Coincident angle-resolved state-selective photoelectron spectroscopy of acetylene molecules: a candidate system for time-resolved dynamics

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range $19...40$ eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the $C_{2}H_{2}^{+}$ cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the $4\pi$-collection geometry of velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.Comment: 12 pages, 7 figures, 5 table

Community mobilization, empowerment and HIV prevention among female sex workers in south India

While community mobilization has been widely endorsed as an important component of HIV prevention among vulnerable populations such as female sex workers (FSWs), there is uncertainty as to the mechanism through which it impacts upon HIV risk. We explored the hypothesis that individual and collective empowerment of FSW is an outcome of community mobilization, and we examined the means through which HIV risk and vulnerability reduction as well as personal and social transformation are achieved

Above-threshold ionization of argon with ultrashort orbital-angular-momentum beams

Light-matter interaction with laser pulses endowed with orbital angular momentum (OAM) raises a fundamental question about the nature of the transfer of this property of light to matter. In this work, a "reaction microscope" is used for precise measurement of the momentum of ionized photoions and photoelectrons from the interaction of Ar atoms with a linearly polarized, ultrashort (similar to 25 fs), moderately intense (10(13)-10(14) W/cm(2)) OAM carrying laser pulses. The angle and energy-resolved photoelectron spectrum is compared with the photoelectron spectrum obtained from the interaction with the laser beams with no OAM at similar intensities. No evidence of angular momentum transfer to the electrons from light is observed in our experiment

Hindered alignment in ultrashort, intense laser-induced fragmentation of O2

Molecules ionized by intense (10–100 TW/cm2) and ultrashort (tens of femtoseconds) laser fields undergo rotation and alignment mediated through their polarizability. The expected alignment is indeed observed in the case of O2 molecules ionized by intense laser pulses of 800 nm wavelength and 25 fs duration, as observed through velocity imaging of the fragments. Strikingly, when 35 fs pulses of 400 nm wavelength of comparable intensity are employed, an anomalous hindering of this alignment is observed. In both cases, we propose dissociation pathways for the energetic ions consistent with the recorded kinetic energy distributions. Using a semiclassical model of induced rotation of the molecular ion that involves polarizabilities of the participating excited states, both behaviors are reproduced. The model suggests that the difference in the observations can be attributed to a transient negative polarizability in an intermediate state of the proposed pathway

Mutations in β′ subunit of Escherichia coli RNA polymerase perturb the activator polymerase functional interaction required for promoter clearance

xi.402 hlm.: - ; 21 cm

Mutations in beta ` subunit of Escherichia coli RNA polymerase perturb the activator polymerase functional interaction required for promoter clearance

P>Transcription activator C employs a unique mechanism to activate mom gene of bacteriophage Mu. The activation process involves, facilitating the recruitment of RNA polymerase (RNAP) by altering the topology of the promoter and enhancing the promoter clearance by reducing the abortive transcription. To understand the basis of this multi-step activation mechanism, we investigated the nature of the physical interaction between C and RNAP during the process. A variety of assays revealed that only DNA-bound C contacts the beta' subunit of RNAP. Consistent to these results, we have also isolated RNAP mutants having mutations in the beta' subunit which were compromised in C-mediated activation. Mutant RNAPs show reduced productive transcription and increased abortive initiation specifically at the C-dependent mom promoter. Positive control (pc) mutants of C, defective in interaction with RNAP, retained the property of recruiting RNAP to the promoter but were unable to enhance promoter clearance. These results strongly suggest that the recruitment of RNAP to the mom promoter does not require physical interaction with C, whereas a contact between the beta' subunit and the activator, and the subsequent allosteric changes in the active site of the enzyme are essential for the enhancement of promoter clearance

Isolation of Biosurfactant-Producing Bacteria and Their Co-Culture Application in Microbial Fuel Cell for Simultaneous Hydrocarbon Degradation and Power Generation

Biosurfactant-producing microorganisms improve the efficacy of hydrocarbon biodegradation as the biosurfactant is essential in making hydrocarbons available for breakdown. The present study reports the isolation of biosurfactant-producing bacteria that can be used for crude oil remediation and to characterize the biosurfactant generated during the breakdown of crude oil. This study also reports evaluating the synergism and potentiality of biosurfactant-producing bacteria for simultaneous hydrocarbon biodegradation and power generation. Two bacterial strains (Bacillus subtilis strain B1 and Pseudomonas aeruginosa strain B2) were isolated from petroleum-contaminated soils, which are found effective in producing biosurfactants and degrading crude oil as the sole carbon source. B. subtilis B1 exhibited a higher potential for biosurfactant production and crude oil degradation than P. aeruginosa B2. The FTIR and GC-MS analysis were conducted for further characterization of the biosurfactant, which revealed that the surfactant produced by strain B1 and B2 was surfactin and rhamnolipid, respectively. The application of the B1 and B2 co-culture in microbial fuel cells (MFCs) showed synergism among them and resulted in a maximum power density production of 6.3 W/m3 with an open circuit voltage of 970 mV while degrading 2.5% v/v crude oil containing anolyte. The findings indicate that the co-culture of isolated crude oil-degrading strains has great potential for enhanced power generation and the bioremediation of hydrocarbon-contaminated environments. Moreover, the synergism of isolated strains in MFCs suggested their potent applicability in environmental, energy, and industrial sectors as an economical and feasible alternative to the existing technologies

Note: An ion imaging spectrometer for studying photo-induced fragmentation in small molecules

A three-dimensional ion imaging spectrometer has been designed and calibrated by ion trajectories simulations. We present a recipe for the verification of the calibration by obtaining kinetic energy (KE) distribution from the recorded flight times alone and consequently correlating the two KE spectra