Dynamical phase shift in x ray absorption and ionization spectra by two delayed x ray laser fields

We study theoretically x ray absorption and ionization spectra of an atom or molecule by two coherent x ray pulses that show a relative phase shift resulting in a time delay of the pulse envelopes. We demonstrate that the phase modulation of the spectra is shifted with respect to the phase oscillation comb of the x ray double pulse. The reason for this shift is the dynamics of the process defined by the interplay of the delay time, the pulse width, the detuning, and the lifetime of the core excited stat

High resolution x ray spectra of carbon monoxide reveal ultrafast dynamics induced by long UV pulse

In theoretical simulations of a UV x ray pump probe UVX PP setup, we show that frequency detuning of the pump UV pulse acts as a camera shutter by regulating the duration of the UVX PP process. This two photon absorption with long overlapping UV and x ray pulses, allowing for high spectral resolution, thereby provides information about ultrafast dynamics of the nuclear wave packet without the requirement of ultrashort pulses and controlled delay times. In a case study of carbon monoxide, the calculated UVX PP spectra of the O1s amp; 8722;12 amp; 960;1 and C1s amp; 8722;12 amp; 960;1 core excited states show different vibrational profiles. The interference of intermediate vibrational states reveals details of nuclear dynamics in the UVX PP process related to a variable duration time controlled by the UV detuning. Both O1s amp; 8722;12 amp; 960;1 and C1s amp; 8722;12 amp; 960;1 pump probe channels display a splitting of the spectral profile, which however is associated with different physical mechanisms. At the O1s amp; 8722;12 amp; 960;1 resonance, the observed dispersive and non dispersive spectral bands intersect and result in destructive interferenc

Time resolved study of recoil induced rotation by X ray pump X ray probe spectroscopy

Modern stationary X ray spectroscopy is unable to resolve rotational structure. In the present paper, we propose to use time resolved two color X ray pump probe spectroscopy with picosecond resolution for real time monitoring of the rotational dynamics induced by the recoil effect. The proposed technique consists of two steps. The first short pump X ray pulse ionizes the valence electron, which transfers angular momentum to the molecule. The second time delayed short probe X ray pulse resonantly excites a 1s electron to the created valence hole. Due to the recoil induced angular momentum the molecule rotates and changes the orientation of transition dipole moment of core excitation with respect to the transition dipole moment of the valence ionization, which results in a temporal modulation of the probe X ray absorption as a function of the delay time between the pulses. We developed an accurate theory of the X ray pump probe spectroscopy of the recoil induced rotation and study how the energy of the photoelectron and thermal dephasing affect the structure of the time dependent X ray absorption using the CO molecule as a case study. We also discuss the feasibility of experimental observation of our theoretical findings, opening new perspectives in studies of molecular rotational dynamic

Acute effects of nicotine on visual search tasks in young adult smokers

Rationale Nicotine is known to improve performance on tests involving sustained attention and recent research suggests that nicotine may also improve performance on tests involving the strategic allocation of attention and working memory. Objectives We used measures of accuracy and response latency combined with eye-tracking techniques to examine the effects of nicotine on visual search tasks. Methods In experiment 1 smokers and non-smokers performed pop-out and serial search tasks. In experiment 2, we used a within-subject design and a more demanding search task for multiple targets. In both studies, 2-h abstinent smokers were asked to smoke one of their own cigarettes between baseline and tests. Results In experiment 1, pop-out search times were faster after nicotine, without a loss in accuracy. Similar effects were observed for serial searches, but these were significant only at a trend level. In experiment 2, nicotine facilitated a strategic change in eye movements resulting in a higher proportion of fixations on target letters. If the cigarette was smoked on the first trial (when the task was novel), nicotine additionally reduced the total number of fixations and refixations on all letters in the display. Conclusions Nicotine improves visual search performance by speeding up search time and enabling a better focus of attention on task relevant items. This appears to reflect more efficient inhibition of eye movements towards task irrelevant stimuli, and better active maintenance of task goals. When the task is novel, and therefore more difficult, nicotine lessens the need to refixate previously seen letters, suggesting an improvement in working memory

Young's double-slit experiment using two-center core-level photoemission: Photoelectron recoil effects

Core-level photoemission from N-2 can be considered an analogue of Young's double-slit experiment (YDSE) in which the double-slit is replaced by a pair of N 1s orbitals. The measured ratio between the 1 sigma(g) and 1 sigma(u) photoionization cross-sections oscillates as a function of photoelectron momentum, due to two-center YDSE interference, exhibiting a remarkable dependence on the vibrational sub-levels of the core ionized state. We theoretically demonstrate that the recoil of the photoelectron given to the ionized N atom strongly influences this interference pattern. The reason for this is that the momentum transfer affects the phases of the photoionization amplitudes. (c) 2007 Published by Elsevier B.