Robert T. Ross

Efficiency of hot‐carrier solar energy converters

A single‐threshold quantum‐utilizing device in which the excited carriers thermally equilibrate among themselves, but not with the environment, can convert solar energy with an efficiency approaching that of an infinite‐threshold device. Such a hot‐carrier flat‐plate device operated under typical terrestrial conditions (AM 1.5 illumination, 300 K) can convert solar energy with an efficiency of 66%, substantially exceeding the 33% maximum efficiency of a quantum device operating at thermal equilibrium, and the 52% maximum efficiency of an ideal thermal conversion device. This high efficiency is achieved in part through an unusual inversion, in which the chemical potential of the excited electronic band is below that of the ground band. This negative potential difference reduces radiation losses, permitting a low threshold energy, and a high Carnot efficiency resulting from a high carrier temperature.

Some Thermodynamics of Photochemical Systems

A limit on the thermodynamic potential difference between the ground and excited states of any photochemical system is established by evaluating the potential difference at which the rate of photon absorption and emission are equal; the relationship between absorption and emission is given by a Planck‐law relation, provided that there is thermal equilibrium between the sublevels of each electronic band. The actual potential developed may be evaluated if the quantum yield of luminescence is known. The maximum amount of power storage obtainable is evaluated by lowering the potential difference until the product of the potential difference and the fraction of the quanta retained is maximized. The history and applications of the Planck‐law relation between absorption and emission spectra are discussed briefly, and applications of the potential difference calculation are mentioned.

Thermodynamics of Light Emission and Free-Energy Storage in Photosynthesis

A Planck law relationship between absorption and emission spectra is used to compute the fluorescence spectra of some photosynthetic systems from their absorption spectra. Calculated luminescence spectra of purple bacteria agree well but not perfectly with published experimental spectra. Application of the Planck law relation to published activation spectra for Systems I and II of spinach chloroplasts permits independent calculation of the luminescence spectra of the two systems; if the luminescence yield of System I is taken to be one-third the yield of System II, then the combined luminescence spectrum closely fits published experimental measurement.Consideration of the entropy associated with the excited state of the absorbing molecules is used to compute the oxidation-reduction potentials and maximum free-energy storage resulting from light absorption. Spinach chloroplasts under an illumination of 1 klux of white light can produce at most a potential difference of 1.32 ev for System I, and 1.36 ev for System II. In the absence of nonradiative losses, the maximum amount of free energy stored is 1.19 ev and 1.23 ev per photon absorbed for Systems I and II, respectively. The bacterium Chromatium under an illumination of 1 mw/cm(2) of Na D radiation can produce at most a potential difference of 0.90 ev; the maximum amount of free energy stored is 0.79 ev per photon absorbed.The combined effect of partial thermodynamic reversibility and a finite trapping rate on the amount of luminescence is considered briefly.

Limits on the yield of photochemical solar energy conversion

Entropy and unavoidable irreversibility place a limit on the efficiency of photochemical solar energy conversion which is substantially lower than that placed by the first law of thermodynamics alone. Shockley and Queisser’s ’’detailed balance limit’’ on the efficiency of p‐n‐junction photovoltaic devices is a special case of this general thermodynamic limit on the efficiency of all quantum‐utilizing solar energy converters. For a single photochemical system operating at 20 °C in sunlight not attenuated by the atmosphere, this efficiency cannot exceed 29%. Under the same conditions, the efficiency of a solar converter composed of two photochemical systems can reach 41%.

Thermodynamic Limitations on the Conversion of Radiant Energy into Work

The fraction of radiant energy incident on an absorber which may appear as work is limited by the radiation entropy and entropy gained in irreversible transfer from the radiation field to an absorber. Irreversibility may result from directionality of the radiation field, and some irreversibility is necessary to cause a net flow of energy from a radiation absorber into work or free‐energy storage. Impedance in the conversion apparatus may further limit the efficiency. Maximization of power storage under these constraints is discussed, and the general arguments are then applied to photoelectrical and photochemical systems; in these systems nonresonant decay of the excited state represents a major source of inefficiency, which may be minimized by appropriate choices for the Boltzmann temperature and optical density of the absorber. The relationships are developed for narrow‐band absorption, and application to broad‐band systems is discussed only briefly.

Dipolar Broadening of EPR Spectra Due to Solute Segregation in Frozen Aqueous Solutions

EPR spectra of Mn2+ and Gd3+ in a number of frozen aqueous solutions show that ice crystallization causes extensive segregation of solutes. The concentration of paramagnetic ions thus induced results in dipolar broadening of EPR spectra, and published spectra of frozen aqueous Mn2+ are reinterpreted accordingly. This segregation effect may be reduced by the addition of quantities of experimentally inert solute.

[27] Component resolution using multilinear models

Publisher Summary This chapter focuses on the application of trilinear models in fluorescence spectroscopy. However, these trilinear models are also applicable to other kinds of excited-state spectroscopy, such as transient absorption spectroscopy. Along the way, this chapter also discusses bilinear and other models, including global analysis. When the dependence of the spectroscopic intensity from every chromophore on at least one experimental variable can be described by a highly specific mathematical function, then the approach known as global analysis is preferred. When this condition is not known to be met, but spectroscopic intensity is separately linear in functions of two or more experimental variables, then the multilinear models described in this chapter are valuable. The field of nonlinear regression, of which multilinear models are one part, is an area of statistics under rapid development, the results of which are likely to improve the utility of multilinear methods in the future.

RESOLUTION OF THE FLUORESCENCE SPECTRA OF PLANT PIGMENT-COMPLEXES USING TRILINEAR MODELS

The intensity of fluorescence from a pigment is separately linear in functions of excitation wavelength, emission wavelength, and any chemical treatment which alters overall fluorescence yield. This multiple linearity permits the use of an extension of principal components analysis to resolve overlapping spectra without the use of any additional information. The method is used to resolve the spectra of pigment complexes in pea thylakoids, using the concentration of Mg 2+ as the chemical treatment variable. Two components could be resolved accurately. One has little or no dependence on Mg 2+ ; the other, with an ecitation spectrum resembling LHC II, has a dependence on Mg 2+ which follows the Hill equation with a binding constant of 0.4–0.6 mM and a Hill coefficient of 2.4–3.1.

RADIATIVE LIFETIME AND THERMODYNAMIC POTENTIAL OF EXCITED STATES

Abstract— To clarify their similarities and common assumptions, a unified derivation is presented of the methods of evaluating the radiative lifetime of an electronically‐excited state by integration of the absorption spectrum or of the fluorescence spectrum. Previous examinations of the accuracy of these methods are briefly reviewed. The radiative lifetime of eleven aromatic molecules is computed by the fluorescence spectrum method, and compared with published lifetimes determined experimentally and by integration of the absorption spectrum; from this it is concluded that the fluorescence spectrum method is accurate to about 25 per cent, and that the common assumption of equal partition functions for the ground and excited states is often not true. Extension of the derivation of formulas for the radiative lifetime shows that the standard chemical potential of the excited state may be determined from the experimental radiative lifetime and appropriately scaled integration of the fluorescence spectrum.

Resolution of component spectra for spinach chloroplasts and green algae by means of factor analysis

Abstract Using a multivariate analysis method known as factor analysis, we have studied two distinct components in the room temperature fluorescence of broken spinach chloroplasts and of two species of algae. This method provides excitation and emission spectra of the components without physical separation and with a minimum of assumptions about the shape of the spectra. In spinach chloroplasts, both components had excitation maxima at 680 nm. Component 1, containing mostly Photosystem I, had a stronger far-red absorbance, and had an emission maximum at 683 nm with a very pronounced shoulder from 710 to 750 nm. Component 2, containing mostly Photosystem II, had a slightly stronger absorbance at 650 nm. Its emission maximum was at 682 nm with a weak band from 710 to 750 nm, similar to those of isolated Photosystem II particles. The spectra from Chlorella pyrenoidosa and Scenedesmus quadricauda were different from those of spinach. Component 1 in algae had an excitation maximum of 680 nm and an emission maximum of 690 nm, without the strong 710–750 nm shoulder. Algal component 2 had peak excitation at 670 nm and peak emission at 685 nm. In both spinach chloroplasts and the algae, the integrated fluorescence from component 1 was 20–40% of the total. The Stepanov temperature of component 2 was within 10 K of the ambient temperature, providing strong evidence for good equilibration within and between the excited states of Photosystem II. The Stepanov temperature of component 1 was less well defined, with a median value of 330 K or more, suggesting a lack of equilibration in Photosystem I. Thus, calculations of free-energy yield based on an assumption of excited-state equilibrium should be valid for Photosystem II, but may not be valid for Photosystem I.