We report the 364-nm negative ion photoelectron spectra of CHX2− and CDX2−, where X = Cl, Br, and I. The pyramidal dihalomethyl anions undergo a large geometry change upon electron photodetachment to become nearly planar, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce qualitatively the experimental data using physically reasonable parameters. Specifically, the harmonic normal mode analysis using Cartesian displacement coordinates results in much more C-H stretch excitation than is observed, leading to a simulated photoelectron spectrum that is much broader than that which is seen experimentally. A (2 + 1)-dimensional anharmonic coupled-mode analysis much better reproduces the observed vibrational structure. We obtain an estimate of the adiabatic electron affinity of each dihalomethyl radical studied. The electron affinity of CHCl2 and CDCl2 is 1.3(2) eV, of CHBr2 and CDBr2 is 1.9(2) eV, and of CHI2 and CDI2 is 1.9(2) eV. Analysis of the experimental spectra illustrates the limits of the conventional normal mode approach and shows the type of analysis required for substantial geometry changes when multiple modes are active upon photodetachment.
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See supplementary material at http://dx.doi.org/10.1063/1.3585606 for a table of the calculated frequencies (TABLE S1) and equilibrium structures (TABLE S2) of CHBr2 and CDBr2; for a table of the calculated frequencies (TABLE S3) and equilibrium structures (TABLE S4) of CHI2 and CDI2; for two-dimensional cuts through the CHCl2− and CHCl2 potential energy surfaces (Fig. S1); for a comparison of the CHCl2− spectrum to its normal mode simulation in internal coordinates and to its calculated spectrum using multidimensional coupled-mode analysis (Fig. S2); for a comparison of the CHBr2− and CDBr2− (Fig. S3) and the CHI2− and CDI2− (Fig. S4) spectra to their simulations using the Sharp-Rosenstock-Chen method in internal coordinates; and for plots of the CHCl2 and CHCl2− wavefunctions (Fig. S5).
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