Francis K. Fong

Quantum efficiency of diffusion limited energy transfer in La1−x−yCexTbyPO4

In this paper, we present a detailed study of the concentration and temperature dependences of the quantum efficiency of energy transfer between Ce3+ and Tb3+ in La1−x−yCexTbyPO4. The determination of the transfer quantum efficiency in terms of the donor luminescence lifetimes is critically compared with that in terms of the donor luminescence intensities. It is shown that for the latter determination it is important to take into account any overlap in the donor and acceptor absorption spectra in the excitation wavelength region. From the exponential behavior, the concentration, and thermal dependences of the donor luminescence decay, it is shown that the diffusion of donor excitation plays an important role in the energy transfer process.

Theory of Radiationless Relaxation of Rare‐Earth Ions in Crystals

The rate of multiphonon relaxation of rare‐earth ions in crystals has been calculated using the Kubo representation of the rate constant in terms of linear response time correlation functions. The nonadiabatic electron‐phonon operator arising from the effect of the kinetic energy of the ions on the Born‐Oppenheimer basis functions is employed in the first‐order perturbation theory. The calculations yield the explicit dependence of the decay rate on the temperature, the transition energy gap, the phonon energy, and the displacements of the adiabatic potentials. Excellent agreement between theory and experiment has been obtained for the multiphonon relaxation phenomenon observed in LaF3:Er3+, LaCl3:Dy3+, LaCl3:Nd3+, and LaBr3:Dy3+. Although the comparison of theory and experiment is specialized to the nonradiative relaxation of rare earth ion excited states in crystals, the treatment given in the present paper is also generally valid for the relaxation of excited states by internal conversion in large molec...

In vitro preparation and characterization of a 700 nm absorbing chlorophyll-water adduct according to the proposed primary molecular unit in photosynthesis☆

1. This study characterized chlorophyll a-H2O adducts in vitro in order to establish their generic relationship to the recently proposed [15, 18-20, 31] primary molecular adduct in photosynthesis. The effects of water titration and temperature on the absorption, fluorescence, excitation, and redox properties of the various in vitro chlorophyll a aggregate species are investigated. 2. From fluorescence measurements, we conclude that the driest chlorophyll a sample contains an equimolar amount of water. This conclusion is consistent with earlier experimental work [2, 3, 14, 17, 31], and clarifies the origin of the controversial [15] Katz model [14] of chlorophyll a-H2O interactions. 3. With increasing water concentration or as the temperature is lowered below room temperature, the A-663 monohydrate chlorophyll a-H2O (species absorbing at 663 nm) is favored at the expense of the A-678 anhydrous aggregate according to the equilibrium 2H2O+chlorophyll a2in equilibrium2 chlorophyll a-H2O. Under excess water conditions, A-663 is converted to A-743 (chlorophyll a-2H2O)n. 4. On slow sample cooling to T less than or approximately 200 degrees K, we observe the growth of A-700 at the expense of A-663. There is a direct correspondence between the increasing (decreasing) absorption by A-700 (A-633) and increasing (decreasing) fluorescence at 720 nm (664 nm). 5. It is concluded that A-700 is most probably the dimer participating in the equilibrium 2 chlorophyll a-H2O in equilibrium (chlorophyll a-H2O)2. The A-700 band consists of two exciton components (separated by approximately 280 cm1) that are interpretable in terms of the dimeric origin of A-700. 6. The deconvoluted A-700 absorption spectrum and the excitation spectrum of the 720 nm fluorescence are compared with the light-minus-dark spectra of P-700. 7. It is found that A-700 is reversibly bleached by I2 (E0 equals 0.54 V). The significance of this observation is discussed in terms of the redox properties of monomeric chlorophyll a and P-700.

Many‐Body Processes in Nonradiative Energy Transfer between Ions in Crystals

It is shown that many‐body interactions can play a dominant role in nonradiative energy transfer processes between ions in crystals. These interactions formally arise from the Coulomb and exchange interactions between the electrons of the donor ion and two or more acceptor ions. In particular, three‐body transfer processes arising from the dipole‐dipole perturbation Hamiltonian through virtual transitions are considered in some detail. Such processes are manifested by the quadratic concentration dependence of the per‐ion nonradiative energy transfer rate observed in concentration quenching and sensitized luminescence experiments. Many‐body interactions are important in rare‐earth ions because of the narrow widths of the 4fn crystal states and because of the large moments of the virtual transitions involving the opposite parity 4fn−1 5d states.

Growth of single crystals of anhydrous lanthanide halides

Abstract A consistently successful procedure for growing optically pure single crystals of the anhydrous lanthanide halides is described. It involves drying the hydrated lanthanide halide with the corresponding ammonium halide, purification by vacuum distillation, and subsequent growth by the Bridgman-Stockbarger method. All critical steps in the procedure are explained, and the relatively simple apparatus is described in detail.

Optical study of ion‐defect clustering in CaF2:Er3+

The effect of ion‐defect clustering in CaF2:Er3+ has been investigated by means of fluorescence spectroscopy and lifetime measurements. The fluorescence spectra of CaF2:Er3+ exhibit a sensitive dependence on the dopant concentration and on the temperature history of the crystal. At low concentrations (≲ 0.01 mole%), only fluorescence lines arising from single Er3+–F− interstitial pairs are observed. As the concentration increases, a large number of new lines are observed. From the explicit dependence of the line intensity on the dopant concentration, the new lines are attributed to the dimerization of the Er3+–F− interstitial pairs. At higher concentrations (> 0.5 mole%), the total observed fluorescence intensity departs dramatically from a linear increase with concentration, indicative of the formation of high‐order Er3+–F− interstitial clusters. The dissociation of the dimeric clusters to single pairs occurs readily at ≳ 1200 K. The present findings help to clarify the origin of the frequently observed ...

Energy transfer upconversion in LaF3:Pr3+

Studies of infrared quantum counter upconversion in LaF3:Pr3+ and LaCl3:Pr3+ have led to the discovery of a process in which a red excitation photon results in the emission of a blue photon. When the 1D2 level of Pr3+ is excited by a cw rhodamine 6‐G dye laser, blue fluorescence from the 3P0 level is observed. From the flux dependence and lifetime of the induced fluorescence, we conclude that the observed phenomenon arises from excitation annihilation involving pairs of ions in the 3H6 and the 1D2 states.

Excited singlet‐state lifetimes of hydrated chlorophyll aggregates

A time‐correlated single photon counting technique, coupled with a red sensitive photomultiplier tube, was used to determine the singlet‐state lifetimes and energy transfer mechanisms of Chl a⋅H2O, (Chl a⋅H2O)2, and (Chl a⋅2H2O)n. The use of low concentrations and low incident light fluxes permitted measurements in a regime where nonlinear quenching effects are absent. The lifetimes exhibit a pronounced dependence on aggregate size. The results are examined in terms of exciton interactions in the excited singlet state of the Chl a aggregate. It is shown that in the absence of exciton annihilation effects, the rate of singlet‐state decay via radiative coupling and intersystem crossing of a Chl a aggregate containing n equivalent monomeric units is equal to n times that of monomeric Chl a.

Molecular symmetry and exciton interaction in photosynthetic primary events

In this paper, the molecular details of the recently proposed energy upconversion theory of photosynthesis are reviewed. The primary light reactions are explained in terms of aC2 symmetrical structure of the reaction center involving a (Chl−H2O)2 adduct. It is shown that exciton interaction within the (Chl−H2O)2 complex leads to an antisymmetric triplet state which may act as an energy trap. The presence of the energy trap in the reaction center suggests that the trigger step for the photoionization of active chlorophylls may involve the summation of two red excitation photons. Under normal conditions, the steadystate one-photon-per-electron quantum requirement is obtained. The functional properties of the various molecular constituents of the Chl-a molecule, such as the Ring V β-ketoester group, the phytyl tail, the central Mg atom, and the π-system of the macrocycle are explained within the present theoretical framework. A detailed analysis is given of the postulates and the consequences of the proposed model. The ramifications of the theory are probed, and their biological consequences are suggested for future study.

Laser optical double resonance and efficient infrared quantum counter upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+

In this paper, we report the first laser‐excited optical double‐resonance experiments in rare‐earth‐doped crystals. Infrared upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+ has been achieved by means of a tunable cw dye laser pump. The quantum efficiency of the upconversion process at 2.03 μ in LaCl3 : Pr3+ is 1.2×10−3, which is an improvement of 4×105 over the highest efficiency found previously with incoherent sources. It is shown that an efficiency of 1 can be easily obtained with higher fluxes and improved crystal quality. Direct excitation of the output fluorescence from the 3P0 level is observed in both LaCl3 : Pr3+ and LaF3 : Pr3+ when the dye laser frequency is resonant with the states of the lower‐lying 1D2 manifold in the absence of the infrared input. Detailed assignments of the observed double‐resonance transitions have been made from wavelength and polarization measurements.