engleski

Molecules detected in laser induced plume by time-resolved cavity ring-down spectroscopy

Laser ablation is a process of great importance in different applications from pulsed laser deposition of thin films to element detection by laser induced breakdown spectroscopy. Recently it is also considered as efficient source of molecules at high densities and sub thermal internal energies needed for cooling and trapping. Usually a colorful plumes are observed and emission analyzed by gated detectors. This shows mainly presence of atoms and their ions in the plume. The observations of molecules directly in the plume are not that common. Cavity ring-down spectroscopy (CRDS) is a direct laser absorption method based on measurement of absorption losses within the cavity. In the plume, absorption changes rapidly during the ring-down time. Therefore we apply analyses over time window segments [1]. Target with a sample of metal 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). The laser beam is normal to the target. Target can be precisely moved in respect to the cavity axis by means of a PC controlled stepping motor synchronously with the lens used to focus the 308 nm laser beam on the target. High reflectivity mirrors for CRDS are mounted in special holders which permits 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 spectral range (460-490) nm. Measured CRDS waveforms are fitted to the exponential function in different time windows which depend on plume dynamics. Recently we observed high densities of Li2, Na2, K2, NaLi molecules with sub thermal internal energies in the plumes of alkali targets [2]. In present work we show example of laser ablation of Mn in presence of 2 torr methane. In addition to the processes related to target ablation, decomposition of methane by the presence of laser induced plasma occurs. In early time window (250 ns) only absorption lines from the excited Mn levels are observed. Later time windows shows build up of absorption steaming from C2 molecules (Swan band at 473 nm) and new molecular products containing Mn. The spectral analysis of emission shows only presence of atomic Mn lines. Figure 1: Cavity loss for the case of laser ablation of Mn at 2 Torr of methane evaluated in different time windows. By applying CRDS to the laser ablated plume we observed time evolution of both atom and molecular densities. The velocities of particles and their spatial distribution perpendicular to the cavity axis could be examined as well. References [1] I. Labazan and S. Milošević, Chem. Phys. Lett. 352 226 (2002). [2] I. Labazan and S. Milošević, to be published

laser ablation; cavity ring-down spectroscopy; manganese; methane

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