Optogalvanic Effect Research Articles

Laser Optogalvanic Spectroscopy: Experimental Details and Potential Applications in R and D

Laser Optogalvanic Spectroscopy (LOGS) is an extremely sensitive detection technique based on the 'Optogalvanic Effect'. The impedance change of gaseous discharge following the absorption of resonant laser light by the discharge species has proved to be a powerful spectroscopic tool to investigate all kinds of matter in vapour phase in discharge plasmas and flames. LOGS does not require any optical detector and the signal-to-noise ratios are often greater than 10/sup 3/. The advent of tunable lasers has pushed LOGS as a unique detection technique, efficient over a wide frequency spectrum from UV through visible to IR. As a simple and most sensitive detection technique, LOGS can have diverse practical applications in science and technology. The experimental details with different discharge excitations and the potential applications of LOGS are briefly described. Some prominent practical applications like wavelength calibration, laser linewidth determination, trace element detection, isotope analysis, material characterisation, laser frequency and power stabilisation, Rydberg atom spectroscopy and combustion and plasma diagnostics are briefly discussed.

The optogalvanic effect in a neon Townsend discharge

The optogalvanic effect for the 1s5-2p2 transition (Paschen notation) in a low current (0.5 mu A) neon Townsend discharge has been studied for three pressures (1, 4 and 10 Torr). The optogalvanic effect occurs by irradiating the discharge with an expanded laser beam, in a variable plane parallel to the electrodes. By moving the discharge tube perpendicular to the electrodes and by using a dye laser with a continuously adjustable frequency, measurements of the optogalvanic effect could be carried out for a number of irradiation positions as a function of the laser frequency. In calculating the optogalvanic effect, secondary ionization processes are dominant and therefore one needs to know the probabilities that resonant photons and metastables produced in the discharge reach the cathode. The first probability has been calculated using Monte Carlo simulation methods, with the method developed by Lee to calculate the frequency redistribution of a resonant photon after the interaction with an atom. The second probability has been found by solving the transport equation of the metastables. The optogalvanic effect measurements agree to first order with the theoretical calculations.

Zirconium isotope shift measurements using optogalvanic detection

Isotope shift measurements are reported for more than 300 zirconium lines in the spectral range 14 160–18 660 cm−1. Most of these lines are predicted transitions between known energy levels, although several of them have not yet been reported. The zirconium vapor is produced in a homemade hollow-cathode lamp, and the atomic transitions are excited by a ring dye laser (dyes used: DCM, Rhodamine 6G, and Rhodamine 110). Light absorption in the vapor is detected by using the optogalvanic effect when either one or the other of two sub-Doppler techniques (intermodulation or saturation) is used, depending on the intensity of the line under study. Numerical values aside, our results also show that the optogalvanic effect is a useful tool for systematic high-resolution spectroscopic investigations.

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>

Photoemission optogalvanic effect studies in N2, NO2 and Ar discharges under pulsed laser excitation

A two-photon induced photoemission optogalvanic effect which brings about a change in the discharge voltage when a pulsed dye laser beam is focused on a tungsten electrode has been described. The experiment is performed with N2, NO2 and Ar discharges. The magnitude of the signal voltage is studied as a function of laser energy and discharge current. The effective quantum efficiency in the discharge is found to be larger than that in the vacuum condition.

Measurements of Atmospheric Water Vapor with an Alexandrite Lidar.

A differential absorption lidar (DIAL) for the detection of atmospheric water vapor was developed based on a tunable alexandrite laser. Theoretical estimation shows that the bandwidth of laser radiation and accuracy of the wavelength tuning critically affect detection errors. We propose a new accurate tuning method on the resonance line of H2O at 725.8 nm using the opto-galvanic effect in a Xe discharge tube, and compared it with an absorption method and a photo-acoustic method. The DIAL experiment demonstrates the detection range up to 9.5 km with a detection error of less than 50%. The H2O distributions measured were agreed with sonde data.

A simple method for the optogalvanic detection of argon ion transitions in hollow-cathode lamps

We describe a simple method for the observation of argon ion transitions in hollow-cathode lamps, using the optogalvanic effect, and report the first optogalvanic detection of the six ArII transitions: 457.9 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm and 514.5 nm. We also study the optogalvanic signal as a function of the incident laser power and the lamp current.

The double-resonance optogalvanic effect of neon as a sensitive photon detector

The double-resonance optogalvanic effect (OGE) of neon in a commercial hollow cathode lamp is used as a sensitive photon detector. The signal photons, emulated by a highly attenuated laser, served as the first step of the two-step, sequential excitation, while a high intensity laser saturated the second, connected transition. The limiting experimental noise was found to be the shot noise on the lamp operating current. The detector exhibited a high quantum efficiency (> 1) over approximately 5 orders of magnitude of energy. The minimum detectable peak energy was 1.3× 10 −15 J (4.2 × 10 3 photons at 640.225 nm).

Studies of the optogalvanic effect in a commercial caesium hollow cathode lamp and its use for tuning pulsed dye lasers

The use of the optogalvanic effect (OGE) in a hollow cathode lamp for wavelength tuning of dye lasers is an established technique at laser wavelengths longer than ∼ 560 nm, corresponding to photon energies less than ∼ 2.2 eV. This work describes the first successful optogalvanic studies on caesium at higher photon energies. Once set up, OGE in a standard commercial hollow cathode lamp provides a quick and cheap means of tuning a continuously tunable laser to a chosen atomic transition. The 6 2S 1 2 → 7 2P 3 2 , 1 2 lines in caesium at 455.55 nm and 459.32 nm were resolved. This work is also believed to be the first to describe the detection of any caesium transition using the optogalvanic effect in a commercial hollow cathode lamp, rather than a specially constructed tube. The method used is applicable to other materials which have previously proved difficult.

Optogalvanic detection of resonant two-photon transitions from the 6 3P J excited states of Hg I

Resonant two-photon transitions from the 6 3P J excited states of Hg I have been observed for the first time. The excited states of Hg I were generated in a low-pressure, mercury-dosed, hollow cathode discharge lamp; the two-photon transitions were stimulated using a pulsed dye laser and detected via the optogalvanic effect. Each of the three Hg I 6 3P J →7 3P J (Δ J=0; J=0, 1, 2) transitions and the Hg I 6 3P 2→8 3P 2 transition were observed. The latter transition was used as the basis of experiments to map the spatial distribution of the 6 3P 2 state in the discharge volume. Attempts to observe the two-photon excited LIF transition Hg I 8 3P 2→7 3S 1 at 690.7 nm in a hot cathode, low-pressure, mercury-argon discharge were unsuccesful.

Use of the Optogalvanic Effect to Examine the Laser Power Dependency of Several Excitation/Ionization Mechanisms in a Hollow Cathode Discharge

Several examples of the relationship between resonant and nonresonant collisional ionization and photoionization, laser intensity, and the ability to saturate both resonant and nonresonant transitions are illustrated. The effects of energy deficit and effective energy overshoot in determining the dominant ionization pathway are also discussed. An approximation of the lifetime of the virtual level is made, and its relationship to the probability of two-photon nonresonant absorption is discussed. Finally, the theoretical model described above is fitted to the optogalvanic data for two single-step resonance transitions of Ne to determine the applicability of the model developed for flame LEI to the hollow cathode discharge environment (under our experimental conditions) and to obtain estimates of the collisional ionization rate constant, k2i, and photoionization cross section, σ2i.

Optogalvanic effect in neon hollow cathode discharge

About 300 transitions have been recorded in the optogalvanic spectrum of neon in the visible wavelength region 410–670 nm by axially irradiating a hollow cathode discharge with an excimer pumped dye laser. Several weak lines have been recorded including the 632.8 nm neon laser wavelength. Temporal evolution of the optogalvanic signals was studied at several wavelengths. An explanation is given for the occasional absence of correlation in intensities observed in optogalvanic effect with those measured in emission. An explanation is also given for the voltage dependent polarity changes of optogalvanic signals. Energy transfer collisions were also detected in iron-neon discharge.

Dynamic behavior of the optogalvanic effect in a CO_2 laser

We observe the dynamic behavior of the optogalvanic effect in a CO(2) laser during the transient process due to the switching of the cavity losses. A three-level model for the laser and a differential relaxation equation based on thermodynamic considerations for the temperature are used to interpret the phenomenon.

Saturation measurements of excited-state transitions in noble gases using the optogalvanic effect

Saturation intensities have been measured for 20 transitions between excited states of argon, krypton, and neon in the 1.3- and 1.5-microm regions. Values of I(sat), corrected for an incident beam with a circular Gaussian intensity profile, range from 20 to 2000 mW/cm(2).

Rate-Equation Analysis of Optogalvanic Effect in CO2 Laser Medium

We propose a novel rate-equation model to describe the optogalvanic effect in a CO2 laser medium. In this model, the spontaneous emission from the upper laser level and the translational energy dissipation in the gas medium are included in addition to the vibrational relaxation processes. The calculation quantitatively reproduces the characteristic dependence of the optogalvanic signal on the modulation frequency of the incident laser beam and the gas pressure. The vibration-to-translation relaxation rates of the upper and lower laser levels are determined.

Erratum: Optogalvanic effect in a CO laser [Sov. J. Quantum Electron. 21, 24–27 (January 1991)

Erratum: Optogalvanic effect in a CO laser [Sov. J. Quantum Electron. 21, 24–27 (January 1991)

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.

A model of the excitation discharge of fast transversal flow lasers

A model of fast transversal flow CO/sub 2/ laser discharges is presented. Electron density and vibrational kinetics equations are solved simultaneously with one-dimensional inviscid flow equations, in order to study the effects of the evolution of hydrodynamic variables on discharge stability. The current-voltage characteristic is calculated, including the influence of laser radiation (optogalvanic effect). A relationship is given between fluid residence time and maximum input power density in the active medium, taking the maximum allowable electron density as a parameter. The results are in satisfactory agreement with the experiments of W.J. Wiegand et al. (1975). Expressions for the optimum length of the discharge zone and maximum power input to the active medium are derived.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency locking of laser diode using metallic vapor optogalvanic spectrum: UI

The frequency of a AlGaAs diode laser has been locked to the 8118-cm(-1)(f(3)d(2)s(5)M(0)(7))-20,218-cm(-1) transition of UI at 826.20570 nm using the optogalvanic effect. A hollow cathode vapor generator has been utilized to produce a density of 10(12) atoms/cm(3) of uranium in vapor phase. The absolute frequency stability for a 10-min run was estimated to be better than 500 kHz P-P at an integration time of 1 s. This preliminary result shows that the rich optogalvanic spectrum of uranium can be efficiently used for the frequency-locking of semiconductor lasers.

Anomalous line shapes associated with the 3 (3)P state of atomic oxygen.

The 3${\mathrm{}}^{3}$P--5${\mathrm{}}^{3}$S transition in atomic oxygen at 725.4 nm has been studied in a weak discharge using the optogalvanic effect with a cw tunable dye laser. The transition appears as a superposition of at least two components with different centroids and widths. The anomalous line shape is probably due to the long-range interaction of $^{3}$P oxygen with $^{4}$${\mathit{S}}_{0}$ oxygen ions.