Beams effusion

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... 

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.

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... 

Fig. 4.1 Laser excitation spectroscopy with reduced Doppler width in a collimated molecular beam (a) schematic experimental arrangement (b) collimation ratio (c) density profile n x) in a collimated beam effusing from a point source A...

Figure B2.3.3. Crossed-moleciilar beam apparatus employed for die study of the F + D2 —> DF + D reaetion. Indieated in the figure are (1) the effusive F atom soiiree (2) slotted-disk veloeity seleetor (3) liquid-nitrogen-eooled trap (4) D2 beam souree (7) skimmer (8) ehopper (9) eross-eorrelation ehopper for produet veloeity analysis and (11) rotatable, ultralrigh-vaeuum, triply differentially pumped, mass speetrometer deteetor ehamber. Reprinted with pemrission from Lee [29], Copyright 1987 Ameriean Assoeiation for the Advaneement of Seienee.

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... 

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

Those fixed-angle measurements reported to date have all used either a heated effusive inlet, or heated gas cell for sample admission [55, 56, 61, 62, 65]. Probably the higher sample number densities these sources generate, compared to a supersonic beam source, provides some compensation for the reduced collection efficiency in the fixed-angle measurement. 

Gopinath, C. S. and Zaera, F. (2000) Transient kinetics during the isothermal reduction of NO by CO on Rh(lll) as studied with effusive collimated molecular beams , J. Phys. Chem. B, 104, 3194. 

For this purpose a special Knudsen cell with exchangeable orifice plates has been designed. When the equilibrium pressure, which corresponds to the saturation pressure ps is reached, effusion of a part of the vaporizing substance takes place through the orifice. The amount depends strongly on the shape and length of the orifice (Fig. 68). From the substance which vaporizes in all directions, only a partial beam can pass freely through the orifice. 

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. 

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