engleski

Absorption spectra of the lithium dimer in the region of X 1Sigmag+ - B 1Piu band by cavity ringdown spectroscopy

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

absorption spectra; lithium

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