Expansion dynamics of laser ablated carbon plasma plume in helium ambient

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Abstract

Time resolved emission spectroscopic measurements on a plasma plume generated by pulsed laser ablation of carbon in helium ambient atmosphere have been performed. Temporal profiles of electronically excited C2 species show a twin peak distribution beyond a threshold laser irradiance. The emission features of C2 species are found to be significantly influenced by the ambient helium atmosphere. It is observed that the helium ambient pressure and laser irradiance have opposite effects on the expansion dynamics of C2 species.

Introduction

Laser ablation of carbon has been extensively used for the deposition of diamond-like carbon (DLC) thin films and for the production of carbon clusters and nanotubes [1], [2], [3]. Carbon clusters like C60 and higher fullerenes are well known to be formed as a product of laser ablation of carbon in an ambient helium atmosphere [4]. However, the underlying physics and chemistry of the processes such as carbon cluster formation or their dissociation are less than well understood. Several spectroscopic studies of carbon plasma have been carried out using a variety of laser wavelengths [5]. It has been shown that shorter wavelengths are more effective for penetration into the sample, mainly because of large ablation rates possible at these wavelengths [6]. However, the main advantage in the use of near infrared (NIR) low energy photons is that, they are less likely to invoke photochemistry into the ablation phenomenon.

In the context of laser deposition of DLC films, it has been reported that high quality DLC films are obtained at moderate laser irradiances where molecular C2 formation is prominent as revealed by its emission spectrum [7]. Laser ablation has the unique advantage that most of these molecules are formed in their excited states and hence spectroscopic measurements offer an excellent means to investigate their evolution and dynamics. Despite considerable experimental and theoretical progress, the studies on laser produced carbon plasma have not yet yielded a clear-cut picture on plasma dynamics of the cluster formation and such a situation arises mainly due to the complexity of the phenomena involved. In this paper, the emission features of laser ablated carbon plume generated in a helium ambient atmosphere are investigated using time resolved diagnostic technique. The effect of laser irradiance and helium ambient pressure on the expansion of electronically excited C2 species in the laser ablated carbon plasma is discussed.

Section snippets

Experimental set up

The experimental set-up used for the present study is similar to the one described elsewhere [8]. The carbon plasma is generated by laser ablation of the high purity polycrystalline graphite sample using 1064 nm radiation pulses from a Q-switched Nd:YAG laser with repetition rate 10 Hz and pulse width 9 ns. The laser beam is focused normal to the target surface with an estimated spot size ∼200 μm. The target in the form of a disc is placed in an evacuated chamber provided with optical windows for

Results and discussion

The electron temperature (Te) and density (ne) of the carbon plasma are measured by the relative intensities of the successive ionization states of carbon atom and Stark broadening method, respectively. Dependence of these parameters on the distance from the target surface, delay time after the plasma initiation, and laser irradiance are discussed [9]. An initial temperature and density of about 3.6 eV and 3.5×1017 cm−3 are observed and they decay rapidly to much lower values within 300 ns time.

Conclusions

A radiation of 1064 nm from a Q-switched Nd:YAG laser is focused onto a carbon target where it produced a transient and elongated plasma. Time resolved diagnostic technique has been used to characterize emission from electronically excited C2 species. The present results show that the ambient gas pressure and laser irradiance have opposite effects on the temporal profiles of C2 species. The broadening and narrowing of the multiple peaks with these parameters is expected due to collision and

Acknowledgements

The author is thankful to Prof. H.J. Kunze, Prof. V.P.N. Nampoori and Prof. C.P.G. Vallabhan and Dr. C.V. Bindhu for their suggestions and support. Financial support from Alexander von Humboldt foundation (Germany) and Council of Scientific and Industrial Research (India) is gratefully acknowledged.

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