Optogalvanic Research Articles

Time dependent optogalvanic reaction and stability to disturbance of an optogalvanic circuit with hollow cathode discharge

An anomalous time dependent optogalvanic (OG) response detected in Ne/As hollow cathode discharge is analysed. The observed damped oscillation is shown to be close in form to a partial solution of the proposed time dependent equation describing the optogalvanic circuit. This oscillating solution takes place for negative dynamic resistance of the OG detector. It was found that the measured i– V curves contain negative resistance parts. The stability of the OG circuit to disturbances at negative resistance is analysed relative to slope steepness of the dynamic resistance and other circuit parameters.

Sputtering—atomizing and optogalvanic properties of an Ar–Mg Hollow cathode discharge

The atomisation and optogalvanic (OG) properties of Ar–Mg a hollow cathode discharge (HCD) plasma are analysed. Under operating conditions of an Ar gas pressure of 39.8 Pa and a discharge current of 10 mA the real sputtering coefficient and the density of the magnesium atoms are calculated to be S k = 3× 10 −2 atoms per ion and 1.45×10 11 cm −3 respectively. The characteristics and a new point of view of the OG signal from the Ar + ion are discussed.

Polarity of Laser Excited Optogalvanic Transitions in Neon

Abstract Irradiation of a gaseous discharge with a tunable laser produces observable voltage changes at wavelengths that correspond to electronic transitions for species within the discharge. Optogalvanic (OG) transitions of neon have been investigated in the visible and near UV regions by axially irradiating a hollow cathode discharge with an excimer-pumped dye laser. A variety of OG transitions that originate from metastable states have been recorded. A digital oscilloscope was used to record the waveforms of the OG transitions. The polarities of 29 OG transitions recorded in the near UV and visible are explained in terms of processes that affect the population of neon atoms in metastable states.

Influence of plasma discharge operating regime and laser beam intensity on optogalvanic spectra

The optogalvanic (OG) spectrum — over the tuning range of R 6G dye (568 nm to 605 nm) — of a Pr/Ne discharge tube is studied. The positive OG signals are approximately two orders of magnitude larger than the negative OG signals. The magnitude of the signals depend mainly on the operating regime of the discharge. Specifically, operation of the discharge in the abnormal glow regime leads to an enhancement in the negative OG signals. The operating point on theV-I characteristic of the discharge should be stipulated. To study the variation of the OG signal with the intensity of the laser beam two OG signals, of different polarities, were examined. We observed that with respect to positive OG signals, there is a trend towards pumping saturation. A minimum threshold of 0.3 kW laser power is required to generate negative OG signals. For the same attenuation in the laser beam there is a ten times more efficient reduction in the negative OG signals than in the case of positive OG signals. This confirms the important role of metastable atoms in the conduction in gas discharges.

Calibration of pulsed laser wavelength by optogalvanic spectroscopy in hollow cathode discharges

The wavelength calibration of pulsed dye lasers using uranium or neon atomic absorption lines detected by the optogalvanic (OG) effect is examined. Measured wavelength shifts are of the same magnitude as the laser spectral width. For a reproducible calibration, all of the parameters regarding the OG recording should be kept constant. The highest wavelength calibration accuracy is obtained by setting the boxcar parameters to minimize OG line shifting. OG line wavelength shifts of up to+or-0.02 nm are measured.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optogalvanic method for the measurement of negative ions in plasmas

A plasma is irradiated by laser beams (argon ion, YAG, dye and diode lasers) and electrons photodetached from negative ions are detected either by a fixed electrode or by a movable probe immersed in the plasma. A hollow cathode tube (galvatron) is used as a test tube in oxygen. Densities of negative ion species are determined from the intensity and the wavelength dependence of the optogalvanic (OG) signal. In the case of detection from the probe, the maximum sensitivity is obtained at a probe voltage above the space potential; the signal-to-noise ratio (SRN) is more enhanced than in the case with an electrode at a fixed position; the spatial resolution is also better. In the case of pulsed lasers, transient phenomena were observed. The peak height plotted against laser power variation showed a linear increase at low powers and then a saturated profile at high powers. Densities of negative ion species are determined from the peak height and the wavelength dependence of the OG signal.

Optogalvanic detection of negative ions in discharges

With the aid of rf, microwave and dc discharges, an optogalvanic (OG) signal, due to the photodetachment of negative oxygen ions, has been observed. The OG signal intensity in an rf discharge was successfully estimated from the discharge parameters. Thus, the OG technique may be potentially useful as a plasma diagnostic method. Furthermore, the OG signal from negative oxygen ions is found to be strong in the diffusion-controlled positive column, while the OG signal due to the excited levels of atomic oxygen is strong in the cathode fall region. Preliminary results for the observation of the OG signal in discharges of H2, CO, H2O2, and (COCH3)2 are also described.

Fir light detection by using optogalvanic effect.

FIR laser light detection by using an optogalvanic (OG) effect was demonstrated in the same laser material gas discharge for a 118.8μm CH3OH laser. The relationship between OG signals and laser power is shown to be linear.

Population Difference and Pulse-Mode Optogalvanic Effect on Hel(23S–33P) 388.9-nm

Pulse-dye-laser-induced optogalvanic (OG) effect has been investigated on Hel(2 3 S–3 3 P) 388.9-nm in the positive column of a He glow discharge. It has been found that the peak OG current is approximately in proportion to the steady-state population difference between the 2 3 S and 3 3 P states. A rate equation analysis is made on the ion density and the population of the excited levels relevant to the optical pumping on the basis of the mechanism in which the excess charges responsible for the OG signal are generated and decay via associative ionization collision of the 3 3 P state atoms optically pumped from the 2 3 S state and via recombination governed by the ion wall current as described by Tonks and Langmuir, respectively.

Detection of the optogalvanic effect in flames with a microwave resonant cavity

A new method for the detection of the optogalvanic (OG) effect in a flame has been developed. In the experimental scheme, the reflected microwave power is measured for the detection of the OG effect inside a microwave resonant cavity. In a preliminary experiment, the OG signal of sodium atom aspirated into a propane/O2 flame was observed at a concentration below 10 ng/ml.

Optogalvanic effect in plasmas and gases

The status of optogalvanic (OG) spectroscopy, involving a change in the impedance of a gas or plasma, with tunable lasers is examined. The main advantage of this approach over the usual absorption spectroscopy is its high sensitivity. The OG effect in plasmas, glow discharges, hf discharges, hollow cathodes, obstructed discharges, neutral gases, etc., is studied. Optogalvanic studies of the spectra of both the ground and excited states of atoms and vibrational-rotational and electronic transitions in molecules, nonlinear spectroscopic phenomena, and interference of degenerate states and the use of the optogalvanic effect for stabilization of laser frequencies are described. A great deal of attention is given to the physical mechanisms involved in the formation of the OG signal. The possibilities for employing the optogalvanic effect in quantitative spectroscopy are evaluated.

Relaxation of the pulsed opto-galvanic current signal in a helium positive column

Transient behaviour of an opto-galvanic (OG) current signal induced by pulsed dye laser excitation has been investigated on the He I 388.9-nm (2 3S-3 3P) transition in a He positive column. The OG current signal is composed of a sharp spike at the early time stage followed by a slowly-varying component. The decay time of the sharp OG current signal has been found to be almost independent of the dc current substaining the discharge and to depend sensitivity on a product of pressure and column-radius. The experimental results show that the dominant decay process of the OG current consists in an ion current to the walls, as has been previously referred to in a mmodel of a cw OG effect in a He positive column. A model for the pulse OG effect in the early time stage is presented on the basis of the experimental results. The decay times of the OG current signals calculated with the model are in agreement with the measurements within an accuracy of 30%.

Spatial characteristics of the optogalvanic effect in striated rare-gas discharges

The spatial variation of the optogalvanic (OG) signal resulting from visible laser irradiation of rare-gas positive column discharges has been investigated for the case of standing striations. The OG signal was observed to periodically change polarity as the laser was scanned transversly across the bright and (relatively) dark regions of the column. Transitions involving the 3P2,0 metastable levels were found to be everywhere opposite in polarity to those originating on freely radiating levels. The effects of trapped resonance level emission, and collisions of the second kind involving metastables are proposed to be important in determining the OG signal polarity for excitation in certain discharge regions.

Systematic trends in optogalvanic signals of some transitions in a uranium hollow cathode discharge

A study of optogalvanic (OG) signals in a uranium hollow cathode discharge using a tunable pulsed dye laser with intensities above saturation revealed certain systematic trends. The peaks of OG signals, for a given hollow cathode current, due to all transitions involving the ground state are within a certain range of values. The same is true of the OG signals due to transitions involving the 620 cm -1 and 3800 cm -1 lower metastable levels. For all transitions involving the same lower state, the peaks of OG signal vary linearly with the discharge current. The slopes of such straight lines are different for transitions involving different lower levels of uranium. Effects of radiation trapping are seen in the OG signals due to transitions involving the ground state of uranium at currents above 60 mA. Details of our observations are presented and discussed.

Spatial variation of the linewidth of Kr atomic lines as observed on optogalvanic signals in cathode fall region of hollow cathode discharge

An abrupt change in the spectral width of optogalvanic (OG) signals of Kr atomic lines has been observed at a distance from the cathode wall where the signal itself changes its sign, by using a hollow cathode lamp whose cathode has its both ends open. The width is broader near the cathode wall with its magnitude varying by discharge current while the narrower width which is obtained on passing the laser beam in the central region of cathode cylinder is independent of the current. It is suggested that a well-defined region, where the particles of larger cross section of collisions with Kr atom are confined, is present in front of the cathode wall.

Atomic physics and atomic spectroscopy: mother and daughter?

In Part 1 the roots of analytical atomic emission (AES), absorption (AAS), fluorescence (AFS), optogalvanic (OG) and forward-scattering (FS) spectroscopy in atomic physics are exposed. The essential differences between FS and AAS or AFS are stressed. The links of plasma-source mass-spectrometry with atomic spectroscopy and with flame ionization research are also indicated. Part 2 deals with various aspects of the continual interrelation between atomic physics and atomic spectroscopy. The usage of AAS in atomic physics is exemplified by shock-tube experiments on the oxidation of Cr atoms and the curve-of-growth of the Cr resonance lines in argon. Examples of the usage of AFS in physical experiments are given that relate to the collisional mixing of Zeeman substates and the far-wing profiles of the collisionally broadened Na- D lines. Other aspects of this interrelation are discussed that relate to the sources of atomization and excitation, to the detection or measuring techniques, and to the research or development of analytical methods and sources. In Part 3 the revival of AES and the prospects of some laser-based atomic spectroscopies are summarized, whereas a further exploitation of anomalous dispersion in analytical atomic spectroscopy is suggested. General conclusions and exhortations are collated in Part 4.