Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spontaneous emission and level shifts in absorbing disordered
  dielectrics and dense atomic gases: A Green's function approach   Spontaneous emission and Lamb shift of atoms in absorbing dielectrics are
discussed. A Green's-function approach is used based on the multipolar
interaction Hamiltonian of a collection of atomic dipoles with the quantised
radiation field. The rate of decay and level shifts are determined by the
retarded Green's-function of the interacting electric displacement field, which
is calculated from a Dyson equation describing multiple scattering. The
positions of the atomic dipoles forming the dielectrics are assumed to be
uncorrelated and a continuum approximation is used. The associated unphysical
interactions between different atoms at the same location is eliminated by
removing the point-interaction term from the free-space Green's-function (local
field correction). For the case of an atom in a purely dispersive medium the
spontaneous emission rate is altered by the well-known Lorentz local-field
factor. In the presence of absorption a result different from previously
suggested expressions is found and nearest-neighbour interactions are shown to
be important.

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The categories of this paper are: alg-geom
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The categories of this paper are: q-alg
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The categories of this paper are: nucl-ex
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th