T. Gebre

Optical emission in magnetically confined laser-induced breakdown spectroscopy

Magnetically confined laser-induced breakdown spectroscopy was investigated by studying the optical emission from laser-induced plasma plumes expanding across an external transverse magnetic field. KrF excimer laser pulses with a pulse duration of 23ns and a wavelength of 248nm were used to produce plasmas from Al, Cu, and Co targets. Various optical emission lines obtained from Al and Cu targets show an obvious enhancement in the intensity of optical emission when a magnetic field of ∼0.8T is applied, while the optical emission lines from Co targets show a decrease in the optical emission intensity. The enhancement factors of optical emission lines were measured to be around 2 for the Al and Mn (impurity) lines from Al targets, and 6–8 for Cu lines from Cu targets. Temporal evolution of the optical emission lines from the Al samples shows a maximum enhancement in emission intensity at time delays of 8–20μs after the incident laser pulse, while from the Cu targets it shows a continuous enhancement at time...

Enhanced chemical vapor deposition of diamond by wavelength-matched vibrational excitations of ethylene molecules using tunable CO2 laser irradiation

Wavelength-matched vibrational excitations of ethylene (C2H4) molecules using a tunable carbon dioxide (CO2) laser were employed to significantly enhance the chemical vapor deposition (CVD) of diamond in open air using a precursor gas mixture of C2H4, acetylene (C2H2), and oxygen (O2). The CH2-wag vibration mode (ν7) of the C2H4 molecules was selected to achieve the resonant excitation in the CVD process. Both laser wavelengths of 10.591 and 10.532 μm were applied to the CVD processes to compare the C2H4 excitations and diamond depositions. Compared with 10.591 μm produced by common CO2 lasers, the laser wavelength of 10.532 μm is much more effective to excite the C2H4 molecules through the CH2-wag mode. Under the laser irradiation with a power of 800 W and a wavelength of 10.532 μm, the grain size in the deposited diamond films was increased by 400% and the film thickness was increased by 300%. The quality of the diamond crystals was also significantly enhanced.

Laser-induced resonant excitation of ethylene molecules in C2H4/C2H2/O2 reactions to enhance diamond deposition

Vibrational resonant excitation of ethylene (C2H4) molecules using a carbon dioxide laser was employed to promote reactions in precursors of ethylene, acetylene (C2H2), and oxygen to enhance diamond deposition. One of the vibrational modes (CH2 wag mode, v7) of the C2H4 molecules was selected to achieve the resonant excitation in the reactions. Optical emission spectroscopy was used to study the effects of laser resonant excitation on the reactions for diamond deposition. The optical emissions of CH and C2 species were enhanced with the laser excitation, indicating that there are more active species generated in the reactions. Thicknesses and grain sizes of the deposited films were increased correspondingly. Temperature calculations from the line set in the R-branch of CH emission spectra indicated that a nonthermal process is involved in the enhanced diamond deposition.

Spectroscopic determination of rotational temperature in C2H4/C2H2/O2 flames for diamond growth with and without tunable CO2 laser excitation.

Optical emission spectroscopy (OES) and spectroscopic temperature determination were carried out to study C(2)H(4)/C(2)H(2)/O(2) flames used for diamond deposition with and without an excitation by a wavelength-tunable CO(2) laser. Strong emissions from C(2) and CH radicals were observed in the visible range in all the acquired OES spectra. When the flames were irradiated by using a continuous-wave (CW) CO(2) laser at a wavelength of 10.591 microm, the emission intensities of the C(2) and CH radicals in the flames increased owing to the laser excitation. The CO(2) laser was also tuned to a wavelength of 10.532 microm to precisely match the resonant frequency of the CH(2)-wagging vibrational mode of the C(2)H(4) molecules. OES spectroscopy of the C(2) and CH radicals were performed at different laser powers. The rotational temperatures of CH radicals in the flames were determined by analyzing the spectra of the R branch of the A(2)Delta-->X(2)Pi (0,0) electronic transition near 430 nm. The deposited diamond thin-films were characterized by scanning electron microscopy, stylus profilometry, and Raman spectroscopy. The deposition mechanism with and without the CO(2) laser excitation was discussed based on the OES spectral results.

Optical Emission Spectroscopy Study of Premixed C2H4/O2 and C2H4/C2H2/O2 Flames for Diamond Growth with and without CO2 Laser Excitation

Optical emission spectroscopy (OES) measurements were carried out to study premixed C2H4/O2 and C2H4/C2H2/O2 combustion flame for diamond deposition with and without a CO2 laser excitation. Strong emissions from radicals C2 and CH were observed in the visible range in all the OES spectra acquired. By adding a continuous-wave CO2 laser to irradiate the flame at a wavelength of 10.591 μm, the common CO2 laser wavelength, it was discovered that the emission intensities of the C2 and CH radicals were increased due to the laser beam induced excitation. OES measurements of the C2 and CH radicals were performed using different gas combinations and laser powers. The rotational temperatures in the flame were determined by analyzing the spectra of the R-branch of the A2Δ→X2Π (0, 0) electronic transition near 430 nm (CH band head). Information obtained from the OES spectra, including the emission intensities of the C2 and CH radicals, the intensity ratios, and the rotational temperatures, was integrated into the study of diamond deposition on tungsten carbide substrates for mechanism analysis of the laser induced vibrational excitation and laser-assisted diamond deposition.

C2 and CH rotational temperatures in diamond growth using CO2 laser-assisted combustion-flames

Excited C2 and CH species occur abundantly in diamond growth using C2H2/O2, C2H2/C2H4/O2 and C2H4/O2 flames. The irradiation of some flames by a continuous-wave (CW) CO2 laser beam has resulted in increased optical emission intensity from the excited species and a change in the physical appearance of the flames due to resonant absorption of laser energy. Gas temperature in the flames is one of the most important parameters in the application of diamond growth. In atmospheric plasmas, the gas kinetic temperature is closely related to the rotational temperature of radical species in the plasmas. Optical emission spectroscopy (OES) was used to obtain molecular spectra of the excited C2 and CH species in the flames for a fixed gas of C2H2/C2H4/O2 flame at several laser energies. The rotational temperatures of CH were calculated using the Boltzmann plot method. In addition, synthetic C2 molecular spectra were compared with the experimental spectra to obtain temperature by the intensity ratio of selected spectrum components. For each condition, the temperatures obtained using these methods were correlated with the quality, grain size, and growth speed of diamond films on cemented tungsten carbide (WC-Co) substrates.

Laser-assisted diamond deposition on copper substrates using combustion-flame method

Diamond was deposited on copper (Cu) substrates in open atmosphere using a C2H2/O2 combustion flame. Graphite powders were used as seeds. The graphite-seeded Cu substrates were heated by a CW CO2 laser to about 750 °C within 1 min at the initial stages of deposition. It was found that diamond nucleation density after graphite seeding and laser irradiation was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4μm were obtained in 5 min. The enhancement of diamond nucleation was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO2 laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth. Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize the deposited diamond films. A mechanism for diamond nucleation and growth on Cu substrates with graphite seeding was proposed.Diamond was deposited on copper (Cu) substrates in open atmosphere using a C2H2/O2 combustion flame. Graphite powders were used as seeds. The graphite-seeded Cu substrates were heated by a CW CO2 laser to about 750 °C within 1 min at the initial stages of deposition. It was found that diamond nucleation density after graphite seeding and laser irradiation was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4μm were obtained in 5 min. The enhancement of diamond nucleation was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO2 laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth. Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize the deposited diamond films. A mechanism for diamond nu...

Magnetically-confined laser-induced breakdown spectroscopy

Magnetically-confined laser-induced breakdown spectroscopy was investigated by time-resolved optical spectroscopy of magnetically-confined plasmas. An obvious enhancement caused by magnetic field in the optical emission from laser-induced Al and Cu plasmas was observed.

Study of magnetic confinement in laser-induced plasmas by optical emission spectroscopy and fast photography

The dynamics and magnetic confinement effects of laser-induced plasmas expanding across an external transverse magnetic field were investigated by both optical emission spectroscopy and fast photography. KrF excimer laser pulses with a pulse duration of 23 ns and a wavelength of 248 nm were used to produce plasmas from Cu targets. Various optical emission lines show an obvious enhancement in the intensity of optical emission when a magnetic field of 0.8 Tesla is applied. Temporal evolution of the optical emission lines shows a continuous enhancement in emission intensity at time delays of 3-20 µs after the incident laser pulse. The enhancement in the optical emission from the Cu plasmas was presumably due to the increase in the effective plasma density as a result of magnetic confinement. Fast photography of the laser-induced Cu plasmas was performed using a Nikon macro lens and Andor intensified CCD camera. It shows that the plasma splits into two parts after gate delay of around 4 µs both with and without a magnetic field. With the presence of a magnetic field, the two parts of the plasma were confined and recombined with each other after gate delay of around 18 µs. While without the presence of a magnetic filed, the two parts kept separated as gate delay increased.The dynamics and magnetic confinement effects of laser-induced plasmas expanding across an external transverse magnetic field were investigated by both optical emission spectroscopy and fast photography. KrF excimer laser pulses with a pulse duration of 23 ns and a wavelength of 248 nm were used to produce plasmas from Cu targets. Various optical emission lines show an obvious enhancement in the intensity of optical emission when a magnetic field of 0.8 Tesla is applied. Temporal evolution of the optical emission lines shows a continuous enhancement in emission intensity at time delays of 3-20 µs after the incident laser pulse. The enhancement in the optical emission from the Cu plasmas was presumably due to the increase in the effective plasma density as a result of magnetic confinement. Fast photography of the laser-induced Cu plasmas was performed using a Nikon macro lens and Andor intensified CCD camera. It shows that the plasma splits into two parts after gate delay of around 4 µs both with and witho...

Spectroscopic study of magnetically-confined laser induced aluminium plasma

The characteristics of laser-induced Al plasmas under the confinement of a magnetic field with different pressures and laser fluences have been investigated using an optical multichannel analyzer (OMA). A magnetic field up to 0.26 Tesla was produced by two permanent magnets. A KrF excimer laser with a wavelength of 248 nm and a pulse width of 23 ns was used to produce plasmas which expanded across the magnetic field. The spectral intensity evolution of different Al spectral lines as functions of delay times, air pressures and laser fluences were reported and the mechanisms behind the observations were analyzed. It was observed that the magnetic field had obvious effect on the Al plasma spectra.The characteristics of laser-induced Al plasmas under the confinement of a magnetic field with different pressures and laser fluences have been investigated using an optical multichannel analyzer (OMA). A magnetic field up to 0.26 Tesla was produced by two permanent magnets. A KrF excimer laser with a wavelength of 248 nm and a pulse width of 23 ns was used to produce plasmas which expanded across the magnetic field. The spectral intensity evolution of different Al spectral lines as functions of delay times, air pressures and laser fluences were reported and the mechanisms behind the observations were analyzed. It was observed that the magnetic field had obvious effect on the Al plasma spectra.