The Photophysics Of Polycyclic Aromatic Hydrocarbons Adsorbed On Silica Gel Surfaces: A Lifetime Distribution Analysis Approach

The relaxation process of excited molecules, in homogeneous systems, is routinely studied through the measurements of fluorescence lifetimes and fluorescence quantum yields. From these data, the radiative decay rate, and hence the non-radiative decay rate, can be easily derived. Effort has been made, in the present work, to conduct such a fundamental study with molecules adsorbed on silica gel surfaces.;The photophysics of six polycyclic aromatic hydrocarbons (PAH), comprising phenanthrene, chrysene, pyrene, perylene, benzoperylene and coronene, adsorbed on variously dehydroxylated silica gel surfaces, has been investigated in this work. It has been observed that the fluorescence lifetimes of these adsorbed PAHs are dispersed into distributions, and that the recovery of these distributions is highly unstable, known as an ill-conditioned problem. In this thesis is discussed, in detail, the problem of the instability of lifetime distribution analysis. The regularization method of Phillips is used to obtain stable solutions. Based on these studies, a bimodal lifetime distribution, which represents the PAH molecules adsorbed on two types of surface sites, is suggested to describe the photophysical behaviour of the PAHs on the silica gel surfaces.;The fluorescence quantum yields of these PAHs have been measured by a diffuse reflectance method and also by an integrating sphere method. These results are then used, in conjunction with the measured lifetime distributions, to derive the radiative decay rates and the non-radiative decay rates for the PAH molecules adsorbed on each type of surface sites.;The so obtained lifetime distributions and quantum yields, as well as the derived radiative and non-radiative decay rates, seem to be physically plausible and self-consistent, which suggests that the techniques used are justifiable. The photophysics of the PAH adsorbed on silica gel surfaces is then discussed on the basis of these data

Empirical extinction coefficients for the GALEX, SDSS, 2MASS and WISE passbands

Using the "standard pair" technique of paring stars of almost nil and high extinction but otherwise of almost identical stellar parameters from the SDSS, and combing the SDSS, GALEX, 2MASS and WISE photometry ranging from the far UV to the mid-IR, we have measured dust reddening in the FUV-NUV, NUV-u, u-g, g-r, r-i, i-z, z-J, J-H, H-Ks, Ks-W1 and W1-W2 colors for thousands of Galactic stars. The measurements, together with the E(B-V) values given by Schlegel et al. (1998), allow us to derive the observed, model-free reddening coefficients for those colors. The results are compared with previous measurements and the predictions of a variety of Galactic reddening laws. We find that 1) The dust reddening map of Schlegel et al. (1998) over-estimates E(B-V) by about 14 per cent, consistent with the recent work of Schlafly et al. (2010) and Schlafly & Finkbeiner (2011); 2) All the new reddening coefficients, except those for NUV-u and u-g, prefer the R(V) = 3.1 Fitzpatrick reddening law rather than the R(V) = 3.1 CCM and O'Donnell (O'Donnell 1994) reddening laws. Using the Ks-band extinction coefficient predicted by the R(V) = 3.1 Fitzpatrick law and the observed reddening coefficients, we have deduced new extinction coefficients for the FUV, NUV, u, g, r, i, z, J, H, W1 and W2 passbands. We recommend that the new reddening and extinction coefficients should be used in the future and an update of the Fitzpatrick reddening law in the UV is probably necessary. We stress however that the FUV- and NUV-band coefficients should be used with caution given their relatively large measurement uncertainties. Finally, potential applications of the "standard pair" technique with the LAMOST Galactic surveys are discussed.Comment: 13 pages, 9 figures, accepted to MNRA

Potential Application of the CRISPR/Cas9 System against Herpesvirus Infections.

The CRISPR/Cas9 system has been applied in the genome editing and disruption of latent infections for herpesviruses such as the herpes simplex virus, Epstein⁻Barr virus, cytomegalovirus, and Kaposi's sarcoma-associated herpesvirus. CRISPR/Cas9-directed mutagenesis can introduce similar types of mutations to the viral genome as can bacterial artificial chromosome recombination engineering, which maintains and reconstitutes the viral genome successfully. The cleavage mediated by CRISPR/Cas9 enables the manipulation of disease-associated viral strains with unprecedented efficiency and precision. Additionally, current therapies for herpesvirus productive and latent infections are limited in efficacy and cannot eradicate viruses. CRISPR/Cas9 is potentially adapted for antiviral treatment by specifically targeting viral genomes during latent infections. This review, which focuses on recently published progress, suggests that the CRISPR/Cas9 system is not only a useful tool for basic virology research, but also a promising strategy for the control and prevention of herpesvirus latent infections

A Submillimeter Burst of S255IR~SMA1 - The Rise And Fall Of Its Luminosity

Temporal photometric variations at near infrared to submillimeter wavelengths have been found in low-mass young stellar objects. These phenomena are generally interpreted as accretion events of star-disk systems with varying accretion rates. There is growing evidence suggesting that similar luminosity flaring also occurs in high-mass star/cluster-forming regions. We report in this Letter the rise and fall of the 900 ${\mu}$m continuum emission and the newly found 349.1 GHz methanol maser emission in the massive star forming region S255IR~SMA1 observed with the Submillimeter Array and the Atacama Large Millimeter/submillimeter Array. The level of flux variation at a factor of $\sim$ 2 at the submillimeter band and the relatively short 2-year duration of this burst suggest that the event is probably similar to those milder and more frequent minor bursts seen in 3D numerical simulations.Comment: 8 pages, 4 figures, Accepted for publication in The Astrophysical Journal Lette

First Abundance Measurement of Organic Molecules in the Atmosphere of HH 212 Protostellar Disk

HH 212 is one of the well-studied protostellar systems, showing the first vertically resolved disk with a warm atmosphere around the central protostar. Here we report a detection of 9 organic molecules (including newly detected ketene, formic acid, deuterated acetonitrile, methyl formate, and ethanol) in the disk atmosphere, confirming that the disk atmosphere is, for HH 212, the chemically rich component, identified before at a lower resolution as a "hot-corino". More importantly, we report the first systematic survey and abundance measurement of organic molecules in the disk atmosphere within $\sim$ 40 au of the central protostar. The relative abundances of these molecules are similar to those in the hot corinos around other protostars and in Comet Lovejoy. These molecules can be either (i) originally formed on icy grains and then desorbed into gas phase or (ii) quickly formed in the gas phase using simpler species ejected from the dust mantles. The abundances and spatial distributions of the molecules provide strong constraints on models of their formation and transport in star formation. These molecules are expected to form even more complex organic molecules needed for life and deeper observations are needed to find them.Comment: 12 pages, 4 figure