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The Journal of Chemical Physics

Photon bunching in the fluorescence from single molecules: A probe for intersystem crossing

J. Bernard, L. Fleury, H. Talon, Michel Orrit

Published Jan 15, 1993• 227 citations
Publisher

Abstract

Fluorescence photons emitted by a single molecule trapped in a solid at low temperature are correlated in time. A simple theory of the correlation pattern is presented, including in the same formalism photon antibunching from coherent Rabi oscillations and photon bunching due to incoherent quantum transitions between electronic levels. The correlation method is applied to single pentacene molecules in a para-terphenyl crystal. A clear photon bunching allows us to determine the ISC rates for each molecule. We attribute the scatter of our results for the transition rate between excited singlet and triplet states, and the difference with the average value taken from the literature, to molecular distortions induced by crystal defects in our small sample. We conclude that the correlation method associated to single molecule spectroscopy has a great potential to study dynamical processes on intermediate time scales in condensed matter.

DOI
10.1063/1.464249

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Fluorescence photons emitted by a single molecule trapped in a solid at low temperature are correlated in time. A simple theory of the correlation pattern is presented, including in the same formalism photon antibunching from coherent Rabi oscillations and photon bunching due to incoherent quantum transitions between electronic levels. The correlation method is applied to single pentacene molecules in a para-terphenyl crystal. A clear photon bunching allows us to determine the ISC rates for each molecule. We attribute the scatter of our results for the transition rate between excited singlet and triplet states, and the difference with the average value taken from the literature, to molecular distortions induced by crystal defects in our small sample. We conclude that the correlation method associated to single molecule spectroscopy has a great potential to study dynamical processes on intermediate time scales in condensed matter.
By severely reducing the number of solute molecules in the illuminated sample, the optical resonances of individual molecules can be resolved in a fluorescence excitation spectrum. Single molecule lines can be studied as a function of time and temperature: sudden jumps of their resonance frequencies are due to spectral diffusion processes. The signal from a single molecule displays specific correlations which makes time-resolved studies possible. Here, emphasis is put on photon-bunching arising from intersystem crossing (ISC). ISC rates are deduced from the observed decay rates of the correlation and are found to differ from molecule to molecule. A single molecule is a truly local probe of its environment by means of which fundamental studies of the matrix dynamics as well as nanophysics experiments may be undertaken.
The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of interphoton time distributions, second order correlation functions, and the variance of the photon number probability distribution. Antibunching at short times and bunching behavior for longer times is observed, associated with sub- and super-Poissonian statistics, respectively. A rate-equation analysis of the molecular level populations indicates an accelerated reverse intersystem crossing.
Experiments with single quantum systems are at the frontier of modern physics. Probing single molecules allows for very precise control of light-matter interaction and for quantum optical studies. Moreover, a single guest molecule in a solid host matrix is a very sensitive probe of local dynamics in its nano-environment. Fluorescence excitation spectroscopy on single molecules is a powerful tool for spectroscopic characterizations at highest resolution, as information can be revealed that is not accessible in experiments on ensembles of molecules. The detection of the fluorescence signal of a single absorber is achieved by exploiting the high spectral selectivity given by the ratio (10 10) of homogeneous to inhomogeneous linewidths of optical transitions at cryogenic temperatures (zero-phonon transitions). Two-photon fluorescence excitation, a fundamental nonlinear optical process, was observed on a single quantum system [1]. Single diphenyl-octatetraene molecules trapped in an n-tetradecane matrix were excited from the 1Ag ground to the lowest excited singlet state by simultaneous absorption of two photons using a single-mode cw laser. Excited molecules were observed through weak one-photon emission resulting from a symmetry breaking induced by the crystal field. The fundamental principles and the experimental technique of single molecule spectroscopy will be introduced briefly and some particular single molecule experiments presented. This work was done in collaboration with T. Plakhotnik, A. Renn and U.P. Wild
Small volumes of a pentacene-doped p-terphenyl crystal are studied by fluorescence excitation. Excitation lines of single molecules of impurities are resolved, as is shown by the features of the single molecule lines and confirmed by the autocorrelation properties of the emitted light. Following the excitation lines as the temperature is changed, we observe various behaviors. In particular, one behavior is most simply explained by postulating a coupled two-level system in the molecule’s neighborhood.
Spontaneous fluorescence has been used to measure the coherent femtosecond response of the organic, binary crystal pentacene/p-terphenyl. By using two-pulse excitation with phase-randomized pulses in an interferometric setup and analyzing the variance of the fluctuating intensity of correlated fluorescence photons, femtosecond beatings have been observed. The pattern of these terahertz oscillations is strongly dependent on the detuning frequency range of the exciting pulses, but is rather invariant with regard to the spectral position of the fluorescence probe window. In the interferometric regime of freely propagating pulses novel, ultrafast fluorescence carrier-wave oscillations superimposed to the beat structure have been obtained. The oscillatory signals evolve from a coherent superposition of optical free induction decays, caused by the different electronic transition energies of the pentacene absorber sites O1, O2, O3, and O4, respectively, are monitored as intrinsic, heterodyne beats by the fluorescence square detector. The major part of oscillations is thus considered to result from typical polarization interferences, but vibrational quantum beats are also extractable from the interferogram in the ultimate frequency regime of the pentacene S0→S1ν=1, S0→S1ν=2 resonances. The early picosecond-decay of the fluorescence beats reveals the mechanism of the loss of coherence to be mainly controlled by inhomogeneous dephasing at low phonon temperatures. A model of uncoupled two-level systems that includes Gaussian inhomogeneous broadening of the individual absorber sites and a Gaussian frequency distribution for the excitation pulses can account for the experimental data, quite adequately.