Molecular beams experimental methods

Figure 3-18. Experimental demonstration of the molecular beam deflection method, from Buck et al. (1985). The Newton circles show the peak positions associated with the different clusters S) denotes the velocity of the center-of-mass for different clusters. The TOF data were recorded at a lab angle of 10° note the correspondence between peaks 2 and 3 and the corresponding intersection points on the Newton circles.

Recently, TDWP method was developed to compute DCSs for four-atom reactions and applied to the prototypical HD- -OH H2O-b D [70, 145] and D2-b OH-> HOD-b D reactions [69]. Excellent agreements were achieved for the first time for a four-atom reaction between the full-dimensional DCS and high-resolution crossed-molecular beam experimental results on the HD-bOH —> H2O -b D reaction [145]. Figure 4.13 compares the theoretical energy... 

An important method for producing semiconductor layers is the so-called molecular beam epitaxy (MBE) (see [3,12-14] and [15-19]). Here, atoms of the same or of a different material are deposited from the vapor source onto a faceted crystal surface. The system is always far from thermal equilibrium because the deposition rate is very high. Note that in this case, in principle, every little detail of the experimental setup may influence the results. 

The value of the magnetic hyperfine interaction constant C = 22.00 kHz is supposed to be reliably measured in the molecular beam method [71]. Experimental data for 15N2 are shown in Fig. 1.24, which depicts the density-dependence of T2 = (27tAv1/2)-1 at several temperatures. The fact that the dependences T2(p) are linear until 200 amagat proves that binary estimation of the rotational relaxation rate is valid within these limits and that Eq. (1.124) may be used to estimate cross-section oj from... 

Our experimental techniques ooitprise static techniques such as TiKKD, thermal desorption lectrosoppy (TDS) and work functicn measurements (A p) and < namic techniques like scattering of and D molecular beams. Details of the experimental methods are ven elseihere (2,3). 

In the following, an overview of the experimental approaches is presented, including the production and detection methods of free radicals and the techniques for studying free radical photodissociation in the molecular beam. The photochemistry of the free radical systems investigated recently will then be discussed in detail. 

Gold s method has been used by a number of workers, including Siska (1973), who applied it to molecular-beam scattering data, MacNeil and Dixon (1977), who applied it to photoelectron spectra, and Jones et al. (1967), who restored infrared spectra of condensed-phase samples. The author is unaware of any experimental results with this method, however, that illustrate the full potential achievable by constrained methods to be described later in this chapter. In the work of Jones et al., the resulting resolution is probably limited by the inherent breadth of spectral lines observed with condensed-phase samples. 

TRPES has been recently reviewed and details of the experimental method and its interpretation can be found elsewhere [5], Trans-azobenzene was introduced via a helium supersonic molecular beam into the interaction region of a magnetic bottle photoelectron spectrometer. The molecules were photoexcited by a tunable femtosecond laser pulse (pump pulse) with a wavelength of 280-350nm. After a variable time delay, the excited molecules were ionized by a second femtosecond laser pulse (probe pulse) with a wavelength of 200 or 207nm. The emitted photoelectrons were collected as a function of pump-probe time delay and electron kinetic energy. 

In this chapter we summarize the current status of the low-energy scattering of noble-gas metastable atoms in molecular beams. A brief summary of potential scattering theory that is relevant to the understanding of collision dynamics, as well as a description of the experimental method, precedes the presentation of experimental findings. The experimental results presented are mainly from the authors laboratories. 

The reaction 140 has been studied by Margitan et al. (69) a molecular beam study of reaction 141 has been carried out by McDonald et al. (70). Note that for these reactions there is a direct transformation of the initial vibrational degree of freedom into a final translational degree of freedom and vice versa. For reaction 1A1 a peak in the translational distribution of the products has been obtained by the present method, in agreement with experimental data. 

In this chapter we consider the physics of the positronium atom and what is known, both theoretically and experimentally, of its interactions with other atomic and molecular species. The basic properties of positronium have been briefly mentioned in subsection 1.2.2 and will not be repeated here. Similarly, positronium production in the collisions of positrons with gases, and within and at the surface of solids, has been reviewed in section 1.5 and in Chapter 4. Some of the experimental methods, e.g. lifetime spectroscopy and angular correlation studies of the annihilation radiation, which are used to derive information on positronium interactions, have also been described previously. These will be of most relevance to the discussion in sections 7.3-7.5 on annihilation, slowing down and bound states. Techniques for the production of beams of positronium atoms were introduced in section 1.5. We describe here in more detail the method which has allowed measurements of positronium scattering cross sections to be made over a range of kinetic energies, typically from a few eV up to 100-200 eV, and the first such studies are summarized in section 7.6. 

WCo2 = r = h4Pco00 corresponding to the E-R mechanism is not satisfied. At present the pendulum has swung to the opposite side and most research workers [98] are sure that, over a wide range of the reaction parameters (T = 450-950 K, P = 10-7 to 10 5 Torr), only the adsorption mechanism (L-H) is valid. This belief is based on the data obtained in unsteady-state experiments and using modern physical methods, in particular the molecular beam technique [98, 52, 107]. But a fairly good qualitative description on the basis of the L-H mechanism has been obtained in only a few cases [56, 57] and this description concerns rather limited experimental... 

Thin semiconductor films (and other nanostructured materials) are widely used in many applications and, especially, in microelectronics. Current technological trends toward ultimate miniaturization of microelectronic devices require films as thin as less than 5 nm, that is, containing only several atomic layers [1]. Experimental deposition methods have been described in detail in recent reviews [2-7]. Common thin-film deposition techniques are subdivided into two main categories physical deposition and chemical deposition. Physical deposition techniques, such as evaporation, molecular beam epitaxy, or sputtering, involve no chemical surface reactions. In chemical deposition techniques, such as chemical vapor deposition (CVD) and its most important version, atomic layer deposition (ALD), chemical precursors are used to obtain chemical substances or their components deposited on the surface. 

It is likely that theoretical methods, both ab initio and MD simulations, will be needed to resolve the complicated chemical decomposition of energetic materials. There are species and steps in the branching, sequential reactions that cannot be studied by extant experimental techniques. Even when experiments can provide some information it is often inferred or incomplete. The fate of methylene nitramine, a primary product observed by Zhao et al. [33] in their IRMPD/molecular beam experiments on RDX, is a prime example. Rice et al. [99, 100] performed extensive classical dynamics simulations of the unimolecular decomposition of methylene nitramine in an effort to help clarify its role in the mechanism for the gas-phase decomposition of RDX. 

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