Velocity selection

In addition to obviating the need of velocity selection, the increased backing pressure over that attainable with an eflfiisive source leads to significantly higher downstream beam densities. 

The velocity distribution/(v) depends on the conditions of the experiment. In cell and trap experiments it is usually a Maxwell-Boltzmann distribution at some well defined temperature, but /(v) in atomic beam experiments, arising from optical excitation velocity selection, deviates radically from the nonnal thennal distribution [471. The actual signal count rate, relates to the rate coefficient through... 

Molybdenum and tungsten are unique in that they are resistant to sulfur, and, in fact, are commonly sulfided before use. The Bureau of Mines tested a variety of molybdenum catalysts (32). They are moderately active but relatively high temperatures are required in order to achieve good conversion, even at low space velocities. Selectivity to methane was 79-94%. Activity is considerably less than that of nickel. Although they are active with sulfur-bearing synthesis gas, the molybdenum and tungsten catalysts are not sufficiently advanced to be considered candidates for commercial use. 

We present here a summary of recent work with light-pulse interferometer based inertial sensors. We first outline the general principles of operation of light-pulse interferometers. This atomic interferometer (Borde et al., 1992 Borde et al., 1989) uses two-photon velocity selective Raman transitions (Kasevich et al., 1991), to manipulate atoms while keeping them in long-lived ground states. 

Brook and co-workers were able to excite the HC1 molecules to v = 1 state using a chemical laser and showed that HC1 (v = 1) was 100 times more reactive than HC1 (v = 0) on collision with K atom. In a velocity selected beam experiment it was also observed that for the same amount of energy reagent, vibration was ten times more effective than translational in bringing out the reaction... 

For systems like He -Ar, with real parts Vt(R) having well depths D% smaller or comparable to thermal energies, elastic differential scattering data obtained with thermal velocity selected beams yield, in connection with the rather well-developed theory, the most reliable information on to date. The systems such as He -H,Li with well depths D% considerably larger than thermal energies cannot, however, be studied in this way. The most direct information on in these cases is obtained... 

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... 

From Penning electron energy distributions measured with velocity selected metastables in the thermal range the ratio is directly obtained as the ratio of the areas of the distributions defined by conditions (II.4a) and (II.4b), respectively. 

The most straightforward way of reducing the collision velocity is to velocity select the atoms in an atomic beam,17 and a natural method of velocity selection is... 

Fig. 14.13 Experimental arrangement for velocity selecting and focusing the atomic beam. The rotating slotted disc and the pulsed laser beam select atoms in a velocity group, and the hexapole magnet focuses them where they cross the laser beam (from ref. 18).

The first and most obvious question is whether or not a narrower velocity distribution leads to narrower collisional resonances. In Fig. 14.14 we show the Na 26s + Na 26s — Na 26p + Na 25p resonances obtained under three different experimental conditions.20 In Fig. 14.14(a) the atoms are in a thermal 670 K beam. In Figs. 14.14(b) and (c) the beam is velocity selected using the approach shown in Fig. 14.13 to collision velocities of 7.5 X 103 and 3.8 X 103 cm/s, respectively. The dramatic reduction in the linewidths of the collisional resonances is evident. The calculated linewidths are 400, 28, and 10 MHz, and the widths of the collisional resonances shown in Figs. 14.14(a)-(c) are 350,40, and 23 MHz respectively. The widths decrease approximately as l/v3/2 until Fig. 14.14(c), at which point the inhomogeneities of the electric field mask the intrinsic linewidth of the collisional resonance. 

Fig. 14.14 Na 26s + Na 26s —> Na 26p + Na 27p collisional resonances observed with (a) no velocity selection, collision velocity 4.6 x 1(T cm/s, (b) velocity selection to a collision velocity 7.5 x 103 cm/s, (c) velocity selection to a collision velocity 3.8 x 103 cm/s (from...

As we pointed out above, an attractive feature of the velocity selected K collisions is that we can examine the region o) 2n < 1/r. We consider first the case in which there is no control over the phase of the field at which the collisions occur. This case is exemplified by Figs. 15.11(b)-(d), which show the effect of adding progressively larger amplitude 1 MHz rf fields of uncontrolled phase. Since the laser fires at an uncontrolled phase of the rf field, the observed cross section can be calculated using Eq. (15.34). As shown by Fig. 15.11 the cross section is broadened in approximately the manner shown in Fig. 15.7. As shown by a... 

We wish to add that there exists a wide variety of literature that considers the opposite case of monochromatic excitation by an infinitely narrow line causing velocity selection, such as [261, 268, 269, 320, 362] and the sources quoted therein. This description has been developed basically in connection with laser theory it refers most often to stabilized single-mode excitation. The intermediate case between monochromatic and broad line excitation is the most complex one, requiring integration over the modal structure of the laser inside the bounds of the absorption contour [28, 231, 243]. 

The subject of molecular beam kinetics is very extensive and in this section, therefore, we will deal only briefly with the relevant aspects of the topic. Molecular beam sources are often thermal, operating as a flow system with a gas or a vapour from a heated oven. The velocity distribution of species in such beams is Maxwell—Boltzmann in form. For many experiments, this does not provide sufficient definition of initial translational energy and some form of velocity selection may be used [30], usually at the expense of beam intensity. 

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.

As in M + X2, the reactive scattering is strongly anisotropic, but now the diatomic product is found predominantly in the backward hemisphere. The early experiments [2,43, 70], performed without velocity selection or velocity analysis, were difficult to interpret quantitatively because of the unfavorable kinematics, which arose because (a) mKI mcH, (b) considerable blurring was introduced by the spread in the incident velocities, and (c) the v lu f factor in the Jacobian distorts the lab distribution and cannot be... 

Blythe, Grosser, and Bernstein [151 ] have used crossed molecular beams to observe the J = 2 - 0 rotational deexcitation process in D2. A velocity-selected atomic beam of potassium was made to impinge on a modulated Da beam from an effusive (T = I8PK) source. The scattered K atoms were detected by surface ionization on a hot Pt-W ribbon, from which the ions were drawn into an electron multiplier equipped with lock-in amplification. 

Schottler and Toennies [152] have employed a velocity-selected Li+ beam to study inelastic collisions giving rise to vibrational excitation in H2. Although individual vibrational levels were not completely resolved in these experiments, it was demonstrated that as the incident kinetic energy of the ions was... 

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