Small hydride molecules in laser vaporized plume

I. Labazan, N. Krstulovic, S. Miloševic
Institute of physics, Zagreb, Croatia

Hydrogen is the main constituent of the Universe. Beside atomic and molecular hydrogen, many other molecules containing hydrogen were identified in interstellar medium and stellar atmospheres [1]. A lot of hydrides spectroscopic data are still missing. Laboratory conditons are far from conditions in, for example, stellar atmospheres where hydrides were observed. To study simple hydride molecules spectroscopically, one needs to find efficient production method. One possible way is laser vaporization of carefully prepared solid targets. Molecules can be vaporized directly from the target surface or through numerous interactions between vaporized particles and background gas. Laser vaporization can circumvents usage of different high-temperature ovens. Previously, we showed that alkali molecules emerging directly from target surface were rotationaly and vibrationaly cold [2]. In this work we focus on the laser vaporization of pure manganese target and LiAlH_4 complex target. Laser induced plume was studied by the cavity ring-down spectroscopy (CRDS) and time-resolved emission measurements. We studied dynamics of the vaporized plume and measured velocities of atoms and molecules. Molecules were not visible in the emission spectra of vaporized plume, while absorption measurements by CRDS resulted with rich molecular ro-vibrational bands. By CRDS, we observed several MnH(a^5S^+ - d^5P) ro-vibrational bands within the electronic a^5S^+ - d^5P transition in the pure manganese vaporization plume spreading into CH_4 atmosphere. MnH molecules were formed in reaction with methane and its dissociation products, CH_n, n = 1...4. In vaporization plume of the LiAlH_4 target, AlH (0,0) ro-vibrational band within the X^1S^+ - A^1P electronic transition was observed. From measured ro-vibrational line-positions we calculated rotational constants for the X^1S^+ and A^1P electronic states.
These results encourage us in searching for other simple hydrides, like LiH, in laser vaporization experiments.

References:

[1] J. Tennyson, Molecules in Space in Handbook of Molecular Physics and Quantum
Chemistry Vol. 3, Ch.14, Wiley&Sons, Chichester, 2003
[2] I. Labazan, S. Miloševic, Phys. Rev. A 68 (2003) 032901

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