Surface ionization detector

Using a surface ionization detector and a tunable dye laser, Van Veen and his colleagues (202) studied the photolysis of KI between 265 and 335 nm using the TOF technique. They were able to show from the TOF spectra that only ground state were... 

The essential elements.of the experiment are a) an effusive molecular beam source, b) inhomogeneous deflecting electric polefaces, c) surface ionization detector, capable of translation in order to obtain the deflected beam pattern. 1, 2, are the distance from the source to the front of the polefaces, the length of the polefaces and the distance from the back of the polefaces to the detector, respectively. A general review of deflection methods for determining polarizabilities is given by Miller and Bederson (8). 

Fig. 18. Total cross sections Q for the production of negative ions in collisions of K, Na and Li on Br2 and Cl2, divided by the unknown energy dependent surface ionization detector efficiency / , vs. the CM-energy. The thresholds are indicated by arrows. The insets show enlargements of the thresholds. Notice the small structure at about 4eV in the Na-Cl2 cross section. The constant signal below threshold is due to the presence of dimers in the alkali beam (Baede and Los, 1971).

Despite its tremendous power, Tonks5 arrangement is still limited to the study of easily ionizable metal atoms, inasmuch as it depends on a surface ionization detector. This limitation was removed by Apker (4) who substituted a hot filament ionization gauge as a detector and was therefore, for the first time, able to measure the evolution of electronega-... 

Obviously, due to the many possible reaction channels which are open in this energy range, a simple surface ionization detector (see Section... 

Dudley R. Herschbach (1932- ) began a series of molecular beam experiments at Berkeley starting in 1960. He employed two small ovens with slits producing crossed beams of molecules or atoms under very low pressure that collide in a vacuum reaction chamber. The early apparatus was built upon a turntable directing the stream of products toward a surface ionization detector sensitive to alkah metals and their salts. His first successful experiment crossed beams of potassium atoms and CH3I molecules ... 

Fig. 5-8 Scattering of K -I- CH3I. (a) Intensity of scattered signal as measured by Pt ( ) and W (O) surface-ionization detectors. Pt measurements are normalized to W measurement in main K beam. Main K beam, shown at laboratory angle = 0, is attenuated by CH3I crossed beam, (b) Detected KI distribution for two experiments. (Adapted from Herschbach [22].)...

It may be assumed that (a) the intensity along the length of the beam is constant (b) the efficiency of the surface-ionization detector is unity. 

At the end of the apparatus the beam of atoms is monitored by either a surface ionization detector or a simple mass spectrometer. 

The detector, which usually can be rotated to various scattering angles, must be able to differentiate among the species present. Surface-ionization detectors with efficiencies of approximately 10% are employed for reactions of metal atoms yielding metal halide products. Laser-induced fluorescence has proved to be an excellent detector for group Ilb halides and a few other systems. In general, products can be detected by electron-bombardment ionization with subsequent analysis by a mass spectrometer. Unfortunately, these mass spectrometer or universal detectors have a low detection efficiency ( 0.1%,) thus, beam machines with such detectors require several differentially pumped chambers to reduce the background pressure (of the mass of interest) in the detector to torr. Modulation tech-... 

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