Resonance Raman spectra of twenty parasubstituted azobenzene derivatives are compared, enabling a more complete assignment of bands to be made. Since all the spectra are rather similar, the various ring substituents are not resonance Raman active, and all the observed bands can be assigned to vibrational modes of the aromatic rings and to the azo group itself. Frequency shifts in the PhN stretching region are opposite to those expected based on delocalization along the PhNNPh moiety. From the complexity of band pattern around the NN stretching region and from the above described frequency shifts, several types of vibrational coupling are evident. On the basis of the band assignments, some useful conclusions can be reached for interpreting the observed spectral changes when azobenzene derivatives are used as chromophore probes in biochemical system studies.

Resonance Raman spectra of azobenzene and some hydroxyazobenzene derivatives, in strong acid solution (HCI 6 M), are analysed. All the observed bands between 900 and 1700 em-' can be assigned to vibrational modes of the aromatic rings and the azo group, and give evidence for a trans azo type structure both for azobenzene and the para substituted hydroxyazobenzene derivatives studied. Resonance Raman spectra of six substituted 2-(4'-hydroxypheny1azo)-benzoic acid derivatives in strong acid solutions are investigated. The spectral features are rather different from the above studied hydroxyazobenzene derivatives, but can be explained on the basis of an azo type structure. A strong perturbation effect on the N=N stretching mode is demonstrated using deuterium substitution. From the complexity of frequency shifts obtained on deuterium substitution, vibrational coupling between NH bending and many other vibrational modes is evident. A tentative assignment of the observed resonance Raman bands is proposed.

The resonance Raman spectra of aminoazobenzene derivatives in acidic solution are investigated. The marked spectral changes observed on decreasing pH are related to transformations between azo and quinoid forms. The pK, value of 4-amidino, 4'-dimethylaminoazobenzene obtained from intensity measurements is in good agreement with the value obtained from visible absorption measurements. Comparison of resonance Raman spectra of some substituted aminoazobenzene derivatives and the quinoid form of an hydroxyazobenzene derivative enable a more complete assignment of the bands observed between 900 and 1700 cm-' to be made. All the spectra are rather similar and the various ring substituents are not resonance Raman active. The influence of the -NH-N= vibration on resonance Raman spectra of the quinoid forms is demonstrated using deuterium substitution. In strong acid solution a second protonation occurs; a tautomeric equilibrium between azo and quinoid forms of aminoazobenzene derivatives is demonstrated using different exciting laser lines.

in the generation of short-lived free radical cations which have strong visible absorptions. These have been used for the excitation of resonance-enhanced Raman spectra. The DMOB+' radicals were produced ia a stopped-flow apparatus linked with an optical multichannel Raman instrument. The DABCO" species was produced by rapid mixing and its spectrum determined by rapid scanning of a conventional spectrometer. Both kinetic data and vibrational spectra are produced by these methods.

We report the realization of very small area resonant tunneling diodes (RTD's) for integrated microwave applications. Diodes with areas down to 3×3 μm2 were fabricated with the necessary coplanar lines for microwave wafer probing. Peak to valley ratios up to 3.5 with peak current densities ranging from 0.6 A/cm2 to 1.5 kA/cm2 have been achieved for Al0.4Ga0.6 As/GaAS double barrier heterostructures grown by MBE on Semi-Insulating substractes. Reflection gain versus frequency measurements in the 1.5-26.5 GHz range, at room temperature, were performed. The smaller devices exhibit a maximum cut off frequency for the negative differential resistance of 18 GHz. For high current versions, we expect significant increase in frequency limit

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