Background subtraction Beam, molecular

Hastie [131] coupled for the first time a quadrupole mass spectrometer with a Knudsen cell. One of the quadrupole mass spectrometer - Knudsen cell systems used at our laboratory is shown in Fig. 4. The system has been developed to study small alkali metal clusters under equilibrium conditions (see Sect. 3.2). Broad-band photoionization by a 1 kW Hg/Xe lamp is used for the first time in Knudsen effusion mass spectrometry to reduce fragmentation. Other quadrupole mass spectrometer - Knudsen cell systems have for example been developed by Hilpert [132, 133], Fraser and Rammensee [134], Plante [135], Ono et al. [136], Kematick et al. [137], and Edwards et al. [138]. Cryogenic pumping is used in the device by Hilpert to reduce mercury background ion intensities for the study of amalgams [132], The instruments described in Refs. 134,135 use a chopper to modulate the molecular beam from the Knudsen cell. Interfering background ion intensities can, thereby, be subtracted. The apparatus developed by the authors of Refs. 137, 138 renders possible the simultaneous application of Knudsen effusion mass spectrometry and the mass-... 

When an electron beam passes through a gas, the electrons are scattered and give rise to a diffraction pattern, which contains information on the molecular structure of the gas molecules. The diffraction pattern is recorded on a photographic plate, and can be analysed by a computer treatment, in which the intensity distribution is compared with theoretically derived patterns given by various structural models. The undulation pattern can be ascribed to the sharply defined scattering from the nuclear positions, and has to be subtracted from the background ascribed to the much less well-defined contribution from the continuous distribution of electron density in the molecule. For molecules that may assume more than one conformation, an overlap of the pattern from the various conformers is obtained. The radial distribution function contains peaks rather than lines, and the peak width... 

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