Spontaneous Excitation Of An Atom In A Spacetime With A Reflecting Plane Boundary

It is generally considered that there exists in quantum radiation theory an in-determination in the separation of the respective effects of cacuum fluctuations and radiation reaction. J. dalibard, J. Dupont-Roc and C. Cohen-Tannoudji have showed that such an indetermination can be removed by imposing the corresponding rates of variation to be Hermitian. J. Audretsch and R. Müller applied this procedure to an atom interacting with a massless scalar quantum field and analyzed quantitatively the distinct contributions of vacuum fluctuations and radiation reaction to the spontaneous excitation of a uniformly accelerated atom.We study a two-level atom in interaction with a real massless scalar quantum field in a spacetime with a reflecting boundary. The presence of the boundary modifies the quantum fluctuations of the scarlar field, which in turn modifies the radiative properties of atoms. We calculate the rate of change of the mean atomic energy of the atom for both inertial motion and uniform acceleration.In the case of an inertial atom, it found that the modifications induced by the presence of a boundary make the spontaneous radiation rate of an excited inertial atom to oscillate near the boundary. At distances far from the plane (zω_ab >> 1), the corrections become negligible as one would expect. It is interesting to note that close to the plane(zω_ab << 1), the corrections becomes so large that the total radiation rate of the atom diminishes to zero in a oscillatory manner as the boundary is approached.In the case of a uniformly accelerated atom, the transitions from ground states to excited states are found to be possible even in vacuum due to changes in the vacuum fluctuations induced by both the presence of the boundary and the acceleration of atoms. There are some interesting features to be noted as compared to the case without any boundary. First, the rate of change of the mean atomic excitation energy is now a function of the distance to the boundary and it dies off in an oscillatory way the boundary is approached. Second, the contribution of radiation reaction is now dependent on the acceleration of the atom, in sharp contrast to the unbounded Minkowski space where it has been shown that for accelerated atoms onarbitrary stationary trajectory, the contribution of radiation reaction is generally not altered from its inertial value.