Time dependence of Bragg forward scattering and self-seeding of hard x-ray free-electron lasers

Free-electron lasers (FELs) can now generate temporally short, high power x-ray pulses of unprecedented brightness, even though their longitudinal coherence is relatively poor. The longitudinal coherence can be potentially improved by employing narrow bandwidth x-ray crystal optics, in which case one must also understand how the crystal affects the field profile in time and space. We frame the dynamical theory of x-ray diffraction as a set of coupled waves in order to derive analytic expressions for the spatiotemporal response of Bragg scattering from temporally short incident pulses. We compute the profiles of both the reflected and forward scattered x-ray pulses, showing that the time delay of the wave $\tau$ is linked to its transverse spatial shift $\Delta x$ through the simple relationship $\Delta x = c\tau \cot\theta$, where $\theta$ is the grazing angle of incidence to the diffracting planes. Finally, we apply our findings to obtain an analytic description of Bragg forward scattering relevant to monochromatically seed hard x-ray FELs.Comment: 11 pages, 6 figure

Protein folding rates correlate with heterogeneity of folding mechanism

By observing trends in the folding kinetics of experimental 2-state proteins at their transition midpoints, and by observing trends in the barrier heights of numerous simulations of coarse grained, C-alpha model, Go proteins, we show that folding rates correlate with the degree of heterogeneity in the formation of native contacts. Statistically significant correlations are observed between folding rates and measures of heterogeneity inherent in the native topology, as well as between rates and the variance in the distribution of either experimentally measured or simulated phi-values.Comment: 11 pages, 3 figures, 1 tabl

Establishment of norms in specific areas of the visual field for critical fusion frequency as determined by a modified stroboscope

Establishment of norms in specific areas of the visual field for critical fusion frequency as determined by a modified stroboscop

Cardiovascular emotional dampening and blood pressure: Is there a relationship between diastolic blood pressure and risk-taking behavior?

Persons with higher blood pressure have emotional dampening, a reduced response to emotionally meaningful stimuli. Dampening of perceived threat could influence risk-related decision-making. The present study examined the relationship between cardiovascular emotional dampening and risk behavior. We measured resting diastolic blood pressure (DBP), emotion recognition, and risk-taking behavior in 44 healthy, self-identified males and 44 females with an average age of 22.5 years. Participants judged the type of emotion depicted in faces and sentences as an index of emotion recognition. We measured risk with a modified Youth Risk Behavior Survey. Results showed a significant correlation between risk-taking behaviors and resting DBP [r(88) = .408, p \u3c.001.]. Sex was also correlated with resting DBP, [r(88) = -.230, p = .031]; however, a multiple regression showed no main effects or interactions with sex in prediction of risk. These results indicate that young adults with higher DBP report more risk-taking behavior and show less accuracy in recognition of emotion in faces, suggesting that cardiovascular emotional dampening may reduce threat appraisal and increase high-risk behavior. The effect of blood pressure on risk behavior could contribute to increased disease risk later in life

Effective Hamiltonian for Excitons with Spin Degrees of Freedom

Starting from the conventional electron-hole Hamiltonian ${\cal H}_{eh}$, we derive an effective Hamiltonian $\tilde{\cal H}_{1s}$ for $1s$ excitons with spin degrees of freedom. The Hamiltonian describes optical processes close to the exciton resonance for the case of weak excitation. We show that straightforward bosonization of ${\cal H}_{eh}$ does not give the correct form of $\tilde{\cal H}_{1s}$, which we obtain by a projection onto the subspace spanned by the $1s$ excitons. The resulting relaxation and renormalization terms generate an interaction between excitons with opposite spin. Moreover, exciton-exciton repulsive interaction is greatly reduced by the renormalization. The agreement of the present theory with the experiment supports the validity of the description of a fermionic system by bosonic fields in two dimensions.Comment: 12 pages, no figures, RevTe