ps 5 solution - Chemistry420 Problemset#5(80pointstotal)Key

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Chemistry 420 Problem set #5 (80 points total) – Key Due at the beginning of class December 8 No late assignments will be accepted. Assignments are due at beginning of class—starts at 12 Noon! For full credit assignments must be in box when the lecture begins. Any assignment turned in between noon and 12:05 PM will automatically be deducted 5 points. Assignments will NOT be accepted for credit after 12:05 PM. Score = 0 No name, no grade. Show all of your work for each answer. Partial credit will be given and NO credit will be awarded if no work is shown. Be careful to use correct significant figures. 1. (15 points) Compare and contrast the methods in which X ray photoelectron spectroscopy and Auger electron spectroscopy probe surface composition. What are advantages and disadvantages of the two techniques? Key points to be discussed : In XPS, surface is irradiated with X ray beam (1 point) In AES, surface is irradiated with electron beam (1 point) In XPS, the kinetic energy of the ejected electron is measured (3 points) In AES, the Auger electron is measured; the electron beam displaces a core electron and an (Auger) electron is emitted when an electron from a higher energy level fills the vacancy (4 points) Since it interrogates the surface with X rays, XPS can be used to analyze non conductive surfaces (3 points) AES is not applicable to non conductive surfaces because it interrogates with an electron beam that charges non conductive surface. However, the electron beam can be focused using electron optics and rastered across the surface giving high spatial resolution chemical composition maps. (3 points) 2. (5 points) A common method of distinguishing X ray photoelectrons from Auger electrons (in a XPS spectrum) is to analyze the sample with multiple sources having different energies. What is the change (if any) in XPS and Auger electrons when the source is changed? In terms of the electron origin, why does this method work?
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