Molecular beam techniques

Molecular beam techniques provide powerful tools for investigating ion-molecule reactions. The angular dependence of cross-sections gives direct insight into the physical nature of the reaction complex. Turner, Fineman and Stebbings (64) have shown that the reaction... 

The advancement of the application of lasers in combination with the molecular beam technique has made a great impact in the understanding of primary photodissociation processes. For state-specific detection of small fragments, laser-induced fluorescence, multiphoton ionization, and coherent laser scattering have provided extremely detailed information on the dynamics of photodissociation. Unfortunately, a large number of interesting... 

Throughout this section on pressure transients we have emphasized electron spectroscopy as a procedure for directly detecting surface species, and, with difficult calibration, their concentration. It is important to keep in mind that the detection limit for these is about 0.01 of a monolayer. Using flash desorption as a complementary technique this limit can be extended to 0.001 monolayer in certain cases. The fact remains that extremely labile chemisorbed species may be present in kinetically important but undetectable concentrations. Since residence times as short as 2 x 10 - seconds can be determined, molecular beam techniques, as described below, afford an alternative but indirect method of measuring the properties of these very reactive species. 

Combining molecular beam techniques with laser state-resolved detection techniques has allowed state-resolved scattering measurements. 

The activated dissociation of H2 (D2) on Cu(l 11) and other single crystal Cu surfaces has played a special role in the development of reactive gas-surface dynamics. Early experiments and theory by Cardillo and collaborators [217-219] first demonstrated the power of molecular beam techniques to probe activated adsorption and the theoretical methodology developed by them (6D quasi-classical dynamics on a model PES) only differs from modem treatments in the use of DFT based PES. 

State-resolved inelastic scattering for a wide range of incident conditions ( ), d,) are measured for this system by combining molecular beam techniques with (2 + 1) ion TOF REMPI detection of the scattered molecules [58]. Energy transfer parallel to the surface is measured from the Doppler broadening of the REMPI spectra. Trapping... 

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

Supermolecular spectra could perhaps be studied with state-selection using adequate molecular beam techniques. That would not be easy, however, because of the smallness of the dipole moments induced by in-termolecular interactions. For the purpose of this book, we will mostly deal with bulk spectra, or interaction-induced absorption of pure and mixed gases. A great variety of excellent measurements of such spectra exists for a broad range of temperatures, while state-selected supermolecular absorption beam data are virtually non-existent at this time. Furthermore, important applications in astrophysics, etc., are concerned precisely with the optical bulk properties of real gases and mixtures. 

An extensive table of values of dipole moments for gases and solute molecules has been published.2 During recent years some very accurate values have been determined by microwave spectroscopy and molecular-beam techniques. 

Molecular beam techniques have been used mostly with small molecules, and most of the information about the dynamics of elementary photochemical processes are restricted to such species. It is however also possible to bring relatively large molecules (e.g. aromatics) into molecular beams through seeding in a carrier gas such as He. 

Following the initial work by Langmuir (7) the catalytic oxidation of carbon monoxide on platinum has been investigated frequently with polycrystalline samples (132, 183-190) as well as with single crystal surfaces (149, 151, 191-197) by applying stationary and nonstationary methods including the molecular beam technique. The general trends observed are... 

The development of supersonic molecular beam techniques created new opportunities to study tunneling effects in gas-phase isolated molecules and dimers at ultralow translational and rotational temperatures. Modern low-temperature chemistry, therefore, includes the study of chemical dynamics of molecules in various states of aggregation. 

Reactions between light molecules have been extensively studied in the last two decades, generally by molecular beam techniques (see Chapter 4, Section 4.2), and these have allowed detailed testing of the predictions made from calculated potential energy surfaces. There are three typical mechanisms for gas phase reactions. 

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... 

Modelling of kinetic dependences. Calculation of steady state kinetic dependences according to the model (4)-(5) cannot be performed without knowing the rate constants. Let us use the parameters (Table 6) for the two-route mechanism (1), the complete set of which was first given by Cassuto et al. [49]. The kinetics and mechanism for CO oxidation over polycrystalline platinum were studied [48] using the molecular beam technique. 

Many elementary chemical reactions have been investigated via molecular-beam techniques. An example is the reaction... 

The interface used today between the atmospheric-pressure plasma and the low-pressure mass spectrometer is based on a differentially pumped two-stage interface similar to those used for molecular beam techniques [89-91]. The key to successful development of ICP-MS instruments was the use of a relatively large ( l-mm-diameter) sampling orifice so that continuum flow was attained with an unrestricted expansion of the plasma to form a free jet. When small orifices were used, a cold boundary layer formed in front of the orifice, resulting in substantial cooling of the plasma, including extensive ion-electron recombination and molecular oxide formation. The smaller orifices were also susceptible to clogging. 

Surface reaction rate data were determined in independent studies in which the diffusion constraint was removed by molecular beam techniques. Predicted values for the overall reaction rate, computed by coupling this data with diffusion rates from boundary layer theory, are in excellent agreement with experimental values for ribbons and wires. 

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