COLLISIONAL APPROACH TO DYNAMICS OF RESONANCE ATOMIC STATES IN AN EXTERNAL-FIELD

The following aspects of the dynamics of an atomic state in an externa l stationary field are assessed: (i) the rearrangement problem; (ii) t he description of the appropriate final-channel wavefunctions; (iii) t he analytical properties of the transition amplitude into the continuu m. The rearrangement problem was solved by the introduction of the eff ective Hamiltonian,the eigenstates of which include both the initial s tate and final states ('modified states of continuum spectrum' MSCS) w hich describe the potential part of the exact wavefunction of the scat tering problem. It is shown that the amplitude of decay and transition into MSCS as functions of time have an exact representation as a sum of resonance terms defined by a set of resonance states and the matrix elements of the shift R-matrix operator. Analysing the following expa nsion basis sets: stationary states, Volkov-Keldysh states and coheren t states it is shown that the most appropriate one is a modified set o f Volkov-Keldysh states.