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We consider tunnel ionization of an atom or molecule in a strong field withinan analytical treatment of the R-matrix method, in which an imaginary boundaryis set up inside the classically forbidden region that acts as a source ofionized electrons. These electrons are then propagated in the semiclassicalapproximation, and relying on a numerical solution of the inner region, whichis accessible using quantum chemical techniques, we describe the subsequentevolution of the ionized electron and the ionic core.  Importantly, we show that correlation interactions between the ionizedelectrons and those left behind in the core can play a role during ionization,and that interactions can occur while the electron is still inside theclassically forbidden region that enable tunnelling from channels normallysubject to far greater exponential suppression since they are subject to higherand wider tunnelling barriers.  This interaction can be described analytically using saddle-point methodsthat find the dominant contributions to the temporal, spatial and momentumintegrals that make up the expressions for the ionization yield. However, thesemethods yield results for the angular distributions of the ionized electronthat do not necessarily match experiment and which yield physicallyunsatisfactory solutions.  In this report we develop a formalism to evaluate these integrals exactly,which yields more precise calculations of the angular distributions while atthe same time providing the language, in terms of exchange of angular momentum,with which to understand their origins and their differences from previouscases, thus giving an insight into the fundamental physics of the correlationinteraction process.

Under-the-barrier electron-ion interaction during tunnel ionization