Fast excitation-driven fluctuations in the fluorescence emission of yellow-shifted green fluorescent protein mutants T203Y and T203F, with S65G͞S72A, are discovered in the 10 ؊6 -10 ؊3 -s time range, by using fluorescence correlation spectroscopy at 10 ؊8 M. This intensity-dependent flickering is conspicuous at high pH, with rate constants independent of pH and viscosity with a minor temperature effect. The mean flicker rate increases linearly with excitation intensity for at least three decades, but the mean dark fraction of the molecules undergoing these dynamics is independent of illumination intensity over Ϸ6 ؋ 10 2 to 5 ؋ 10 6 W͞cm 2 . These results suggest that optical excitation establishes an equilibration between two molecular states of different spectroscopic properties that are coupled only via the excited state as a gateway. This reversible excitation-driven transition has a quantum efficiency of Ϸ10 ؊3 . Dynamics of external protonation, reversibly quenching the fluorescence, are also observed at low pH in the 10-to 100-s time range. The independence of these two bright-dark flicker processes implies the existence of at least two separate dark states of these green fluorescent protein mutants. Time-resolved fluorescence measurements reveal a single exponential decay of the excited state population with 3.8-ns lifetime, after 500-nm excitation, that is pH independent. Our fluorescence correlation spectroscopy results are discussed in terms of recent theoretical studies that invoke isomerization of the chromophore as a nonradiative channel of the excited state relaxation.

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This paper reports spectroscopic investigations of the chromophore terrylene embedded in a matrix of crystalline p-terphenyl. While this system is particularly well suited for single molecule spectroscopy, little is known about the guest site configuration of terrylene. To shed some light on this issue, we employed absorption and fluorescence spectroscopy and compared the experimental data to the results of theoretical calculations. Based on this comparison we suggest a substitution scheme which is in agreement with all the spectroscopic evidence. The dispersed fluorescence spectra of single molecules in the wings of the inhomogeneous distribution deviate significantly from the bulk spectra. This observation is discussed in terms of a host-induced change of the structure and a possible C13 isotopic substitution of the chromophores. Finally we investigated the dynamic host–guest interactions via the temperature-dependent shift and broadening of single molecule excitation lines and found these processes dominated by coupling to characteristic pseudolocal phonon modes of the host, although chromophores in the wings of the inhomogeneous distribution exhibit additional contributions which we attribute to thermal matrix expansion.

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We have measured the ac-Stark effect on single terrylene molecules in p-terphenyl crystals at superfluid helium temperatures.With the pump beam far enough from resonance, the molecular transition shifts as expected, i.e. , proportional to the pump intensity and to the inverse pump detuning.Experiments closer to resonance with stronger beams show Autler-Townes-like structures that can be well reproduced by means of Bloch equations with two intense laser fields. These experiments are a demonstration of nonlinear optical effects with the localized electronic states of a single molecule trapped in a solid. PACS numbers: 42.50.Hz, 33.80.b, 78.55.Kz Quantum optics, the study of the interaction of simple quantum systems with coherent light, has long been a testing ground for basic quantum mechanics. The many concepts it has bred may become crucial to the working of future nanoelectronic devices at the molecular scale. This remote aim requires experimental tests of the validity of quantum concepts in complex systems such as are found in condensed matter. Many of the quantum optical experiments done to date were performed on atoms or ions in vapors, beams, or traps. Atoms have the advantage of being simple, isolated from outside perturbations, and fairly well understood theoretically. Nonlinear spectroscopy [1] and, more recently, laser cooling [2] developed largely thanks to atoms. Here, we consider a particular class of experiments, those where two laser beams (a pump and a probe) act simultaneously on a quantum system. Several experiments in atomic physics fall into this category. For instance, in sub-Doppler spectroscopy, a strong pump beam "tags" atoms by saturating one of their transitions, while the probe beam reads the absorption change. More generally, a strong pump beam, by coupling two resonant levels, "dresses" atoms with laser photons, leading to changes in the response of the dressed atom to a probe beam [3] that appear as shifts of the levels and changes of their relaxation rates. For a slightly detuned pump, the transition of the dressed atom is shifted in proportion with pump intensity, and multiphoton resonances appear, which may exhibit gain for the probe beam [4]. For a resonant pump, transitions between dressed atom levels lead to three components in the emission spectrum [5] (the Mollow triplet), while the probe absorption spectrum shows a complex shape with amplification at strong pump saturation. The absorption spectrum from the excited state to a third level shows a splitting known as the Autler-Townes effect. The shift of atomic levels under pump light, often called light shift, can be interpreted as an ac-Stark effect, where the quadratic effect of the relatively weak pump laser field is amplified by near resonance with the transition. While being perhaps the simplest manifestation of the dressing of atoms by laser photons, the light shift is the cornerstone of powerful laser-cooling methods [2] and of proposed nondestructive quantum measurements of photons [6]. For all their importance as model...

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