Eur. Phys. J. D (2015) 69: 270
https://doi.org/10.1140/epjd/e2015-60161-1

Regular Article

Time and species dependent ambient air’s effects on carbon clusters generated during femtosecond laser ablation of highly oriented pyrolytic graphite

Institute of Modern Optics, Nankai University, Key Laboratory of Optical Information Science and Technology, Ministry of Education, Tianjin 300071, P.R. China

^a e-mail: zhangn@nankai.edu.cn

Received: 11 March 2015
Received in final form: 29 August 2015
Published online: 8 December 2015

Abstract

Near infrared femtosecond laser is employed to ablate highly oriented pyrolytic graphite (HOPG) in ambient air and in vacuum respectively. The recorded transient emission spectra of the ablated plume with a time resolution of 2 ns indicate that the effects of air on the plume are dependent on both time and species. This finding provides important insights into the generation and decay mechanisms of different carbon radicals or clusters. At 1 or 2 ns after the laser pulse strikes the target, air weakens the Swan bands of C₂ compared with the case in vacuum, an effect that may be caused by the quenching collisions between air molecules and C₂ radicals. This result shows that C₂ may be mainly generated by direct spallation from the target surface at the early stage of ablation. Emission spectra at large time delays present that the existence time of the Swan bands in air is longer than the lifetime of the upper electronic state of the Swan system, suggesting that the air-involved three-body recombination and collisional excitation that enhance the generation of emitting C₂ overcome quenching collisions at large time delays. A spectral band at ~416 nm assigned to the transition from ¹Σ_u⁺ to X¹Σ_g⁺ of C₁₅ is more intense in air than in vacuum, indicating that C₁₅ clusters are generated at least partially by the combination of smaller clusters in air. It is also found that air-assisted heat transfer makes the temperature of carbon clusters decrease more quickly in air than in vacuum, leading to a much shorter lifetime of the continuum in air.