Species resolved analysis of the expansion of hydroxyapatite laser ablation plumes

Abstract

The plume generated by ablation of hydroxyapatite targets under ArF excimer laser irradiation has been investigated by means of fast intensified charge coupled device (CCD) imaging and optical emission spectroscopy. Results have shown that the plume splits into two plasma clouds as it expands. Time and spatial resolved spectra have revealed that under the experiment conditions emission is mostly due to calcium neutral atoms and calcium oxide molecular radicals. Imaging of the plume with the aid of bandpass filters has demonstrated that the emissive species in the larger and faster plasma cloud are calcium neutral atoms whereas in the smaller and slower one are calcium oxide molecular radicals.Pulsed laser deposition (PLD) is a good technique to deposit thin films of complex compounds, and many authors [1][2][3][4][5][6] have used it to obtain hydroxyapatite (HA) coatings for medical prostheses. This motivated us to study the HA laser ablation process through the characterization of the plume by fast intensified CCD imaging. 7,8 The resultant pictures of the plasma generated by ArF excimer laser irradiation of HA targets revealed the presence of two different emissive regions. Thus, in this paper we develop a more comprehensive analysis of the HA laser ablation plume by means of time and space resolved optical emission spectroscopy in addition to fast intensified CCD imaging in order to characterize better the ablation process and to identify the nature of both regions.An ArF excimer laser (Lambda Physik LPX 205i) provides pulses at a wavelength of 193 nm, with duration of 23 ns, and energy of 300 mJ. The beam is limited by a 12 3 2 mm 2 mask and impinges at an angle of 45 ± onto a pellet made from HA powder pressed at 6 3 10 8 Pa at room temperature. The fluence on the target is about 1.5 J͞cm 2 . All the experiments are carried out in a high vacuum chamber evacuated to a base pressure of 5 3 10 25 mbar. Images are taken with a gated CCD camera (ANIMATER-VI from ARP France), 288 3 384 pixels, 8 bits dynamic range, intensified by a microchannel plate (MCP) whose aperture is delayed by a pulse generator triggered by a fast photodiode that is activated by the laser pulse. Bandpass filters (MicroCoatings Inc.) centered at 520 and 600 nm with a FWHM of 10 nm are also used in some of the recorded images and, in these cases, accumulations of 20 shots are needed to acquire an image. Otherwise, pictures are taken in a single shot mode.The optical emission spectra are acquired by means of a 0.5 m spectrometer (Chromex 500IS/SM). The laser-generated plume is imaged onto the entrance slit in such a way that the propagation axis of the plume remains parallel to this slit. This allows good spatial resolution (0.03 mm) to be obtained with the CCD camera, with no lens/mirror translation system needed. A 600 g͞mm grating blazed at 400 nm provides a spectral resolution of 0.3 nm. Each spectrum consists of the accumulation of the light emitted by the plume after 20 laser shots. The synchronization system is the same as that descri...

Evidence of chemical reactions in the hydroxyapatite laser ablation plume with a water atmosphere

“…1͑b͒. This filter intercepts the emission line at 519 nm of neutral calcium [10][11][12] and the images as obtained reveal the spatial distribution of the excited state of this emissive spe-FIG. 1.…”

“…1͑b͒, as observed under vacuum. [9][10][11] The position of the leading edge of the emission pattern of Ca versus time is shown in Fig. 2͑b͒.…”

“…This filter intercepts a 10-nm-wide portion of the second emission band of the orange system of calcium oxide (Ca x O y ). [10][11][12] The analysis of this species is especially interesting since the recorded spectra have revealed that the intensity of emission from the orange system is much higher in a water atmosphere 12 than in vacuum. 11 A spherical front develops after 400 ns but it does not correspond to an intensity maximum, that is, to a shock wave.…”

“…6 This motivated us to undertake a detailed study of HA laser ablation process in order to obtain deeper knowledge of the phenomena involved and to optimize the deposition conditions. Therefore, a complete analysis was developed under high vacuum conditions [8][9][10][11] in order to characterize the process in the plainest environment. The study was then extended to a water atmosphere 8,12 in similar conditions to those normally used in the deposition experiments.…”

“…In the former case, the distribution of the emissive species along the plasma was also analyzed with the aid of interferential filters in the collection of the images with the CCD camera. [9][10][11] Here we use interferential bandpass filters to analyze the distribution of emissive species along the plume in a water atmosphere. The scope of this study is not only to discover the different dynamic behavior of the diverse species in the plume, but also to investigate the presence of chemical reactions with the water atmosphere.…”