Energetics, molecular electronic structure, and spectroscopy of forming Group IIA dihalide complexes

Multiple-collision relaxed (helium) chemiluminescence and laser-induced flu orescent spectroscopy have been used to demonstrate the highly efficient co llisional stabilization of electronically excited Group IIA dihalide collis ion complexes formed in M (Ca,Sr) + X-2 (XY) (Cl-2, Br-2, ICI, IBr, I-2) re active encounters. The first discrete emission spectra for the CaCl2, CaBr2 , SrCl2, SrBr2, and SrICI dihalides are observed and evaluated; however, th e low-pressure 'continuous' chemiluminescent emission observed for forming barium dihalide (BaX2) complexes is quenched under these experimental condi tions. The reactions of the Group IIA metals with molecular fluorine do not readily produce the corresponding dihalide. While the lowest-lying observe d dihalide visible transition is, as predicted, found to result in an exten ded progression in a dihalide complex bending mode (SrCl2), the observed pr ogression suggests the presence of a residual halogen (Cl-Cl) bond. Two hig her-lying transitions are dominated by a vibrational mode structure corresp onding to progressions in the symmetric stretching mode or, for nominally f orbidden electronic transitions, odd quanta of the asymmetric stretching mo de. Some evidence for sequence structure associated with the dihalide bendi ng mode is also obtained. These observations are consistent with complex fo rmation as it is coupled with a modified valence electron structure (correl ation diagram) associated with the highly ionic nature of the dihalides. Th e bonding in the Group IIA dihalides (and their complexes), whose atomizati on energies are more than twice the metal monohalide bond energy, strongly influences the evaluation of energetics and the determination of monohalide bond energies from chemiluminescent processes. Discrepancies between those bond strengths determined by mass spectrometry and chemiluminescence are d iscussed with a focus on energy partitioning in dihalide complex formation and its influence on chemical vapor deposition. (C) 1999 Published by Elsev ier Science B.V. All rights reserved.