Laser Optogalvanic Signal Research Articles

Optogalvanic detection of oxygen negative ions in reactive sputtering process

Spatial distributions of oxygen negative ions in a parallel-plate 0.2 Torr Ar/O 2 d.c. glow discharge are measured by using probeassisted laser optogalvanic (LOG) spectroscopy. The oxygen negative ions are photodetached with a laser pulse from a pulsed YAG/harmonic generator system. Photodetached electrons are detected by a probe set close to the laser beam, giving an LOG signal. Saturation characteristics of the LOG versus laser energy is in good agreement with theory. The LOG signal increases linearly with increasing the discharge current and with increasing the oxygen partial pressure. Both the spatial profile and the O 2 pressure dependence of the LOG signal are similar to those of O 2 + emission intensity due to the fact the positive and negative ions are generated by the electron impact ionization and attachment respectively with different threshold energies. Both the saturation characteristics and the wavelength dependence of the LOG signal indicates that the concentration ratio of O − to O 2 − is 2±1:7 for 100% O 2 discharge. The presence of negative ions is independently confirmed by single probe characteristics. As for the O 2 partial pressure dependence, the probe data is qualitatively in good agreement with that from the probe-assisted LOG method.

Role of photoacoustics in optogalvanics

Time-resolved laser optogalvanic (LOG) signals have been induced by pulsed laser excitation (lsj→2pk, Paschen notation) of a ∼30 MHz radio-frequency (rf) discharge in neon at ∼5 torr. Dramatic changes of the shape/polarity of certain parts of the LOG signals occur when the rf excitation frequency is scanned over the electrical resonance peak of the plasma and the associated driving/detecting circuits. These effects are attributed to ionization rate changes (i.e., laser-induced alterations of the plasma conductivity), with concomitant variations in the plasma resonance characteristics. In addition to ionization rate changes, it is shown that photoacoustic (PA) effects also play a significant role in the generation of the LOG signal. Those parts of the LOG signal that are invariant with respect to the rf frequency are attributed to a PA effect. The similarity of LOG signal shapes from both rf and dc discharges suggests that photoacoustics play a similar role in the LOG effect in dc discharges. Contrary to common belief, most reported LOG signal profiles, ones produced by excitation to levels that do not lie close to the ionization threshold, appear to be totally mediated by the PA effect.

Electric field reversals in dc negative glow discharges.

Electric field reversals play an important role in maintaining glow discharges by determining the partitioning of ion flux between cathode and anode. For years, the existence of field reversals in the negative glow has been the subject of controversy. For a range of pressures, gas compositions, and electrode spacings, we observe field reversals using laser optogalvanic (LOG) spectroscopy: The discharge current is increased by excitation of molecular ions to a state with larger mobility. The sign of the LOG signal is a direct measure of the sign of the electric field. A single-beam electron model, where it is assumed that the glow is sustained by a monoenergetic, unidirectional beam of electrons emanating from the cathode, is in good agreement with most experimental observations. The model accurately predicts the existence of field reversals and the spatial dependence of both LOG and laser-induced fluorescence signals. By including momentum and energy dispersion in the beam, the model could be further improved.

Time-resolved laser optogalvanic spectroscopy of iodine in a radio frequency discharge

Pulsed laser optogalvanic (LOG) spectra of iodine vapor in a ∼32 MHz rf discharge were excited at 14 900–17 100 cm−1. Two distinct, time-resolved components were observed: a fast component, synchronous with the laser pulse, width ∼1 μs, followed by a slow component, width ∼100 μs, delayed relative to the laser pulse. The fast component exhibits atomic transitions of I(I) and I(II). The slow component reproduces the B̃←X̃ photoacoustic (PA) spectrum of molecular I2. The signal delay of the slow component accords with the velocity of acoustic waves in iodine vapor. The rf electrode region is the ‘‘sensitive’’ region where the acoustic wave generates the slow LOG signal. Two mechanisms of signal generation and propagation are involved. The fast signal originates in a two-step laser photoionization of plasma-excited atoms, the first-step being resonant, and/or in changes of the atomic collisional ionization rates. These processes occur on time scales shorter than the laser pulse and generate an ‘‘instantaneous’’ LOG signal by creating additional electron–ion pairs. The delay of the molecular LOG signal, which is mediated by the PA effect, indicates that local heating produces a negligible perturbation of the discharge impedance. This is contrary to common belief. The polarity of the slow LOG signal depends on the direction of the PA wave, suggesting that the signal is generated by an actual physical movement of charged species by the pressure wave. Thermal effects are involved, but only as precursors to the PA wave.

One- and two-photon optogalvanic spectrum of Ne: 610–730 nm

The laser optogalvanic (LOG) spectrum of Ne in the region 610–730 nm has been recorded using Nd-YAG laser pumped dye laser and a Hamamatsu FeNe hollow cathode lamp. Forty-five lines have been observed in the LOG spectrum and have been explained as originating from one- and two-photon transitions in Ne. The negative signal for a few lines and the observed variation in LOG signal with lamp voltage and laser power have been explained.

Population monitoring in the B-X system of HgBr by laser optogalvanic spectroscopy

Laser optogalvanic (LOG) signals have been detected from a dc discharge in flowing HgBr 2 vapour. Different laser lines from a 4 WAr + laser have been used for excitation. The signals are identified with transitions in the B-X system of this molecule and variation in the signal strength with discharge voltage and input laser power have been investigated. The LOG signal is seen to vary with laser power and the variation is wavelength dependent. An explanation for this dependence is offered.

INFRARED LASER OPTOGALVANIC SPECTROSCOPY OF NH3 AND NO2

Infrared laser optogalvanic (LOG) spectra of portions of the NO2 ν3 band at 6.2 µm and the NH3 ν2 band at 9.5 µm have been recorded at Doppler-limited resolution using cw tunable diode lasers. A novel cell design, using adjustable electrode positions and an orthogonal geometry between the probe laser and discharge axis allows two contributions to the LOG signal to be identified, one associated only with the negative glow.