  In this paper, single-molecule spectroscopy experiments based on continuous
laser excitation are characterized through an open quantum system approach. The
evolution of the fluorophore system follows from an effective Hamiltonian
microscopic dynamic where its characteristic parameters, i.e., its electric
dipole, transition frequency, and Rabi frequency, as well as the quantization
of the background electromagnetic field and their mutual interaction, are
defined in an extended Hilbert space associated to the different
configurational states of the local nano-environment. After tracing out the
electromagnetic field and the configurational states, the fluorophore density
matrix is written in terms of a Lindblad rate equation. Observables associated
to the scattered laser field, like optical spectrum, intensity-intensity
correlation, and photon-counting statistics, are obtained from a
quantum-electrodynamic calculation also based on the effective microscopic
dynamic. In contrast with stochastic models, this approach allows to describe
in a unified way both the full quantum nature of the scattered laser field as
well as the classical nature of the environment fluctuations. By analyzing
different processes such as spectral diffusion, lifetime fluctuations, and
light assisted processes, we exemplify the power of the present approach.
