Pulsed Main Beams of Ions

Although the above has considered only the use of a continuous main ion beam, which is then pulsed, it is not necessary for the initial beam to be continuous it too can be pulsed. For example, laser desorption uses pulses of laser light to effect ionization, and the main ion beam already 

Diagram showing a flow of ions of m/z a, b. c, etc. traveling in bunches toward the front face of a microchannel array. After each ion strikes the inside of any one microchannel, a cascade of electrons is produced and moves toward the back end of the microchannel, where they are collected on a metal plate. This flow of electrons from the microchannel plate constitutes the current produced by the incoming ions (often called the ion current but actually a flow of electrons). The ions of m/z a, b, c, etc. are separated in time and reach the front of the microchannel collector array one set after another. The time at which the resulting electron current flows is proportional to V(m/z).  

Since the microchannel plate collector records the arrival times of all ions, the resolution depends on the resolution of the TOP instrument and on the response time of the microchannel plate. A microchanne] plate with a pore size of 10 pm or less has a very fast response time of less than 2 nsec. The TOF instrument with microchannel plate detector is capable of unit mass resolution beyond m/z 3000. 

Ion detectors can be separated into two classes those that detect the arrival of all ions sequentially at one point (point ion collector) and those that detect the arrival of all ions simultaneously along a plane (array collector). This chapter discusses point collectors (detectors), while Chapter 29 focuses on array collectors (detectors). 

Other types of mass spectrometer may use point, array, or both types of collector. The time-of-flight (TOF) instrument uses a special multichannel plate collector an ion trap can record ion arrivals either sequentially in time or all at once a Fourier-transform ion cyclotron resonance (FTICR) instrument can record ion arrivals in either time or frequency domains which are interconvertible (by the Fourier-transform technique). 

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Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

Recent review articles ([16,17] and references therein) allow the interested reader to get a broader picture of this exciting research domain and related applications. In the following (Sect. 10.2), we will mainly devote ourselves to the principles of the main ion acceleration mechanism, and to the way the temporal profile of the laser pulse, and more specifically the beam contrast ratio, can influence it. In particular, we will briefly review the main theoretical and experimental published work concerning the action of a plasma gradient on ion acceleration characteristics. Section 10.3 presents the contrast improvement device we have implemented for our laser beam, and the related temporal profile measurements. In Sect. 10.4, we will show and discuss the main results obtained using ultra high contrast (UHC) laser pulses in laser-driven ion acceleration experiments. Finally, an example of the exploitation of the particular features of UHC pulses in laser-driven ion acceleration will be given in Sect. 10.5. 

Table 8.2 compares the main parameters of three mass analyzers. From Table 8.2 we can understand the reasons that the ToF analyzer has become so popular for static SIMS it provides high resolution, high transmission and high sensitivity. The major shortcoming of the ToF analyzer is its use of pulse primary ions. The ratio of primary beam on- to off-time is only about 10-4. Thus, it is not efficient for analysis such as depth profiling of chemical elements. 

The orthogonal acceleration (oa) feature of a TOF mass analyzer enables it to be used with continuous-beam ion sources [28-31]. The ion beam from the external source enters an ion acceleration region from a direction perpendicular to the main axis of the TOF instrument (see Figure 3.13). A short pulse of an orthogonal accelerating field is applied to eject the ions efficiently in a section of... 

The technical principle of MALDl imaging is summarized in Figure 4.1. A pulsed laser beam is focused to the size of the aspired lateral resolution. To date, mainly lasers with ultraviolet wavelengths (337, 355, and 266 nm) and pulse lengths of a few nanoseconds have been used. The focused laser beam is directed to the surface of the sample, which is then moved in steps in order to scan the sample according to the intended lateral resolution of the system. Before analysis, the sample must be prepared in such a way that the biomolecular ions can be desorbed and ionized by the laser beam, as in regular MALDl analyses. To achieve this, the sample (e.g., a biological tissue sample) must be covered with a suitable matrix, such as 2,5-dihydroxybenzoic acid (DHB), sinapinic acid (SA) or... 

Dr. Bernhard Holzapfel is head of the superconducting materials group at the Leibniz Institute for Solid State and Materials Researdi (IFW) Dresden, Gamany. His main area of research is pulsed laser deposition of functional thin films and superconductivity. Currently he works on the develr ment of HTSC high Jc coated conductors using ion beam assisted deposition or highly textured metal substrates. His work is supported by a number of national and European founded research projects. 

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