Theoretical study of photodetachment of HOCO$^-$ near threshold

Continetti and Lu (Phys.~Rev.~Lett. 99, 113005 (2007)) have shown that the photoelectron kinetic energy distribution from HOCO$^-$ exhibits two sharp peaks near threshold accompanied by a broader feature raging from 0.4 to 1.9 eV. They have argued that the threshold peaks are s- and p- wave shape resonances. In contrast, here we show that these resonances can be attributed to the vibrational levels of a dipole-bound state of trans-HOCO$^-$. Our MP2 calculations give a value of 2.50 Debye for the dipole moment of trans-HOCO$^-$. The electron binding energy for the dipole-bound anion was found to be 0.0012 eV at the configuration-interaction level of theory with single and double excitations. We have also calculated the fixed-nuclei photodetachment amplitudes using variationally obtained electron-molecule scattering wavefunctions, which identifies the broad feature as an A$''$-shape resonance.