Atomic beams

The nature of reaction products and also the orientation of adsorbed species can be studied by atomic beam methods such as electron-stimulated desorption (ESD) [49,30], photon-stimulated desoiption (PDS) [51], and ESD ion angular distribution ESDIAD [51-54]. (Note Fig. VIII-13). There are molecular beam scattering experiments such... 

Since this state is so low in energy, it is likely to be populated in the F atom beams typically used in scattering experiments (where pyrolysis or microwave/electrical discharges are used to generate F atoms), so the issue of its reactivity is important. The molecular beam experiments of Lee [43] and Toennies [45] showed no evidence for... 

Rettner C T 1994 Reaction of an H-atom beam with Cl/Au(111)—dynamics of concurrent Eley-Rideal and Langmuir-Hinshelwood mechanisms J. Chem. Phys. 101 1529... 

An electron or atomic beam of (projectile or test) particles A with density N, of particles per cm travels with speed V and energy E tln-ongh an infinitesimal thickness dv of (target or fielc0 gas particles B at rest with... 

Dharmasena G, Copeland K, Young J H, Lasell R A, Phillips T R, Parker G A and Keil M 1997 Angular dependence for v /-resolved states in F + H2 -> HF(v /) + H reactive scattering using a new atomic beam source J. Rhys. Chem. A101 6429—40... 

Faubel M, Martinez-Haya B, Rusin L Y, Tappe U and Toennies J P 1996 An intense fluorine atom beam source J. Rhys. D Appl. Rhys. 29 1885-93... 

Early experiments witli MOT-trapped atoms were carried out by initially slowing an atomic beam to load tire trap [20, 21]. Later, a continuous uncooled source was used for tliat purjDose, suggesting tliat tire trap could be loaded witli tire slow atoms of a room-temperature vapour [22]. The next advance in tire development of magneto-optical trapping was tire introduction of tire vapour-cell magneto-optical trap (VCMOT). This variation captures cold atoms directly from the low-velocity edge of tire Maxwell-Boltzmann distribution always present in a cell... 

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... 

Morinaga M, Yasuda M, Kishimoto T and Shimizu F 1996 Holographic manipulation of a cold atomic beam Phys.Rev.Lett. 77 802-5... 

Instead of the fast-atom beam, a primary ion-beam gun can be used in just the same way. Generally, such an ion gun emits a stream of cesium ions (Cs ), which are cheaper to use than xenon but still have large mass (atomic masses Cs, 139 Xe, 131). Although ion guns produce no fragment ions in the primary beam, they can contaminate the mass spectrometer by deposition with continued use. 

A gun is used to direct a beam of fast atoms (often Xe) or fast ions (often Cs+) onto a small metal target area where the solution of interest is placed. Production of an atom beam is described in Figure 13.1. 

The use of molecular and atomic beams is especially useful in studying chemiluminescence because the results of single molecular interactions can be observed without the complications that arise from preceding or subsequent energy-transfer coUisions. Such techniques permit determination of active vibrational states in reactants, the population distributions of electronic, vibrational, and rotational excited products, energy thresholds, reaction probabihties, and scattering angles of the products (181). 

J. Villain, A. Pimpinelli. Physique de la croissance cristalline. Paris Editions Eyrolles, 1995 (in French) J. Villain. Continuum models of crystal growth from atomic beams with and without desorption. J Phys (France) I 7 19, 1991. 

Furthermore, LandS s theory only represents a first-order approximation, and the L and S quantum numbers only behave as good quantum numbers when spin-orbit coupling is neglected. It is interesting to note that the most modem method for establishing the atomic ground state and its configuration is neither chemical nor spectroscopic in the usual sense of the word but makes use of atomic beam techniques (38). 

For a typical sodium atom, the initial velocity in the atomic beam is about 1000 m s1 and the velocity change per photon absorbed is 3 crn-s. This means that the sodium atom must absorb and spontaneously emit over 3 x 104 photons to be stopped. It can be shown that the maximum rate of velocity change for an atom of mass m with a photon of frequency u is equal to hu/lmcr where h and c are Planck s constant and the speed of light, and r is the lifetime for spontaneous emission from the excited state. For sodium, this corresponds to a deceleration of about 106 m s"2. This should be sufficient to stop the motion of 1000 m-s 1 sodium atoms in a time of approximately 1 ms over a distance of 0.5 m, a condition that can be realized in the laboratory. 

In more recent experiments using a cesium atomic beam, temperatures as low as 2.5 pK have been reported.1111 Enthusiastic workers in the field predict that the temperature may be decreased by as much as another factor of 106 before the final minimum temperature is realized, and perhaps temperatures lower than 2.5 pK have been claimed by now.bb... 

To obtain single photon pulses, one can use the emission by a single dipole as shown below in section 21.3.1. The experiment was performed in 1977 by Kimble, Dagenais and Mandel (Kimble et al., 1977). They showed that single atoms from an atomic beam emitted light which, at small time scales, exhibited a zero correlation function. This result can not be explained through a semiclassical theory and requests a quantum description of light. 

When FAB is utilized for FC-MS, often known as continuous-flow FAB, a matrix material is added to the HPFC eluent, either pre- or post-column, and this mixture continuously flows to the tip of a probe inserted into the source of the mass spectrometer where it is bombarded by the atom beam (Figure 3.3). 

Preliminary activation may be performed not only by means of dissociation of the components being analyzed, but also by electronic and vibrational excitation, either in the gaseous phase, or even better, directly on the film of semiconductor sensor. It should be also noted that this method is applicable to dissociation in the adsorbed layer. Excitation of the molecules in adsorbed layer (we are referring to physically adsorbed particles) can be performed optically, by an electron (ion) beam, or by an electronically excited atom beam, by Hg, for example [10, 11]. 

Fig. 3.8. Experimental set-up to examine interaction of atom particles with the surface of a solid body by means of atom beam reflection. I - Chamber with atom particles source installed II, III - Intermediate and main chambers / -Pyrolysis filament 2 - Collimation channel 3 - Beam chopper 4 - Titanium atomizer 5 - Collimation slot 6 - Target 7 - Deflector 8 - To vacuum pump pipe 9 - Filament 10 - ZnO semiconductor sensor...

Fig. 3.9. Intensity profile I of H-atoms beam incident on the target. Beam pressure in the main chamber is 6.3xl0 5 Torr temperature of H-atoms generator pyrolysis filament is 1550 C.

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