A. Hedin

Plasma desorption mass spectrometry (PDMS). Limitations and possibilities

Abstract The present status of fast-ion-induced desorption-ionization and time-of-flight analysis of secondary ions of biomolecules (PDMS) is reviewed briefly. Emphasis is on an evaluation of various analytical parameters of the method such as resolution, precision and sensitivity. It is concluded that, although not yet fully demonstrated experimentally, molecular weight determination with picomole amounts, and 0.1% precision is possible in the mass range up to 25000 u. The principal limitations in improving these figures of merit are discussed.

On the detection of large organic ions by secondary electron production

Abstract A particle detector based on the detection of secondary electrons ejected at the impact of the particle on the detector surface has been constructed. It has been utilized in plasma desorption time-of-flight mass spectrometry to achieve post-acceleration yielding enhanced detection efficiency and identification of metastable decay channels in the spectrometer. Threshold velocities for electron ejection at the impact of large ions on a surface have been determined. Further, the multiplicity of secondary electrons ejected on the impact of the detected ion on the surface has been determined by energy measurements of the ensemble of ejected electrons.

Sputtering of biomolecules by fast heavy ions

Abstract The ejection of large intact molecular ions from a sample of biomolecules when bombarded with fast heavy ions is discussed. The mass spectrometric technique based on this effect and time of flight mass analysis is called plasma desorption mass spectrometry (PDMS). Recently small proteins and mixtures of such molecules have been studied by this method. The analytical potential of PDMS motivates the search for a deeper understanding of the mechanisms involved. A strongly nonlinear behaviour of the yield of large molecular ions versus energy deposited by the fast primary ion has been experimentally observed. A secondary electron multihit ion track model will be used to analyze the data. Furthermore new experimental data on sputtering yields of neutral intact molecules as well as data on the influence of sample film thickness on secondary ion yields will be presented.

Radial velocity distributions of secondary ions ejected in electronic sputtering of organic solids

Abstract Initial radial velocity distributions of large organic molecular ions as well as low mass contaminant and fragment ions ejected from a solid by impact of 72.3 MeV 127 I 13+ have been measured by deflecting the secondary ions by means of a pair of deflection plates placed in the field-free region of a time-of-flight mass spectrometer. The radial velocity distributions have been studied as a function of the angle of incidence of the primary ion for different masses and charge states of the secondary ions. The results indicate that different ejection mechanisms may be involved, applicable in different regions of the secondary ion mass range.

Secondary ion multiplicities and correlations in fast heavy ion-induced desorption of biomolecules

Abstract Secondary ion multiplicity distributions in fast heavy ion-induced desorption are determined for several biomolecules using the time-of-flight technique. The shapes of the distributions are discussed and it is shown that high-mass molecules are associated with high-multiplicity events. Secondary ion pair correlations are shown to be weak for low-mass molecules. A general correlation between high-mass molecules, presumably caused by metastable decay is demonstrated. Using correlation techniques, delayed emission of ions in CsI is demonstrated. It is suggested that this effect is due to delayed decay of excitons produced by the fast primary ion.

Incident angle dependence of electronic desorption and sputtering by energetic ions

Abstract It is shown that deviations of the dependence of sputtering yield on the incident angle, θ, from the standard (cos θ) −1 dependence can be indicative of a depth dependence in the net energy deposition in the surface region. Information about the sputter ejection mechanism is shown to be obtained only secondarily. We give expressions that allow one to relate the measured dependence of the yields on angle of incidence to the dependence of energy deposition on depth. This is used to analyze results for condensed gas sputtering and heavy ion desorption of organic molecular ions. This analysis indicates that the spatial distribution of excitations produced by the secondary electrons is important in determining these yields. This is also confirmed by observed differences in the yields for transmission and back-sputtering which is a closely related effect.

A comparison of fast heavy ion induced desorption of biomolecules from multilayer films and monolayers adsorbed to nitrocellulose backings

Abstract Fast heavy ion induced desorption yields of biomolecular ions as a function of the electronic stopping power has been studied for different sample preparation techniques. A comparison has been made between multilayer films prepared with the electrospray technique and monolayers adsorbed to nitrocellulose backings. The yield data, obtained with the time-of-flight technique, have been analyzed in an ion track model. The results show that molecular ions from the nitrocellulose adsorbed samples require less energy for ionization-desorption than those from multilayer samples.

Electronic effects in MeV ion tracks affecting thin film adhesion

An ion track model was applied to describe MeV ion induced adhesion improvement of Au thin films on amorphous SiO_2. Good agreement with experimental data was found when assuming that ion track energy densities above and below a certain interval do not contribute to the adhesion enhancement; damage effects detrimental to adhesion may be associated with the high energy densities in the vicinity of the ion path.

Heavy-Ion and Laser-Pulse Induced Ejection of Large Organic Molecules

Ejection from the solid state of large, thermally-labile organic molecules occurs due to the energy deposited by fast heavy ions or by laser-pulse excitation of a matrix material. Laser-pulse-induced ejection is described using the ‘pressure pulse’ model developed for heavy-ion-induced ejection and its relationship to sublimation is considered. Since the rate of energy deposition for ‘nano-second lasers’ is slow compared to that for heavy-ion-induced ejection, processes that affect the ability to form a pressure pulse prior to significant energy dissipation and dispersion are considered. An expression for analyzing yield data is given in which ion yields depend on ion formation/destruction via the local energy density and the total yield depends on the ejection mechanism.

On the observation of charged particles in cold fusion

With the aim to confirm or reject the recent claim of observation of cold d-d fusion, an experimental effort has been made to try to observe MeV protons which should be emitted as a result of d-d fusion. Pd foils, thin enough to allow all protons produced to escape the foil, were electrolytically charged with deuterium. A Si(SB) detector was placed close to the Pd foil during charging in order to detect any protons emitted. The deuterium content was measured to be the expected 0.7 D per Pd. Monte Carlo simulations were made to estimate the detection efficiency of 3.02 MeV protons produced in the Pd foil. The background in the experiment was so low that fusion rates considerably lower than those reported on by Jones et al. could be detected. A number of experiments have been performed where the charging conditions were varied. In spite of that and the good sensitivity of the experiment no evidence for cold fusion has been found.