelength of 6728 Å. Laser energies varied from 430 to 645 mJ and were focused to a spot diameter of 300 m with a pulse length of 850 ns. Light from the plasma was focused onto the slit of a Hilger spectrograph, which incorporated a Pellin-Broca prism whose output was focused onto a charge-coupled device camera. Electron densities of 10 16 -10 18 cm Ϫ3 were deduced from Stark width and shift measurements of the Li II 2s -2p line at 5485 Å and from the Li I 2 p -4d line at 4603 Å. In this way the electron density could be mapped as a function of distance from the target. By observing emission at 5485 Å the electron density was observed to increase when the laser was tuned on resonance compared to off. This was not the case when the neutral Li I 2 p -4d line was used as a diagnostic. The anomaly is accounted for by referring to both the ionization rates involved and the temporal profile of the emission.

Effects of resonant pumping on the temporal and spatial evolution of a laser produced lithium plasma

“…It also explains why in other work on lithium plasmas, a higher electron density was measured at the end of the plasma pulse when the laser was detuned. 8,18 These effects, when combined with the fact that the ion and neutral emission peak at different times, can present difficulties in using lines from a single ion stage, without knowledge of their time history, for plasma diagnostics. 18 Indeed, in a full 3D visualization, emission from higher stages of ionization can occur from a hot inner core, while emission from lower stages may primarily originate from cool outer layers where the ion and electron densities are much lower.…”

“…30 From a study of Stark shifts of the 2s -2p line, the electron density within 2 mm from the surface, under very similar experimental conditions, was found to be greater than 1 ϫ10 17 cm Ϫ3 . 18 …”

The combined effect of optical laser and microwave radiations on a metal surface

“…(following from inequality (11)), above which MR can no longer penetrate the plasma region (in the figures, this instance is indicated by a vertical dashed line t = τ c ). The electrons are further heated by laser radiation only.…”

“…To avoid this happening before the critical concentration n * e has been achieved, from Eqs. (10) and (11) we obtain that ω m must satisfy the condition…”

“…Moreover, this model enables us to predict the effect of the screening of the target by the resulting plasma [3] , which is later confirmed in the experiments on a lithium target. [10,11] We now deal with the heating of electrons by MR. Part of this radiation penetrates the vapor layer, while part of the radiation is reflected from it. The reflection coefficient r m is dependent on the thickness of the layer H, its complex dielectric permeability ε, the vapor density, and the extent of vapor ionization.…”