Velocity selector

These equations indicate that the energy of the scattered ions is sensitive to the mass of the scattering atom s in the surface. By scanning the energy of the scattered ions, one obtains a kind of mass spectrometric analysis of the surface composition. Figure VIII-12 shows an example of such a spectrum. Neutral, that is, molecular, as well as ion beams may be used, although for the former a velocity selector is now needed to define ,. ... 

Diagram of ihe rotating disks lhal serve as a velocity selector in a molecular beam apparatus... 

Molecular beams provide the answer. We first met molecular beams in Box 4.1, where we saw how a velocity selector is constructed. A molecular beam consists of a stream of molecules moving in the same direction with the same speed. A beam may be directed at a gaseous sample or into the path of a second beam, consisting of molecules of a second reactant. The molecules may react when the beams collide the experimenters can then detect the products of the collision and the direction at which the products emerge from the collision. They also use spectroscopic techniques to determine the vibrational and rotational excitation of the products. 

Henglein (23) has constructed a machine for studying stripping reactions which does not fall into any of the above categories. It consists of an ion gun followed by a flight tube which also serves as a reaction chamber. A velocity selector scans the ions which have suffered little or no change in direction, and energy analysis of the secondary ion beam is used to deduce cross-sections and reaction mechanisms in chosen simple cases. 

Fio. 7. Ionization efficiency curve for oxygen (a) obtained using monoenergetic electrons (= + 0-06 e.v.) from an electrostatic velocity selector. The positions of thresholds due to the ground state and vibrationally excited states of the 77g ion are indicated by the arrows (b). (Reproduced with permission from Brion, 1964.)... 

Cross-section curves ) obtained in this way constitute probably the most reliable and most detailed experimental information on total ionization cross sections available at present. It is available for the Pgl systems He(2 5)-Ar,Kr,Xe,N2 and He(23S)-Ar,Kr,Xe,N2.34 Unnormalized cross-section curves arel(t>) are available for the systems He(21S ) Hg,Ba He(23S)-Hg,Ba Ne(3/>20)-Hg and ArC -Hg34 44. Cross-section curves directly measured on an absolute scale, using a mechanical velocity selector, are available for the systems Ne( 2/3) -Ar,Kr,Xe 45 As examples, we show cross-section curves for He(2 S), He(235)-Ar, He(2 S), He(23S)-Hg, and Ne -Kr in Figs. 6 to 8. In Fig. 8 results of the TOF method34 are compared with results using mechanical velocity selection 45 For the system He(23S)-Ar, it is shown in Fig. 9 that the temperature... 

The intensity of signal transmitted to the detector is greatly improved by using time-of-flight methods instead of mechanical velocity selectors. The beam of product molecules is chopped into a sequence of short pulses and the molecules then travel a known distance before being detected. The time-of-arrival spectrum at the detector gives the velocity distribution of the products [30]. This method of velocity analysis is now widely used in studies of crossed-beam reactions [111]. 

For the reactions K + HBr and K + DBr, the KBr recoil energy distribution has been determined in a crossed-molecular beam experiment using a mechanical velocity selector. No difference was found in the form of the translational energy distributions for the two reactions for which a value of 0.30 may be derived. Although all the angular momentum appears in the product rotation, the moments of inertia for the alkali halides are large, which implies that the mean product rotational energy is quite small ( 0.21, 0.21 and 0.09 for K, Rb, Cs + HBr, respectively [3] these values are derived from the rotational temperatures obtained by electric deflection analysis). 

Details of vacuum chamber and velocity selector not shown... 

Analyzers positioned after the accelerator remove scattered particles accepted by the injector analyzer, molecular fragments, and unwanted charge states. Magnetic analyzers alone are not sufficient. An electrostatic analyzer or velocity selector is necessary to remove particles that have different mass but would otherwise have the correct mass-energy product to pass through the magnetic analyzers. 

The total pressure of NaF(g) and Na F Cg), in equilibrium with NaF(cr, l) at temperatures 1020 - 1974 K, have been determined by many investigators, using manometric (, 2, 5), torsion-effusion (6, 8), Knudsen-effusion (3, 7), transpiration (4), and molecular-beam velocity-selector (9, 10) methods. In order to evaluate (LiF. g) we have used a trial and error variation of... 

Although the absolute values of the vapor pressure measurements are not used for evaluation, the ratios of the numbers of dimeric and monomeric NaF molecules that effuse from the oven at the temperatures 1115 - 1191 K determined by Eisenstadt et al. (1 ) by use of the electron-beam velocity-selector method are used. Based on the reported equilibrium data, we evaluate the enthalpy change of the reaction (A) Na F Cg) = 2 NaF(g) by the 2nd and 3rd law methods. The results are presented in the table below. 

To see more fringes we have to increase the coherence length and therefore decrease the velocity spread. For this purpose we employ a mechanical velocity selector, as shown after the oven in Fig. 1. It consists of four slotted disks that rotate around a common axis. The first disk chops the fullerene beam. Only those molecules are transmitted which traverse the distance from one disk to the next in the same time that the disks rotate from one open slot to the next. Although two disks would suffice for this purpose, the additional disks decrease the velocity spread even further and help to eliminate velocity sidebands. By varying the rotation frequency of the selector, the desired velocity class of the transmitted molecules can be adjusted. To measure the time of flight distribution we chopped the fullerene beam with the chopper right behind the source (see Fig. 1). The selection is of course accompanied by a significant loss in count rate, but we can still retain about 7% of the unselected molecules. 

It should also be pointed out that by using the velocity selector we can now choose a slower mean velocity centered about 136 m/s, which corresponds to a de Broglie wavelength of 4.1 pm. It is obvious that this increase in wavelength results in a wider separation of the diffraction peaks, which can be seen by comparing Figs. 2 and 4. 

An example of a similar spectrometer is the one used by Champion et al.9 Primary ions were produced by electron bombardment, accelerated and mass analysed by a 60°, 13-3 cm magnetic mass spectrometer. The ions were then retarded to the desired energy and energy selected by a 127° electrostatic cylindrical velocity selector. The energy resolution was 5 %. The beam half angle is reported to be 18°. The ions then entered a reaction chamber at a pressure of about 10 4torr. The chamber had an exit slit which could be rotated with the detecting system which consisted of another 127° velocity selector, a quadrupole mass spectrometer and an electron multiplier. 

Fig. 1. Schematic diagram of a crossed molecular beam apparatus. (1) primary beam oven. (2) velocity selector. (3) secondary beam oven. (4) velocity analyser for the secondary beam, (5) detector for measuring the differential cross section, (6) monitor detector. (7) detector for measuring the total cross section.

In Fig. 3 the apparatus is shown with which Stolte (1972) has performed measurements of this type NO molecules were selected with the help of electrostatic sixpole fields, in accordance with their linear Stark effect in strong fields. The source slit of 0 05 mm width has an image formed by the selected NO molecules in the plane of the detector slit which has an experimental width of 1-4 mm f.w.h.m. this width includes all disturbing effects like the magnification factor (about 18), the imperfect linear Stark effect of the NO molecules in the selected state, the finite width of the transmission of the velocity selector (Av/v = 7% f.w.h.m.) in combination with the chromatic lens errors and the directional dependence of the maximum transmitted velocity of the velocity selector. The j = nij = Q = 3/2 state was selected where 1 is the projection of the electronic angular momentum on the molecular axis. The hyperfine structure of NO influences the situation only slightly. 

A serious disadvantage of the sputtered beams is the low intensity caused by the need of a velocity selector. 

As we have discussed above, the direct measurement of the amount of energy disposed into relative kinetic energy of the products is restricted to molecular beam experiments. The earliest measurements of the product recoil velocity used a velocity selector consisting of a series of rotating slotted discs, where the slots act as phased choppers such that only a narrow range of velocities are transmitted for a given rotation frequency... 

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