We relate transitions in galaxy structure and gas content to refueling, here defined to include both the external gas accretion and the internal gas processing needed to renew reservoirs for star formation. We analyze two z = 0 data sets: a high-quality ∼200-galaxy sample (the Nearby Field Galaxy Survey, data release herein) and a volume-limited ∼3000-galaxy sample with reprocessed archival data. Both reach down to baryonic masses ∼10 9 M ⊙ and span void-to-cluster environments. Two mass-dependent transitions are evident: (i) below the "gas-richness threshold scale (V ∼ 125 km s −1 ), gas-dominated quasi-bulgeless Sd-Im galaxies become numerically dominant, while (ii) above the "bimodality scale (V ∼ 200 km s −1 ), gas-starved E/S0s become the norm. Notwithstanding these transitions, galaxy mass (or V as its proxy) is a poor predictor of gas-to-stellar mass ratio M gas /M * . Instead, M gas /M * correlates well with the ratio of a galaxys stellar mass formed in the last Gyr to its preexisting stellar mass, such that the two ratios have numerically similar values. This striking correspondence between past-averaged star formation and current gas richness implies routine refueling of star-forming galaxies on Gyr timescales. We argue that this refueling underlies the tight M gas /M * vs. color correlations often used to measure "photometric gas fractions. Furthermore, the threshold and bimodality scale transitions reflect mass-dependent demographic shifts between three refueling regimes -accretion dominated, processing dominated, and quenched. In this picture, gas-dominated dwarfs are explained not by inefficient star formation but by overwhelming gas accretion, which fuels stellar mass doubling in 1 Gyr. Moreover, moderately gas-rich bulged disks such as the Milky Way are transitional, becoming abundant only in the narrow range between the threshold and bimodality scales.

The observed Raman spectra for single crystals of rubrene and tetracene are compared with the calculated spectra for the isolated molecules. The Raman measurements presented are of the bulk properties of the material, and they confirmed that the vapour growth process yields very pure, unstrained rubrene crystals. Finally, Raman measurements indicate that rubrene, unlike many other oligoacenes, has very weak intermolecular coupling and no observable intermolecular Raman vibrational modes. We discuss the apparent conflict between the high mobility and the weak π-electron overlap in this material.

The structure changes of thin films of amorphous ͑a͒ to microcrystalline (c) silicon are studied by Raman scattering in terms of three deposition parameters: the silane flow rate, the hydrogen flow rate, and the total gas pressure in hot-wire chemical vapor deposition. The Raman transverse optical ͑TO͒ mode is deconvoluted into two Gaussian functions for a-Si:H and intermediate components and one Lorenzian function for the c-Si component. We found that ͑a͒ in general, the change in structure is a function of the ratio of hydrogen to silane gas flow, R, but also depends on the SiH 4 flow rate and total gas pressure; ͑b͒ there is a narrow structural transition region in which the short-range order of the a-Si:H network improves, i.e., the variation in bond angle of the a-Si network decreases from ϳ10°to ϳ8°once the c-Si grains start to grow; and ͑c͒ when the films were deposited using a high SiH 4 flow rate of 22 sccm, the narrow TO mode with low peak frequency could be related to the column-like structures.

Resonant Raman measurements of ␣-hexathiophene at 33 K have identified over ten vibrational modes that couple to electronic excitations in the single crystal. These excitations at 2.066 eV and 2.068 eV with a width of 2meV have been tentatively identified as bound singlet Frenkel excitons or a triplet Frenkel exciton.

ABSTRACTa-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.