Bruno Fanconi

Quantum Theory of the Intensities of Molecular Vibrational Spectra

Formulas for the transition probabilities and hence the absolute intensities of molecular vibrational spectra are obtained from a unified quantum field treatment. The theory of infrared, Raman, and hyper‐Raman spectroscopy of molecular vibrations is developed by assuming these processes occur as time‐ordered steps involving the creation or destruction of one quantum of vibrational energy and changes in the occupation number of one, two, or three photons, respectively. The formulas obtained by this method for ir transitions become equivalent to the earlier treatment of Jones and Simpson if the energy difference of the ground and first excited electronic energy levels are very large relative to that of the vibrational quantum. The formulas obtained for Raman transitions are very similar to those obtained by the method originated by Albrecht and developed further by Savin; we get not only the original terms of Albrecht but also the trace terms obtained by Savin. Furthermore by using third‐order time‐dependen...

Raman Spectra and the Phonon Dispersion of Polyglycine

The Raman spectra of the two crystalline modifications of polyglycine, I and II, and of N‐deuterated polyglycine II have been recorded. The results of a normal‐coordinate analysis of polyglycine II and N‐deuterated polyglycine II are presented along with the phonon dispersion curves of polyglycine II. Assignments are made on a number of bands not observed in the infrared. Some previous band assignments are found to be inconsistent with the Raman data.

Polarized Laser Raman Studies of Biological Polymers

A group‐theoretical analysis of the Raman tensor for helical polymers is given and illustrated for the Pauling α helix which possesses 18 residues in five turns. A detailed determination of the polarized Raman spectra is given for oriented α‐helical poly‐l‐alanine fibers and for four polyribonucleotides in solution. Tentative assignments to A, E1, and E2 symmetry are made on certain Raman bands of poly‐l‐alanine based on the polarization of the Raman light. A detailed comparison of monomer and polymer bands are given for the four polynucleotides. A theory for the angular dependence of Raman light scattered from helical molecules is given.

Influence of Exciton Phonon Interaction on Metallic Reflection from Molecular Crystals

Abstract Reflection spectra of a number of polymethine dyes are presented and analysed. Characteristic features of these spectra appear to be outside the framework of the Lorentz-Lorenz theory. An alternate approach based on an appropriate linear response function for surface excitation is developed. The linear response at a particular frequency involves a sum over all k-dependent Greens functions. Damping is brought in through the exciton-phonon interaction. A model Hamiltonian is introduced which can be diagonalized exactly for the single molecule. For the crystal, this permits the treatment of single phonon effects in the strong coupling limit almost as if they were multiple phonon effects. Structure, mainly associated with damping, in the reflection spectrum is expected and found in those regions of the exciton dispersion curve where the density of momentum states is large. Reasonably good agreement is found between the theoretical and experimental reflection curves.

Angular Dependence of Raman Scattering Intensity

The angular distribution of the intensity of Raman scattered light from a sample of randomly oriented molecules is calculated for all Raman active vibrational transitions for the common molecular point groups. Experiments of Damen, Leite, and Porto are presented.

Hyperpolarizability of Helical Polymer Molecules and the Sampling of the ir‐Active Optical Phonon Dispersion Curves

The selection rules for the inelastic three‐photon scattering of light by helical molecules is obtained from group theoretical analysis. It is shown that the dispersion curve (frequency vs wave vector) for ir‐active optical phonons may be sampled at four points throughout the Brillouin zone by measuring the frequency of the scattered light as a function of the polarization of both the incident (laser) and the scattered light provided that proper orientation of the laser light relative to the helix axis is maintained.

Characterization of Biological Polymers by Laser Raman Scattering

From Raman scattering we can measure certain vibrational frequencies of biological polymers, many of which cannot be obtained from infrared spectroscopy because of selection rules, intensity differences, or interference from solvent bands. The frequencies can be used to determine force fields of the polymers, and various spectral features can be used to perform structural studies on the polymers.