Spectroscopy optogalvanic

Optogalvanic spectroscopy is an excellent and simple technique to perform laser spectroscopy in gas discharges. Assume that the laser beam passes through part of the discharge volume. When the laser frequency is tuned to a transition E. - E between two levels of atoms or ions in the discharge, the population densities (E. ) and n (E ) are changed by optical pumping. Because of the different ionization probabilities from the two levels, this population 

Already with moderate laser powers (a few milliwatts) large signals (yV to mV) can be achieved in gas discharges of several milliamperes [8.25]. Since the absorbed laser photons are detected by the optically induced current change, this very sensitive technique is called aptogalvanio spectroscopy [8.26]. 

Both positive and negative signals are observed, depending on the levels E., E involved in the laser-induced transition E. E - If IP(E ) is the 

There are several competing processes which may contribute to ionize atoms in level E., such as direct ionization by electron impact 

A more detailed representation of ionization spectroscopy and its various applications to sensitive detection of atoms and molecules can be found in [92-94, 99, 100, 103]. 

There are several competing processes that may contribute to the ionization of atoms in level /, such as direct ionization by electron impact A( /) + e A+ + 2e, collisional ionization by metastable atoms A( /) + B A+ + B + e , or, in 

Molecular spectra can also be measured by optogalvanie spectroscopy [121]. In particular, transitions from highly excited molecular states that are not accessible to optical excitation but are populated by electron impact in the gas discharge can be investigated. Furthermore, molecular ions and radicals can be studied. Some molecules, called excimers (Vol. 1, Sect. 5.7.6) are only stable in their excited states. They are therefore appropriate for this technique because they do not exist in the ground state and cannot be studied in neutral-gas cells. Examples are He orH [122, 123]. 

Optogalvanic spectroscopy is a suitable technique for studies of excitation and ionization processes in flames, gas discharges and plasmas [6.82]. Of particular interest is the investigation of radicals and unstable reaction products which are formed by electron-impact fragmentation in gas discharges. These species play an important role in the extremely rarefied plasma in molecular clouds in the interstellar medium. 

For more details on optogalvanic spectroscopy see the reviews [6.58, 75-78] and the book [6.88], which also give extensive reference lists. 

Sensitive and accurate measurements of atomic and molecular Rydberg levels have been performed [6.87-6.89] with thermionic diodes. With a special arrangement of the electrodes, a nearly field-free excitation zone can be realized that allows the measurement of Rydberg states up to the principal quantum numbers n = 300 [6.89] without noticeable Stark shifts. 

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Some of the most powerful tools for in situ discharge diagnostics are optical (62). Plasma-induced emission spectroscopy, laser-induced fluorescence, laser absorption, and laser optogalvanic spectroscopy have all been... 

Optogalvanic spectroscopy (11.12) is the detection of changes in the degree of ionization of the flame upon irradiation with a... 

Ion-beam photodetachment spectrometer used to measure high-resolution photodetachment spectra and autodetachment of C2 ions Laser optogalvanic spectroscopy used to determine photodetachment threshold for CN ... 

Barshick C. M., Shaw R. W., Young J. P. and Ramsey J. M. (1994) Istopic analysis of uranium using glow discharge optogalvanic spectroscopy and diode lasers, Anal Chem 66 4154-4158. 

Fig. 1.42 Experimental arrangement for optogalvanic spectroscopy in a hollow cathode lamp...

In hollow cathodes the cathode material can be sputtered by ion bombardment in the discharge. The metal vapor, consisting of atoms and ions, can be investigated by optogalvanic spectroscopy. Figure 1.43b illustrates a section of the optogalvanic spectrum of aluminum, copper, and iron atoms, and ions A1+, Fe , measured simultaneously in two hollow cathodes irradiated with a tunable pulsed dye laser [120]. 

For the spectroscopy of atoms or ions in gas discharges, optogalvanic spectroscopy (Sect. 1.5) is a very convenient and experimentally simple alternative to fluorescence detection. In favorable cases it may even reach the sensitivity of excitation spectroscopy. For the distinction between spectra of ions and neutral species velocity-modulation spectroscopy (Sect. 1.6) offers an elegant solution. 

C.R. Webster, C.T Rettner, Laser optogalvanic spectroscopy of molecules. Laser Focus 19, 41 (1983) ... 

J.C. Travis, Analytical optogalvanic spectroscopy in flames, in Analytical Laser Spectroscopy, ed. by S. Martellucci, A.N. Chester (Plenum, New York, 1985), p. 213 D. King, P. Schenck, K. Smyth, J. Travis, Direct calibration of laser wavelength and bandwidth using the optogalvanic effect in hollow cathode Itimps. Appl. Opt. 16,2617 (1977)... 

Stewart RS, Lawler JE (eds). 1991. Optogalvanic Spectroscopy. Institute of Physics Conference Series, vol. 113. lOP Publishing Bristol. 

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

Often calibration spectra that are taken simultaneously with the unknown spectra are used. Examples are the h spectrum, which has been published in the iodine atlas by Gerstenkom and Luc [5.95] in the range of 14 800 to 20 000 cm" For wavelengths below 500 nm, thorium lines [5.96] measured in a hollow cathode by optogalvanic spectroscopy (Sect. 6.5) or uranium lines [5.97] can be utilized. 

Optogalvanic spectroscopy is a suitable technique for studies of excitation and ionization processes in flames, gas discharges, and plasmas [6.97]. Of par-... 

If the discharge cell has windows of optical quality, it can be placed inside the laser resonator to take advantage of the -fold laser intensity (Sect. 6.2.2). With such an intracavity arrangement. Doppler-free saturation spectroscopy can also be performed with the optogalvanic technique (Sect. 7.2 and [6.101]). An increased sensitivity can be achieved by optogalvanic spectroscopy in thermionic diodes under space-charge-limited conditions (Sect. 6.4.5). Here... 

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