Experimental and theoretical study of photo-dissociation spectroscopy of pyrene dimer radical cations stored in a compact electrostatic ion storage ring.

In this paper, we present an experimental and theoretical study of the photo-dissociation of free-flying dimer radical cations of pyrene (C16H10)2+. Experimentally, the dimers were produced in the plasma of an electron cyclotron resonance ion source and stored in an electrostatic ion storage ring, the Mini-Ring for times up to 10 ms and the photo-dissociation spectrum was recorded in the 400 to 2000 nm range. Two broad absorption bands were observed at 550 (2.25 eV) and 1560 nm (0.79 eV), respectively. Theoretical simulations of the absorption spectrum as a function of the temperature were performed using the Density Functional based Tight Binding approach within the Extended Configuration Interaction scheme (DFTB-EXCI) to determine the electronic structure. The simulation involved all excited electronic states correlated asymptotically with the five lowest excited states D1-D5 of the monomer cation and a Monte Carlo exploration of the electronic ground state potential energy surface. The simulations exhibit three major bands at 1.0, 2.1 and 2.8 eV respectively. They allow assigning the experimental band at 1560 nm to absorption by the charge resonance (CR) excited state correlated with the ground state of the monomer D0. The band at 550 nm is tentatively attributed to dimer states correlated with excited states D2-D4, in the monomer cation. Simulations also show that the CR band broadens and shifts towards longer wavelength with increasing temperature. It results from the dependence on the geometry of the energy gap between the ground state and the lowest excited state. The comparison of the experimental spectrum with theoretical spectra at various temperatures allows us to estimate the temperature of the stored (C16H10)2+ in the 300-400 K range, which is also in line with the expected temperatures of the ions deduced from the analysis of the natural decay curve.

Photo-dissociation of naphthalene dimer cations stored in a compact electrostatic ion storage ring.

Naphthalene dimer cations [C10H8]2 + have been produced by using an electron cyclotron resonance plasma ion source and stored in a compact electrostatic ion storage ring. We show that the radiative cooling of these cations is much slower than the isolated monomer naphthalene cations. We also report on photo-dissociation studies in the gas phase of naphthalene dimer cations at high internal energy. The dissociation energy is estimated to 0.5 eV in close agreement with previous measurements but a factor of 2 smaller than recent (density functional theory (DFT) and ab initio) theoretical studies. As uncertainties on theory as well as on the experiment cannot be as large as this difference, we conclude that this discrepancy may be due to temperature effects with possible isomerization. As an interpretation of the photo-dissociation spectrum of naphthalene dimer cations, we propose a tentative simple analytical model based on effective Morse potentials. These effective potentials are expected to "average" temperature effects that would apparently result in a smaller energy difference between the fundamental and dissociation states due to the twisting vibration modes of the naphthalene dimer cations.

Fragmentation of singly charged adenine induced by neutral fluorine beam impact at 3 keV.

The fragmentation scheme of singly charged adenine molecule (H(5)C(5)N(5)(+)) has been studied via neutral fluorine impact at 3 keV. By analyzing in correlation the kinetic energy loss of the scattered projectile F(-) produced in single charge transfer process and the mass of the charged fragments, the excitation energy distribution of the parent adenine molecular ions has been determined for each of the main dissociation channels. Several fragmentation pathways unrevealed in standard mass spectra or in appearance energy measurements are investigated. Regarding the well-known hydrogen cyanide (HCN) loss sequence, we demonstrate that although the loss of a HCN is the dominant decay channel for the parent H(5)C(5)N(5)(+) (m = 135), the decay of the first daughter ion H(4)C(4)N(4)(+) (m = 108) involves not only the HNC (m = 27) loss but also the symmetric breakdown into two dimers of HCN.

Excitation and dissociation of tungsten hexacarbonyl W(CO)6: statistical and nonstatistical dissociation processes.

We have studied the excitation and dissociation processes of the molecule W(CO)(6) in collisions with low kinetic energy (3 keV) protons, monocharged fluorine, and chlorine ions using double charge transfer spectroscopy. By analyzing the kinetic energy loss of the projectile anions, we measured the excitation energy distribution of the produced transient dications W(CO)(6)(2+). By coincidence measurements between the anions and the stable or fragments of W(CO)(6)(2+), we determined the energy distribution for each dissociation channel. Based on the experimental data, the emission of the first CO was tentatively attributed to a nonstatistical direct dissociation process and the emission of the second or more CO ligands was attributed to the statistical dissociation processes. The dissociation energies for the successive breaking of the W-CO bond were estimated using a cascade model. The ratio between charge separation and evaporation (by the loss of CO(+) and CO, respectively) channels was estimated to be 6% in the case of Cl(+) impact.

Age-related decline in sensory processing for locomotion and interception.

The ability to control locomotion through the environment and to intercept, or avoid objects is fundamental to the survival of all locomotor species. The extent to which this control relies upon optic flow, visual direction cues or non-visual sensory inputs has long been debated. Here we look at the use of sensory information in young and middle-aged participants using a locomotor-driven interceptive task. Both groups of participants were asked to produce forward displacements in more or less impoverished environments by manipulating a joystick and to regulate, if necessary, their displacement velocity so as to intercept approaching targets. We show that the displacements produced by the middle-aged participants were more nonlinear in comparison with young participants. The errors in the middle-aged group can be accounted for by a constant bearing angle (CBA) model that incorporates a decrease in the sensitivity of sensory detection with advancing age. The implications of this study to a better understanding of the mechanisms underlying the detection of the rate of change in bearing angle are discussed.

A "tabletop" electrostatic ion storage ring: Mini-Ring.

We report on the design, construction, and commissioning of a novel electrostatic ion storage ring of small dimensions--in the following referred to as "Mini-Ring." Mini-Ring consists of four horizontal parallel-plate deflectors and two conical electrostatic mirrors. Ions are injected through the two deflectors on the injection side and off axis with respect to the conical mirrors which face each other. The first injection deflector, originally at zero voltage, is switched to its set value such that the ions after one turn follow stable trajectories of lengths of roughly 30 cm. This design reduces the number of electrodes necessary to guide the ion beam through the ring in stable orbits. The six elements (deflectors and mirrors) are placed on a common grounded plate--the tabletop. Here, we present the design, ion trajectory simulations, and results of the first test experiments demonstrating the successful room-temperature operation of Mini-Ring at background pressures of 10(-6) - 10(-7) mbar.

A robust solution for dealing with environmental changes in intercepting moving balls.

The authors tested whether a simple model based on the cancellation of the rate of change in bearing angle could account for the behavioral adaptations produced when individuals intercept moving balls while walking. In Experiment 1, the place of arrival of the ball and the angle of approach were varied. In accord with the model, velocity regulations were earlier and more pronounced the larger the angle of approach. In Experiment 2, ball speed unexpectedly changed during a trial, once again highlighting participants' functional velocity adaptations. A direct test of the model on the basis of each individual trial (N = 256) revealed that, on average, 70% of the total variance could be explained. Together, those results confirm the usefulness of such a robust strategy in the control of interceptive tasks.

The perceptual control of goal-directed locomotion: a common control architecture for interception and navigation?

Intercepting a moving object while locomoting is a highly complex and demanding ability. Notwithstanding the identification of several informational candidates, the role of perceptual variables in the control process underlying such skills remains an open question. In this study we used a virtual reality set-up for studying locomotor interception of a moving ball. The subject had to walk along a straight path and could freely modify forward velocity, if necessary, in order to intercept-with the head-a ball moving along a straight path that led it to cross the agent's displacement axis. In a series of experiments we manipulated a local (ball size) and a global (focus of expansion) component of the visual flow but also the egocentric orientation of the ball. The experimental observations are well captured by a dynamic model linking the locomotor acceleration to properties of both global flow and egocentric direction. More precisely the changes in locomotor velocity depend on a linear combination of the change in bearing angle and the change in egocentric orientation, allowing the emergence of adaptive behavior under a variety of circumstances. We conclude that the mechanisms underlying the control of different goal-directed locomotion tasks (i.e. steering and interceptive tasks) could share a common architecture.

The visual control of ball interception during human locomotion.

According to the required velocity model, on-line modulations of movement acceleration are performed on the basis of an optically specified difference between required and current behavior. Can this model account for observed displacement regulations in an interceptive task requiring locomotive displacements? In the present study, a virtual reality set-up was coupled to a treadmill. Subjects walking on the treadmill were required to intercept a virtual ball approaching at eye-level by adjusting their velocity, if necessary. While the required velocity model could partially account for displacement regulation late in the interception, it was ineffective to explain early regulations. The possible use of a bearing angle strategy to control displacement regulation and the possible degree of complimentarity of these strategies are discussed.

Temporal information for spatially constrained locomotion.

How is locomotion adapted to spatial environmental constraints? The control of this everyday behavior is claimed to be based on information that specifies either spatial or temporal properties of the actor-environment system. Although studies on open-loop locomotor pointing (i.e., the positioning of a foot on a target on the floor while walking) agree on the use of spatial information, studies on closed-loop locomotor pointing propose the use of temporal information. Here, we test the hypothesis of closed-loop locomotor pointing based on temporal information, by dissociating spatial and temporal information in a virtual reality setup (virtual environment connected to a treadmill). The results support this hypothesis and shed some light on the type of temporal information that is used. The performed dissociation between spatial and temporal information, however, does not rule out a control based on a continuous updating of spatial information. Therefore, our conclusion on the use of temporal information was moderated.

Perception--action coupling model for human locomotor pointing.

How do humans achieve the precise positioning of the feet during walking, for example, to reach the first step of a stairway? We addressed this question at the visuomotor integration level. Based on the optical specification of the required adaptation, a dynamical system model of the visuomotor control of human locomotor pointing was devised for the positioning of a foot on a visible target on the floor during walking. Visuomotor integration consists of directly linking optical information to a motor command that specifically modulates step length in accordance with the ongoing dynamics of locomotor pattern generation. The adaptation of locomotion emerges from a perception-action coupling type of control based on temporal information rather than on feedforward planning of movements. The proposed model reproduces experimental results obtained for human locomotor pointing.

Rotor synchronization of radiofrequency and gradient pulses in high-resolution magic angle spinning NMR.

We have investigated the extent to which rotor synchronization of radiofrequency pulses leads to spectral improvement in high-resolution magic angle spinning NMR experiments. Several pulse sequences were tested, and the effect was found to be maximal in homonuclear TOCSY spectra. The physicochemical nature of the sample plays a role in the phenomenon, as rotor synchronization allows the refocusing of residual anisotropic interactions. However, even in a liquid sample the effects were visible. Radial inhomogeneities of the radiofrequency field were identified as an important source of the problem.

Spatially constrained locomotion under informational conflict.

This study investigates the informational based that supports intentional adaptation of locomotion to spatial environmental constraints. A virtual reality setup was used to present subjects with targets providing normal as well as abnormal optical expansion during locomotor pointing (i.e. positioning of a foot on a visible target on the floor during walking). The manipulation dissociated two variables providing temporal information about time-to-passage (TTP): TTP(beta alpha) which encompasses target expansion, and TTP(alpha) which is independent of target expansion. While a previous study showed TTP(alpha) to be sufficient, the present results reveal that TTP(beta alpha) may be used when it is available. This finding indicates that both variables play a role that varies according to the circumstances. Furthermore, the present results provide evidence of the operation of a security principle for action in conflicting situations.

A perception-action coupling type of control in long jumping.

The authors' goal was to identify the control mechanisms used by long jumpers (N = 6) to precisely position their foot at the board. In addition to the intertrial method usually used in previous research, an original method based on a trial-by-trial analysis was also implemented. If the approach to the board in long jumping encompasses two distinct sequences separated by a key step that marks the initiation of visual control, then a trial-by-trial analysis should reveal those sequences, regardless of the amount of adjustment: The step number at which regulation is initiated should be the same irrespective of the amount of adjustment. If, in contrast, a perception-action coupling mechanism operates, then the step number at which regulation is initiated should be a function of the amount of adjustment: A linear relation between those 2 variables should emerge. The results of the present study are compatible with continuous control mechanisms based on a perception-action coupling.

The control of human locomotor pointing under restricted informational conditions.

Since a perception-action coupling type of control (Kugler, P.N. and Turvey, M.T., Information, Natural Law, and Self-Assembly of Rhythmic Movements, Erlbaum, Hillsdale, 1987, 481 pp.) continuously operates during locomotor pointing tasks (e.g. long jumping) (Montagne, G., Cornus, S., Glize, D., Quaine, F. and Laurent, M., A 'perception-action coupling' type of control in long-jumping. J. Motor Behav., (2000) in press), the information sources underlying this control have to be dealt with. Under the assumption that subjects use information about the first-order time remaining before they pass the target, we identify in the literature four different sources of information that specify this physical property. Only one of these sources is inevitably present under all possible environmental conditions containing at least a continuously visible target on the floor. This study aimed to test its sufficiency to perform a locomotor pointing task. The use of a virtual reality set-up permitted us to compare locomotor pointing executed with all four information sources or only with the aforementioned one. The likeness found between those two conditions, as far as the pointing performance and the mode of control are concerned, expresses the evoked sufficiency.

The study of locomotor pointing in virtual reality: the validation of a test set-up.

The goal of this experiment was to validate an experimental set-up for studying locomotor pointing. The specific and also original element of this set-up was the interactive nature of virtual reality and movement production. This interaction was achieved through the coupling of a treadmill and a Silicon Graphics system. This latter system generated on a screen (3 x 2.3 m) an environmental array that moved according to the action produced by subjects on a treadmill. The task was to place either foot on a spatial target that appeared on the floor in front of the subject's displacement trajectory. We analyzed the step length patterns of subjects approaching these targets, along with the current target-subject relationship. The results are in agreement with a perception-action coupling type of control mechanism that operates continuously as the subject approaches the desired target. Apparently, these findings mirror observations of real-life locomotion, indicating that the present set-up provides a valid and useful tool for examining human locomotion.

The regulation of externally paced human locomotion in virtual reality.

The goal of the present experiment was to study regulation of human locomotion under externally paced temporal constraints. On a screen placed in front of a treadmill a virtual hallway was projected (Silicon Graphics systems) in which a pair of doors were presented that continuously opened and closed at a rate of 1 Hz. Subjects were attached to a locometer and instructed to regulate walking pace such that the doors were passed correctly. Performance outcome, movement kinematics (stride duration, stride length and synchronization of stride and door cycles) and flow patterns (change in visual angle of door aperture) were used to examine the data. The analysis of the synchronization patterns indicates that stride cycles were not linked to the period of door oscillation. Moreover, results for stride duration reveal that subjects walked at their preferred speed up to the final phase of the approach. This observation is supported by the inspection of the flow patterns, revealing a final increase in variability as a result of regulation. In sum, regulation of locomotion under externally paced temporal constraints seems to generate a specific functional behavior. It appears that regulations are postponed until the final stage of the approach during which adaptations are made according to the requirements of the current situation.

Movement reversals in ball catching.

This study aimed to determine whether the interception of a moving object is achieved by implementing a predictive or a prospective strategy. We examined the kinematics of catching movement in a situation in which the catching hand was constrained to move along a single dimension. In line with predictions based on a prospective strategy, the results obtained indicated that, for the same interception point and the same initial hand position, modification of the spatiotemporal characteristics of the ball's trajectory (via modification of the angle of approach of the ball) gave rise to systematic changes in the kinematics of catching movement. Moreover, the production of movement reversals when the hand was already positioned at the interception point, while in line with the predictions of the prospective strategy formalized by Bootsma et al. (1997), allowed for rejection of a predictive strategy.

Three cases of serious infection caused by Aerococcus urinae.

Three cases of serious infection caused by Aerococcus urinae are presented: a patient with endocarditis and two patients with soft-tissue infection (phlegmon and balanitis respectively). The literature on Aerococcus urinae infections is reviewed and the antibiotic therapy discussed. Aerococcus urinae is a pathogen isolated primarily from urine specimens of elderly patients with local or systemic predisposing conditions. Most infections are mild, but serious infections such as endocarditis and septicemia/urosepsis have been described. Penicillin or ampicillin in combination with an aminoglycoside and close monitoring of the patient's clinical status and laboratory results would seem to be the best strategy for management of cases of serious infection.

Bilateral intravesical duplex system ureteroceles with multiple calculi in an adult patient.

We present a case report of an adult female patient with bilateral duplex system ureteroceles, containing multiple small stones. There was no history of urinary tract infections or stone disease. Treatment consisted of right upper pole heminephrectomy because of non-function and transurethral incision of the left ureterocele, resulting in almost complete removal of the calculi. Furthermore no vesicoureteral reflux was seen 3 months postoperatively. Transurethral incision is a safe and effective treatment in removing stones in ureteroceles without necessitating further treatment because of vesicoureteral reflux.