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On the relaxation to quantum‐statistical equilibrium of the Wigner–Weisskopf atom in a one‐dimensional radiation field. VIII. Emission in an infinite system in the presence of an extra photon

### Abstract

In this paper we study the emission of a two‐level atom in a radiation field in the case where one mode of the field is assumed to be excited initially, and where the system is assumed to be of infinite extent. (The restriction to a one‐dimensional field, which has been made throughout this series, is *n* *o* *t* essential: It is made chiefly for ease of presentation of the mathematical methods.) An *e* *x* *a* *c* *t* expression is obtained for the probability ρ (*t*) that the two‐level quantum system is in the excited state at time *t*. This problem, previously unsolved in radiation theory, is tackled by reformulating the expression found in VII [J. Math. Phys. 16, 1013 (1975)] of this series for the time evolution of ρ (*t*) in a finite system in the presence of an extra photon, and then constructing the infinite‐system limit. A quantitative assessment of the role of the extra photon and of the coupling constant in influencing the dynamics is obtained by studying numerically the expression derived for ρ (*t*) for a particular choice of initial condition. The study presented here casts light on the problem of time‐reversal invariance and clarifies the sense in which exponential decay is universal; in particular, we find that: (1) It is the infinite‐system limit which converts the time‐reversible solutions of VII into the irreversible solution obtained here, and (2) it is the weak‐coupling limit that imposes exponential form on the time dependence of the evolution of the system. The anticipated generalization of our methods to more complicated radiation‐matter problems is discussed, and finally, several problems in radiation chemistry and physics, already accessible to exact analysis given the approach introduced here, are cited.

© 1978 American Institute of Physics

Published online 11 August 2008

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2008-08-11

2016-10-24

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