Knudsen cells molecular beams

The central KEMS equation can be derived now that the Knudsen cell vapor source and mass spectrometer have been described. This follows directly from the vapor flux in the molecular beam selected from the distribution of material effusing from the Knudsen cell (molecular beam flux equation) and the definition of the ionization cross section (Equation 48.18). However, in accordance with the aim of identifying factors that affect the measured ion intensity and that are unrelated to sample temperature and composition, it useful to rewrite Equation 48.18 in terms of the number of ions produced per second in the elementary volume dv in the region defined by the intersection of the molecular and electron beams, ni(E) [71,80] (this is prior to the formation of the ion beam) ... 

Fig. 4. Schematic of a high vacuum molecular beam epitaxy (MBE) chamber containing four effusion (Knudsen) cells. Also shown is a high energy electron...

A molecular beam generated by reaction in a Knudsen cell—a... 

Figure 1 shows a magnetic-type sector field mass spectrometer coupled with a Knudsen cell. The most important part of the instrument is the Knudsen cell. It can be heated up to temperatures above 2500 K. The temperatures are measured with an optical pyrometer or a thermocouple. There would be thermodynamic equilibrium in the Knudsen cell if it were closed. However, real Knudsen cells have an effusion orifice (typical diameter 0.1 to 1 mm) through which a small fraction of the molecules effuse without practically disturbing the equilibrium in the cell. A molecular beam representing the equilibrium vapor in... 

The identification of the ions in the mass spectrum originating in the vapor species over the sample can be easily carried out by their mass, their isotopic distribution, and by interrupting the molecular beam from the Knudsen cell with the shutter. The mass spectra for molecular ions can be computed from those of the component elements. The essential step for the identification of the gaseous species in the equilibrium vapor is the assignment of the ions observed in the mass spectrum to their neutral precursors. This can be difficult due to fragmentation. The following rules are useful for the assignment ... 

Modulation of the molecular beam from the Knudsen cell with phase analysis of the ion signal is useful for the assignment of ions to noncondensible gaseous species in the cell [37]. 

The pressure calibration factor k takes into account losses of species caused for example by the intensity distribution of the molecular beam from the Knudsen cell or the transmission of the ion source and the analyzer. Three different methods are generally used [86] ... 

Different mass spectrometer-Knudsen cell systems are used at various laboratories. Single-focusing magnetic sector field or quadrupole mass spectrometers are used in most cases for the analysis of the molecular beam. 

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-... 

Gomez et al. [145] describe skimmer diaphragms to define the molecular beam originating in the Knudsen cell for the analysis thereby increasing the potential for Oj and partial pressure measurements in the cell. 

Multiple Knudsen cells for the determination of chemical activities have been reported by Chatillon and coworkers [148-151], Fraser and Rammensee [134], Zimmermann [152], Paulaitis and Eckert [153], and Hackworth et al. [154]. Multiple cells have practically the same temperature. One cell contains a reference material, the other for instance samples of different compositions. A comparison of the vapor in the different cells is generally rendered possible by their displacement from outside so that the molecular beams originating in the different cells can be analyzed successively. The quadrupole mass spectrometer and not the multiple Knudsen cell is displaced in the Knudsen cell-mass spectrometer system described by Ref. 153. No displacement of the mass spectrometer or the cell is necessary if the triple cell by Hackworth et al. [154] is used. Backdiffusion through the orifice has, however, to be considered for the two cells containing sample and reference. 

Mass spectrometric analysis of a molecular beam selected from the effusate distribution coming from a Knudsen cell provides information about the identity of... 

KNUDSEN CELL VAPOR SOLfRCES AND MOLECULAR BEAMS 1147... 

KNUDSEN CELL VAPOR SOURCES AND MOLECULAR BEAMS... 

The major advantage of defining the molecular beam this way is calibration. Consider two Knudsen cells on a multiple-cell fiange—one with a pure material that has a known vapor pressure and one with an alloy with a component that has an unknown vapor pressure. Ideally we can use the cell with the pure material to determine the calibration constant that relates measured ion intensity to vapor pressure. However, this determination of a calibration constant only gives a reliable value if the molecular beam from each cell is sampled in exactly the same way. 

The UHV system shown in Fig. 6.8 has different analysis and preparation devices (including two Molecular Beam Epitaxy systems) among which also a facility for Conversion Electron Mbssbauer Spectroscopy measurements. It is the lead-wrapped facility on the right of Fig. 6.9. The tube in the middle of Fig. 6.9 is an ultra high vacuum transport tube that can be used to transport samples from one facility to another. The facility on the front left of Fig. 6.9 is a MBE-system. It contains a Knudsen cell allowing to evaporate Fe onto surfaces. Other layers can be evaporated on top of this layer so that samples can be prepared for Fe Mbssbauer studies at surfaces and interfaces. 

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