Probing dipole-forbidden autoionizing states by isolated attosecond pulses

We propose a general technique to retrieve the information of dipole-forbidden resonances in the autoionizing region. In the simulation, a helium atom is pumped by an isolated attosecond pulse in the extreme ultraviolet (EUV) combined with a few-femtosecond laser pulse. The excited wave packet consists of the $^1S$, $^1P$, and $^1D$ states, including the background continua, near the $2s2p(^1P)$ doubly excited state. The resultant electron spectra with various laser intensities and time delays between the EUV and laser pulses are obtained by a multilevel model and an ab initio time-dependent Schr\"odinger equation calculation. By taking the ab initio calculation as a "virtual measurement", the dipole-forbidden resonances are characterized by the multilevel model. We found that in contrast to the common assumption, the nonresonant coupling between the continua plays a significant role in the time-delayed electron spectra, which shows the correlation effect between photoelectrons before they leave the core. This technique takes the advantages of ultrashort pulses uniquely and would be a timely test for the current attosecond technology.Comment: 10 pages, 6 figure

New Methods To Assess Protein Folding And Conformational Dynamics

A protein’s folding and conformational energy landscape depends on a large number of molecular degrees of freedom and interactions. As a result, different proteins can follow different sequences of events moving toward the native state along the course of folding. For example, the underlying structural organization and ordering can occur locally first and then globally, or vice versa. In addition, the associated conformational transitions can take place over a wide range of timescales. Because of these complexities, arriving at a detailed assessment and understanding of the folding dynamics and mechanism of any protein via a single type of experiment is challenging, and sometimes impossible. As such, over the past two decades, many different experimental methods have been employed to study how proteins fold among which, the laser-induced temperature-jump (T-jump) technique has emerged as a powerful tool to measure protein folding kinetics occurring on the nanosecond and microsecond timescales. Herein, we further expand the utility of the T-jump technique. First, we introduce a new form of the T-jump technique (referred to as VIPT-jump) that can be used to distinguish between different folding mechanisms. Second, we apply the VIPT-jump concept to better understand the folding dynamics of an alanine-based -helix, and, in conjunction with theoretical modeling, we are able to determine the long-sought microscopic rate constants of the helical nucleation and propagation processes. Third, we develop a new method to extend the time window of observation in a T-jump experiment to the millisecond timescale. In a parallel effort, we demonstrate that quenching the fluorescence of a dye molecule by a tryptophan residue via photoinduced electron transfer mechanism can be used to interrogate the conformational dynamics of proteins that are crucial for function. Applying this method to the M2 proton channel of the Influenza A virus allow us to determine, for the first time, the gating dynamics of the tryptophan tetrad in this membrane protein

Against the grain: the battle for public service broadcasting in Taiwan

Over the last two decades public service broadcasting (PSB) around the world has faced increasing pressures from accelerating commercialisation and the fragmentation of the broadcasting landscape. This has led a number of media commentators in the system’s traditional heartlands to ask whether the idea has now outlived its usefulness. Against the grain of this international trend, Taiwan has moved in the opposite direction, democratising its state-owned television system and introducing a form of public broadcasting for the first time. Against the grain of growing enthusiasm for a privatised solution supporters presented PSB as a necessary counter to the perceived deficiencies of the existing system, in serving a society moving from authoritarian to competitive party rule. This study sets out to explore how the expansion of PSB in Taiwan has been socially defined and constructed, and by whom. The various constructions in play were mapped through in-depth interviews with a range of claim-makers involved in the process. A systematic content analysis of the mainstream Taiwanese press was then conducted to explore the ways contending positions and issues were presented in the public domain and to identify the key voices given a public platform. This analysis demonstrated that the opinions and concerns of the general public were largely missing from a debate dominated by political and academic elites. Against the grain of their own claims to be representing the public key actors constructed public debate as a series of monologues, advancing their own sectional and paternalistic interpretations of the public interest. These findings point to the supremely ironic conclusion that a process ostensibly dedicated to reconstructing broadcasting as key element in a new, democratic, public sphere, excluded the public from active participation and relegated them to the role of spectators