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The search for the bound-free emission from the LiH C 1sigma+ state

In spite of great interest in LiH molecule, because of its importance for astrophysics and other applications there is almost no knowledge about any other higher potential curves beside two lowest singlet states.1 Recent ab initio theoretical calculations2 revealed a very complex structure of LiH singlet potential curves, where ionic-covalent interaction causes many avoided crossings. Thus LiH C 1?+ state appears as a double minimum state with its inner minimum lying entirely above the dissociation limit. The outer minimum is below the dissociation limit, but its equilibrium position is at internuclear distances where lower LiH A 1?+ and X 1?+ states are almost horizontal. Spectral transitions C?A and C?X in the region of the outer minimum of the C 1?+ state are of bound-free type in the infrared and green part of the spectrum, respectively. Both spectra should be governed by the strong enhancement due to the extremum in the corresponding difference potential curves C-A and C-X. In our experimental search for the bound-free emission from the LiH C 1?+ state we employed a crossed heat-pipe oven with cylindrical electrodes inserted in two arms. The electrodes were isolated from the main oven body. The electric discharge in a Li+Li2+H2 mixture produced LiH molecules in large quantity so that the A 1?+ ? X 1?+ could be easily observed. By increasing the partial pressure of hydrogen gas we succeeded to enhance LiH A 1?+ ? X 1?+ emission and to entirely eliminate emission spectra of Li2 dimers. Possible C-A and C-X features in the observed spectra will be discussed in connection with spectral simulations. Quite recently we learned about the successful optical-optical double resonance depletion fluorescence experiment in which LiH C 1?+ was found.3 References: 1. W. C. Stwalley and W. T. Zemke J. Phys. Chem. Ref. Data 22 (1993) 87. 2. A. Boutalib and F. X. Gadea J. Chem. Phys. 97 (1992) 1144. 3. W. C. Lin, J. J. Chen and W. T. Luh, J.Phys.Chem. 101 (1997) 6709.

LiH molecule; bound-bound; bound-free emission; heat-pipe oven

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