Effusive beam technique

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... 

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... 

We limit the presentation of experimental data to those experiments utilizing supersonic molecular beam expansions in combination with UHV techniques. Supersonic beam experiments control the incident kinetic energy and angle of the molecule to a high degree, and thus are generally easier to interpret at the microscopic dynamical level than those experiments utilizing effusive beams. 

The experimental results have been obtained almost entirely from LEED, UPS and LEELS measurements for coverages ranging from a fraction of a monolayer to 20 monolayers, but an important point has been the use of molecular beam techniques for metal deposition to minimze contamination effects. (The metal was effused from clean Knudsen sources.) If we consider first the LEED observations, the initial state is a thermally produced 111 7 x 7 structure. The 7x7 periodicity is retained up to 1 monolayer of metal deposit, but as an extrinsic 7x7 pattern induced by the metal rather than a simple decrease in intensity of the Si 7 x 7 reflections. This was quoted as evidence of an essentially two-dimensional morphology for the metal deposit, since the formation of three-dimensional nuclei with clean silicon between them would have only reduced the intensity of the intrinsic 7x7 pattern. Beyond one monlayer, growth could follow a Stranski-Krastanov mode, however. 

Related to the Surface Science approach to catalysis is the use of molecular beam techniques . With Mass Spectroscopy and other detection techniques the reactivity of small metal particles effusing into a vacuum is studied. Again reaction conditions are well defined, but the temperature is close to zero Kelvin. This approach enables the study of chemical reactivity 2ls a function of particle size. Knowledge on the reactivity of isolated particles is relevant especially for theoretical studies since these data refer to well-defined particles without interaction with a support. Theoretical studies can be conveniently done with isolated small clusters. As with surface science studies, molecular beam experiments are fundamental to the study of elementary reactions as a function of surface or particle structure. We will frequently refer to data collected by these approaches. 

Figure 24.1 shows a schematic layout of a crossed-beam apparatus in which the Na (FCH3) (n = 1 to 5) clusters were produced by the pick-up technique. For this, a hot effusive beam of Na atoms is crossed with a pulsed, cold supersonic beam of FCH3. 

The first experiments were carried out in 1983 [13.99,13.100]. The H atoms were produced by photodissociation of HJ molecules in an effusive beam using the fourth harmonics of Nd YAG lasers. Since the dissociated iodine atom is found in the two fine-structure levels /(P1/2) and /(P3/2), two groups of H atoms with translational energies kin = 0.55 eV or 1.3 eV in the center-of-mass system H-f-D2 are produced. If the slower H atoms collide with D2 they can reach vibrational-rotational excitation energies in the product molecule up to (u = 1, / = 3), while the faster group of H atoms can populate levels of HD up to (v = 3, J = 8). The internal-state distribution of the HD molecules can be monitored either by CARS (Sect. 8.3) or by resonant multiphoton ionization [13.99]. Because of their fundamental importance, these measurements have been repeated by several groups with other spectroscopic techniques that have improved signal-to-noise ratios [13.101]. 

The mass analysis of an effusive beam of metal particles from a Knudsen cell allows us to calculate the equilibrium constants for the formation of dimers, trimers, etc. Performing similar studies over a wide range of temperatures then allows us to obtain the complete set of thermodynamic quantities free energy, enthalpy, and entropy of formation. Unfortunately, the multimer concentrations are typically so small that this technique is limited to an analysis of the thermodynamics of only dimers except in favorable cases. Nevertheless, these studies provide a convenient check for spectroscopic measurements of bond energies. 

Trapping techniques have been and still are the basis of many new experiments in physics and chemistry. All these experiments make use of inherent advantages such as extremely long interaction times, the possibility to accumulate weak beams, phase space compression, laser cooling or interaction with buffer gas. This contribution has focused on the use of rf fields to explore collisions at low temperatures or with low relative velocities. The examples have shown that it is now possible to study collisions at energies of 1 meV or at temperatures of lOK. As already mentioned, there are activities to cool ions in traps to temperatures below 1 K using the slow tail of a cold effusive beam for buffer gas cooling. There are also efforts to heat ions with a laser in order to access temperatures above 2000 K. ... 

Figure 1 Schematic illustration of an atomic-beam technique to reduce Doppler broadening. Atomic vapour effuses from a small orifice of an oven. The angular divergence of atoms in the beam is limited to 6Q = Brr bldi by a slit whose aperture is 2d placed at a distance b from the orifice.

The problems associated with the formation and detection of molecular beams have already been referred to. They are interrelated and have largely determined which reactions have been studied with this technique. The simplest method to form a beam is to collimate the effusive flow occurring from a low-pressure source, conventionally called an oven, although its temperature may be subambient. Unfortunately, this yields low beam intensities, and the velocities in the beam are thermally distributed. As a result, even for the accurate assessment of the incident-beam intensity, a highly sensitive detector is required. Moreover, the relatively low beam temperature requires that the reaction has a small threshold energy so that an appreciable proportion of the scattering is reactive. 

The improvements in the sensitivity of CARS (Sect. 3.3) have made this nonlinear technique an attraetive method for the investigation of molecular beams. Its spectral and spatial resolution allow the determination of the internal-state distributions of molecules in effusive or in supersonic beams, and their dependence on the location with respect to the nozzle (Sect. 3.5). An analysis of rotationally-resolved CARS spectra and their variation with increasing distance z from the nozzle allows the determination of rotational and vibrational temperatures Tlotiz), TVibiz), from which the cooling rates can be obtained [457]. With cw CARS realized with foeused cw laser beams the main contribution to the signal comes from the small focal volume, and a spatial resolution below 1 mm can be achieved [458]. 

In addition to absolute activities, the multiple-cell KEMS technique allows relative activities to be determined directly by comparing the relative partial pressure of species in equilibrium with different samples, with compositions I and II, and in adjacent effusion cells in a single experiment, according to Equation 48.53a, where any difference in flux distribution of the molecular beams is again represented by the GFR. Relative activities are the most direct measure of any differences between the solution behavior and phase equilibrium of two samples. According to Equation 48.39, relative activities provide a direct measure of the difference in chemical potential between the two compositions, as in Equation 48.53b ... 

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