New PDF release: Chemistry and Physics of Solid Surfaces VII

By Gert Ehrlich (auth.), Professor Ralf Vanselow, Dr. Russell Howe (eds.)

ISBN-10: 3642739024

ISBN-13: 9783642739026

ISBN-10: 3642739040

ISBN-13: 9783642739040

This quantity comprises assessment articles written through the invited audio system on the 8th foreign summer time Institute in floor technology (ISISS 1987), held on the college of Wisconsin-Milwaukee in August of 1987. in the course of the process ISISS, invited audio system, all the world over well-known specialists within the numerous fields of floor technological know-how, current educational evaluation lectures. furthermore, those specialists are requested to jot down evaluation articles on their lecture subject. Former ISISS audio system function advisors about the collection of audio system and lecture issues. Em­ phasis is given to these components that have now not been coated intensive by means of contemporary summer season Institutes, in addition to to parts that have lately received in importance and within which vital growth has been made. as a result of area obstacles, no person quantity of Chemistry and Physics of stable Surfaces can very likely conceal the total sector of modem floor technological know-how, or perhaps supply an entire survey of contemporary professional­ gress within the box. besides the fact that, an try out is made to offer a balanced review within the sequence as a complete. With its entire literature references and vast topic indices, this sequence has develop into a valu­ capable source for specialists and scholars alike. The accumulated articles, which pressure rather the gas-solid interface, were released less than the subsequent titles: floor technological know-how: contemporary growth and views, Crit. Rev. stable kingdom Sci. four, 125-559 (1974) Chemistry and Physics of stable Surfaces, Vols. I, II, and III (CRC Press Boca Raton, FL 1976, 1979, and 1982); Vols.

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Although it is true that for fast chemisorption from the precursor the sticking coefficient is expected to fall with rising surface temperatures, this fall constitutes a reliable test for unactivated chemisorption via a precursor only if measurements span a sufficiently large temperature range. 48]. l s~ ex+ Ek~~d]. + (111) Here the contribution from direct chemisorption out of the gas phase at temperature T g is given by the first term on the right, which previously was just represented by sa e f .

2 it was noted that for experiments of the type done here, in which both the surface and the gas are kept at the same temperature, the activation energy deduced from the temperature dependence is given' by Ec-Ed , the difference between the barrier to precursor conversion and to desorp- 25 tion. If vibrational excitation of the molecules were a requirement for chemisorption, then the energy that must be imparted to the incoming molecules should be comparable to ~Ec. From Brass's experiments, the barrier to conversion Ec is ~ll kcal/mol, significantly larger than the energy supplied to CH4 in the various unsuccessful attempts at laser excitation.

24 are quite striking: the activation energy for chemisorption obtained when only the gas is heated is 11 kcal/mol, 6 kcal/mol higher than in previous experiments with gas and surface at the same temperature. This difference is just equal to the molecular desorption energy Ed for methane. The conclusion is that vibrational excitation of the molecular gas plays a dominant role in overcoming the barrier to chemisorption. Linear MOl Ion Feed Through Reoctlon Tube Fig. 23. 0 Fig. 24. 60,74]. ) (mol/keel) All these measurements were made on evaporated films of uncertain surface structure.

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Chemistry and Physics of Solid Surfaces VII by Gert Ehrlich (auth.), Professor Ralf Vanselow, Dr. Russell Howe (eds.)


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