Structural dynamics around a hydrogen bond: Investigating the effect of hydrogen bond strengths on the excited state dynamics of carboxylic acid dimers.

The photochemical dynamics of the acetic acid and trifluoro-acetic acid dimers in hexane are studied using time-resolved infrared absorption spectroscopy and ab initio electronic structure calculations. The different hydrogen bond strengths of the two systems lead to changes in the character of the accessed excited states and in the timescales of the initial structural rearrangement that define the early time dynamics following UV excitation. The much stronger hydrogen bonding in the acetic acid dimer stabilizes the system against dissociation. Ground state recovery is mediated by a structural buckling around the hydrogen bond itself with no evidence for excited state proton transfer processes that are usually considered to drive ultrafast relaxation processes in hydrogen bonded systems. The buckling of the ring leads to relaxation through two conical intersections and the eventual reformation of the electronic and vibrational ground states on a few picosecond timescale. In trifluoro-acetic acid, the weaker hydrogen bonding interaction means that the dimer dissociates under similar irradiation conditions. The surrounding solvent cage restricts the full separation of the monomer components, meaning that the dimer is reformed and returns to the ground state structure via a similar buckled structure but over a much longer, ∼100 ps, timescale.

Electron correlation driven non-adiabatic relaxation in molecules excited by an ultrashort extreme ultraviolet pulse.

The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Ultrashort extreme ultraviolet pulses offer a means to reveal molecular dynamics at ultrashort timescales. Here, we report the use of time-resolved electron-momentum imaging combined with extreme ultraviolet attosecond pulses to study highly excited organic molecules. We measure relaxation timescales that increase with the state energy. High-level quantum calculations show these dynamics are intrinsic to the time-dependent many-body molecular wavefunction, in which multi-electronic and non-Born-Oppenheimer effects are fully entangled. Hints of coherent vibronic dynamics, which persist despite the molecular complexity and high-energy excitation, are also observed. These results offer opportunities to understand the molecular dynamics of highly excited species involved in radiation damage and astrochemistry, and the role of quantum mechanical effects in these contexts.

Influence of long-range Coulomb interaction in velocity map imaging.

The standard velocity-map imaging (VMI) analysis relies on the simple approximation that the residual Coulomb field experienced by the photoelectron ejected from a neutral or ion system may be neglected. Under this almost universal approximation, the photoelectrons follow ballistic (parabolic) trajectories in the externally applied electric field, and the recorded image may be considered as a 2D projection of the initial photoelectron velocity distribution. There are, however, several circumstances where this approximation is not justified and the influence of long-range forces must absolutely be taken into account for the interpretation and analysis of the recorded images. The aim of this paper is to illustrate this influence by discussing two different situations involving isolated atoms or molecules where the analysis of experimental images cannot be performed without considering long-range Coulomb interactions. The first situation occurs when slow (meV) photoelectrons are photoionized from a neutral system and strongly interact with the attractive Coulomb potential of the residual ion. The result of this interaction is the formation of a more complex structure in the image, as well as the appearance of an intense glory at the center of the image. The second situation, observed also at low energy, occurs in the photodetachment from a multiply charged anion and it is characterized by the presence of a long-range repulsive potential. Then, while the standard VMI approximation is still valid, the very specific features exhibited by the recorded images can be explained only by taking into consideration tunnel detachment through the repulsive Coulomb barrier.

Shaping of ultraviolet femtosecond laser pulses by Fourier domain harmonic generation.

We present a method to finely tailor ultraviolet femtosecond laser pulses using a pulse shaper with ability in the infrared/visible spectral range. We have developed to that end a frequency doubling module in which the up-conversion mechanism is carried out in the Fourier plane of a 4 f -line. The pulse shaper is used to imprint a spectral phase and/or amplitude onto the fundamental pulse. The shaped pulse is then frequency doubled through the module which transfers the applied spectral shaping to the second harmonic field in a predictable manner. The relevance of the method is demonstrated by synthesizing and characterizing shaped pulses at a central wavelength of 400 nm. The results demonstrate a full control over the spectral phase and amplitude of the harmonic field. The experimental setup is simple and features interesting prospects for the polarization shaping of ultraviolet pulses and the production of shaped ultraviolet pulses requested for the seeding of free-electron lasers.

Orientation and alignment echoes.

We present one of the simplest classical systems featuring the echo phenomenon-a collection of randomly oriented free rotors with dispersed rotational velocities. Following excitation by a pair of time-delayed impulsive kicks, the mean orientation or alignment of the ensemble exhibits multiple echoes and fractional echoes. We elucidate the mechanism of the echo formation by the kick-induced filamentation of phase space, and provide the first experimental demonstration of classical alignment echoes in a thermal gas of CO_{2} molecules excited by a pair of femtosecond laser pulses.

Polarization shaping for unidirectional rotational motion of molecules.

Control of the orientation of the angular momentum of linear molecules is demonstrated by means of laser polarization shaping. For this purpose, we combine two orthogonally polarized and partially time-overlapped femtosecond laser pulses so as to produce a spinning linear polarization which in turn induces unidirectional rotation of N2 molecules. The evolution of the rotational response is probed by a third laser beam that can be either linearly or circularly polarized. The physical observable is the frequency shift imparted to the probe beam as a manifestation of the angular Doppler effect. Our experimental results are confirmed by theoretical computations, which allow one to gain a deep physical insight into the laser-molecule interaction.

Formation of molecular halide ions from alkyl-halide clusters irradiated by ps and fs laser pulses.

We report on the interaction of alkyl-halide clusters with 35 ps and 20 fs laser pulses at λ = 266, 532, and 1064 nm and 400 and 800 nm, respectively. Particularly, we examine by means of time-of-flight mass spectrometry the intracluster photochemical processes, which give rise to the formation of molecular halogen ions. The efficiency of molecular halogen ion formation is found to depend strongly on the laser wavelength and pulse duration. The ionization/excitation schemes involve in both cases the multiphoton absorption by the clusters and the combined action of the laser and the intracluster electric field. Intracluster energy transfer processes seem to have a significant contribution to the molecular halogen ion formation in the ps domain, while in the fs region, this is probably facilitated by a rescattering process and/or by photon absorption. A physical mechanism for the interpretation of our experimental results is proposed.

The ejection of triatomic molecular hydrogen ions H(3) (+) produced by the interaction of benzene molecules with ultrafast laser pulses.

The ejection process of triatomic molecular hydrogen ions produced by the interaction of benzene with ultrafast laser pulses of moderate strong intensity ( approximately 10(14) W/cm(2)) is studied by means of TOF mass spectrometry. The H(3) (+) formation can only take place through the rupture of two C-H bonds and the migration of hydrogen atoms within the molecular structure. The H(3) (+) fragments are released with high kinetic energy (typically 2-8 eV) and at laser intensities >or=10(14) W/cm(2), well above that required for the double ionization of benzene, suggesting that its formation is taking place within multiply charged parent ions. The relative ejection efficiency of H(3) (+) molecular hydrogen ions with respect to the atomic ones is found to be strongly decreasing as a function of the laser intensity and pulse duration (67-25 fs). It is concluded that the H(3) (+) formation is only feasible within parent molecular precursors of relatively low charged states and before significant elongation of their structure takes place, while the higher multiply charged molecular ions preferentially dissociate into H(+) ions. The ejection of H(2) (+) ions is also discussed in comparison to the production of H(3) (+) and H(+) ions. Finally, by recording the mass spectra of two deuterium label isotopes of benzene (1,2-C(6)H(4)D(2), 1,4-C(6)H(4)D(2)) it is verified that the ejection efficiency of some molecular fragments, such as D(2)H(+), DH(+), is dependent on the specific position of hydrogen atoms in the molecular skeleton prior dissociation.

Acute renal failure in patients over 80 years old: 25-years' experience.

OBJECTIVE: To determine the epidemiological trends, spectrum of etiologies, morbidity and mortality of acute renal failure (ARF) in patients over 80 years old. DESIGN: Historical cohort analysis. SETTING: Intensive care unit (ICU) of nephrology, Tenon Hospital, Paris. PATIENTS AND PARTICIPANTS: The criteria of inclusion was ARF, defined on the basis of a creatinine value over 120 mumol/l, in patients over 80 years of age admitted between October 1971 and September 1996. When moderate chronic nephropathy was pre-existing, ARF was defined by the increase of at least 50% over the basal creatininemia. MEASUREMENTS AND RESULTS: Three hundred and eighty-one patients over 80 years of age were included. The etiology and mechanism of ARF are detailed. 29% of the patients received dialysis. Global mortality at the hospital was 40%. Factors significantly associated with a poor prognosis are identified. Mean survival after hospitalization was 19 months. CONCLUSION: The frequency of admission to ICUs for ARF in patients older than 80 years seems to be on the increase. Mortality is less severe than expected. These patients could benefit from the renal replacement therapy of modern intensive care medicine.

Macroeconomic determinants of migration: the case of Germany, 1964-1988.

PIP: Macroeconomic determinants of immigration are analyzed with pooled cross-country and time series data on net immigration to Germany from European countries during 1964-88. It presents a combined focus on short- and long-run patterns of net immigration. Time series studies show that the flow of migration is affected by the business cycle in both sending and receiving countries, whereas cross-country analyses emphasize the importance of income levels and standards of living. In this paper, cross section and time series data were pooled in order to maximize the identifying power of the sample and simultaneously estimating both low and high frequency phenomena. The findings demonstrated that both high and low frequency determinants have been important. Long run trends are determined by the degree and speed of per capita income convergence between the sending and receiving countries, while year-to-year changes in net immigration flows are dominated by cyclical economic conditions.^ieng

Skeletal muscle blood flow and venous capacitance in patients with severe sepsis and systemic hypoperfusion.

Alterations in peripheral vascular tone are presumed to contribute to circulatory failure during severe sepsis. Decreased venous tone with venous pooling may decrease effective circulatory blood volume, while decreased arterial tone with redistribution of systemic blood may compromise tissue nutrient flow. We compared forearm arterial and venous tone and forearm blood flow in ten patients with and ten patients without sepsis. The FVT, MVC, and FBF were measured by air plethysmography. In the septic patients, MCV was 1.4 +/- 0.1 ml compared with 3.1 +/- 0.2 ml in nonseptic patients (p less than 0.01). The FVT was 13.4 +/- 1.0 mm Hg/ml in septic patients versus 7.0 +/- 0.5 mm Hg/ml in nonseptic patients (p less than 0.01). The ratio of FBF to cardiac output was 0.28 +/- 0.07 percent in septic patients and 0.31 +/- 0.07 percent in nonseptic patients. These data suggest that increased peripheral venous capacitance and redistribution of skeletal muscle blood flow are not present in patients with sepsis.

Inotropic response to digoxin and dopamine in patients with severe sepsis, cardiac failure, and systemic hypoperfusion.

We studied the inotropic response to dopamine and digoxin in 20 patients with severe sepsis and left ventricular failure. Left ventricular failure was defined as a left ventricular stroke work index less than or equal to 40 g.m/m2 at a pulmonary artery wedge pressure greater than or equal to 15 mm Hg. Hemodynamic assessment was obtained before and following administration of digoxin 10 micrograms/kg IV or dopamine, 5 to 12 micrograms/kg/min IV. Patients treated with digoxin demonstrated a significant increase in LVSWI. The LVSWI increased 13 +/- 10 percent in the dopamine-treated patients compared with 74 +/- 16 percent in the digoxin patients (p less than 0.02). We conclude that digoxin exhibited significant inotropic activity in patients with sepsis.