Probing electron and hole colocalization by resonant four-wave mixing spectroscopy in the extreme ultraviolet.

Extending nonlinear spectroscopic techniques into the x-ray domain promises unique insight into photoexcited charge dynamics, which are of fundamental and applied interest. We report on the observation of a third-order nonlinear process in lithium fluoride (LiF) at a free-electron laser. Exploring the yield of four-wave mixing (FWM) in resonance with transitions to strongly localized core exciton states versus delocalized Bloch states, we find resonant FWM to be a sensitive probe for the degree of charge localization: Substantial sum- and difference-frequency generation is observed exclusively when in a one- or three-photon resonance with a LiF core exciton, with a dipole forbidden transition affecting details of the nonlinear response.

Photon-recoil imaging: Expanding the view of nonlinear x-ray physics.

Addressing the ultrafast coherent evolution of electronic wave functions has long been a goal of nonlinear x-ray physics. A first step toward this goal is the investigation of stimulated x-ray Raman scattering (SXRS) using intense pulses from an x-ray free-electron laser. Earlier SXRS experiments relied on signal amplification during pulse propagation through dense resonant media.

Dissecting Strong-Field Excitation Dynamics with Atomic-Momentum Spectroscopy.

Observation of internal quantum dynamics relies on correlations between the system being observed and the measurement apparatus. We propose using the c.m.

Limit on Excitation and Stabilization of Atoms in Intense Optical Laser Fields.

Atomic excitation in strong optical laser fields has been found to take place even at intensities exceeding saturation. The concomitant acceleration of the atom in the focused laser field has been considered a strong link to, if not proof of, the existence of the so-called Kramers-Henneberger (KH) atom, a bound atomic system in an intense laser field. Recent findings have moved the importance of the KH atom from being purely of theoretical interest toward real world applications; for instance, in the context of laser filamentation.

Neural correlates of instrumental responding in the context of alcohol-related cues index disorder severity and relapse risk.

The influence of Pavlovian conditioned stimuli on ongoing behavior may contribute to explaining how alcohol cues stimulate drug seeking and intake. Using a Pavlovian-instrumental transfer task, we investigated the effects of alcohol-related cues on approach behavior (i.e.

Unified Time and Frequency Picture of Ultrafast Atomic Excitation in Strong Laser Fields.

Excitation and ionization in strong laser fields lies at the heart of such diverse research directions as high-harmonic generation and spectroscopy, laser-induced diffraction imaging, emission of femtosecond electron bunches from nanotips, self-guiding, filamentation and mirrorless lasing during propagation of light in atmospheres. While extensive quantum mechanical and semiclassical calculations on strong-field ionization are well backed by sophisticated experiments, the existing scattered theoretical work aiming at a full quantitative understanding of strong-field excitation lacks experimental confirmation. Here we present experiments on strong-field excitation in both the tunneling and multiphoton regimes and their rigorous interpretation by time dependent Schrödinger equation calculations, which finally consolidates the seemingly opposing strong-field regimes with their complementary pictures.

Strong-field excitation of helium: bound state distribution and spin effects.

Using field ionization combined with the direct detection of excited neutral atoms we measured the distribution of principal quantum number n of excited He Rydberg states after strong-field excitation at laser intensities well in the tunneling regime. Our results confirm theoretical predictions from semiclassical and quantum mechanical calculations and simultaneously underpin the validity of the semiclassical frustrated tunneling ionization model. Moreover, since our experimental detection scheme is spin sensitive in the case of He atoms, we show that strong-field excitation leads to strong population of triplet states.

Strong-field Kapitza-Dirac scattering of neutral atoms.

Laser induced strong-field phenomena in atoms and molecules on the femtosecond (fs) time scale have been almost exclusively investigated with traveling wave fields. In almost all cases, approximation of the strong electromagnetic field by an electric field purely oscillating in time suffices to describe experimental observations. Spatially dependent electromagnetic fields, as they occur in a standing light wave, allow for strong energy and momentum transfer and are expected to extend strong-field dynamics profoundly.

Observing Rydberg atoms to survive intense laser fields.

The idea of atoms defying ionization in ultrastrong laser fields has fascinated physicists for the last three decades. In contrast to extensive theoretical work on atoms stabilized in strong fields only few experiments limited to intermediate intensities have been performed. In this work we show exceptional stability of Rydberg atoms in strong laser fields extending the range of observation to much higher intensities.

Acceleration of neutral atoms in strong short-pulse laser fields.

A charged particle exposed to an oscillating electric field experiences a force proportional to the cycle-averaged intensity gradient. This so-called ponderomotive force plays a major part in a variety of physical situations such as Paul traps for charged particles, electron diffraction in strong (standing) laser fields (the Kapitza-Dirac effect) and laser-based particle acceleration. Comparably weak forces on neutral atoms in inhomogeneous light fields may arise from the dynamical polarization of an atom; these are physically similar to the cycle-averaged forces.

Strong laser field fragmentation of H2: Coulomb explosion without double ionization.

We observe fragmentation of H2 molecules exposed to strong laser fields into excited neutral atoms. The measured excited neutral fragment spectrum resembles the ionic fragmentation spectrum including peaks due to bond softening and Coulomb explosion. To explain the occurrence of excited neutral fragments and their high kinetic energy, we argue that the recently investigated phenomenon of frustrated tunnel ionization is also at work in the neutralization of H+ ions into excited H atoms.

Strong-field tunneling without ionization.

In the tunneling regime of strong laser field ionization we measure a substantial fraction of neutral atoms surviving the laser pulse in excited states. The measured excited neutral atom yield extends over several orders of magnitude as a function of laser intensity. Our findings are compatible with the strong-field tunneling-plus-rescattering model, confirming the existence of a widely unexplored neutral exit channel (frustrated tunneling ionization).

Effects of rivastigmine on actigraphically monitored motor activity in severe agitation related to Alzheimer's disease: a placebo-controlled pilot study.

Acetylcholinesterase inhibitors (AChEIs) are effective in the treatment of cognitive symptoms in Alzheimer's disease (AD). Because the behavioral and psychological symptoms of dementia (BPSD) have also been attributed to central cholinergic deficits, we examined whether the AChEI rivastigmine can reduce motor activity as measured in a rater-independent manner by wrist actigraphy in agitated AD patients. A total of 20 consecutive AD inpatients (13 females, 7 males, 80.

Delta-9-tetrahydrocannabinol for nighttime agitation in severe dementia.

Rationale: Nighttime agitation occurs frequently in patients with dementia and represents the number one burden on caregivers today. Current treatment options are few and limited due to substantial side effects.

Objectives: The aim of the study was to measure the effect of the cannabinoid dronabinol on nocturnal motor activity.

Core relaxation in atomic ultrastrong laser field ionization.

We have investigated atomic ionization dynamics in Kr in the transition regime from nonrelativistic to relativistic laser intensities (10(16) to 10(18) W/cm2) by measuring yields of highly charged ions stemming from an inner shell. Interpretation of the data is focused on the applicability of the single active electron description, which requires fully relaxed core states between successive ionization steps. In particular, we are concerned with transient core polarization or alignment effects originating from the strong dependence of the ionization rates on the magnetic quantum number.

Fano line shapes reconsidered: symmetric photoionization peaks from pure continuum excitation.

In a photoionization spectrum in which there is no excitation of the discrete states, but only the underlying continuum, we have observed resonances which appear as symmetric peaks, not the commonly expected window resonances. Furthermore, since the excitation to the unperturbed continuum vanishes, the cross section expected from Fano's configuration interaction theory is identically zero. This shortcoming is removed by the explicit introduction of the phase shifted continuum, which demonstrates that the shape of a resonance, by itself, provides no information about the relative excitation amplitudes to the discrete state and the continuum.

Association between response to lithium augmentation and the combined DEX/CRH test in major depressive disorder.

Although lithium augmentation is the foremost and most well-documented treatment strategy for treatment resistant depression, knowledge of factors related to response remains scanty. Findings with the combined dexamethasone/corticotropin-releasing hormone (DEX/CRH) test are associated with response to treatment with a tricyclic antidepressant. This study investigated the potential predictive value of the DEX/CRH test for lithium augmentation response in major depressive disorder.

Hypothalamic-pituitary-thyroid system activity during lithium augmentation therapy in patients with unipolar major depression.

Objective: Lithium augmentation is an established strategy in the treatment of refractory depression, but little is known about predictors of response and its mode of action. There is increasing evidence that low thyroid function indices within the normal range are associated with a poorer treatment response to antidepressants, but previous studies on the hypothalamic-pituitary-thyroid (HPT) system during lithium augmentation provide inconclusive results and have methodological limitations. This study aimed at exploring the role of thyroid function in lithium augmentation and used a prospective design that included a homogeneous sample of inpatients with unipolar major depressive disorder.

Lithium augmentation increases post-dexamethasone cortisol in the dexamethasone suppression test in unipolar major depression.

Although considerable evidence exists on the efficacy of lithium as an augmenting agent in refractory depression, the underlying neurobiology of this phenomenon is unknown. In patients with major depression, changes of the hypothalamic-pituitary-adrenocortical (HPA) system have been detected by means of the dexamethasone suppression test (DST), when administered during treatment with tricyclic antidepressants. We investigated whether the DST also reveals alterations of the HPA system during lithium augmentation.

Lithium augmentation increases the ACTH and cortisol response in the combined DEX/CRH test in unipolar major depression.

Lithium augmentation is a well established strategy for treatment-resistant depression. The exact mode of its action is unknown, but an enhancement of serotonergic transmission is hypothesized. The authors investigated changes in the hypothalamic-pituitary-adrenocortical (HPA) system during lithium augmentation and their correlation to clinical response by means of the combined dexamethasone/CRH test (DEX/CRH test).

Collective multielectron tunneling ionization in strong fields.

We investigate the quantum mechanical process of two-electron tunneling in strong external electric fields. Numerical solution of a two-electron s-wave model reveals the existence of collective tunneling ionization in a mode where both electrons stay at equal distance from the nucleus. Otherwise the lagging electron is immediately recaptured.