Optogalvanic Spectroscopy Research Articles

Perturbations of conduction in liquids by pulsed laser-generated plasma

The observation of optical perturbations of electrical conduction processes in aqueous solutions by laser-produced plasmas is reported. The irradiation of a liquid in a conduction cell with a pulsed tightly focused Q-switched Nd/sup +3/:YAG laser produces plasma that modifies the conductivity of the medium and generates a voltage pulse that can be recorded at the cathode using the same procedure, as in optogalvanic spectroscopy. At low voltages across the cell, this electrical pulse is a positive voltage change, which returns to the equilibrium potential in times much longer than the laser pulse duration. Increasing the cell voltage, this electrical pulse becomes a damped oscillation. Beyond those values, the pulse inverts its sign at the cathode. Results also show that there is a change in cell conductivity in a long-term regime. An interpretation with a first-order model suggests that this behavior could be explained as an excess of ionization, followed by diffusion controlled by recombination in the bulk of the solution.

High-resolution measurements and multichannel quantumdefect analysis of the Kr(4p5(2P1/2)nd', J = 2,3)autoionizing resonances

Using two-step laser excitation of metastable Kr(4p55s 3P2,0) atoms via selected Kr(4p55p J = 1,2) levels, we haverecorded the low-lying autoionizing resonances Kr(2P1/2 nd', J = 2,3) (n = 6,7) at high resolution in two different experiments(atomic beam spectroscopy and optogalvanic spectroscopy in a discharge).Accurate values for the resonance positions (quantum defects) and the(reduced) widths have been determined and profile indices are reported.For comparison, multichannel quantum defect theory (MQDT) analyses,using energy-dependent MQDT parameters, have been carried out for theodd Kr(J = 1,2,3) levels, yielding good agreement in the quantumdefects and partial agreement in the predicted reduced widths for thend' resonances with the experimental values. Moreover, simplifiedphase-shifted quantum defect theory analyses of the measured bound oddKr(J = 2 and 3) Rydberg levels were carried out with the aimof providing improved insight into the dominant decay mechanism of the resonances.We also present a summary and comparative discussion of the reducedwidths for the nd'[K]J resonances of all the rare-gas atoms Ne,Ar, Kr and Xe.

Method to measure the electric field vector in an argon glow discharge using laser polarization spectroscopy.

A method for measurement of the direction of the electric field in a glow discharge is reported. This method uses the dependence of the electronic excitation spectrum of argon atoms on the polarization of the laser radiation. In this research, laser radiation was used to excite argon atoms in a plasma from the 4s [3 / 2](2) metastable level to Rydberg levels, and excitation spectra were measured using laser optogalvanic (LOG) spectroscopy. In addition, LOG spectra of argon atoms interacting with an electric field were calculated by solving the Schrödinger equation. Good agreement was found between experimental and theoretical LOG spectra obtained for different polarizations of the laser radiation.

Laser optogalvanic and jet spectroscopy of germylene (GeH2): New spectroscopic data for an important semiconductor growth intermediate

The à 1B1–X̃ 1A1 electronic transition of germylene has been reinvestigated. A room temperature absorption spectrum of the central portion of the 000 band of GeH2 has been obtained using the technique of laser optogalvanic spectroscopy. A rotationally resolved spectrum of the 000 band of jet-cooled GeD2 has been recorded with a pulsed discharge source. Analysis of these spectra has yielded ground and excited state rotational constants for the GeH274, GeH272, GeH270, GeD276, GeD274, GeD272, and GeD270 isotopomers and approximate equilibrium structures of: r″(Ge–H)=1.5883(9) Å, θ″(H–Ge–H)=91.22(4)°, r′(Ge–H)=1.5471(6) Å, and θ′(H–Ge–H)=123.44(2)°. The ground state ν1 and ν2 vibrational frequencies have been determined from wavelength-resolved fluorescence spectra of jet-cooled GeH2 and GeD2. There is good evidence that GeH2 rotational levels with Ka′>1 are so strongly predissociated that lifetime broadening makes them diffuse, severely restricting the information that can be obtained from absorption and laser-induced fluorescence experiments.

Measurement method for electric fields based on stark spectroscopy of argon atoms

We report the development of a method for the measurement of electric fields in glow discharge plasmas, based on Stark spectroscopy of argon atoms. The method is based on laser excitation of transitions in atomic argon. The key feature of the method is that the electric field is determined by matching experimentally obtained absorption spectra to theoretically calculated spectra. The dependence of the positions of energy levels of argon atoms on the strength of the electric field was calculated by solving the Schrodinger equation for the argon atom. Measurements of Stark spectra were made in the sheath region of a glow discharge using laser optogalvanic spectroscopy. The wavelength of the laser radiation was tuned to the transitions 4s-->nf (n=7,8,ellipsis,14) of the argon atom. For n=11, the lower limit for electric field measurements was estimated to be 14 V/mm.

One and two photon optogalvanic spectroscopy of argon and neon for the wavelength calibration in the near infrared

The one photon and two photon optogalvanic spectra of argon and neon have been investigated in the near infrared region between 735 and 781 nm. About 35 transitions have been recorded by illuminating a hollow cathode discharge with a pulsed Ti:sapphire laser. These transitions can serve as a convenient way to calibrate Ti:sapphire laser or optical parametric oscillator radiation in this important spectral range.

Study of Odd-Parity Autoionization States of Praseodymium Atom by Optogalvanic Spectroscopy

Autoionization states of neutral praseodymium atom (Pr I) were investigated using two-color two-step laser resonance ionization spectroscopy (RIS). A Pr hollow cathode lamp was used as an atomic source and optogalvanic signals were measured. The Pr I was excited from the ground state to four first step excitation levels (21120.69, 21471.81, 22014.11 and 23067.34 cm -1 ) by the first step laser. These four channels were used to study odd-parity autoionization states. Four dyes were used in the second step excitation. The 108 odd-parity autoionization states were found by optogalvanic spectroscopy and their total angular momentum ( J ) values were estimated.

Model for cw laser collisionally induced fluorescence in low-temperature discharges

A perturbed steady-state rate-equation model has been developed for the cw laser collisionally induced fluorescence (LCIF) produced by excitation on one of the 1s-2p noble gas transitions. This work is one part of a wider complementary modeling program which includes cw optogalvanic spectroscopy, optical emission spectroscopy, and optical absorption spectroscopy, with the overall aim of testing all of these models with the same stringently assembled atomic and discharge data set. Our aim here is to demonstrate the principal features of our cw LCIF model by using it to describe our experimental observations produced by pumping transitions originating on the 1s(5) metastable and 1s(4) resonance states of neon atoms in the positive column of a normal glow discharge at 2.0 Torr and a discharge current of 5 mA. The model shows that these cw LCIF spectra are dominated by 1s-2p excitation and electron collisional coupling among the 2p states. We show that the model allows us to quantify explicitly the various individual contributions to each line in the cw LCIF spectra. The theory and analyses presented here apply equally well to other noble gases and we believe can be modified appropriately for trace noble gases in atomic-molecular mixtures.

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.

Rigorous theoretical analysis of the continuous wave optogalvanic effect in the neon positive column

A rate-equation model with greatly improved quantitative rigour is detailed for the CW optogalvanic effect on the 1s-2p transitions of neon atoms in the positive column of a dc normal glow discharge. This work constitutes one part of a wider complementary programme which also includes CW laser collisionally-induced fluorescence, optical emission spectroscopy and optical absorption spectroscopy for the excited-state populations, all employing the same atomic and discharge data set. Our aim has been to produce a theoretical model and test it with stringently collected data, to demonstrate that tunable laser CW optogalvanic spectroscopy (OGS) can provide a truly quantitative diagnostic of the excited-state kinetics in low-temperature discharges. The model is deliberately restricted to just six essential perturbed rate equations, four for the 1s states, one for the charged particles and one for the discharge current. Our formulation, via the 1s and 2p CW-induced pump rate perturbations, allows very direct identification of the important excited-state kinetics producing the OGE. The principles and scope of our theory are demonstrated for a neon filling pressure of 2.0 Torr and currents of 1-10 mA, based on fitting the model for three carefully selected transitions, the 1s5-2p4 (594.5 nm), 1s4-2p4 (609.6 nm) and 1s5-2p9 (640.2 nm). Results show that the magnitudes of the CW optogalvanic effect on the 1s-2p transitions are strongly dependent on the discharge pumping rates of the 1s states and their coupling, and confirms that cascade effects must be included in the 1s excitation rate coefficients.

Laser assisted ratio analyser 13C-urea breath testing, for the detection of H. pylori: A prospective diagnostic European multicentre study.

Novel technology based on laser optogalvanic spectroscopy called the LARA (Laser Assisted Ratio Analyser) system was developed to measure 12C/13C ratios in breath samples using stable 13C isotopes, to detect Helicobacter pylori infection. To determine the sensitivity and specificity of the 13C-LARA-urea breath test in the detection of H. pylori infection in a prospective European multicentre trial; FDA-and EMEA-approved. Consecutive dyspeptic patients underwent diagnostic gastroscopy with biopsies for culture and histopathology, to detect H. pylori infection (gold standard). Subsequently, the LARA-urea breath test was performed using either a system without a cold trap (part I) or a system with a cold trap (part II). In both instances baseline, 30-min and 60-min breath samples were collected. The optimum cut-off level for 12C/13C ratios was determined by Receiver Operator Characteristics analysis. In part I, 544 out of 604 patients were evaluable (low CO2: 47; withdrawn: 13). 284 out of 544 patients (52%) were H. pylori-positive according to the gold standard. The sensitivity of the LARA-urea breath test was 95% and the specificity 94%. In part II, 257 out of 272 were evaluable (low CO2: 14; withdrawn: 1). Sensitivity and specificity were 93% and 96%, respectively. The LARA-technology represents an accurate and non-invasive testing system for the detection of H. pylori infection. Its major advantages are the use of stable 13C isotope, the high throughput of samples and the easy means of collecting, storing and transporting the samples, thus making the system convenient to both patient and clinician.

Laser Optogalvanic Analysis in a Radiofrequency Plasma: Detection of Iodine Atoms and Molecules

The optogalvanic effect (OGE) may be used to detect specific species in a plasma (ions, atoms, radicals, or molecules) by selective laser excitation of the plasma. The plasma itself is merely the reservoir of electronically excited, ionized, and atomized species. Compared with conventional ICP-AES, OGE has many advantages: no external detector, zero background, no interferences, and greater versatility. Since the photoacoustic (PA) and ionization rate change (IRC) components of the OGE signal can be separated, optogalvanic spectroscopy can exhibit a further selectivity based on this discriminatory ability. The OGE method has been applied to iodine analysis. By careful selection of the operating conditions, pure atomic and pure molecular iodine signals constitute the PA and IRC profiles, respectively. The best detection limit, in a somewhat primitive experiment, is ∼10−7μg/ml of I2, which is comparable to laser-induced fluorescence. This detection limit can be improved by at least three orders of magnitude by optimizing the experimental conditions.

Laser optogalvanic spectroscopic studies of xenon

We report new data on the bound odd-parity Rydberg series of xenon using single-colour two-photon laser optogalvanic spectroscopy. These series are excited from the metastable level. A total of eight odd-parity , , , , , , and Rydberg series are observed with an accuracy of . The , and series for high-n values are reported for the first time. We have also observed an even-parity (two-photon parity-forbidden) Rydberg series excited from the metastable level by focusing the laser in the region (10 mm away from the cathode) of discharge where the electric field is stronger. The multichannel quantum defect theory analysis for the odd-parity bound levels of J = 0, 2 and 3 has been performed. Besides these two-photon Rydberg series we also report a number of single-photon transitions. Some of them can be grouped into Rydberg series .

Application of Diode Lasers to Determine Excitation Temperature in Hollow Cathode Discharges by Optogalvanic Spectroscopy

The excitation temperatures of sputtered gadolinium and uranium atoms in an argon hollow cathode discharge have been determined by diode laser-excited optogalvanic spectroscopy. These results have been compared to those determined by conventional emission spectroscopy. It was found that the temperatures derived from each method do not differ very much, but the optogalvanic method revealed a better standard deviation uncertainty due to the good signal-to-background ratios and excellent spectral resolutions. Temperature variations with discharge currents ranging from 15 to 50 mA have been examined.

Saturation measurements of the a 3P 0–y 3D 1o Ti I transition by optogalvanic spectroscopy

The optogalvanic signal (OGS) induced in a Ti–Ar hollow-cathode discharge, stimulated by modulated cw laser light tuned to the a 3P 0–y 3D 1 o Ti I transition, was measured as a function of the light intensity. By solving analytically the rate equations of a three-level system, which include the Ti I ground state, the lower metastable a 3P 0 and the upper y 3D 1 o levels, an expression was obtained that relates the magnitude of the OGS to the laser intensity and the saturation parameter (1/ σ 0 τ). The σ 0 τ product was determined by fitting the theoretical curve to the experimental data.

Atomic hyperfine structure studies using temperature/current tuning of diode lasers: An undergraduate experiment

We present a simple and inexpensive experimental arrangement for hyperfine structure studies in atoms using commercially available laser diodes and hollow cathode lamps. The experiment is highly suitable for the undergraduate laboratory. This technique can be employed to investigate the hyperfine structure of rare earth and other elements such as Ta and Nb which have large nuclear magnetic and or quadrupole moments. In this paper, we report well-resolved hyperfine structure spectra recorded for holmium employing optogalvanic spectroscopy. We also report Doppler limited hyperfine structure measurements on the ground state of rubidium using injection current/temperature tuning of the diode laser. This involves a simple experimental arrangement suitable for undergraduate laboratories. The hyperfine coupling constants for the level at 31 443.26 cm−1 in Ho I are reported for the first time. Details of the data analysis to obtain accurate hyperfine structure coupling constants from the observed spectra are presented. A number of commercially available diode lasers in the visible and the near infrared regions and simple in-house developed or commercially available low cost current and temperature controllers can be employed for the present studies. We employ simple cooling/heating or current modulation for tuning the output wavelength of the diode laser. The presently proposed experimental arrangement can be assembled easily and requires no machine/glass shop facilities.

The effect of the laser–atom interaction time on the influence of velocity-changing collisions in Doppler-free spectroscopy

Remarkable reduction of the influence of velocity-changing collisions in Doppler-free optogalvanic spectra has been achieved by using a square pulse train produced by the intensity modulation of a cw ring dye laser in conventional intermodulated optogalvanic spectroscopy. Decreasing the pulsewidth and the repetition rate of the square pulse train, the Gaussian collisional parameter, C, for the 1S 0 → 3P 1 transition of Yb atoms at low discharge current is reduced by two orders of magnitude when compared to that of a conventional intermodulated optogalvanic spectrum. The line narrowing of the Gaussian pedestals below the theoretical limit was also observed. Reduction of the collisional parameter as well as the line narrowing of Gaussian pedestals enhances the spectral resolution of this method and allows measurements of highly resolved Doppler-free spectra in hollow cathode discharge cells.

Electronic spectroscopy of the C state of NO by laser multiphoton ionization: rotational structure of the C2Pi(v'=0)«-X2Pi(v"=0) two-photon band

Rotationally resolved two-photon electronic spectra of nitric oxide (NO) on the C2Π(ν′= 0) ← X2Π(ν″ = 0) transition have been recorded in the near-UV between 384nm and 375nm by the (2+ 1) REMPI technique. The REMPI spectra of the δ(0,0) band have been measured and assigned for jet-cooled NO, NO at room temperature and for the nascent, highly rotationally excited NO formed by the UV-photodissociation of nitrosyl chloride (ClNO) in a one-colour dissociation-probe experiment. The wavelengths of all major rovibronic transitions have been measured accurately using optogalvanic spectroscopy and interference fringe counting as a calibration technique. The unperturbed transitions have been fitted to term values up to J = 53.5 with rotational energies in excess of 4000cm-1 to a single algebraic energy formula. The absence of certain Λ-doubled sub-branches in the spectra has been explained by different virtual two-photon routes via the electronic A state as the intermediate level. In connection with a discussion of two-photon parity selection rules and rotational line strengths we have shown that (2+1) REMPI spectroscopy of the C ← X transition on the δ(0,0) band will permit convenient and reliable measurements of the rotational and fine-structure populations of most, if not all levels of NO X (ν = 0).

Determination of actinide isotope ratios using glow discharge optogalvanic spectroscopy

Diode laser-excited optogalvanic spectroscopy (OGS) of a glow discharge has been utilized to measure U-235/U-235+U-238 isotope ratios. This `optical mass spectrometric' measurement has been demonstrated for a number of samples including uranium oxide, fluoride, and metal. Various diode laser-accessible atomic transitions in the 775–835 nm region have been evaluated; these transitions were chosen by considering OGS sensitivity and isotope shift. Using the 831.84 nm uranium line, for example, it was possible to measure the U-235/U-235+U-238 isotope ratio (0.0026) of depleted uranium samples. A prototypical field instrument to make these measurements has been assembled and demonstrated. A U-236 spectral line was identified in an enriched uranium sample, and an abundance was measured.

Single-color photoionization optogalvanic spectroscopy in U--Xe and U--Ne hollow-cathode discharges

Resonance ionization spectroscopy is a powerful tool used to study and identify highly excited states of atoms and molecules, but usually requires complex experimental setups. Recently, we described a new method for this spectroscopy based on previous work on the rapid optogalvanic effect. The optogalvanic photoionization spectroscopy method is particularly attractive as a replacement for the atomic beam when refractory elements are considered. As an illustration of the method, we measured the rapid optogalvanic spectra in U--Ne and U--Xe hollow-cathode discharges over the 16500--20500 cm-1 range, where we have identified about 900 lines. Although showing a larger density of lines, the rapid optogalvanic spectrum of UI reproduces the main characteristics of the resonant ionization spectra obtained in field-free low-pressure atomic uranium beams. Over 600 of the observed lines are interpreted as two or three photon photoionization transitions, from which we propose nearly 400 new highly excited levels of UI. Electric field and collision ionization of Rydberg states is also considered but is shown to play only a minor role.PACS Nos.: 32.30, 31.50, 32.80, and 52.70.