engleski

Cavity ringdown spectroscopy in lithium vapor

Cavity ringdown spectroscopy (CRDS) [1] in many varieties, for example integrated CRDS [2], is widely used laser absorption technique for measurements of a very low absorption coefficients. Short laser pulse is injected into an optical resonator with high reflectivity mirrors. In the case of bound–bound transitions, absorption properties of the sample which fills the cavity are related to cavity ringdown times for laser tuned or detuned from absorption resonance. In the case of free–bound transitions (continua) one should know CRD time of an empty cavity. In this experiment, we study application of CRDS methods for absorption of nearly optically thick medium using mirrors of relatively low reflectivity and relatively large laser pulse energies. Our final goal is to study free-bound molecular absorptions in alkali dimers at relatively low atom densities (temperatures) [3]. Laser system consists of an excimer pumped dye laser with about 15 ns pulse length and 1 GHz bandwidth, operating in the region of DCM dye. Cavity of length 1.945 m with mirrors of 2.8 m radius of curvature and reflectivity of about 99.6 % is mounted on the heat-pipe oven. The heat-pipe oven is filled with lithium metal. Helium or argon are used as buffer gas at pressure of about 1 Torr. Temperature range was 523 - 923 K, which corresponds to atom densities from 109 - 1015 cm-3. The length of vapour column is 10 cm. Emergent CRDS signal was detected with a photomultiplier. A 8-bit, 150 MHz digital oscilloscope is used to measure the cavity ringdown times. We tune the laser wavelength and store each waveform on a PC for later analysis. Measurements are taken mainly in the range 639.0 – 639.5 nm where single photon (X 1Sg+ Ž A 1Su+) and two (single) photon (X 1Sg+ Ž A 1Su+ Ž F 1Sg+ ) molecular [4] and atomic 2S – 3D absorption occur [5]. In addition we perform measurements in the range 670.0 – 671 nm, around lithium 2S – 2P atomic resonance lines. We analyse waveforms by performing exponential fits in different time windows. Moving the time windows to delayed times we observe peculiar character of the absorption line profiles. We study these absorption line profiles in respect to the cavity properties, laser pulse energy, atom density etc. We compare signals of CRDS, integrated CRDS and classical (single pass) laser absorption measurements and discuss their applicability for different experimental conditions which occur in the heat-pipe ovens. This work was financially supported by Ministry of Science and Technology of the Republic of Croatia. [1] R. Engeln, G. Berden, R. Peeters and G. Meijer, Rev. Sci. Instrum. 69, 3763 (1998) [2] A. O’Keefe, Chem. Phys. Lett. 293, 331 (1998) [3] D. Veža, S. Milošević and G. Pichler, Chem. Phys. Lett. 93, 410 (1982) [4] J. G. Balz, R. A. Bernheim, L. P. Gold, P. B. Kelly, D. K. Veirs, J. Chem. Phys. 75, 5226 (1981) [5] I. Labazan and S. Milošević, to be published

cavity ringdown spectroscopy; lithium vapours

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