A. C. Albrecht

Polarizations and assignments of transitions: The method of photoselection

Abstract An outline is given of the theory which underlies the concept of the polarization of an electronic transition. Nuclear coordinate dependent electronic functions are employed to bring out the significance of “mixed polarization” in an electronic absorption or emission band. Brief attention is given to various techniques for obtaining polarizations of transitions. A detailed discussion is reserved for the method here called photoselection. This method includes, for example, polarized emission studies and polarized photochemistry. The theory underlying the method is outlined and critically discussed. A table of ideal orientation ratios for a variety of conditions is presented. Practical application of the theory is examined and it is shown how (and under what conditions) it is possible to relate gross observations to intrinsic molecular anisotropy of absorption (or emission). Two specific examples are analyzed, one involving polarized emission, the other polarized photochemistry. Finally, it is shown how when polarization of phosphorescence and fluorescence measurements are made together, additional information can be provided with regard to anisotropy in absorption which is not available when only one of the two types of measurements is made. Comments regarding the variety of applications of photoselection are offered.

On the Dependence of Vibrational Raman Intensity on the Wavelength of Incident Light

Experiments on the wavelength dependence across the visible region of the intensity of the 1334‐cm−1 Raman line of p‐nitroaniline (PNA) are analyzed from a general theoretical viewpoint. When theory is simplified to correspond to the PNA example, two obvious sources of Raman intensity are apparent. One is based on a possible shift of equilibrium nuclear position along the 1334‐cm−1 mode upon exciting PNA into its first excited singlet state (in the near ultraviolet). The other source deals with vibronic coupling of the first excited electronic state with a whole family of states in the deep ultraviolet having similar polarizations. It is seen how this second mechanism appears to be the dominant one in this case. The absolute scattering cross section for the 1334 cm−1 is given and is found to agree with the order of magnitude estimates calculated theoretically.

C–H vibrational states of benzene, naphthalene, and anthracene in the visible region by thermal lensing spectroscopy and the local mode model

Visible (red) absorption spectra of benzene (h6 and d6), methyl substituted benzenes, naphthalene, and anthracene are presented as seen by thermal lensing spectroscopy. It is shown how this highly sensitive laser‐based technique can easily detect transitions having absorption coefficients as small as 10−6 cm−1. The observed transitions are principally due to C–H stretching vibrations at the sixth quantum of excitation. The local mode (LM) model for describing such molecular states is explored in some detail with reference to the C–H excitations in benzene. Based on a two parameter empirical fit, the LM model can give a true C–H stretch ’’mechanical’’ frequency; it successfully predicts isotope effects; and it fully determines the energy level scheme at any quantum of excitation. The LM energy level scheme for the 462 C–H stretching states at the sixth quantum of excitation is presented. A majority of these states are only mildly anharmonic; one state in fact is a perfect harmonic of the fundamental. Howev...

Vibronic—Spin‐Orbit Perturbations and the Assignment of the Lowest Triplet State of Benzene

Second‐order perturbation theory in electron configuration and spin space using both vibronic and spin‐orbit perturbations reveals a number of routes for bringing dipole allowed character into a spin‐forbidden transition. Apart from direct spin‐orbit interaction, there are three additional mechanisms: (1) direct spin‐vibronic coupling (a very weak, though first‐order term), (2) spin‐orbit coupling with vibronic coupling in the singlet manifold, and (3) vibronic coupling in the triplet manifold with spin‐orbit coupling. In the case of benzene, vibrational analysis alone of the phosphorescence spectrum can neither discern the route for bringing allowed singlet character into the triplet state nor can it lead to an assignment of that state. It happens that polarized luminescence data are able to show that route (3) is dominant, and, given this, vibrational analysis leads to a 3B1u assignment. The vibronic characteristics of the various routes are discussed. Estimates are made of the relative importance of th...

Photovoltaic effects of metal–chlorophyll‐a–metal sandwich cells

The microcrystalline chlorophyll−a film prepared by the method of electrodeposition is shown to have strong photovoltaic effects. The photovoltaic cell (M1‖Chl‖M2) has lamellar arrangement with the Chl−a film sandwiched between two metal electrodes. With dissimilar electrodes of different work functions, φM, the cell usually exhibits a dark rectification behavior. A large forward bias current is seen when the metal with a lower φM is the negative electrode. The rectification is small when the same metal is used as both electrodes. A blocking contact or Schottky barrier is evidently present at the Chl−a‖metal junction particularly fo the metal having a low φM. A p−type semiconduction in Chl−a is implicated. In the photovoltaic mode, cells such as (Al‖Chl‖Hg) and (Al‖Chl‖Au) have an open circuit voltage ranging from 200−500 mV. The power conversion efficiency for these cells is on the order of 10−3% which is among the highest in photovoltaic cells using organic materials. The direction of the photovoltaic c...

Femtosecond energy-transfer processes in the B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1

The B800-to-B850 energy transfer time in the purified B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1 is determined to be 0.7 ps at room temperature. The electronic state dynamics of the principal carotenoid of this species, spheroidene, are examined, both in vivo and in vitro, by direct femtosecond time-resolved experiments and by fluorescence emission yield studies. Evidence is presented which suggests that carotenoid-to-bacteriochlorophyll energy transfer may occur directly from the initially excited carotenoid S2 state, as well as from the carotenoid S1 state. Further support for this conjecture is obtained from calculations of energy transfer rates from the carotenoid S2 state. Previous measurements of in vivo carotenoid and B800 dynamics are discussed in light of the new results, and currently unresolved issues are described.

Depletion layer studies in organic films: Low frequency capacitance measurements in polycrystalline tetracene

The study of the electrical properties of (metal 1‖ organic ‖metal 2) cells has not led to a consistent model of their origin. On the one hand, conductivity measurements suggest that many organic materials behave as insulators (low carrier concentrations) while photovoltaic properties of some of these same materials may be rationalized using the concepts of semiconductor physics, suggesting that the organic film has a high carrier concentration. In this paper the voltage dependence of the capacitance of a (Al‖tetracene‖Au or Nesatron) cell is investigated in an attempt to better understand this contradiction. The electrical properties of this cell are explained in terms of a model in which depletion layers are made from a high density of immobile trapped charge. The trapped charge can be mobilized by light and then evidence is found for a Schottky depletion region (∼2000 A thick) in the tetracene film.

Determination of the S2 lifetime of β-carotene

Abstract The S2 lifetime of β-carotene has been determined by both femtosecond transient absorption experiments and emission yield studies. For β-carotene dissolved in ethanol and in CS2, the initially excited S2 was found to live for 200 to 250 fs at room temperature. Implications of this result for carotenoid-to-chlorophyll energy transfer in photosynthetic systems are discussed.

Vibronic Calculations in Benzene

A group‐theoretical reduction incorporating normal‐coordinate parameters is applied to obtain expressions for the oscillator strengths of forbidden B2u, B1u, and E2g transitions in benzene in terms of the minimum number of electronic integrals. The simplification obtained encourages the testing of wave functions within the Herzberg‐Teller framework as well as empirical evaluation of the theory. An investigation is made of the possible effect of isotopic substitution on forbidden intensities. The group‐theoretical reduction is carried out for 1,4 dideuterobenzene, sym‐benzene‐d3, 2,3,5,6‐tetradeuterobenzene, benzene‐d6, and a hypothetical molecule p‐di‐X‐benzene (where (X) = 20 and C—X = 1.40 A). It is observed that the B2u and B1u transitions are very insensitive to mass effects independent of the nature of the electronic functions. On the other hand, E2g transitions may or may not show a significant sensitivity depending on the details of the wave functions. By way of illustration, oscillator strengths a...

Advances in calculating Raman excitation profiles by means of the transform theory

Powerful transform methods for calculating Raman excitation profiles from the resonant absorption spectrum have recently been introduced. In the present work, previous results are extended to include both Condon and non‐Condon sources of scattering and individual transform laws are presented for a number of commonly encountered symmetry conditions. In contrast to time correlator based methods, here the Kramers–Heisenberg polarizability expression, itself, serves as the starting point. Furthermore, the adiabatic approximation is limited to only one dimension, that of the scattered vibration. The remaining space of the resonant molecular eigenstates is formally free of many of the encumbering effects which find expression within the usual application of the adiabatic approximation in 3N‐6 dimensions. This leads to a set of approximations which formally differ from those previously identified with the transform method. Their nature is less restrictive and encourages the view that the transform method is quit...