The temperature dependence of internal conversion in model compounds of the chromophore of the green fluorescent protein and one of its mutants has been measured. The strong temperature dependence persists in all charge forms of the model compounds, in all solvents and in a polymer matrix. The ultrafast internal conversion mechanism is thus an intrinsic property of the chromophore skeleton, rather than one of a specific charge or hydrogen-bonded form. An isoviscosity analysis shows that the coordinate which promotes internal conversion is essentially barrierless at room temperature. At reduced temperatures (or high viscosity) there is evidence for the formation of a small barrier. This may reflect a change in the nature of the microscopic solvent dynamics close to the glass transition temperature. In all cases the viscosity dependence of the rate constant for internal conversion is very weak, being approximately proportional to viscosity raised to the power of 0.25 ( 0.06. This suggests weak coupling between the relevant coordinate and macroscopic solvent viscosity. It is suggested that a potential candidate for the coordinate which promotes internal conversion is the volume-conserving "hula twist".

The fluorescence and ultrafast ground-state recovery times of the isolated chromophore of the green fluorescent protein have been studied in basic alcohol solutions. The fluorescence quantum yield increases more than 10 3 times between 295 and 77 K. The major part of the increase occurs in the supercooled liquid range, and continues below the glass transition. The ground-state recovery at 295 K is essentially (95%) complete in under 5 ps, is nonexponential, and only weakly dependent on solvent viscosity. These results are inconsistent with a viscosity-controlled radiationless process involving large scale intramolecular reorganization. If intramolecular motion is involved it must be of small scale. Alternative mechanisms are discussed. A thermally activated radiationless decay process is consistent with the present data, but the mechanism is unclear. For either mechanism the high quantum yield in the intact protein must arise through protein-chromophore interactions which effectively suppress the radiationless channel.

One of the great successes of quantum physics is the description of the long-lived Rydberg states of atoms and ions. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr-Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. Manipulation of Rydberg states in free atoms and ions by single and multiphoton processes has been tremendously productive since the development of pulsed visible laser spectroscopy. The analogous manipulations have not been conducted for donor impurities in silicon. Here, we use the FELIX pulsed free electron laser to perform time-domain measurements of the Rydberg state dynamics in phosphorus-and arsenicdoped silicon and we have obtained lifetimes consistent with frequency domain linewidths for isotopically purified silicon. This implies that the dominant decoherence mechanism for excited Rydberg states is lifetime broadening, just as for atoms in ion traps. The experiments are important because they represent a step toward coherent control and manipulation of atomic-like quantum levels in the most common semiconductor and complement magnetic resonance experiments in the literature, which show extraordinarily long spin lattice relaxation times-key to many well known schemes for quantum computing qubits-for the same impurities. Our results, taken together with the magnetic resonance data and progress in precise placement of single impurities, suggest that doped silicon, the basis for modern microelectronics, is also a model ion trap.coherence ͉ free electron laser ͉ quantum information ͉ picosecond population dynamics ͉ hydrogenic donor impurity H omogenous lifetime-broadened two-level atoms in ion traps (1) have become favorite objects of study for quantum optics with a view toward both fundamental physics and the eventual development of a quantum computer. Among the many schemes proposed (2), the states of ions in trap systems are attractive for the realization of quantum information ''qubits'' (quantum bits) because they are well isolated from the decohering effects of the environment and can be coherently controlled by lasers. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr-Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. As with atoms in traps the ground states are tightly confined and well isolated from the environment, giving rise to extraordinarily sharp transitions (3-5) and very long spin coherence times (6, 7), measured with magnetic resonance experiments. There are several proposals for quantum information processing based on the spin of silicon do...

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Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10 5 T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8 T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H 2 analogues, and for investigation of He 2 , a bound molecule predicted under extreme field conditions.