Secondary electron multiplier SEM

Ion intensities up to a count rate of 2 x 10 are measured using a secondary electron multiplier (SEM). When it becomes saturated above that value, it is necessary to switch to a Faraday cup. Its ion-current amplification must be adjusted to fit to the electron multiplier response. 

Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly employed in photometry. These are detectors with an external photo-effect . 

Figure 4.2 Basic principles of the secondary electron multiplier (SEM).

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

To intensify the ion current, an amplification via electron multiplication is advantageous. For this purpose a MCP is introduced into the PIMMS. An MCP rectangular cut in shape from a standard MCP [25,26] works as a secondary electron multiplier (SEM) as shown in Fig. 11. The MCP is hybrid-integrated into the spectrometer by clamping with flexible silicon springs 20 pm in thickness. They serve both as mechanical fixture and electrical contact. The MCP is operated between 400 and 1,300 V and amplifies the current by more than 1,000, which allows for measurements into the <100 ppm region. 

Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly i are detectors with an external photo-effect .-------------------------------... 

Most process analyzers utilize either a Faraday cup or a secondary electron multiplier (SEM) for detection. The Faraday cup is the simpler and more mgged and stable of the two, but is generally useful for detection of species at higher concentrations (100 ppm to 100%). The SEM is much more sensitive, capable of measurements in the ppb range. It is quite common to configure a process MS with both detectors, along with a set of electrostatic lenses to switch the mass-filtered ion beam between the two detectors. This results in a single process analyzer that is capable of quantitation from 1 ppb to 100 % ... 

Secondary electron multiplier (SEM) detectors replaced Faraday cup detectors for scanning mass spectrometers. The electron multiplier is based on the concept of a photomultiplier except that there is no glass membrane, so ions (electrons) can enter the amplification region of the detector. Because electron multipliers are not sealed and are open to the atmosphere, they must be operated under vacuum conditions and therefore cannot be used directly in atmospheric pressure IMS. 

The exit radiation is measured in a secondary electron multiplier (SEM) used as a detector as the photons hit the photocathode. The latter usually consists of alkali-metal alloys, and is of varying sensitivity, depending... 

The secondary electron multiplier (SEM) detector is the key to the role of mass spectrometry as an extremely sensitive analytical technique with wide dynamic range and compatibility with on-line coupling to fast chromatographic separations. The SEM was a natural development from the invention of the photomultiplier (Zworkin 1936, 1939), in which photoelectrons produced by photons falling on a conversion dynode with a photo-sensitive surface are amplified in an avalanche fashion by accelerating the original (first strike) photoelectrons on to a... 

FIGURE 28.1 Schematic diagram of the PTR-MS instrument that contains a hollow cathode (HC), a source drift (SD) region, an intermediate chamber (IC), and a secondary electron multiplier (SEM). 

Fig. 2.2. Experimental setup (as used for the investigations on NasB). An argon ion laser (ps mode-locked fs all lines, visible) pumps either a femtosecond laser system (a) (OPO synchronously pumped optical parametric oscillator SHG second-harmonic generator) or a picosecond laser system (b) (taken from [178]). The pulse duration and spectral width of the laser pulses are measured by an autocorrelator (A) and a spectrometer (S) respectively. A Michelson arrangement allows the probe pulses to be delayed At) with respect to the pump pulses. A quadrupole mass filter (QMS) enables the selection of the ensemble of investigated molecules ionized by a pump probe cycle. A secondary electron multiplier (SEM) detects the intensity I of the ions as a function of the delay time At. A Langmuir-Taylor detector (LTD) measures the total intensity /o of the cluster beam. The ratio I/Iq as a function of the delay time At is called the real-time spectrum...

Fig. 2.18. Side elevation of the molecular beam machine s two-chamber setup. Chi vacuum chamber to generate the molecular beam by adiabatic expansion. A seeded supersonic beam source is placed here. Further details are shown in Fig. 2.19. Ch2 here, the molecules interact with the laser pulses. Detection is performed by a quadrupole mass spectrometer (QMS) with a 90° ion deflector between the mass filter and secondary electron multiplier (SEM) (Fig. 2.20) or the Langmuir-Taylor detector (Fig. 2.22)...

The ion currents in Table 46 (from fourth to eighth columns) were corrected for the isotopic composition of the chemical elements. When a secondary electron multiplier (SEM) was used, corrections were applied for the dependence of the multiplier gain on the ion mass. We found that this gain is proportional to where M is the mass of the ion... 

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