Microscopic theory of photoemission in the quadratic response formalism

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Abstract
A decoupling scheme for the three-current correlation function describing the external photocurrent within the quadratic response formalism is introduced. Terms attributed to 'simple scattering processes', which do not contain the three-particle interaction vertex, are analysed explicitly applying many-body Green function techniques. It is possible to separate from these terms one part, which describes the excitation of the dressed electron by the external field through renormalized current vertices and the transport of this electron to the detector in terms of a Bethe-Salpeter equation for a particle-hole function. The latter equation is solved by the scattering-time approximation. The resultant expression is compared with commonly used formulae for photoemitted primary electrons.