Beams effusive beam

An effusive beam of atoms or molecules (see Ramsey, 1956 in fhe bibliography) is produced by pumping fhem fhrough a narrow slif, fypically 20 pm wide and 1 cm long, wifh a pressure of a few forr on fhe source side of fhe slif. The beam may be further collimated by suifable apertures along if. 

Such beams have many uses, including some imporfanf applications in specfroscopy. In particular, pressure broadening of specfral lines is removed in an effusive beam and, if observations are made perpendicular to fhe direction of fhe beam, Doppler broadening is considerably reduced because fhe velocify componenf in fhe direction of observation is very small. 

Why are fhese beams, or jefs, distinguished from effusive beams by fheir description as supersonic In some ways fhis description is rafher misleading, firsf because particles in an effusive beam may well be fravelling af supersonic velocities and, second, because fhe name implies fhaf somefhing special happens when fhe particle velocities become supersonic whereas fhis is nof fhe case. Whaf supersonic is meanf fo imply is fhaf fhe particles may have very high Mach numbers (of fhe order of f 00). The Mach number M is defined as... 

Effusive beam technique, 157-158 Electron bombardment flow radiolysis, 238 Electrospray ionization and ionic clusters, 168 Enantiomers, separation techniques, 154-155 Enantioselectivity of enzymes, 148 Enthalpy-entropy compensation plots, 261 Enthalpy of activation, and quantum tunneling, 67, 70-71... 

Fig. 15.21 The experimental setup including the laboratory-scale coffee roaster with a sampling unit and a laser mass spectrometer. The homebuilt mobile device consisted of a Reflectron TOFMS analyser, an effusive beam inlet system and a built-in laser operated at 266 nm (Continuum NcL YAG laser SURELIGHT", 266 nm). (Adapted from [203])...

Knudsen effusion mass spectrometry is used to measure the composition of the effusing beam (including the composition of the sample investigated, fragments or newly formed species) and in particular to determine the thermochemical data. The instrumentation is briefly described in Section 5.2.10.137... 

In conventional gas electron diffraction experiments, an effusive beam is used in which vibrational levels of molecules are thermally populated and the width of a peak in a radial distribution curve is determined by thermally averaged mean amplitudes. When a molecular beam or a free jet is used, mean amplitudes could become small, since the contribution from the vibrationally excited levels is reduced significantly. As a consequence, sharper peaks are expected in the radial distribution curve, and the spatial resolution of the snapshot could be improved. However, it seems that the observed peaks in the radial distribution curve are considerably broad even though a molecular beam is used. There could be some reasons to have such broadened peaks in the radial distribution curve. 

Recent measurements utilizing the crossed-beam technique have been performed as follows.37 A metastable helium beam is formed by electron-impact excitation of a thermal helium beam effusing from a multichannel array. The optical quenching method12 described earlier is applied to obtain results for He(2 5 ) and He(23S) separately. The target gas beam is... 

In Section 2.3.5.1 we have seen that pumping of atoms or molecules through a narrow slit, or small pinhole, whose width (or diameter) is about 20 pm at a pressure of a few torr on the high-pressure side of the aperture produces an effusive beam. Removal of pressure... 

A static fill, an effusive beam or a supersonic nozzle can be used for the introduction of the gas sample,. The supersonic nozzle has the advantage of refrigeration, and one may study rotationally and vibrationally cold molecules, or even stabilize otherwise inaccessible van der waals molecules. Finally, when operated in a pulsed mode, the nozzle cycle can be matched to the duty cycle of the laser to optimize the measuring conditions and improve the background vacuum. 

Figure 1.2 Representation of a simple crossed-molecular-beam source [16]. The primary beam effusing from an oven source (A) is velocity selected (S) and then crosses the thermal beam issuing from a second source (B). This diagram shows the detector (D) positioned at the lab angle 0.

Previous experimenters have found dimerization in all alkali halides at vapor pressures corresponding to source temperatures appropriate for effusive beams ( 5, 6, T). The percent dimerization increases as molecular weight decreases towards lighter alkali halides, with lithium fluoride possessing the largest fractional dimerization. 

An additional effusive beam source, operated with pure O2 as the precursor geis, is used for some studies. This source, shown schematically in Fig. 7, is a low pressure ( 100 mTorr) RF discharge tube, with a small hole (0.5 mm) in the end. The inductively coupled plasma is typically operated at a power of 50 W. The effluent (beam) from this source has been characterized by aligning the detector with the plasma source and placing... 

The barium atom beam effuses from a molybdenum oven (orifice diam. 0.3 cm) that is resistively heated to between 1000 and 1100 K using tantalum wire windings. The oven is... 

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