Infrared diode laser spectroscopy of lithium hydride

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Abstract
The fundamental and hot bands of the vibration–rotation transitions of 6 LiH, 7 LiH, 6 LiD, and 7 LiD were observed by infrared diode laser spectroscopy at Doppler-limited resolution. Lithium hydride molecules were produced by the reaction of the Li vapor with hydrogen at elevated temperatures. Some 40 transitions were observed and, after combined with submillimeter-wave spectra reported by G. M. Plummer et al. [J. Chem. Phys. 81, 4893 (1984)], were analyzed to yield Dunham-type constants with accuracies more than an order of magnitude higher than those published in the literature. It was clearly demonstrated that the Born–Oppenheimer approximation did not hold, and some parameters representing the breakdown were evaluated. The Born–Oppenheimer internuclear distance rBOe was derived to be 1.594 914 26 (59) Å, where a new value of Planck’s constant recommended by CODATA was employed. The relative intensity of absorption lines was measured to determine the ratio of the permanent dipole moment to its first derivative with respect to the internuclear distance: μe /[(∂μ/∂r)e re ]=1.743(86). The pressure broadening parameter Δνp /P was determined to be 6.40 (22) MHz/Torr by measuring the linewidth dependence on the pressure of hydrogen, which was about four times larger than the value for the dipole–quadrupole interaction estimated by Kiefer and Bushkovitch’s theory.