We have studied the ionization behaviour close to threshold for the fragment ion production from CF 4 , CHF 3 , CH 4 and C 3 H 8 . In particular, we use a reliable method to extrapolate weak ion signals towards the appearance energy. Our evaluation is based on an extension of Wannier's law for the ionization of atoms towards the case of polyatomic molecules. In the experiment we use a hemispherical electron monochromator with an energy resolution of ≈135 meV. The vibrationally cold target molecules are prepared in a supersonic expansion and the resulting ions are mass analysed using a quadrupole mass spectrometer. We observe several disagreements with recommended ionization energy values, reaching up to 5 eV in the CHF 3 case. This is in contrast to a number of recent reliability tests that we have performed on Ne, Ar, Kr, Xe, H 2 , D 2 , N 2 , O 2 and N 2 O and that have reproduced the tabulated literature data within the resolution of the monochromator.

This preclinical study suggests that ultrasound is a useful guiding tool for facet joint injections in the cervical spine.

SUMMARY

Aiming devices enable the use of neuronavigation systems for rigid instrument guidance mimicking the possibilities of a frame-based system without having a stereotactic frame affixed to the skull. The aim of the presented work was to investigate the phantom targeting accuracy of the Vertek aiming device (Medtronic Inc., Louisville, USA) and whether it can be safely and accurately applied in a concept of minimally invasive brain biopsy in which multi-modal image fusion, image-to-patient registration and head immobilization were based on a non-invasive vacuum mouthpiece. A plastic model of a head with 20 target beads broadly distributed around the head volume was used for determination of CT-based targeting accuracy. Every target was punctured 5 times totaling 100 needle positionings. Accuracy was evaluated on postoperative CT scans with the needles in place. The mean normal deviation (n = 100) was 1.5 +/- 0.8 mm and the mean angle of deviation was 1.1 +/- 0.7 degrees. In a preliminary clinical series in ten patients diagnostic biopsy sampling of intracranial lesions with a median diameter of 28 mm (range: 12-90 mm) could be achieved in all patients and no biopsy related complications were recorded. The experimental results showed a similar accuracy to frame-based stereotaxy. The device facilitates trajectory alignment via two pivot joints and the actual depth and location of the biopsy needle can be monitored. Within the limitations of a preliminary study, brain biopsy may be accurately and safely performed for lesions > or = 12 mm.

We report the results of the experimental determination of the appearance energy values AE(Xn + /X) for the formation of multiply charged Ne, Ar and Xe ions up to n = 4 (Ne), n = 6 (Ar) and n = 8 (Xe) following electron impact on Ne, Ar and Xe atoms using a dedicated high-resolution electron impact ionization mass spectrometer. The data analysis uses the Marquart-Levenberg algorithm, which is an iterative, nonlinear least-squares-fitting routine, in conjunction with either a two-function or a three-function fit based on a power threshold law. This allows us to extract the relevant AEs and corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and spectroscopic values of the AEs and the extracted exponents are compared with other available experimental data and with the predictions of the various Wannier-type power law models.

No abstract

The occurrence of two different chemical reactions initiated by the surface impact of acetone dimer, trimer, and tetramer cations (energy 20–70 eV) on a stainless-steel surface (covered with hydrocarbons) was observed. The reaction product is the protonated acetone ion, formed in (i) an intracluster ion–molecule reaction, and in (ii) a hydrogen pickup reaction of the cluster ion with the surface material. Only the monomer product ions (and small amounts of their dissociation products) could be observed; the spectra did not show any presence of clustered product ions. A simple model based on the Brauman double-well potential is suggested to explain the formation of the two product ions. In accordance with predictions from molecular dynamics simulations, this appears to be the first observation of competitive chemical reactions of a cluster ion driven by energy transfer in a surface collision.

Using two different crossed-beams high-resolution electron attachment instruments (employing either a trochoidal electron monochromator or a hemispherical electron monochromator) we have determined the cross section curve for H − production from H 2 via the 4 eV resonance at two different temperatures. These relative partial cross sections have been calibrated by comparing present values for the 14 eV resonance with absolute total cross sections determined previously. Taking into account the experimental energy distribution and the rotational excitation and its influence on the cross section shape we obtain very good agreement with theoretical predictions in terms of both the shape and magnitude of this resonance peak.