VIRTUAL TRANSITIONS, SELF-DRESSING AND INDIRECT SPECTROSCOPY

The time-dependent self-dressing of an initially bare two-level atom is studied in the framework of non-relativistic quantum electrodynamics. The long-time behavior of the system is investigated. It is shown that self-dressing is accompanied by the emission of a real photon resonant with the transition frequency of the atom. The self-dressing is thus an irreversible process. The probability of this emission is calculated and the detectability of this new effect is discussed. In the case of a multi-level system, the possibility of ''indirect spectroscopy'' is proposed. In addition to the usual Weisskopf-Wiper ''direct'' decay from one state to a lower one, first a virtual transition brings the system to a higher excited level, from which it subsequently decays by a resonant transition. These new phenomena also bring a correction to Einstein's A coefficient.