Nonoptical excited state spectroscopy of CHF2Cl: Characterization of nondipolenrarr;sgr;ast; valence transitions by anglehyphen;resolved electron energy loss spectroscopy

摘要

A nondipole lowhyphen;lying excitation feature at 8.0 eV in difluorochloromethane (CHF2Cl) has been observed in anglehyphen;resolved electron energy loss spectra obtained at 2.5 keV impact energy. Based on the results of a singlehyphen;excitation configuration interaction calculation, we assign this feature predominantly as electronic transitions from Cl nonbonding 14arsquo;and 7alsquo;orbitals lsqb;the highest occupied molecular orbital (HOMO) and second HOMO, respectivelyrsqb; to an antibonding sgr;Cndash;Clast;15arsquo;orbital (the lowest unoccupied molecular orbital), the sohyphen;callednrarr;sgr;ast; type transitions. The generalized oscillator strength (GOS) profile of this transition has been determined and found to be similar in shape and magnitude to that of a recently reportednClrarr; sgr;Cndash;Clast;(7erarr;11a1) transition in CF3Cl lsqb;Yingetal., Chem. Phys. Lett.212, 289 (1993)rsqb;. Both GOS profiles are found to have a shape characteristic of a quadrupole transition, with a maximum at momentum transfer of sim;1 a.u. The similarity in the nature of thesenClrarr; sgr;Cndash;Clast;transitions in CHF2Cl and CF3Cl to that of aphyphen;tohyphen;ptransition in a chlorine atom can be demonstrated qualitatively by using contour maps of the density functions of the dominant initialhyphen;state and finalhyphen;state orbitals generated fromabinitioselfhyphen;consistent field wave functions. The homology of transitionhyphen;related properties (including the GOS profile) among thenClrarr; sgr;Cndash;Clast;transitions in lsquo;lsquo;simplersquo;rsquo; polyatomic molecules that contain a single highly localized Cndash;Cl bond is also discussed. Furthermore, the calculated potential energy diagram for CHF2Cl along the Cndash;Cl bond direction suggests that electronic transitions from the 14arsquo;and 7alsquo;orbitals to the 15arsquo;orbital may lead to dissociation of the Cndash;Cl bond. Such predissociation may represent a possible common consequence of thenClrarr; sgr;Cndash;Clast;transitions in these monochlorohyphen;substituted chlorofluorocarbons and related derivatives. thinsp;