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Molecular-beam experiments

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... [Pg.880]

The time-dependent Schrddinger equation governs the evolution of a quantum mechanical system from an initial wavepacket. In the case of a semiclassical simulation, this wavepacket must be translated into a set of initial positions and momenta for the pseudoparticles. What the initial wavepacket is depends on the process being studied. This may either be a physically defined situation, such as a molecular beam experiment in which the paiticles are defined in particular quantum states moving relative to one another, or a theoretically defined situation suitable for a mechanistic study of the type what would happen if. .. [Pg.268]

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... [Pg.271]

A pattern emerges when this molecular beam experiment is repeated for various gases at a common temperature Molecules with small masses move faster than those with large masses. Figure 5 shows this for H2, CH4, and CO2. Of these molecules, H2 has the smallest mass and CO2 the largest. The vertical line drawn for each gas shows the speed at which the distribution reaches its maximum height. More molecules have this speed than any other, so this is the most probable speed for molecules of that gas. The most probable speed for a molecule of hydrogen at 300 K is 1.57 X 10 m/s, which is 3.41 X 10 mi/hr. [Pg.294]

One of the gas mixtures was used in a pulsed molecular beam experiment. The result of the experiment is shown below. Which of the two gas samples, A or B, was used for this experiment ... [Pg.295]

C05-0015. A molecular beam experiment of the type illustrated in Figure 5J is performed with an equimolar mixture of He and CO2. Sketch the appearance of a graph of the number of molecules reaching the detector as a function of time. [Pg.301]

C05-0118. Molecular beam experiments on ammonia at 425 K give the speed distribution shown in the figure ... [Pg.345]

Kinetics on the level of individual molecules is often referred to as reaction dynamics. Subtle details are taken into account, such as the effect of the orientation of molecules in a collision that may result in a reaction, and the distribution of energy over a molecule s various degrees of freedom. This is the fundamental level of study needed if we want to link reactivity to quantum mechanics, which is really what rules the game at this fundamental level. This is the domain of molecular beam experiments, laser spectroscopy, ah initio theoretical chemistry and transition state theory. It is at this level that we can learn what determines whether a chemical reaction is feasible. [Pg.24]

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. [Pg.298]

Another popular method for studying combustion species is via crossed molecular beams. In this technique, the reactant molecules of interest are propelled as beams toward an intersection where their molecular collisions bring about reactions. For a more complete discussion of crossed molecular beam experiments, see reference 59. [Pg.265]

The previous sections focused on the case of isolated atoms or molecules, where coherence is fully maintained on relevant time scales, corresponding to molecular beam experiments. Here we proceed to extend the discussion to dense environments, where both population decay and pure dephasing [77] arise from interaction of a subsystem with a dissipative environment. Our interest is in the information content of the channel phase. It is relevant to note, however, that whereas the controllability of isolated molecules is both remarkable [24, 25, 27] and well understood [26], much less is known about the controllability of systems where dissipation is significant [78]. Although this question is not the thrust of the present chapter, this section bears implications to the problem of coherent control in the presence of dissipation, inasmuch as the channel phase serves as a sensitive measure of the extent of decoherence. [Pg.177]

The main focus of the molecular beam experiments has been to investigate the kinetic details of the catalytic reduction of NO in the presence of a reducing agent (most often CO) under isothermal steady-state conditions. This type of studies have been carried out on Rh(lll) [29], Rh(110) [30], and Pd(lll) [31] single-crystal surfaces. On Rh(lll), we have reported systematic studies as a function of surface temperature, NO + CO... [Pg.72]

Figure 3.8. Kinetic data from molecular beam experiments with NO + CO mixtures on a Pd/MgO(100) model catalyst [70]. The upper panel displays raw steady-state C02 production rates from the conversion of Pco = PN0 = 3.75 x 10-8 mbar mixtures as a function of the sample temperature on three catalysts with different average particle size (2.8, 6.9, and 15.6 nm), while the bottom panel displays the effective steady-state NO consumption turnover rates estimated by accounting for the capture of molecules in the support. After this correction, which depends on particle size, the medium-sized particles appear to be the most active for the NO conversion. (Reproduced with permission from Elsevier, Copyright 2000). Figure 3.8. Kinetic data from molecular beam experiments with NO + CO mixtures on a Pd/MgO(100) model catalyst [70]. The upper panel displays raw steady-state C02 production rates from the conversion of Pco = PN0 = 3.75 x 10-8 mbar mixtures as a function of the sample temperature on three catalysts with different average particle size (2.8, 6.9, and 15.6 nm), while the bottom panel displays the effective steady-state NO consumption turnover rates estimated by accounting for the capture of molecules in the support. After this correction, which depends on particle size, the medium-sized particles appear to be the most active for the NO conversion. (Reproduced with permission from Elsevier, Copyright 2000).
Thirunavukkarasu, K., Thirumoorthy, K., Libuda, J. et al. (2005) Isothermal kinetic study of nitric oxide adsorption and decomposition on Pd(lll) surfaces Molecular beam experiments , J. Phys. Chem. B, 109, 13283. [Pg.93]

While it is not feasible to measure exponential decay of resonance states in the environment of a molecular beam experiment, in theoretical work the exponential decay law provides a necessary condition that a proposed state, generated by some method, is in fact a resonance state. Furthermore, the rate of exponential decay provides probably the most accurate method for the numerical determination of the lifetime. [Pg.56]

Perhaps the first clear observation of a reactive resonance in a collision experiment was recently made for the F + HD —> HF + D reaction.65-67 This reaction was one isotopomer of the F + H2 system studied in the landmark molecular beam experiments of Lee and co-workers in 1985.58 Unlike the F + H2 case, no anomalous forward peaking of the product states was reported, and results for F + HD were described as the most classical-like of the isotopes considered. Furthermore, a detailed quantum mechanical study68 of F + HD —> HF + D reaction on the accurate Stark-Werner (SW)-PES69 failed to locate resonance states. Therefore, it was surprising that the unmistakable resonance fingerprints emerged so clearly upon re-examination of this reaction. [Pg.60]

The molecular beam experiments of Liu and co-workers65,66 employed the Doppler profile time-of-fiight technique that allowed the ready observation of the excitation function (i.e. the total reactive ICS summed over... [Pg.60]

Fig. 9. The excitation function in A2 for the reaction F + p-H2 —> H + HF versus collision energy. The solid line is the result of quantum scattering calculations done with the SW-PES and the points are the molecular beam experiments. Fig. 9. The excitation function in A2 for the reaction F + p-H2 —> H + HF versus collision energy. The solid line is the result of quantum scattering calculations done with the SW-PES and the points are the molecular beam experiments.
In summary, the H + HD reaction shows little sign of resonance scattering in the ICS. Furthermore, the product distributions without angle resolution show no unusual behavior as functions of energy that might indicate resonance behavior. On the other hand, the forward peaking in the angular product distribution does appear to reveal resonance structure. Since time-delay analysis is at present not possible in a molecular beam experiment, it is the combination of a sharp forward peak with the unusual... [Pg.78]

A close analogy to the localized surface interaction can be found in the field of chemical kinetics, namely, in the spectator stripping mechanism (5, 6) of the gas reactions, as evidenced by the recent crossed-molecular-beams experiments. Here the projectile seems to meet with only a part of the target molecule (that one to be transferred), while the rest of the target behaves as a spectator, in a sense not taking part in the reaction. [Pg.53]

As in the molecular beam experiment, the reactor volume, pumping speed, and rate of introduction of reactants have values which lead to a flux of reactants well defined in time. Strozier, however, simply doses gas into the vacuum system (reactor) rather than using a molecular beam. He studied CO oxidation, which has nonlinearities in the surface rate equation, so that computer rather than analytic solutions are necessary. The results are represented at constant frequency and varying temperature as shown in Fig. 8, which is a computer simulation (37). [Pg.14]


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See also in sourсe #XX -- [ Pg.20 ]




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