Raman Phonon Spectra of Isotopic Mixed Naphthalene Crystals: Librational Exciton Model and the Amalgamation Limit

We present an experimental Raman study of lattice modes in neat and mixed (10%–90%) crystals of naphthalene‐h8 and naphthalene‐d8 at 100°K with 1 cm−1 resolution. The spectral features of the neat crystals are preserved in the heavily doped mixed crystals, with small shifts and broadenings characteristic of an amalgamation limit that assumes weakly coupled excitation bands in the restricted Frenkel‐Davydov limit. Rotation‐translation interaction does not affect the mixed crystal spectra, thus making the Raman technique uniquely suited for the investigation of the librational (rotational) phonon band structure. The evidence is against localized or pseudolocalized phonons in these isotopic mixed crystals.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70356/2/JCPSA6-57-2-863-1.pd

Temperature dependence of a vibrational exciton: Some methyl motions of durene

An intramolecular vibrational frequency in a molecular crystal has been found to show a significant temperature dependence. The low lying intramolecular mode at 272 cm−1 (room temperature value) of durene‐h14 (and the corresponding durene‐d14 frequency at 242 cm−1) shows an increase of [invertedlazys]9 cm−1[invertedlazys]9cm−1 in frequency in going from room temperature to 100°K. A theoretical discussion is presented on the role of various interactions that may give rise to the temperature dependence of the intramolecular mode. The theoretical considerations suggest that, although the thermal expansion of the lattice may be contributing to this temperature dependence, the dominant contribution is due to the cubic anharmonic interactions involving methyl torsions. It is suggested that in complex molecular crystals, where there are some very low lying intramolecular modes, the distinction between lattice modes and intramolecular modes based on the criterion that only lattice modes show frequency shift with temperature may be misleading. In such cases our mixed crystal criterion that the phonons obey the amalgamation limit in isotopic (H�D)(H�D) mixed crystals, whereas intramolecular vibrations are in the separated band limit, should be more reliable. This is clearly demonstrated in the case of durene.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70780/2/JCPSA6-58-11-5031-1.pd

Magnetically modulated optical nanoprobes

We have developed magnetically modulated optical nanoprobes (MagMOONs) to magnetically modulate the signal from fluorescent probes and thus separate it from autofluorescence, electronic offsets, and other background signals. These micro- and nanosized particles emit fluorescence signals, indicating chemical concentrations, and blink in response to rotating magnetic fields. Demodulating the signal dramatically enhances the probe’s signal to background ratio. The probes and methods promise to improve immunoassays, intracellular chemical sensing, and fundamental biochemical research. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69653/2/APPLAB-82-7-1102-1.pd

Aspherical magnetically modulated optical nanoprobes (MagMOONs)

Aspherical magnetic particles orient in a magnetic field due to magnetic shape anisotropy. They also emit different fluxes of light from their different geometric faces due to self-absorption and total internal reflection within the particles. The particles rotate in response to rotating magnetic fields and appear to blink as they rotate. We have made pancake and chain shaped particles and magnetically modulated their fluorescent intensities. Demodulating the signal extracts the probe fluorescence from electronic and optical backgrounds dramatically increasing signal to noise ratios. The probes have applications in sensitive and rapid immunoassays, improved intracellular sensors, and inexpensive single molecule analysis. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70021/2/JAPIAU-93-10-6698-1.pd

Brownian modulated optical nanoprobes

Brownian modulated optical nanoprobes (Brownian MOONs) are fluorescent micro- and nanoparticles that resemble moons: one hemisphere emits a bright fluorescent signal, while an opaque metal darkens the other hemisphere. Brownian motion causes the particles to tumble and blink erratically as they rotate literally through the phases of the moon. The fluctuating probe signals are separated from optical and electronic backgrounds using principal components analysis or images analysis. Brownian MOONs enable microrheological measurements on size scales and timescales that are difficult to study with other methods. Local chemical concentrations can be measured simultaneously, using spectral characteristics of indicator dyes embedded within the MOONs. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70496/2/APPLAB-84-1-154-1.pd

Phonon sidebands of localized excitons in molecular crystals with methyl torsions: Hexamethylbenzene

Fluorescence and phosphorescence phonon sidebands of isotopic mixed hexamethylbenzene crystals at 2°K are presented. The external phonons can be observed separately from the semi‐internal (methyl torsion) ones. The nature of the electronic or vibronic state has observable but not drastic effects on the exciton‐phonon coupling function. Likewise, the exciton delocalization is of minor importance to the exciton‐phonon function in hexamethylbenzene. The coupling between external and internal vibrations is also weak in this system. The exciton‐phonon coupling appears comparable for the optical and acoustic phonons in hexamethylbenzene. The phonon sidebands give some of the phonon singularities of the low‐temperature crystal.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70173/2/JCPSA6-60-6-2365-1.pd

Triplet exciton percolation and superexchange: Naphthalene C10H8–C10D8

The phosphorescence of betamethylnaphthalene doped into a naphthalene−h8/naphthalene−d8 mixed crystal has been measured. The results demonstrate that (1) dynamical exciton percolation does occur (i.e., a transition from an exciton insulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70872/2/JCPSA6-62-1-292-1.pd

Phonon Raman spectra, molecular motions, and phase transitions of dimethylacetylene crystal

The phonon Raman spectra of crystals of dimethylacetylene, perdeutero-dimethylacetylene and their mixtures have been investigated down to 20[deg]K. The high temperature phase shows only one line and the low temperature phase only two. Neither of these is related to the Rz (methyl) rotation, which seems to remain practically free. However, the latter motion is found to be restricted in a newly discovered metastable phase, which is reported here first, displaying 5-6 phonon bands. The well-known [lambda]-point transition appears little related to methyl rotations. The internal rotation of dimethylacetylene seems to stay practically free down to 20[deg]K in the stable phases.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33846/1/0000104.pd

Spectroscopic evidence for a continuous change in molecular and crystal structure: deformation of biphenyl in the low temperature solid

A Raman temperature study of biphenyl-h10 and biphenyl-d10 crystals reveals an unusual spectral change over a wide temperature region. Many internal and external infrared-active modes appear gradually as the temperature is lowered from about 75 to 15[deg]K. We interpret this as a mild and gradual change in both crystal and molecular structure: a loss of center of inversion, probably involving the partial return of the molecule to its non-planar shape (D2) in the vapor phase.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22435/1/0000886.pd