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Abstract— Resonance Raman (RR) bands assignable to the 21Ag excited state of ß‐carotene are recorded using picosecond time‐resolved resonance Raman (PTR3) spectroscopy. The RR spectrum contains bands in both the C‐C (1204 cm−1, 1243 cm−1, and 1282 cm−1) and C=C (1777 cm−1) stretching regions. The time‐dependent intensities of these RR features, measured with ˜ 30 ps. resolution, are found (i) to closely correlate with picosecond transient absorption (PTA) data recorded at 575 nm on the same sample and (ii) inversely correlate with the time‐dependent intensities of RR bands assigned to the 11Ag ground state. Both of these observations support the assignment of these four RR features to the 21Ag excited state. These results remove uncertainties associated with earlier experiments in which excited‐state RR scattering from (3‐carotene was not observed in spite of predicted trends emanating from studies of shorter polyene compounds. The observed C=C band position also agrees with a recent report of this feature.

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con-containing reactive intermediate present in the gas phase in CVD systems with SiH,Cl2 as a source gas, formed in our experiments by heterogeneous decomposition of SiH2CI2 over the decomposition film, while SiHCl is present as a minor species. Since identical results were obtained with quartz and graphite pyrolyzers and results were independent of deposition time, we are confident that similar results would be obtained from silicon surfaces.It is interesting to note from the mass spectra in Figures 4 and 5 that, at temperatures where the precursor chlorosilane is completely decomposed, the relative concentrations of HCI and SiCl, are nearly the same for dichlorosilane and trichlorosilane. The ratio of SiCl,/HCI is higher in the trichlorosilane case, presumably because there is more chlorine carried to the surface by this feed gas. This is in agreement with previous optical and mass spectrometric studies which showed that the gas-phase species present in CVD systems were independent of the chlorosilane source gas! Gilbert and Ban,4 using mass spectrometry, showed that the gas-phase species found in CVD reactions were independent of the feed gas and that their relative concentrations depended only on the temperature and CI/H ratio of the feed gas. The IR multiphoton dissociation results of Sausa and Ronn showed that only H2 and SiCI,, and not HCI, are produced by the homogeneous gas-phase decomposition of dichlorosilane.' The results in this study show that the HCI observed in previous studies of CVD systems using chlorosilanes as source gases is probably generated by heterogeneous surface decomposition of the chlorosilanes, followed by surface reactions. Speculation Regarding Heterogeneous Decomposition Pathways of ChlorosilanesThe results presented in this study show that SiCI, and HCI desorb from silicon surfaces deposited on both quartz and graphite by the heterogeneous decomposition of SiH2C12 and SiHCI, above 600 and 800 "C, respectively. These species are likely to be important in the removal of excess chlorine and hydrogen from the growing polycrystalline thin silicon film. Under conditions, where the dichlorosilane decomposes on the surface and not above it, species 1 and I1 in Scheme I would be likely to form by dis-Spectra for electrons trapped in TiO, have been reported. In this study, kinetic analysis of processes taking place when TiO, colloids are flashed in the presence of three dyes leads to assignment of the spectrum of a trapped hole. Within the duration of a 20-ps pulse at 350 nm, a transient is formed in Ti02 which decays with a second-order rate constant of 2.4 X IO-'' ne s-I, where n, is the number of electrons. The absorbance is probably attributable to electrons in the "conduction band" (a term that must be used cautiously for these very amorphous systems), and the rate constant measures the rate of hole-electron recombination. Upon addition of a dye that may scavenge carriers, a new transient grows with a rate constant of 5 X IO* s-'. This feature, with an absorption maximum at 630 nm...

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Thin film gas sensors are discussed which are produced from polycrystalline organic dyes, such as the phthalocyanines, perylenes, etc. or from metal oxides, such as SnO2, ZnO, etc. Both types of materials are typically used to detect gas phase analytes by virtue of the surface conductance changes which accompany chemisorption of the analyte. In the case of the thin film oxides this process is understood in terms of the change in surface charge density and band bending which accompanies chemisorption of electron donors or acceptors. Defects in the oxide structure, or intentionally added impurities in the near surface region, can enhance the charge density at the surface and provide specific chemisorption sites for various analytes, which are the main means of introducing selectivity to these sensors. In the case of the organic dye thin films, hopping processes, rather than movement of charge through valence and conduction bands, control the flow of charge. Significantly higher defect and impurity densities are anticipated, versus the metal oxide thin films, and movement of charge through trap sites accounts for a significant fraction of the charge transfered. Illumination of certain organic thin films can enhance the sensitivity of the sensor, by a factor of up to 104, depending upon the ratio of dark to photoconductivity in the solid. Addition of electron or hole traps in the near surface region of the thin film can introduce chemisorption sites which interact more selectively with the analyte, and further increase sensitivity. Examples of this approach are given, for the detection of NH3 and NO2, using selected photoconductive phthalocyanine thin films.

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