Modulated molecular beam

MBRS Molecular beam spectroscopy [158] A modulated molecular beam hits the surface and the time lag for reaction products is measured Kinetics of surface reactions chemisorption... 

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]. 

Cycled Feed. The qualitative interpretation of responses to steps and pulses is often possible, but the quantitative exploitation of the data requires the numerical integration of nonlinear differential equations incorporated into a program for the search for the best parameters. A sinusoidal variation of a feed component concentration around a steady state value can be analyzed by the well developed methods of linear analysis if the relative amplitudes of the responses are under about 0.1. The application of these ideas to a modulated molecular beam was developed by Jones et al. ( 7) in 1972. A number of simple sequences of linear steps produces frequency responses shown in Fig. 7 (7). Here e is the ratio of product to reactant amplitude, n is the sticking probability, w is the forcing frequency, and k is the desorption rate constant for the product. For the series process k- is the rate constant of the surface reaction, and for the branched process P is the fraction reacting through path 1 and desorbing with a rate constant k. This method has recently been applied to the decomposition of hydrazine on Ir(lll) by Merrill and Sawin (35). 

Jones, R. H., Olander, D. R., Siekhaus, W. J., and Schwarz, J.A. Investigation of gas-solid reactions by modulated molecular beam mass spectrometry. J. Vac. Sci. Technol. 

The most sophisticated and incisive transient experiments are those derived from modulated molecular beam reactive scattering experiments. 

The polymer sample (35 mg) was pyrolyzed in a quartz cell which was directly attached to the inlet flange of a quadrupole mass spectrometer. Gases evolved from the pol3raier compound were dynamically sampled via a 1.0-mm diameter orifice, formed into a modulated molecular beam, and mass analyzed. Information was obtained on the total yield of volatile products, product composition, and individual product yields as a function of temperature. 

Modulated molecular beam studies show that while trapping-desorption is the dominant process at modest Et and high Ts, there is also some direct or rather indirect inelastic scattering as well [192,201]. The dominance of trapping is anticipated for... 

TMS, (8)]. Both are modulated molecular beam methods with phase sensitive detection, and they allow for accurate measurement of gaseous and condensible species concentrations. The basic differences between the KMS and TMS methods are, the upper pressure A... 

Freedman, A., Wormhoudt, J., and Stewart, G., "ARI Modulated Molecular Beam Mass Spectrometer," Aerodyne Research, Inc. Bedford, MA 01730, Report No. TM-13, Feb. 1981. 

Figure 5.4.6. Schematic diagram of a laser Py-MS system using a modulated molecular beam.

Other techniques utilize lasers for sample evaporation/pyrolysis and excitation such as laser induced desorption (LID) or laser microprobe mass analysis (LAMMA) (see e g. [1]). Some of the sample introduction procedures in Py-MS enhance the information obtained from Py-MS by the use of time-resolved, temperature-resolved, or modulated molecular beams techniques [10]. In time-resolved procedures, the signal of the MS is recorded in time, and the continuous formation of fragments can be recorded. Temperature-resolved Py-MS allows a separation and ionization of the sample from a platinum/rhodium filament inside the ionization chamber of the mass spectrometer based on a gradual temperature increase [11]. The technique can be used either for polymer or for additives analysis. Attempts to improve selectivity in Py-MS also were done by using a membrane interface between the pyrolyzer and MS [12]. 

Molecular Beams—Kinetics and Mechanism. —An outstanding example in this field is by Wachs and Madix who employed a modulated molecular beam source to study the reaction of HCOOH on Ni(llO). Products were examined individually as was the phase-lag in their appearance in their quadrupole MS detector. They could measure reaction rates for coverages of 10 to 1 monolayer and reaction times from 10 to 10 s over a temperature range of 400—800 K. The processes investigated were adsorption desorption of HCOOH, and HCOOH -> CO2, CO, H2, and H2O. Their findings on selectivity and rates for all possible reactions can be found in their paper. 

The differential cross sections of argon and neon have been measured by using refinements of the modulated molecular-beam technique. From these measurements the intermolecular potentials were found. These potentials differ significantly from the Lennard-Jones potential. The neon and argon potentials have different shapes and are not related by any simple scaling factor. The macroscopic properties have been calculated and are in reasonable agreement with experiment. The face-centered cubic structure was found to be the most stable crystal lattice for neon. The effect of the argon potential on the critical properties and saturation pressures is also discussed. 

The background problem can be solved by a careful application of the modulated molecular beam technique. Since electron bombardment detection is required for argon and neon, the relative inefficiency must be compensated for by sharp tuning of the ion current and by extended signal integration. 

In analysing data from modulated molecular beam experiments, it is... 

The impetus for this topic has been provided by the development of molecular beam epitaxy (MBE) as a viable thin film deposition process [ 111]. As a result, the approach has concentrated more on investigations of reaction kinetics than on electronic effects, since kinetic parameters are directly available from modulated molecular beam measurements (see Sect. 2.4.1). We will summarize here only the results for beams of As4 and As2 interacting with 100 GaAs surfaces, but closely similar behaviour is observed for other Group V elements and other Group III—V compound surfaces. The choice of tetramer and dimer beams is dictated by the evaporation behaviour of Group V elements in that elemental sources produce tetramers and Group III—V compound sources produce dimers. Monomeric species are not readily available. 

W.J. Siekhaus, J.A. Schwarz, and D.R. Olander. A Modulated Molecular Beam Study of the Energy of Simple Gases Scattered from Pyrolytic Graphite. Surf. Sci. 33 445 (1972). 

M)MB (laser-induced, collision-induced) desorption (Modulated) molecular beam Yes243.244 No... 

The progress of the reactionis sketched in Fig. 6.16. In this case, adsorbed CO forms a densely packed c2 x 4 layer occupying both the "on top" and "bridge" sites [57], while adsorbed O gives rise to the same 2x2 structure as with Rh(l 11). Consequently, the same mixed structure, as depicted in Fig. 6.15c, will be formed if both adsorbates are present on the surface. The operation of the Langmuir-Hinshelwood mechanism was estabhshed through modulated molecular beam experiments that enabled derivation of the kinetic parameters in the framework of the Langmuir approximation [58]. (The CO2 formed is so weakly held at the surface that it is immediately released into the gas phase [59].)... 

D. Padowitz, K. PeterUnz, and S. J. Sibener, New modulated molecular beam scattering methods for probing nonlinear and coverage-dependent reaction kinetics at surfaces, Langmuir, vol. 7, pp. 2566-2573, 1991. 

A computer-controlled modulated molecular beam source is used to investigate the kinedcs of the surface reactions which occur when bromine is reactively scattered by Pd(lll). The reaction products are atomic bromine and molecular bromine the latter species arises from an adatom recombination process and gives rise to a product vector modulated at twice the frequency of the incident beam (2u.) By making suitable measurements of the temperature dependence of the product vector phase shifts at w and 2u, the four kinetic paranwteis which characterise the first-order and second-order rate processes are obtained. These are A, =2.5X 10 s , = 177 kJ moP, = 3.6X 10 m s , 13 3 mol. The significance of these values is discussed... 

The present paper seeks to clarify these aspects of the Pd-Br2 system by a modulated molecular beam reactive scattering investigation of the surface reaction kinetics. The object is to identify the kinetic order of all overlayer evaporation pathways and to obtain values for all rate parameters which characterise them. 

The basic experimental technique has been described in detail by Madix (ref. [5] and references therein), As normally applied it requires the production of a well-collimated, high intensity amplitude modulated molecular beam. This... 

Behrens, Jr., R., (1986), A New Simultaneous Thermogravimetry and Modulated Molecular Beam Mass spectrometry Apparatus for Quantitative Thermal Decomposition Studies , Rev. Sci. Instrum., 58, pp. 451- 461. 

Zhang, J. Gibson, E.M. Foxon, C.T. and Joyce, B.A. Modulated molecular beam study of group III desorption during growth by MBE. J. Cryst Growth 1991 111 93-97. 

---