Comparative studies of dissociative electron attachment to CH$_2$Cl$_2$ and CF$_2$Cl$_2$ molecules

Experimental studies\footnote{K. Aflatooni and P. D. Burrow, Int. J. Mass Spectrom. {\bf 205}, 149 (2001).} of dissociative electron attachment to dichloroalkanes and chlorofluoromethanes have established a universal relation between the peak value of the cross section and the vertical attachment energy. The CH$_2$Cl$_2$ is the only compound that is substantially removed from the trend observed for all other compounds: its peak cross section is more than one order of magnitude lower than that predicted by the universal curve. In an attempt to understand this difference we calculated energies of the LUMO states for CH$_2$Cl$_2$ and CF$_2$Cl$_2$ anions as functions of the C-Cl distance and performed dissociative electron attachment calculations using one effective reaction coordinate corresponding to the C-Cl stretch. For the potential energy curve calculations we employed MP2 theory with 6-31+G(d) and 6-311++G(3df,3pd) basis sets. The calculations show that the ionic curve for CH$_2$Cl$_2$ is much less repulsive and crosses the neutral curve at substantially larger internuclear separation than for CF$_2$Cl$_2$. This explains qualitatively and semiquantitatively the much lower dissociative attachment cross section for CH$_2$Cl$_2$. This work was supported by the US National Science Foundation. and by a Fullbright fellowship.