Shape of the Absorption and Fluorescence Spectra of Condensed
Phases and Transition Energies
Miguel Lagos and Rodrigo Paredes
General integral expressions for the temperature dependent profile of
the spectral lines of photon absorption and emission by atomic or
molecular species in a condensed environment are derived with no other
hypothesis than: (a) The acoustic vibrational modes of the condensed
host medium constitute the thermodynamic energy reservoir at a given
constant temperature, and local electronic transitions modifying the
equilibrium configuration of the surroundings are multiphonon events,
regardless the magnitude of the transition energy.
(b) Electron--phonon coupling is linear in the variations of the bond
length. The purpose is to develop a theoretical tool for the analysis
of the spectra, allowing to grasp highly accurate information from
fitting the theoretical lineshape function to experiment, including
those spectra displaying wide features. The method is illustrated by
applying it to two dyes, Lucifer Yellow CH and Coumarin1, which
display fluorescent maxima of 0.51 and 0.41 eV FWHM. Fitting the
theoretical curves to the spectra gives that the neat excitation
energies are 2.58 eV ± 2.5% and 3.00 eV ± 2.0%,
respectively.