Tracking Temporal Development of Optical Thickness of Hydrogen Alpha Spectral Radiation in a Laser-Induced Plasma

David M Surmick,Christian G Parigger

doi:10.3390/atoms7040101

David M Surmick, Christian G Parigger

Open Access

https://doi.org/10.3390/atoms7040101

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Abstract

In this paper, we consider the temporal development of the optical density of the H α spectral line in a hydrogen laser-induced plasma. This is achieved by using the so-called duplication method in which the spectral line is re-imaged onto itself and the ratio of the spectral line with it duplication is taken to its measurement without the duplication. We asses the temporal development of the self-absorption of the H α line by tracking the decay of duplication ratio from its ideal value of 2. We show that when 20% loss is considered along the duplication optical path length, the ratio is 1.8 and decays to a value of 1.25 indicating an optically thin plasma grows in optical density to an optical depth of 1.16 by 400 ns in the plasma decay for plasma initiation conditions using Nd:YAG laser radiation at 120 mJ per pulse in a 1.11 × 10 5 Pa hydrogen/nitrogen gas mixture environment. We also go on to correct the H α line profiles for the self-absorption impact using two methods. We show that a method in which the optical depth is directly calculated from the duplication ratio is equivalent to standard methods of self-absorption correction when only relative corrections to spectral emissions are needed.

Highlights

The act of tightly focussing a laser beam of sufficient energy creates a dynamic, micro sized plasma.The temperature and density properties of the decaying plasma depend on the laser, focal, and ambient conditions used at the onset of the plasma
Such plasma characteristics are ideal for use in nano-particle formation [2,3], pulsed laser deposition [4], and laser-induced breakdown spectroscopy (LIBS) [5,6,7]
The temporal development of the self-absorption of the Hα line was considered by studying the duplication method of re-imaging the spectral line onto itself and comparing the line with and without this duplication

Summary

Introduction

The act of tightly focussing a laser beam of sufficient energy creates a dynamic, micro sized plasma. For a nominal nanosecond pulsed laser with 10–100 mJ of energy per pulse, this plasma can have a temperature range of 0.5 to 5 eV and an electron density range between 1015 and 1019 cm−3 depending on the plasma decay conditions and the laser ablation target [1] Such plasma characteristics are ideal for use in nano-particle formation [2,3], pulsed laser deposition [4], and laser-induced breakdown spectroscopy (LIBS) [5,6,7]. We detail the optical density of a laser-induced plasma throughout the plasma decay by monitoring the temporal progression of the hydrogen Balmer series α line (Hα ) This is completed through the standard practice of using a doubling mirror to re-image the plasma onto itself and take ratios of the plasma spectroscopic image collected with and without its doubled image. We go on to apply both methods to measurements of the Hα line and use the corresponding electron density determined from the line to indicate the usefulness of both methods of correcting the opacity of the spectral line

Theory

Experimental Details

Temporal Self-Absorption Behavior

Self-Absorption Impact on Line Shapes

Conclusions