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PTB-Mitteilungen. Band 117 (2007), Heft 3, S. 221 - 293. ISSN 0030-834X

In dieser Arbeit wurde eine einfache laserspektroskopische Messmethode für lokale elektrische Feldstärken im Hinblick auf ihre Messmöglichkeiten und -grenzen untersucht. Als empfindliche optische Feldsensoren dienen dabei Wasserstoffatome, für die die Stark-Aufspaltung der Spektrallinien im elektrischen Feld wohl bekannt und exakt berechenbar ist. Die experimentellen Untersuchungen wurden an einer Niederdruck-Gaszelle durchgeführt, in der ein elektrisch geheizter Wolframdraht für thermische Dissoziation von Wasserstoffmolekülen sorgte. Die Wasserstoffatome wurden durch zwei gegenläufige Laserstrahlen Doppler-frei angeregt. Die Durchstimmung der schmalbandigen Laserstrahlung über den Wellenlängenbereich der Zwei-Photonen-Resonanz lieferte direkt das vom elektrischen Feld hervorgerufene Stark-Spektrum des angeregten Zustands. Weil die Methode im Gegensatz zu ähnlichen, erheblich aufwendigeren Verfahren nur die niedrigsten Wasserstoff-Energieniveaus benutzt, die mit Zwei-Photonen-Anregung direkt aus dem Grundzustand erreichbar sind, kommt sie mit einem einzigen Laser aus. Für das erste angeregte Niveau mit n = 2 wird Strahlung bei 243 nm benötigt, das nächsthöhere Niveau mit n = 3 erfordert 205 nm. Für n = 2 wurden Untersuchungen an Wasserstoff und Deuterium durchgeführt und Stark-Spektren mittels optogalvanischer Detektion gemessen. Schwerpunkt der Arbeit waren aber die Messungen an Wasserstoff für n = 3, bei denen zusätzlich Balmer-alpha-Fluoreszenz im Sichtbaren zur Detektion eingesetzt werden konnte. Bei elektrischen Feldern bis 200 V/cm wurden Stark-Spektren für drei verschiedene Polarisationszustände der Laserstrahlung aufgenommen. Als Ergebnis konnte jeweils ein Paar isolierter Stark-Komponenten in den Spektren identifiziert werden, dessen gut messbarer Frequenzabstand durch Vergleich mit theoretischen Werten die Bestimmung der elektrischen Feldstärke ermöglicht., A method for electric field measurements that observes the Stark spectra of the low excited levels n = 2 and n = 3 of atomic hydrogen has been explored in this work. As advantage these levels can be excited Doppler-free from the ground state by a single laser and the highly resolved Stark spectra are easy to understand and to be calculated. Good sensitivity of electric field measurements is achieved with specially designed solid state laser systems, which provide tuneable pulsed UV radiation with a high pulse peak-power and a narrow bandwidth needed for Doppler-free two-photon excitation. Using hydrogen and deuterium the Stark spectra of the n = 2 level are detected as optogalvanic signal. For three different cases of laser polarization the n = 3 spectra of hydrogen are measured simultaneously with optogalvanic and laser induced Balmer alpha fluorescence detection. Electric fields down to 200 V/cm can be determined from the Stark spectra of n = 2 level, while the spectra of n = 3 level enable measurements of electric fields as small as 50 V/cm in each of the three cases of laser polarization.

This paper presents the methodology for working space assessment in environmental and thermostatic test chambers. The temperature and humidity parameters including: rate of temperature and relative humidity rise, spatial temperature uniformity, spatial temperature-induced relative humidity gradients, short term temperature and humidity fluctuations, and deviations from the actual set points. The temperatures are treated on time-averaged basis. Assessment procedure uncertainties are described and applied to the case when the chamber is used for calibration of humidity sensors. The scope of the method is presented by example assessment of a humidity chamber with results presented in tabular and graphical form.

Quasiresonant laser excitation offers an interesting way to produce highly excited atoms in a vapor mixture. It is generally a process in which laser photon absorption starts from an excited atomic level. The initial excited atomic level population is obtained through the accidental resonance with molecular transitions followed by various processes such as collisionally or radiatively induced dissociation or energy transfer. In the Li+Li2 system, the quasiresonant excitation has not been studied in details, but was used in laser-induced plasma experiments [1,2]. In this work we performed quasiresonant laser excitation of pure lithium vapor and Li-Cd vapor mixtures at 460.3 nm (2p-4d) and 497.2 nm (2p-4s). The initial population of Li(2p) level is caused by three processes: collisional dissociation or photodissociation of the Li2 B1Pu molecules or energy transfer in Li2 (B1Pu) and Li(2s) collisions. The Li(4d,4s) atoms radiatively decay towards Li(3p), from which collisional population transfer to Cd(53PJ) occurs, followed by three-body reactive collisions which produce the LiCd(2 2P ) excimer states [3]. We have used absorption spectroscopy and selective wavelength excitation spectroscopy to study line shapes of the relevant resonances and obtain detials of the excitation processes. Experimental results are suported with detailed spectral simulations. 1. J. T. Bahns, M. Koch, W. C. Stwalley, ''Physics of Ionized Gases''. (Edited by. L. Tanovic, N. Konjevic and N. Tanovic) (Nova Science Publishers, New York 1989), p. 503. 2. A. C. Tam and W. Happer, Opt. Commun. 21, 403 (1977). 3. S. Milosevic, D. Azinovic, I. Labazan and G. Pichler, Physica Scripta, in press (2000).

Cavity ringdown spectroscopy, CRDS, is recently developed laser absorption technique for measurements of small absorbances in the gas phase [1]. In short, laser pulse is injected into an optical resonator formed by pair of high reflectivity mirrors and laser pulse lifetime (ringdown time) is measured. By scanning the laser wavelength a spectrum related to absorption properties of the sample within the cavity is obtained. There are many varieties of this new method, such as integrated cavity output spectroscopy, ICOS [2]. Here we present the use of CRDS on lithium vapor generated in the heat-pipe oven. Cavity mirrors are mounted on one arm of the heat-pipe oven. Cavity length was 1.37 m, with empty cavity ringdown time about 1.5 ms. Laser system consists of an excimer pumped dye laser with pulse duration about 15 ns and 1 GHz bandwidth. Due to the characteristics of such laser system we expected multiexponential decaying curve. Various kind of nonlinear effects in such case can be observed [3]. We compared shapes of the spectral lines obtained from fitting procedure in time windows of different lengths and placed at different positions across the ringdown decay curve. Dips in the line center of strong transitions are observed and analyzed. Molecular Li2 (X1Sg -B1Pu) absorption was measured, and vibrational-rotational transitions are identified by comparison with detailed spectral simulations. We concentrated especially on wavelength intervals in which two-photon Li(2s-5d) and Li(2s-6d) and quasiresonant Li(2p-4s) transitions overlap with single-photon molecular transitions. In addition to CRDS we used selective wavelength excitation spectroscopy at various atomic transitions and we present ICOS spectra in comparison with ‘ordinary’ CRD spectra regarding molecular line shape profiles. 1. M. D. Wheeler, S. M. Newman, A. J. Orr-Ewing, M. N. R Ashfold, J. Chem. Soc., Faraday Trans. 94 (1998) 337-351 2. A. O’ Keefe, Chem. Phys. Lett. 293 (1998) 331-336 3. I. Labazan, S. Rudić, S. Milošević, Chem. Phys. Lett. 320 (2000) 613-623