On the delayed emission from laser produced aluminum plasma under argon environment

Abstract

In this article, we report rather long time emission (∼ 250 µs) from aluminum neutrals (Al I) in ns laser produced plasma in the presence of ambient argon. The study is carried out with varying laser intensity, background pressure, and the distance from the target. Slow and fast peaks components in the emission spectra observed at earlier times are well reported. However, interestingly a very long delayed emission is also observed for the first time which depends on laser intensity, distance from the target, ambient gas and pressure. The emission is observed from Al neutrals only. The most likely mechanism of this emission appears to be the excitation and subsequent emission from Al neutrals as a result of energy transfer from metastables of the ambient gas. I. INTRODUCTIONLaser Produced Plasma (LPP) has drawn substantial attention of scientific community due to its numerous applications like lithography 1-3 , nano-particle generation 4,5 , cluster formation 6,7 , Laser-Induced Breakdown Spectroscopy (LIBS) 8-11 , radiography 12,13 , mass spectroscopy [14][15][16] , thin-film decomposition 17 , generation of ion sources 18,19 , acceleration 20,21 , inertial confinement fusion 22,23 , plasma diagnostics [24][25][26] , etc. Despite numerous experimental studies and extensive theoretical modeling on LPP's, the plasma plume evolution still presents challenges aspects due to its complexity and transient nature. The lifetime of LPP depends upon the duration of pulse width 27,28 , and can be extended by the external factors e.g. background pressure [29][30][31][32] . Investigating plume dynamics in the background medium 33,34 has been performed by various groups to understand processes like dragging 35 , thermalization 36 , attenuation/enhancement of optical emissions 37 , diffusion 38 , recombination 39 , generation of shock waves 40 , etc. In vacuum where it expands adiabatically. On the other hand the expansion dynamics of plasma plume against a background meduim depends on its atomic mass, pressure, etc.Recently, various groups have studied the dynamics of laser-produced plasma in background gas [29][30][31][32]41,42 using multiple diagnostics like Optical Emission Spectroscopy (OES) 43 , imaging 44 , Langmuir Probes 45,46 , etc. to study the confinement and plume parameters. Among these diagnostics, OES is widely used to study LPP plume evolution due to its simplicity and ability to quantify the parameters like density and temperature due to its high density and the validity of Local thermodynamic equilibrium (LTE) condition. Moreover, OES can easily be used to study the evolution of plasma parameters over space and time and provide information about atomic processes e.g. charge exchange process, two and threebody recombinations 47 , etc.