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  • 1
    ISSN: 1572-879X
    Keywords: aluminum oxide ; hydroxyl groups ; rhodium ; growth ; scanning tunneling microscopy ; infrared spectroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier.
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: After UV-laser-induced desorption we observe bimodal velocity distributions independent of internal vibrational excitation [up to v=2 (4%)] applying resonance-enhanced multiphoton ionization techniques. Both contributing desorption channels are of nonthermal origin. We introduce a model where the two desorption channels are correlated with the rupture of the molecule surface bond of the librating molecule either on the way toward or away from the surface. We have performed trajectory calculations to simulate the desorption processes. The calculated momentum distributions of the desorbing molecules show either one or two maxima, depending on lifetime, in agreement with experimental results. The vibrational distribution of the desorbing molecules can be reproduced by assuming transition into a state that is characterized by an altered N–O bond length as it is found, for example, in NO−. The model calculations both for velocity distributions and vibrational excitations result in similar lifetimes of the excited state, even though the translational and the vibrational degree of freedom of the desorbing molecules are decoupled.
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to interpret the experimental results of the state resolved UV-laser-induced desorption of NO from NiO(100) (rotational and vibrational populations, velocity distributions of the desorbing NO molecules, etc.), we have performed ab initio complete active space self-consistent field (CASSCF) and configuration interaction (CI) calculations for the interaction potential between NO and the NiO(100) surface in the electronic ground state and for those excited states which are involved in the desorption process. The NiO(100)–NO distance and the tilt angle between the NO axis and the surface normal have been varied. A cluster model containing a NiO8−5-cluster embedded in a Madelung potential has been used for representing the NiO(100) surface. The excited states which are important for the desorption process, are charge transfer states of the substrate–adsorbate system, in which one electron is transferred from the surface into the NO-2π-orbital. The potential curves of these excited charge transfer states show deep minima (4 eV–5 eV) at surface/NO distances which are smaller than that in the ground state. The angular dependence of these potentials behaves similar as in the ground state. A semiempirical correction to the calculated excitation energies has been added which makes use of the bulk polarization of NiO. With this correction the charge transfer states are considerably stabilized. The lowest excitation energy amounts to about 4 eV which is in reasonable agreement with the onset of the laser desorption observed experimentally at about 3.5 eV. The density of the NO−-like states is rather high, so that probably several excited states are involved in the desorption process. The potential energy curves for all of these states are quite similar, but the transitions from the ground state into different excited charge transfer states show strongly differing oscillator strengths, which are also strongly dependent on the surface/NO distance. This fact is important for the dynamics of the deexcitation process in the sense of a selection criterion for the states involved. The magnitude of the oscillator strengths is large in comparison with the excitation of NO in the gas phase, which might be an indication for the possibility of optical excitation processes. One dimensional wave packet calculations on two potential energy curves using fixed lifetimes for the excited state in each calculation have been performed and enable us to estimate the mean lifetime of the excited state to be 15 fs≤τ≤25 fs. This implies that the dynamics of the system is dominated by the attractive part of the excited state potential. © 1996 American Institute of Physics.
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  • 4
    ISSN: 1432-1106
    Keywords: Eye-arm coordination ; Cerebellum ; Visual feedback ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The initiation of coupled eye and arm movements was studied in six patients with mild cerebellar dysfunction and in six age-matched control subjects. The experimental paradigm consisted of 40 deg step-tracking elbow movements made under different feedback conditions. During tracking with the eyes only, saccadic latencies in patients were within normal limits. When patients were required to make coordinated eye and arm movements, however, eye movement onset was significantly delayed. In addition, removal of visual information about arm versus target position had a pronounced differential effect on movement latencies. When the target was extinguished for 3 s immediately following a step change in target position, both eye and arm onset times were further prolonged compared to movements made to continuously visible targets. When visual information concerning arm position was removed, onset times were reduced. Eye and arm latencies in control subjects were unaffected by changes in visual feedback. The results of this study clearly demonstrate that, in contrast to earlier reports of normal saccadic latencies associated with cerebellar dysfunction, initiation of both eye and arm movements is prolonged during coordinated visuomotor tracking thus supporting a coordinative role for the cerebellum during oculo-manual tracking tasks.
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  • 5
    ISSN: 1432-1106
    Keywords: Key words Parieto-premotor circuit ; Ventral premotor cortex ; Anterior intraparietal sulcus ; Object manipulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Functional magnetic resonance imaging (fMRI) was used to assess cerebral activation during manipulation of various complex meaningless objects as compared to manipulation of a single simple object (a sphere). Significant activation was found bilaterally in the ventral premotor cortex (Brodmann’s area 44), in the cortex lining the anterior part of the intraparietal sulcus (most probably corresponding to monkey anterior intraparietal area, AIP), in the superior parietal lobule and in the opercular parietal cortex including the secondary somatosensory area (SII). We suggest that the cortex lining the anterior part of the intraparietal sulcus and area 44 are functionally connected and mediate object manipulation in humans.
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  • 6
    ISSN: 1432-1106
    Keywords: Optic tract ; Latencies ; Centre-excitation ; Centre-inhibition ; Cat
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Excitatory and inhibitory responses of on- and off-centre neurones were elicited by stimuli restricted to the receptive field centres of optic tract units. The latencies of these responses were measured for various stimulus intensities and durations. Latencies depend upon: 1. the total effective luminance, including stimulus and background; 2. the luminance of the test spot relative to threshold luminance: latency changes for a particular light increment near threshold are larger than in the higher supra-threshold range; 3. the latencies of inhibitory responses are further influenced by the activity of the neurone before the onset of inhibition. A comparison of the latencies of excitatory and inhibitory responses showed that, following incremental light stimuli, the off-centre neurones became inhibited before the on-centre neurones were excited. The actual latency difference was a function of stimulus intensity and previous neuronal activity. Following light-off inhibitory responses were again faster, but the difference in time between the inhibition of on-centre neurones and the excitation of off-centre neurones was smaller than for the corresponding on-responses. It is suggested that the earlier onset of inhibitory responses is important for the release of reciprocal inhibition between on- and off-centre neurones in the lateral geniculate body.
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  • 7
    ISSN: 1432-1106
    Keywords: Voluntary contractions ; Speed control ; Synergistic innervation ; Open-loop movements
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The relationship between the speed of the fastest possible voluntary contractions and their amplitude was examined for several hand- and forearm muscles under isometric and isotonic conditions. The consistent finding was the amplitude dependence of the speed of the fastest voluntary efforts: the larger the amplitude, the faster the contraction. The increase of the rate of rise of isometric tension or of the velocity of isotonic movements with rising amplitude was linear. The slope of this relationship was the same for three different hand- and forearm muscles examined. The duration of the contractions measured from onset to peak was approximately constant for all amplitudes. The duration of the EMG-burst recorded from the contracting muscle was similar as the time from onset to peak of the contraction. These results show that the skeleto-motor speed control system operates by adjusting the velocity of a contraction to its amplitude in such a way that the contraction time remains approximately constant. It is suggested that this type of speed control is a necessary requirement for the synchrony of synergistic muscle contractions.
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  • 8
    ISSN: 1432-1106
    Keywords: Hand movements ; Cutaneous mechanoreceptors ; Active touch ; Motor control ; Sensory control of movement ; Humans
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The measurement of eight manipulative serial hand movements showed a clear distribution of their temporal characteristics into two distinct groups. When the hand was used as a sense organ during active touch the finger movements across objects were restricted to a slow performance range below 2 Hz. Recordings from single mechanoreceptive afferents and calculations of their receptor densities indicated that these movements have to be slow to match the temporal requirements of the sequential sampling process from the mechanoreceptor populations. In contrast, manual skills not associated with the collection of sensory information like handwriting, typing or pencil shading, were performed rapidly. Their frequencies were close to those of fastest possible tapping. Evidence is provided that the different frequency groups are associated with distinct sensory control processes. The low frequency group represents movements involving focal sensory control (Julesz 1984). The high frequency group is not performed open-loop but monitored by preattentive sensory processes. The results indicate a dual sensory control mode operating in separate frequency domains of movement.
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  • 9
    ISSN: 1432-1106
    Keywords: Somatosensory processing ; Somatosensory ; evoked potentials ; Microneurography ; Movement ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Modification of somatosensory processing depending on the behavioral setting was studied. Active alternating movements of the fingers, passive tactile stimuli to the hand, and active exploration of objects were performed during recording of somatosensory evoked potentials (SEPs). SEPs were elicited by compound electrical median nerve stimulation and electrical stimulation at detection threshold of cutaneous median nerve fascicles identified by microneurography. Electrical stimulation was not time-locked to the studied condition. In comparison with SEPs at rest there was attenuation of early cortical potentials up to 25 ms post-trigger in all nonresting conditions. In stimulation of the compound median nerve as well as of isolated cutaneous fascicles of a hand actively exploring an object there was an additional increased negativity, peaking at 28 ms. This facilitory effect was independent of attentional focusing and was absent during exploration using the ipsilateral, non-electrically stimulated hand. In patients with parietal lesions the facilitatory effect was diminished on the affected side. Spline interpolated brain maps at this latency based on 32channel recordings in healthy volunteers showed a shift of local contralateral positive maximum from frontal to parietal during exploration, indicating enhancement of a tangential dipole. It is suggested that in conditions involving close sensorimotor interaction such as exploratory hand movements there is preactivation of a cortical area which is located in the central sulcus and receives cutaneous somatosensory inputs.
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  • 10
    ISSN: 0093-934X
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Linguistics and Literary Studies , Medicine , Psychology
    Type of Medium: Electronic Resource
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