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

Abstract

The spatial and temporal distribution of neutral and singly ionized lithium atoms has been investigated when slab targets are irradiated by ϳ1 s pulses of both resonant and nonresonant radiation from a flashlamp pumped dye laser. The ion population is enhanced when the laser is tuned to resonance with the 2s -2p line of neutral lithium and there is evidence for efficient ion production at distances up to 7.5 mm from the target surface. In particular, the rate of ionization is increased and the maximum ion population is attained between 120 and 200 ns earlier in the resonant case depending on distance above the target surface. However, the on-resonance ion population close to the surface is found to decrease significantly some 200 ns into the laser pulse. Ion and neutral drift velocities were measured by direct observation of the surface of the expanding plasma plume at a laser flux close to 8ϫ10 8 W cm Ϫ2 and were consistent with the value predicted by a dynamic source model for accelerated plasma expansion, as was the value for the electron temperature derived from line intensity ratios. Power density thresholds for ion production have been studied and in particular, the threshold for Li 2ϩ production is established.

Vacuum ultraviolet assessment of ionization temperature by space-resolved emission spectroscopy

“…2 was also observed in previous studies employing spectral lines in the visible range [12,14]. In the first study [12], the ionization temperature of plasmas generated by the fundamental radiation from a Nd:YAG laser focused onto a graphite target under vacuum (ϳ10 −4 mbar) varied between 2.46 eV (28 540 K) at 1.5 mm to 1.66 eV (19 260 K) at about 12 mm away from the target surface.…”

“…In the second investigation [14], the spatial as well as temporal evolution of electron density and ionization temperature for both neutral and singly ionized lithium atoms was investigated under vacuum. The on-target power density was estimated to be very close to 1 GW/cm −2 , which is about five times lower with respect to that used in the present work.…”

Plasma diagnostics for investigating extreme ultraviolet light sources

“…͑1͒ was used, which involves the peak intensity ratio of lines from two successively charged ions ͑all constants were taken from the NIST database 26 The relative intensities of lines originating from adjacent ion stages were used to calculate the ionization temperature ͑T ionz ͒ of the plasma as a function of space and time.…”

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

“…This means that the energy expended by electrons on excitation of atoms feeds back to the electron subsystem, and the channel of loss of the electron energy associated with spontaneous decay is of little importance. Our estimates (for λ ≈ 1 µm within this model) suggest that for condition (10) to be satisfied at atomic concentrations n ∼ 10 17 ÷10 19 cm −3 , the laser radiation intensity needs I 0 ≥ 10 9 W/cm 2 (for an Al target). The required electron concentration due to emission from the surface, as well as due to vapor ionization by electrons, can only be obtained at this kind of intensity.…”

“…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.…”