By using an evaporated silver film that has a continuously varying thickness, we have demonstrated the dependence of surface-enhanced Raman scattering (SERS) on the dielectric properties of the film. These results support local field models that are based on metal-particle resonances for SERS.

Cyclotron resonance of a two-dimensional electron gas at a GaAs/GaAlAs interface is measured in the far infrared at intensities of up to 10 kW/cm2. Both the cyclotron mass and the carrier density are independent of intensity but the relaxation time changes with a dependence which is similar to that observed earlier in high dc field mobility studies.

A new Raman-gain medium suitable for use throughout the infrared spectral region is described. Hollow silica fibers with liquid-bromine cores are shown to have very large Raman-gain coefficients (gamma=1.7x10(-8)cm/W at 1.1 microm with a Raman shift of 312 cm(-1)). The liquid-bromine core, being a homonuclear diatomic material, has no first-order infrared absorption, and transparency between 1 and 16 microm has been verified. Broadband infrared radiationbetween 1.1 and 4 microm was generated by stimulated Raman scattering in bromine-core fibers pumped with 1.06- and 2.11-microm radiation.

The 35Cl n.q.r. frequencies of eleven sulphinyl and sulphenyl chlorides, nine substituted alkyl sulphonyl chlorides, five halo and oxysulphonyl chlorides, four N-substituted N-sulphonyl chlorides, ten halobenzene sulphonyl chlorides, six methylbenzene and p-methoxybenzene sulphonyl chlorides, six nitrobenzene sulphonyI chlorides and seven carboxy and sulphone substituted benzene sulphonyl chlorides are reported for 77 K. The frequencies are interpreted in terms of the sulphur orbital electronegativity, the inductive effects of substituents and possible dz orbitals on sulphur. The most coherent explanation is that the -SO2group behaves like the pseduo-united atom germanium. EXPERIMENTALThe samples were all obtained conmercially, except where noted, and were purified by distillation or sublimation under an atmosphere of nitrogen. The 35Cl n.q.r. frequencies were measured using two modified Dean-type super-regenerative oscillators, with different quench rates,2 externally quenched with a ~ine-wave,~ with sine-wave frequency modulation. The frequency was scanned by mechanical drive of a variable tuning condenser. A spectrum analyzer (Tetronix Pentrix L20) was loosely coupled to the oscillator to ensure that the output amplitude was distributed over as few sidebands as possible, and hence select the most suitable quench rates, and to select the oscillator fundamental. The oscilloscope display was fed to a lock-in amplifier phase sensitive detector (PAR 121) with pre-filtering. The detected frequency on the recorder was related to a peak on the spectrum analyzer, and the signal from a loosely coupled BC221M frequency meter was superimposed on the spectrum analyzer peak to measure it pre~isely.~ The accuracy was 5100 Hz for a line width less than 5 kHz.The frequencies were measured at liquid-nitrogen temperatures, 77.0 f0.5 K4 Samples were measured repeatedly over several months to ensure that the frequencies measured immediately after crystallization or cooling to 77 K were not those of a meta-stable phase, and to find if molecules which did not initially give frequencies would do so after annealing. RESULTS A N D DISCUSSIONThe 35 C1 n.q.r. frequencies of the S-C1 bond for various formal valence states of sulphur range from 29.5 to 42.9 MHz from C2H5SOCl to SCl; respectively.6 This range is slightly less than that for the C-C1 bond which ranges from 28.3 to 43.6 MHz for (CH3)3CCOCl and C12C(CN)2 respectively. Since sulphur exists in several valence states, unlike carbon, it might be expected to display a greater range of 35Cl n.q.r. frequencies for the SCl bond than carbon does in the CC1 bond. Apparently sulphur is a poorer electronic transmitter than carbon, and resembles phosphorus in the PC1 bond.4* 5 * 8 * 9 9 l o