Lecture #9, Friday, April 16th

Today's lecture was entirely devoted to the content of REEL Lab #1. This lab has three main parts:

A. Lattices and Unit Cells
B. Optical Diffraction
C. X-ray Powder Diffraction

The concept of diffraction is typically introduced in physics classes, but in order to solve the problems we are faced with today, we need to combine concepts from various disciplines. X-ray diffraction is the key tool used by chemists to determine the structures of crystalline solids. To learn a little bit more about the concept of diffraction, check out this youtube link featuring University of Minnesota physics professor Jim Kakalious, who was a science consultant for the Warner Brothers film Watchmen. In Watchman Dr. Manhattan has amazing super powers. Dr. Kakalious outlines which physics phenomena are responsible for these super powers. In the movie Dr. Manhattan can appear in more than one place at once and Kakalious asks how he could possible do this. This can be explained by discussing diffraction. At the 2 min 10 sec mark in the clip diffraction is discussed, which is a signature of the wave phenomena of light.

The one thing I want to emphasize is how on earth can a collection of dots from a diffraction pattern relate to the structure of a solid? Diffraction can tell us the similarities and differences on the slides we used in the optical diffraction part of REEL Lab 1. X-rays can be used to measure the same thing in a crystal structure. Since a bond length is on the Angstrom length scale, we need to use electromagnetic radiation of the same magnitude, or X-rays, to determine the arrangement of atoms in a crystal. In my lectures I have been trying to capture the perspective of the Bragg's as they were trying to determine structures of crystals. I found an article from the research Journal Nature that I posted on Carmen and I think it gives an excellent background for what they were trying to study at the time and will give you a good idea of what to keep in mind as you are performing the labs.

On Monday I will discuss how we can use diffraction to identify the structures of solids and if multiple phases are present in our sample. This is the content of REEL Lab #2.