Fraunhofer-type absorption lines in double-pulse laser-induced plasma

Abstract

We studied the confocal double-pulse laser-induced plasma in the very beginning of its life. It was found that the second laser pulse fired 0.7 to 5 µs after the first pulse produces plasma which, during the first 0 to 20 ns, resembles solar configuration. There is a very hot and compact plasma core that radiates a broad continuum spectrum and a much larger and cooler outer shell. The light from the hot core passes through the cold outer shell and is partly absorbed by atoms and ions that are in ground (or close to ground) states. This produces absorption lines that are similar to Fraunhofer lines observed in the sun spectrum. The possibility to use these absorption lines for new direct and calibration free laser-induced breakdown spectroscopy analytical applications, both in laboratory and industrial conditions, is proved.

Fraunhofer-type absorption line splitting and polarization in confocal double-pulse laser induced plasma

“…Integration delay is D = D 1 =0 ns, so that integration coincides with the second pulse. As emphasized in our previous paper [13], this SP-excited plasma radiation is a continuum whose spectrum is very similar to the background radiation in DP-LIP ( Fig. 2 in [13], where the background spectrum I 0 (λ) is represented by a dashed line and I(λ) is represented by a solid line).…”

“…As emphasized in our previous paper [13], this SP-excited plasma radiation is a continuum whose spectrum is very similar to the background radiation in DP-LIP ( Fig. 2 in [13], where the background spectrum I 0 (λ) is represented by a dashed line and I(λ) is represented by a solid line). OD and emission spectra are measured twice: while observing the LIP in viewing directions n v perpendicular to n ex -E ex plane (π plane) and perpendicular to n ex -B ex plane (σ plane) (see Fig.…”

“…The experimental setup used in our previous study [13] is modified for polarization measurements (Fig. 1a, b).…”

“…In contrast with earlier studies, both laser pulses impinge on the same point of a sample. A model was proposed based on a solar-type DP-LIP plume structure, which correctly explains the existence of absorption lines and side absorption bands detected in the early (0-20 ns) plasma [13].…”

Lasing effects in a laser-induced plasma plume

“…2 and very low splitting of the ground energy state indicates that the Al LIP laser has a threelevel generation scheme. In our experimental conditions estimated Al atoms concentration in plasma plume is about 10 16 cm À 3 [15]. This laser involves high optical transition probabilities (about 10 8 s À 1 ) and possess high optical gain α., that lead to strong backward SE when no resonator is used.…”