Vibrations and dissociation of molecules in strong electric fields: N2, NACl, H2O and SF6

The static deformations and vibrations of the title compounds with an applied static electric field have been studied by density functional theory calculations. It is found that for diatomic molecules, bond length and vibrational frequency as a function of the field can be fit very well all the way up to the dissociation limit by an analytical formula derived from a Morse potential model including an additional external field term. Polyatomic molecules show more complex behaviour with a single mode becoming soft at the dissociation limit. The frequency of the soft mode near the critical field is again described well by the analytical model. The vibrational analysis shows that in polyatomic molecules dissociation proceeds as a heterolytic fragmentation process, which can break the symmetry of the molecule in the applied field.