Interatomic Potential in the Nonequilibrium Warm Dense Matter Regime.

We present a new measurement of lattice disassembly times in femtosecond-laser-heated polycrystalline Au nanofoils. The results are compared with molecular dynamics simulations incorporating a highly optimized, embedded-atom-method interatomic potential. For absorbed energy densities of 0.9-4.3 MJ/kg, the agreement between the experiment and simulation reveals a single-crystal-like behavior of homogeneous melting and corroborates the applicability of the interatomic potential in the nonequilibrium warm dense matter regime. For energy densities below 0.9 MJ/kg, the measurement is consistent with nanocrystal behavior where melting is initiated at the grain boundaries.

A single-shot spatial chirp method for measuring initial AC conductivity evolution of femtosecond laser pulse excited warm dense matter.

To study the rapid evolution of AC conductivity from ultrafast laser excited warm dense matter (WDM), a spatial chirp single-shot method is developed utilizing a crossing angle pump-probe configuration. The pump beam is shaped individually in two spatial dimensions so that it can provide both sufficient laser intensity to excite the material to warm dense matter state and a uniform time window of up to 1 ps with sub-100 fs FWHM temporal resolution. Temporal evolution of AC conductivity in laser excited warm dense gold was also measured.

Optical Shaping of X-Ray Free-Electron Lasers.

In this Letter we report the experimental demonstration of a new temporal shaping technique for x-ray free-electron lasers (FELs). This technique is based on the use of a spectrally shaped infrared (IR) laser and allows optical control of the x-ray generation process. By accurately manipulating the spectral amplitude and phase of the IR laser, we can selectively modify the electron bunch longitudinal emittance thus controlling the duration of the resulting x-ray pulse down to the femtosecond time scale. Unlike other methods currently in use, optical shaping is directly applicable to the next generation of high-average power x-ray FELs such as the Linac Coherent Light Source-II or the European X-FEL, and it enables pulse shaping of FELs at the highest repetition rates. Furthermore, this laser-shaping technique paves the way for flexible tailoring of complex multicolor FEL pulse patterns required for nonlinear multidimensional x-ray spectroscopy as well as novel multicolor diffraction imaging schemes.

Pressure-induced non-superconducting phase of ß-Na0.33V2O5 and the mechanism of high-pressure phase transitions in ß-Na0.33V2O5 and ß-Li0.33V2O5 at room temperature.

The crystal structure of ß-Na0.33V2O5 (C2/m, Z = 6) has been studied on compression to 19 GPa at room temperature using synchrotron single-crystal diffraction in a diamond anvil cell. The vanadate bronze undergoes a phase transition to a non-superconducting phase at about 12 GPa due to changes of polyhedral connectivities in the vanadate framework and due to ordering of the Na(+) cations. This novel structure (Cm, Z = 6) is interpreted as an intermediate stage in the sequence of pressure-induced transformations in the ß-A0.33V2O5 bronzes (A: Li, Na) at room temperature. This study reveals the close relation between the loss of the two-leg ladder V-V system and non-superconducting state of the ß-A0.33V2O5 materials.

Femtosecond visualization of lattice dynamics in shock-compressed matter.

The ultrafast evolution of microstructure is key to understanding high-pressure and strain-rate phenomena. However, the visualization of lattice dynamics at scales commensurate with those of atomistic simulations has been challenging. Here, we report femtosecond x-ray diffraction measurements unveiling the response of copper to laser shock-compression at peak normal elastic stresses of ~73 gigapascals (GPa) and strain rates of 10(9) per second. We capture the evolution of the lattice from a one-dimensional (1D) elastic to a 3D plastically relaxed state within a few tens of picoseconds, after reaching shear stresses of 18 GPa. Our in situ high-precision measurement of material strength at spatial (<1 micrometer) and temporal (<50 picoseconds) scales provides a direct comparison with multimillion-atom molecular dynamics simulations.

Anxiety disorders in headache patients in a specialised clinic: prevalence and symptoms in comparison to patients in a general neurological clinic.

Data from several studies indicate an association of headache with anxiety disorders. In this study, we assessed and differentiated anxiety disorders in 100 headache patients by using the PSWQ (Penn State Worry Questionnaire) screening tool for generalised anxiety disorder (GAD) and the ACQ (Agoraphobic Cognitions Questionnaire) and BSQ (Body Sensation Questionnaire) for panic disorder (PD). Control groups were constructed: (1) on the basis of epidemiological studies on PD and GAD in the general population and (2) by including neurological patients. 37.0% of headache patients had a GAD. 27% of headache patients met the score for PD in the BSQ, 4.0% in the ACQ. Significant results were obtained in comparison to the general population (p < 0.001) and with regard to GAD in comparison with a sample of neurological patients (p < 0.005). The BSQ significantly correlated with the number of medication days (p < 0.005). The results confirm the increased prevalence of GAD in headache patients. PD seems to increase the risk of medication overuse.

Spinal DC stimulation in humans modulates post-activation depression of the H-reflex depending on current polarity.

OBJECTIVE: Transcranial direct current stimulation induces long-lasting changes in cortical excitability in humans depending on the current used. Further, transcutaneous spinal application of direct current (tsDCS) induces plastic changes in spinal conduction properties, tested by somatosensory evoked potentials. To verify this thesis on plastic changes in spinal circuitry, we investigated the effects of tsDCS on H-reflex size and post-activation depression. METHODS: Ten healthy subjects participated in the study. The H(max)/M(max) ratio and H-reflex post-activation depression were evaluated before, at current offset, and 15 min after anodal, cathodal or sham tsDCS. Stimulation of the spinal cord (2.5 mA, 0.063 mA/cm(2), 0.056 C/cm(2)) was applied for 15 min at Th11 level. RESULTS: Anodal tsDCS induced a lasting decrease in H-reflex post-activation depression, while cathodal stimulation resulted in a sustained increase. Sham stimulation had no significant effects. The H(max)/M(max) ratio remained unchanged throughout all conditions. CONCLUSION: Anodal and cathodal tsDCS is a non-invasive and painless method that is able to induce lasting changes in the efficacy of the Ia fibre-motoneurone synapse. SIGNIFICANCE: Transcutaneous spinal DC stimulation might be a valuable new tool in modulating spinal motor pathways.

Perceptions of medical school graduates and students regarding their academic preparation to teach.

PURPOSE: How medical students learn and develop the characteristics associated with good teaching in medicine is not well known. Information about this process can improve the academic preparation of medical students for teaching responsibilities. The purpose of this study was to determine how different experiences contributed to the knowledge, skills, and attitudes of medical school graduates and students regarding medical teaching. METHODS: A questionnaire was developed, addressing reliability and validity considerations, and given to first year residents and third year medical students (taught by those residents). Completed questionnaires were collected from 76 residents and 110 students (81% of the sample group). Item responses were analysed using descriptive and inferential statistics. RESULTS: Most residents (n = 54; 71%) positively viewed opportunities they had to practice teaching when they were seniors. Residents rated three activities for learning to teach highest: (1) observing teachers as they teach; (2) reviewing the material to be taught; and (3) directly teaching students; representing both individual and participatory ways of learning. Residents' self ratings of teaching behaviours improved over time and this self assessment by the residents was validated by the students' responses. Comparison between residents' self ratings and students' views of typical resident teaching behaviours showed agreement on levels of competence, confidence, and motivation. The students rated characteristics of enthusiasm, organisation, and fulfilment lower (p<0.002) than residents rated themselves. CONCLUSIONS: The residents and students in this study viewed academic preparation for teaching responsibilities positively and showed agreement on characteristics of good teaching that may be helpful indicators in the process of developing medical teachers.

Mid-infrared cavity leak-out spectroscopy for ultrasensitive detection of carbonyl sulfide.

We present a ringdown absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near lambda = 5 microm. Using a linear ringdown cavity (length, 0.5 m) with R > = 99.99% mirrors, we observed a noise-equivalent absorption coefficient of 7 x 10(-11) cm(-1) Hz(-1/2). This is 2 orders of magnitude improved compared with previous values. With this setup we studied the spectroscopic detection of carbonyl sulfide (here abbreviated OCS) traces in ambient air and in exhaled breath. We achieved a detection limit of 7 parts in 10(12) (parts per trillion) OCS in ambient air, which is unprecedented and shows great promise for environmental and biomedical applications.

A novel navigation principle in computer-assisted surgery.

Novel methods of laser measurements and interventions in the framework of image-guided surgery are presented. The first innovation concerns the basis of error propagation studies in registration chains from CT via OR navigator to the surgical laser tool holder. Here, we propose a holographic ground truth. The second innovation concerns a laser-based inherent sound-guidance principle for burr hole depth measurement.

Holographic face models as planning tool in maxillofacial surgery.

The holographic facial profile scan is a new technique for creation of high-resolution, three-dimensional, realistic facial computer models which can be used for surgical planning and documentation in maxillofacial surgery. First, a holographic image of the patient is recorded using a pulsed laser system. In a second step, called holographic tomography, the real image of the patient's hologram is reconstructed by means of a continuous-wave laser. By moving a screen through the real three-dimensional image, it is sliced into a series of two-dimensional projections which are captured with a digital camera. The slices containing the specific two-dimensional information are superimposed to a three-dimensional surface model using special software. The extremely short exposure time of 35 nanoseconds for taking a holographic image is separated from the time-consuming rendering process of the surface model; thus, the obtained models are not affected by the movements of the patient.

Osteotomy with 80-micros CO2 laser pulses--histological results.

Haemostatic and aseptic effects and intricate cut geometry are beneficial aspects of non-contact laser osteotomy. Collateral thermal damage, however, has severely limited the use of conventional lasers. The purpose of this study was to test the side effects on bone after cutting it with short CO2 laser pulses and simultaneous application of a fine air-water spray. The 10.6 microm CO2 laser emitted 80 micros pulses of 46 mJ energy, f=100 Hz, focused to a spot diameter of 130 ìm. Scan rate amounted to 40 mm/s. To approximate live conditions 10 samples of cortical bone and 10 rib segments were prepared immediately after sacrificing of pigs. A reference cut with a bandsaw and three laser cuts with an increasing number of beam passes (4, 16, 64) were performed on each sample. Half of the samples were decalcified in EDTA. The others were embedded in plastic to cut non-decalcified sections. The laser incisions were not accompanied by carbonisation. The incisions with slightly convergent walls were 150 ìm wide. The depths of the cavities increased with the number of the beam passes from approximately 0.5 mm (4 passes) to 3 mm (64 passes). At the border of the incisions two narrow zones of damage were noted: an amorphous intensively stained zone of 1-3 microm width and a wider, also sharply demarcated but faintly stained zone of 7-10 microm. A broader zone of about 50 microm was characterised by empty lacunae and osteocyte damage. These effects were not predictable; intact osteocytes were also observed near to the cut surface. Polarised light microscopy showed no alterations in the inorganic structure of the bone at the cut borders. The histological results indicated only minimal damage to bone ablated at the specified parameters. The described laser procedure might have advantages over mechanical instruments.

The ablation threshold of Er:YAG and Er:YSGG laser radiation in dental enamel.

The scientific investigation of fundamental problems plays a decisive role in understanding the mode of action and the consequences of the use of lasers on biological material. One of these fundamental aspects is the investigation of the ablation threshold of various laser wavelengths in dental enamel. Knowledge of the relationships and influencing factors in the laser ablation of hard tooth tissue constitutes the basis for use in patients and the introduction of new indications. The present paper examines the ablation threshold of an Er:YAG laser (lambda=2.94 micro m) and an Er:YSGG laser (lambda=2.79 micro m) in human dental enamel. To this end, 130 enamel samples were taken from wisdom teeth and treated with increasing energy densities of 2-40 J/cm(2). The sample material was mounted and irradiated on an automated linear micropositioner. Treatment was performed with a pulse duration of tau(P(FWHM)) approximately 150 micro s and a pulse repetition rate of 5 Hz for both wavelengths. The repetition rate of the laser and the feed rate of the micropositioner resulted in overlapping of the single pulses. The surface changes were assessed by means of reflected light and scanning electron microscopy. On the basis of the results, it was possible to identify an energy density range as the ablation threshold for both the Er:YAG and the Er:YSGG laser. With the Er:YAG laser, the transition was found in an energy density range of 9-11 J/cm(2). The range for the Er:YSGG laser was slightly higher at 10-14 J/cm(2).

Bone tissue ablation with sub-microS pulses of a Q-switch CO(2) laser: histological examination of thermal side effects.

. The goal of this study is an in vitro evaluation of thermal side-effects by the application of short sub- micro s CO(2) laser pulses in combination with an air-water spray on different types of bone tissue. A mechanically Q-switched CO(2) laser delivered 300 ns pulses at 9.6 micro m wavelength, which were focused down to a spot size of 440 micro m on the tissue (a corresponding energy density of 9 J/cm(2)). Bone samples (blocks from pig femur, rib, or cartilage) were moved through the beam repeatedly until 1-5 mm deep cuts were produced. An air driven water spray was applied to prevent the tissue dehydration. Subsequent visual and histological examinations revealed no carbonisation, melting traces or fissuring of the tissue. An extremely narrow, 2-6 micro m thick thermally altered layer was observed at the cut border in compacta and cartilage. No accumulation of the thermal damage occurred with increasing cut depth. Laser incisions in trabecular tissue were accompanied with a 100-200 micro m thick zone of thermal necrosis in bone marrow. The difference from compacta and cartilage can be explained considering the particular character of the spreading of the ablation products in the trabecular meshwork. Minor thermal side effects make the Q-switched and probably other short pulsed CO(2) laser systems interesting for hard tissue surgery.

Hydrodynamic description of laser-induced coulomb explosion for small molecules

We have performed extensive numerical studies of Coulomb explosion of small molecules such as CO2 and N2O with the help of a hydrodynamic model, which allows one to deal with many-electron systems in intense laser fields. The predicted kinetic energy releases of the fragmentation channels are in good agreement with the measured energies, using realistic molecular and laser excitation condition parameters.

Scattering delay time of Mie scatterers determined from steady-state and time-resolved optical spectroscopy.

We have determined the scattering delay time of Mie scatterers (r = 255 nm quartz spheres in polyester resin) from a combination of steady-state (integrating-sphere) and time-resolved (frequency-domain) measurements performed in the multiple-scattering regime. The effective transport velocity of light was derived from intensity and phase measurements at four different wavelengths by using the time-integrated microscopic Beer-Lambert law. We could demonstrate a systematic underestimation of the effective transport velocity compared with the phase velocity in the medium. Assuming that this discrepancy was caused entirely by the transient nature of a single-scattering process, the data presented resulted in time delays of between 18 fs (lambda = 678 nm) and 177 fs (lambda = 1,064 nm) per scattering event. For three out of four wavelengths investigated, the measured values are in excellent agreement with values predicted by a theoretical model for the scattering delay time based on Mie theory.

Real-time detection of 13CH4 in ambient air by use of mid-infrared cavity leak-out spectroscopy.

We report on spectroscopic real-time detection of (13)CH(4) in ambient air. Our measurements were carried out by means of cavity leak-out absorption spectroscopy employing a tunable cw laser in the mid-infrared spectral region near lambda = 3 microm. A CO laser in combination with tunable microwave sideband generation was used as the light source. Using a 50-cm-long ringdown cell with R = 99.98% mirrors, we achieved a detection limit of 290 parts in 10(12) (ppt) (13)CH(4) in ambient air (integration time, 100 s). The corresponding noise-equivalent absorption coefficient was 5 x 10(-9)/cm.

Interpretative difficulties with growth hormone provocative retesting in childhood-onset growth hormone deficiency.

A review of literature demonstrates that there are many ill-understood factors that determine the results of GH provocative (re)testing, so that these results should be interpreted with extreme caution when used for diagnosis or confirmation of diagnosis of GHD. GH provocation tests are probably of no value at all for what has been called 'partial GHD'. The phenomenon of 'normalization' of test results after long-term treatment with GH needs no 'transient GHD' hypothesis as it can be largely explained by the very low reproducibility of the tests and by a regression to the mean effect. Moreover, it is possible that 'normal values' increase with age. Other determinants of normal peak values may also change from childhood to adulthood and contribute to 'normalization'.

Quantitative microspectrophotometry in turbid media.

We present a new technique to determine the scattering coefficient, the absorption coefficient, and the anisotropy factor in turbid media on a microscopic level. To this end, a microspectrophotometer was used to obtain transmission measurements at different solid angles. To extract the optical properties from phantom materials (liquid and solid) and biological tissue (bovine liver) an inverse Monte Carlo algorithm was used. The results obtained with the new microspectrophotometric technique agreed within one standard deviation with the values from Mie theory and within less than 10% with the data derived from conventional spectroscopic measurements. The results suggest that this new method is a valid tool to determine the optical properties of turbid media on a microscopic level.