The conclusion drawn earlier (Oleinik and Belousov 1983) concerning the existence of the intense vacuum polarisation mechanism by the external field, for which the number of electron-positron pairs created in the field is proportional to the energy levels width of elementary electron-positron excitations, is confirmed. The estimates of the pair creation probability allows the authors to think that the atom ionisation in a steady electric field is accompanied by creation of electron-positron pairs which can be quite well registered experimentally at the relatively weak fields. It is shown that at the sudden switching on of an electric field, the value of the tunnel current of electron emission out of the well is affected appreciably by allowance for the positron band.