LASER SPECTROSCOPIC STUDY OF CaH IN THE B2_+ AND D2_+ STATES

Calcium hydride is one of the abundant molecules in the stellar environment, and is considered as a probe of stellar analysis$footnote{B. Barbuy, R. P. Schiavon, J. Gregorio-Hetem, P. D. Singh , C. Batalha , textit{Astron. Astrophys. Sippl. Ser.} textbf{101}, 409 (1993).}$. Ab initio calculations have shown that the electronic excited states of CaH have complex potential curves. It is suggested that the B$^2Sigma^+$ state has an interesting double minimum potential due to the avoided crossing$footnote{P. F. Weck and P. C .Stabcil, textit{J. Chem. Phys.} {textbf{118}}, 9997 (2003).}$. Such a potential leads to drastic change of the rotational constants when the vibrational energy level goes across the potential barrier. Spectroscopic studies on CaH began in the 1920's$footnote{R. S. Mulliken, textit{Phys. Rev.} {textbf{25}}, 509 (1925).}$, and many studies have been carried out since then. Bell et al. extensively assigned the D$^2Sigma^+-$X$^2Sigma^+$ bands in the UV region$footnote{G. D. Bell, M, Herman, J. W. C. Johns, and E. R. Peck, textit{Physica Scripta} {textbf{20}}, 609 (1979).}$. Bernath's group has observed transitions in the IR and visible regions and identified their upper states as the A$^2Sigma^+$, B$^2Sigma^+$ and E$^2Sigma^+$ states$footnote{A. Shayesteh, K. A. Walker, I. Gordon, D. R. T. Appadoo, and P. F. Bernath, textit{J. Mol. Struct.} {textbf{695-696}}, 23 (2004).}$$footnote{R. S. Ram, K. Tereszchuk, I. E. Gordon, K. A. Walker, and P. F. Bernath, textit{J. Mol. Spec.} {textbf{266}}, 86 (2011).}$$footnote{G. Li, J. J. Harrison, R. S. Ram, C. M. Western, and P. F. Bernath textit{Quant. Spectrosc. Rad. Transfer.} {textbf{113}}, 67 (2012).}$$footnote{A. Shayesteh, R. S. Ram, and P. F. Bernath, textit{J. Mol. Spec.} {textbf{288}}, 46 (2013).}$. We have carried out a laser induced fluorescence (LIF) study in the UV region between 360 and 430 nm. We have produced CaH by using laser ablation of a calcium target in a hydrogen gas environment, then molecules have been excited by a second harmonic pulse of dye laser and the fluorescence from molecules have been detected through a monochromator. Detection of the D$^2Sigma^+$-X$^2Sigma^+$ bands already identified by Bell et al. indicates the production of CaH. In addition, many other bands have been also found and a few bands have been assigned by using the combination differences, the lower state of these bands have been confirmed to the vibrational ground state of X$^2Sigma^+$ state. We have tentatively assigned these bands as the B$^2Sigma^+ -$X$^2Sigma^+$ transition. We will discuss the assignment of these bands, together with the rotational constants comparing with those calculated from the ab initio potential