What is going on at higher electric fields

3 WHAT IS GOING ON AT HIGHER ELECTRIC FIELDS Polarization 

The theory described so far is applicable only when the electric field intensity is small. Such a field can polarize (a small deformation) the electronic charge distribution. More fascinating phenomena begin when the electric field gets stronger. 

Of course, the equilibrium configurations of the molecule with and without an electric field differ. In a simple case, say the HCl molecule, the HQ distance increases in an electric field. It has to increase, since the cathode pulls the hydrogen atom and repels the chlorine atom, while the anode does the opposite. In more complex cases, like a flexible molecule, the field may change its conformation. This means that the polarizability results both from the electron cloud deformation and the displacement of the nuclei. It turns out that the later effect (called vibrational polarization) is of great importance.  

A sufficiently strong electric field will destroy the molecules through their ionization. The resulting ions accelerate in the field, collide with the molecules and ionize them even more (these phenomena are accompanied by light emission as in vacuum tubes). Such processes may lead to the final decomposition of the system (plasma) with the electrons and the nuclei finally reaching the anode and cathode. We will have a vacuum. 

Let us keep increasing the electric field applied to the vacuum. Will anything interesting happen We know, from Chapter 3, that when huge electric field intensities 

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