International Journal of Mass Spectrometry and Ion Physics最新文献

Chemically modified chlorophylls and porphyrins have been studied by ²⁵²Cf-plasma desorption mass spectrometry (PDMS). The ²⁵²Cf-PDMS spectra of thin films show intense molecular ions and fragmentation from whicn we make reasonable assignments for compound identification. The ²⁵²Cf-PDMS of two compounds, bis(10-hydroxybutoxy-methyl pheophorbide $\text{a}$)-1,4-dichlorophtalate and linear bis(tetraphenylporphyrin) are discussed.

A multistop time digitizer is described which was designed specifically for ²⁵²Cf-plasma desorption time-of-flight mass spectrometry. The digitizer is a high resolution time intervalometer capable of measuring the times of a number of stop events following a common start. A resolution of 78.125 ps is achieved. The time range can be varied from 5 ms to 320 ms in 16 steps. Stop event processing time takes less than one microsecond, enabling high resolution measurements of closely spaced bursts of events.

The digitizer consists of the following three modules. A clock module provides the accurate time reference including a precision time calibrator. An analog module contains a special tandem stretcher by which the high resolution and low differential nonlinearity are achieved. A digital module comprises the counting, logic and interface circuits. Up to four complete digitizers can be housed in and powered by a standard CAMAC crate.

After a short survey of the problems in desorption kinetics we review the theory of phonon mediated physisorption kinetics at low coverage within the master equation approach (cascade model). A simplified Kramers-Fokker-Planck description is extracted. For finite coverage we review the mean field theory of multilayer physisorption and discuss the predicted compensation effect in the desorption rates around monolayer coverage. Lastly we look at photodesorption of molecules by resonant laser-molecular vibrational coupling.

A review is given of several aspects of charged particle energy losses in condensed matter. Emphasis is placed on the spatial distribution of energy deposition near the surface. The influence of collective bulk and surface modes on absorption processes is sketched. Comparison of coherent energy deposition in the infratrack and delta-ray energy deposition in the ultratrack is made. Theories of surface desorption of atoms and small molecules by ionizing radiation and the use of multi-hit and multi-target theory in the analysis of large biomolecule desorption are discussed.

At the heavy ion accelerator UNILAC/GSI-Darmstadt Phenylalanine films of 2000 Å thickness evaporated onto Al-foils were irradiated by ¹⁶N and ²³⁸U ions having a fixed energy of 1.4 MeV/N. The yields of secondary ions ejected from the target surface were investigated as function of the primary ion charge q. Changing the charge state q/e from 2⁺ to 49⁺ an increase of ion yields is observed, which does not follow a straight qⁿ-dependence. The same targets were also irradiated by ⁴He²⁺ions of 1.37 MeV/N. The corresponding ion yields were 430 times lower than yields measured with ¹⁴Ne². Additionally the mass spectra obtained with heavy ions were compared with laser induced ion desorption. It turned out that the relative yields of laser ejected ions are very sensitive to the amount of energy absorbed in the target. - The results are discussed in the frame work of the modified lattice potential model of Watson and Tombrello (ref.1).

Experiments with heavy ion beams from accelerators have been performed to study the desorption of atomic and secondary ions. The shapes of the yield curves have been measured for energies of projectiles ranging from O.1 to about 5 MeV/u. Secondary electron yields have been measured in coïncidence with projectiles and secondary ions. A Different primary ion velocity dependence is found for electron and secondary ion yields.

The ²⁵²Cf Time-of-Flight/Plasma Desorption Mass Spectroscopy method (²⁵²Cf TOF/PDMS) is an example of how technology transfer from one discipline to another can stimulate new approaches to the solutions of important problems. The ²⁵²Cf TOF/PDMS technique evolved from three technologies originally developed by Macfarlane and his co-workers to study nuclear processes: the electrostatic particle guide, the helium-jet recoil transport method, and the on-line β-recoil mass spectrometer. In this paper, these techniques are reviewed, and their contributions to the ultimate development of ²⁵²Cf TOF/PDMS are discussed.

One problem encountered in the mass spectrometry of solid materials is the complex interactions that take place on the surface during the ion-induced desorption process (PDMS)¹. One possible way to reduce the complexity of this system is to develop a matrix that can control the adsorption site of the molecule of interest. It is possible that ion-containing polymers such as Nafion² (used in this study) can aid in this process.