engleski

Laser vaporized alkali dimers

The plume induced by pulsed laser vaporization of solid targets contains electrons, ions, excited and ground state atoms and molecules, clusters etc. Their concentrations and temperatures are changing rapidly in time, as plume expands. Ionic species are usually detected by mass spectrometry techniques. Ground and excited state neutrals are characterized mainly by optical emission spectroscopy (temporal and spatial evolution) or absorption measurements by means of cw laser tuned to the specific transition. We have demonstrated application of Cavity Ring-Down Spectroscopy (CRDS) for both temporally and spatially resolved absorption measurements in the pulsed laser vaporized plume of lithium metal target &#61531 ; ; 1&#61533 ; ; . CRDS is newly developed absorption method based on measurements of laser photon lifetime in a high finesse optical resonator &#61531 ; ; 2&#61533 ; ; . Small densities of absorption species can be detected because of the long optical path. In the present application absorption losses are not uniform over duration of the ring-down signal, therefore we fit different exponential decays in subsequent temporal windows within the ring-down. Target with a sample of alkali metal (lithium, sodium or potassium) is placed in a high vacuum chamber and is allowed to rotate. Laser vaporization is achieved by a pulsed excimer laser (308 nm, 15 ns). Incidence angle of laser pulse was either 35o or 0o with respect to target normal. High reflectivity mirrors for CRDS are mounted in a special holders which permit easy cavity adjustment. Dye laser pulse (pumped by a part of an excimer laser pulse) is injected into cavity with the delay of 10 ns in respect to the laser vaporization pulse. CRDS measurements are performed in different spectral ranges, (630-690) nm and (460-490) nm. Measured CRDS waveforms are fitted to the exponential function in different time windows which depend on plume dynamics. Vaporization laser fluences are about 1 &#8211 ; ; 2 Jcm-2. Under such conditions optical emission measurements shows mainly presence of excited atoms and their ions &#61531 ; ; 3&#61533 ; ; . By using CRDS we observed both atomic and dimer absorptions. In the lithium case we observe Li(2s-2p), Li(2p-4d) atomic and dimer Li2(X1&#61523 ; ; g+- A1&#61523 ; ; u+), Li2(X1&#61523 ; ; g+-B1&#61520 ; ; u) absorptions. Similary, in the vaporized plume of solid sodium and potassium, we observed the Na2(X1&#61523 ; ; g+- A1&#61523 ; ; u+) and K2(X1&#61523 ; ; g+-B1&#61520 ; ; u) absorptions, respectively. Measured absorption spectra are compared to quantum mechanically simulated spectra. By fitting calculated absorption coefficients to the experimental, we determine rotational and vibrational temperatures and densities. Temperatures are below 175 K and depend on laser fluence and the time window. High densities of dimers are observed (109 cm-3) compared to the densities expected at given temperatures of alkali vapors. The results indicate that alkali dimers in the plume are coming from the target surface and are cooled in the jet like expansion. REFERENCES &#61531 ; ; 1&#61533 ; ; I. Labazan, S. Milošević, Chem. Phys. Lett. 352, 226 (2002) &#61531 ; ; 2&#61533 ; ; G. Berden, R. Peeters, G. Meijer, Int. Rev. Phys. Chem. 19, 565 (2000) &#61531 ; ; 3&#61533 ; ; S. Gogić, S. Milošević, Fizika A 7, 37 (1998)

laser vaporization; cavity ring-down spectroscopy; alkali molecules

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