Excited-State Equilibration and the Fluorescence-Absorption Ratio

Abstract

In any complex system at temperature T the absorption cross-section and fluorescent power at a given photon energy are connected by a simple relation if the system is in thermal equilibrium while occupying one particular electronic excited state. Although this situation is impossible in principle because of finite excited-state lifetimes, it is often approximated to the extent that the simple relation, which is expressed as a linear function of energy with slope -1/kBT, holds in a variety of cases. (The usual symbols for Boltzmann's constant and absolute temperature are used.) Observed deviations are of two principal kinds: a slope characteristic of some temperature T* other than ambient, and departures from a single pure straight line. The latter may include seemingly random variations and in some cases multiple regions of straight-line behavior. We have recently introduced an effective temperature T* (E), derived from the actual local slope of the putative straight line at energy E, which turns out to be a very sensitive detector of deviations from the ideal and, we believe, from equilibrium in the excited state. Plots of T* (E) display a variety of features. An anomaly in the T* (E) spectrum of chlorophyll a can be analyzed on this model, indicating a second weakly fluorescent state about 70 meV below the well-known Qy band. The cases of chlorophyll and many others are included in a selective review of applications of the universal relation to fluorescent systems.

Dynamics and correlations of a Bose–Einstein condensate of photons

“…Experimentally, the scaling has been verified in e.g. liquid dye solutions [58,[90][91][92][93], dye-doped polymers [60], semiconductors [94] or ultra-dense gases [95].…”

Hot photons and open-circuit voltage in molecular absorbers

“…Although the Kennard-Stepanov-Kazachenko analysis of many dyes is in keeping with the conclusion that the vibrational subsystem is at the room temperature, there are frequent reports where the analysis produces elevated temperatures [19][20][21]. No general consensus appears to exist as to the origin of this phenomenon.…”

“…In this paper, we investigate the promise of organic dyes as absorbers in hot carrier solar cells. Whilst the standard model of molecular spectra is founded on the ambient temperature [17,18], elevated temperatures are also frequently observed [19][20][21]. By using BASF red dye 305 we report here on the observation of an equilibrium photon flux at a temperature well in excess of the ambient temperature, generated by the edge emission from a fluorescent collector (FC) with an organic dye layer used as an absorber.…”