Tunable Laser Desorption from Diamond Films and Pyrolytic Graphite
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TUNABLE LASER DESORPTION FROM DIAMOND FILMS AND PYROLYTIC GRAPHITE A. UEDA*, J. T. MCKINLEY*, R. G. ALBRIDGE*, A. V. BARNES*, N. H. TOLK*, J. L. DAVIDSON** AND M. L. LANGUELL** *Centerfor Molecular andAtomic Studies at Surfaces, Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 USA "**Departmentof Electrical Engineering, Vanderbilt University, Nashville, TN 37235 USA
ABSTRACT We report the observation of wavelength-dependence in the early phases of the laser ablation of chemical vapor deposited (CVD) diamond films. The measurements were made with 3-4 gIm infrared radiation from the Vanderbilt Free-Electron Laser (FEL), the brightest tunable mid-infrared FEL in the world. Though we do not propose any specific mechanism for the ablation, the C-H bond-stretching mode at 3.47 gIm is clearly important in the early stages of the damage process, as evidenced by a sharp drop in the ablation threshold near 3.47 [Im. The ablation damage threshold was determined by directing a single FEL pulse at the sample and measuring the diameter of the resulting hole using a scanning electron microscope (SEM). This measurement was repeated for several pulse intensities. The damage threshold was defined as the extrapolation to zero hole diameter. We also looked at the desorption products using a time-of-flight spectrometer and a quadrupole mass analyzer. C2 is the dominant desorbed species. Visible light emission was monitored with a monochromator and photodetector.
INTRODUCTION Diamond's high thermal conductivity, transparency and hardness give it significant technological potential in applications such as protective optical coatings [1]. The diamond films used in this study are grown by chemical vapor deposition (CVD) in a hydrogen plasma [2, 3]. For this reason, there may be hydrogen incorporation [4]. It is reported that the presence of this hydrogen can result in infrared (IR) absorption corresponding to the C-H bondstretching mode (3.47 jIm) [5,6]. Therefore, it is important to assess the role of the C-H stretch mode in the photodamage process. To do this, we have irradiated the diamond films with intense (1-100 GW/cm 2 ) pulsed radiation in the 3-4 Itm region. Both time-of-flight (TOF) and quadrupole mass spectroscopy were used to identify the desorption products. For the purpose of comparison, we carried out parallel measurements on CVD-grown highly oriented pyrolytic graphite, the desorption characteristics of which have already been carefully studied by some of the authors of this paper [7]. We particularly emphasize that the ablation damage threshold was found to be strongly wavelength-dependent and well correlated to the C-H stretch infrared absorption. Many factors influence the photodamage threshold, including: pulse duration and film thickness relative to the wavelength [8]. Film thickness effects are believed unimportant for thicknesses exceeding the wavelength. The photodamage intensity threshold typically increases as the pulse duration decreases. For this reason, pulsed photodamage studie
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