Chopped CW laser-induced optogalvanic effect in a neon hollow cathode discharge

A detailed model for the optogalvanic effect in a neon hollow cathode discharge irradiated by a chopped CW dye laser is presented. A rate equation formalism is used to calculate the evolution of the first and second electronic configuration populations coupled by the laser and of the electric charges number density. Processes as ambipolar-like electrons loss, electronic collisional coupling of level populations and electron emission by the cathode due to VUV radiation from the 1s ₂ resonant level are taken into account and further discussed.The transients and steady-state magnitude of the optogalvanic signal are calculated, compared with experimental data and related to population changes. We predict sign changes of the optogalvanic signal when the laser is tuned over transitions originating from the resonant level with respect to transitions involving the metastables states. The optogalvanic signal is shown to be basically determined by the laser-induced variations of the excited-state populations while changes in the electron temperature, due to laser energy transfer by collisions between electrons and excited atoms, play a negligible role.     

用脉冲电场光电流光谱研究Ne原子的自电离态

本文用脉冲电场光电流光谱的实验方法测定Ne原子ns＇(n=15—31)和nd＇(n=13—30)两通道的35条自电离态能级,用参数拟合得出Ne原子ns＇和nd＇通道的电离阈值,计算了每条能级的量子亏损,实验用脉冲电场代替直流放电,基本上消除了Stark效应在光电流光谱测量中的影响。 关键词：     

Observation of two-photon induced photoemission optogalvanic effect

Observation of laser induced two-photon photoemission optogalvanic (TPPOG) effect from tungsten electrode in a discharge cell using 564 nm radiation obtained from a pulsed dye laser is described. The magnitude of the POG signal is studied as a function of laser energy under various discharge parameters. Competition between one-photon and two-photon processes has been observed when nitrogen gas is used in the discharge cell.     

Anomalous behavior of optogalvanic signal in a miniature neon discharge lamp

A strong optogalvanic effect has been observed in a negative glow of a miniature neon discharge lamp using tunable pulse dye laser pumped by a copper vapor laser. A comparative study on temporal evolution of optogalvanic signal in a positive and negative dynamic resistance region of the discharge is described. Dye laser beam was tuned to various neon transitions 1s_i → 2p_j (Paschen notations) within 570-617 nm wavelength range. Anomalous behavior of optogalvanic signal was observed at 588.2 nm for (1s₅ → 2p₂) neon transition at low discharge current (<220 μA). This anomalous behavior is the attributes of damped oscillations of optogalvanic signal that correlate with negative dynamic resistance (dV/di < 0) of the discharge. Penning ionization at low discharge current and small energy mismatch is assumed to be the main cause of the negative dynamic resistance. Penning ionization process has been explained by resonantly ionizing energy transfer via collisions between neon buffer gas atoms in the lowest metastable state (1s₅) and electrode sputtered atoms in ground state using their partial energy level diagram.     

Noncontact optogalvanic signal detection in DC discharge CO<Subscript>2</Subscript> lasers

A new noncontact method for detecting optogalvanic signals in DC discharge CO₂ lasers is reported. The presented technique is based on the detection of potential difference variations through a capacitor comprised of a discharge plasma column and a coaxial or parallel conductor. The obtained optogalvanic signals exhibit better responsivity and pulse shape in comparison with other conventional techniques; in addition, they are detectable as far as 3 m from a signal source.     

Laser optogalvanic spectroscopy of an indium atom

The specific features of the optogalvanic effect in a hollow-cathode discharge on single- and two-stage excitation of indium atoms are studied. The autoionized states of an indium atom in the region 52,651–52,500 cm⁻¹ have been revealed and investigated by means of the method of optogalvanic spectroscopy. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 396–398, May–June, 2000.     

Magnetic Field Enhancement of the Optogalvanic Signal in a Hollow Cathode Discharge

An optogalvanic signal enhancement was obtained in a magnetic field applied on a hollow cathode discharge-detector. In this magnetic field a mutual correlation was observed between optogalvanic signal behaviour, discharge current change and lower level population of the transition corresponding to the 632,8 nm laser irradiation spectral line.     

Photodetachment as a Control Mechanism for Diffuse Discharge Switches

Photodetachment is considered as a control mechanism for diffuse discharge switches. Experiments have been performed on photodetachment of ions in the flowing afterglow of a dc glow discharge in oxygen. Experiments with different laser wavelengths and the dependence of the optogalvanic signal on the laser energy flux indicate that O- is the dominant negative ion. For an energy flux of 35 mJ/cm2, 50percent of the O- ions can be photodetached.     

Elements of Mechanics,by P.F. Kelly

Applications of optogalvanic spectroscopy developed since the resurgence of interest in optogalvanic detection are reviewed. The optogalvanic effect is a change in the electrical properties of a discharge caused by illuminating the discharge with radiation having a wavelength corresponding to an atomic or molecular transition in the discharge. The general scheme of optogalvanic spectroscopy is presented, followed by a discussion of the gas discharge physics of the optogalvanic effect. Applications of optogalvanic detection in moderate resolution spectroscopy, in Doppler-free spectroscopy, and in analytic studies are 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.     

On the Time‐Resolved Optogalvanic Spectra of Neon and Krypton

In this work time resolved optogalvanic signals associated with transitions excited from the first metastable state of neon and krypton have been studied. These gases have similar energy state configurations and it is of significant interest to study their time resolved optogalvanic waveforms resulting from transitions belonging to the states of same quantum numbers. The experimentally observed optogalvanic signals recorded for different discharge currents have been fitted to a theoretical model to obtain parameters that determine amplitudes, instrumental time constants and decay rates of the 1s levels. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relationship of the laser-induced population perturbation of excited levels with the dynamical optogalvanic signal in a discharge plasma

For a pulsed-laser excitation of various neon transitions (1s_j → 2p_k) in a glow discharge the population perturbations in the upper and lower levels are measured by emission and absorption spectroscopy, and the dynamical optogalvanic signals are observed. We propose that the population perturbation in the lower levels (1s₂–1s₅) as a whole is responsible for the optogalvanic signal, and that metastable-level populations determine its decay characteristics. The sign reversal of the optogalvanic signal that depends on the excitation condition is interpreted in this context.     

The study of dominant physical processes in the time-resolved optogalvanic spectra of neon

We present the dominant physical processes responsible for the production of the optogalvanic signal in the spectra of neon. We have investigated the effects on the optogalvanic signal by scanning a dye laser across the neon transitions in the DC discharge plasma. Time-resolved spectra are obtained at a fixed wavelength of the dye laser resonantly tuned to an optically allowed transition. The temporal evolutions of the signals are registered on a storage oscilloscope. Three transitions from the 3s[1/2]₂ metastable state corresponding to the ΔJ = ΔK = 0, ±1 dipole selection rules have been selected to investigate the dominant physical processes responsible for the optogalvanic signals. The change in the signal amplitude as a function of the discharge current has been registered. In addition the electron collisional ionization rate parameter ratios have been determined for the transitions corresponding to dipole selection rules ΔJ = ΔK = -1, ΔJ = ΔK = 1 and ΔJ = ΔK = 0, as 1.63, 1.75 and 1.0 respectively. The effective lifetimes of the upper levels involved in the aforementioned transitions are also calculated as 62.5 μs, 31.25 μs and 12.85 μs respectively.     

Optogalvanic spectroscopy of sputtered atoms

Optogalvanic Spectroscopy (OGS) is finding wide ranging applications in atomic structure studies, laser wavelength calibration, intensity and frequency stabilization of lasers and analytical chemistry. Sputtered atoms produced in a hollow cathode lamp by the bombardment of a rare gas discharge is a convenient source for optogalvanic spectroscopy work. Here, we discuss the sputtered ion/atom optogalvanic spectroscopy applications to low resolution atomic spectroscopy, laser wavelength calibration, studies of radioactive samples available in limited quantities, studies of atoms in highly excited states and Rydberg atoms and high resolution laser spectroscopy. For the sake of completeness, we list other applications of OGS without going into details.     

Optogalvanic effect in CO2 amplifier

In this paper, we deal with optogalvanic CO₂ laser frequency stabilization and the improvement obtained by detecting the optogalvanic signal on the discharge of a small auxiliary low pressure CO₂ amplifier illuminated by the laser to be stabilized. Then we compare the characteristics of the signals supplied by both the laser and the amplifier versus the amplifier gas pressure.     

Observation of penning ionization in Zr-Ne discharge by optogalvanic effect

The pulsed optogalvanic (OG) effect is used to observe Penning ionization in a Zr-Ne hollow-cathode discharge, which was axially irradiated by a pulsed dye laser pumped by copper vapor laser. The effect of discharge current on the temporal evolution of the OG signal is studied at 588.2, 594.5, 597.6 and 614.3 nm. The double humped temporal profile of the OG signals, corresponding to transitions of neon, and closeness of energy levels between that level of neon (Ne) and excited levels of zirconium (Zr) ions confirmed that Penning type of ionizing energy transfer collisions were taking place in the Zr-Ne discharge at lower discharge currents (≤5 mA). The disappearance of the double humped feature in the temporal profile of the OG signals indicated that the Penning contribution became negligible at higher discharge currents (≥10 mA).     

Two-photon time-resolved optogalvanic signals of neon

In this work, temporal evolution of two-photon laser optogalvanic signals of neon has been studied. Optogalvanic signals for four transitions from the metastable 2p⁵3s[3/2]₂ state to 2p⁵4d′[3/2]₁, 2p⁵4d′[3/2]₂, 2p⁵4d′[5/2]₃ and 2p⁵4d′[5/2]₂ states were recorded over a range of discharge currents (3.4–9 mA). It was found that the shape of the optogalvanic signal was strongly dependent on the discharge current so that its peak shifted to shorter times and its amplitude increased with the discharge current. The decay rates of the 4d states, calculated from the optogalvanic signals, were found to increase linearly with the discharge current in the range of 6.2–9 mA. However, for the range of 3.4–5.4 mA, the decay rates were observed to slightly decrease with the discharge current.     

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.     

Spatial Dependence of the Strong Optogalvanic Effects Due to Metastable Quenching in a DC Helium Discharge

The optogalvanic effects of the He 2.058-?m line (21P-21S) from He lamps illuminating a weak dc He discharge (current density ?10 ?A/cm2, field/pressure ~25 V/cm - torr) are reported. For illumination at the positive column, we have made quantitative measurements of the decreases in the discharge current, electron density, and metastable densities, as weli as the increase in the electric field in the positive column, as the intensity of illumination increases. We have also observed that for sufficiently strong illumination (using He lamps only), the optogalvanic effect is catastrophic, i.e., the discharge is switched off; this clearly shows that a sufficiently large metastable density (which is reduced by the illumination) is necessary to maintain a weak He discharge. For illumination at the cathode regions, the optogalvanic effects are "anomalous": the discharge current is strongly suppressed by illumination at the cathode dark space next to the cathode, but is enhanced by iumination at the adjacent negtive glow region.