Wikipedia is your friend! Dispersion is defined as the change in phase velocity with wavelength (or wave number, frequency etc), see here: http://en.wikipedia.org/wiki/Dispersion_relation .
The phase velocity is directly related to the refractive index: http://en.wikipedia.org/wiki/Refractive_index so dispersion is also used to describe the variation of refractive index with wavelength etc. 'Normal' and 'anomalous' are merely labels which describe whether the sign of the change is +1 or -1, you could equally well call it positive and negative dispersion (I forget which is which). There is nothing anomalous about anomalous dispersion! So the term dispersion should really be reserved for those folks who actually do measure the variation of r.i. with wavelength (using e.g. an interferometer: the refractive index of X-rays is barely distinguishable from 1). Dispersion is experimentally only relevant to the forward scattered beam (theta=0), and of course as crystallographers we couldn't care less about this forward scattering, in fact we routinely block it with a lump of lead! Scattering (http://en.wikipedia.org/wiki/Scattering) is defined as the amplitude of the radiation (which may suffer a phase shift and so may need to be described by a complex number) which bounces off some material, relative to that scattered by a hypothetical stationary free electron in the case of X-ray scattering. Whereas dispersion really only relates to the forward scattered beam, as far as we crystallographers are concerned scattering relates only to the side beams, which are of course responsible for diffraction when the beams hit an X-ray detector. Anomalous scattering (a.k.a. anomalous diffraction, a term used only by crystallographers and I believe coined by Wayne Hendrickson) is the variation of this complex amplitude with wavelength. Interestingly my dictionary defines dispersion as "the scattering of objects over a wide area" so at least in the colloquial sense they perhaps are synonymous, but of course in science we like to have much narrower meanings so we can use terms more precisely. George Sheldrick is the best person to address your Shel-X questions to! HTH! -- Ian > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of DebajyotiDutta > Sent: 22 July 2009 12:06 > To: [email protected] > Subject: SAD questions > > Dear Sir, > > I have very little knowledge about anomalous dispersion > method. The thing is that I have just started to deal with a > case of SAD. I have consulted several text books to acquire > knowledge about it though I have some queries. CCP4BB, I > think is the best place to place my questions. > > 1.Is there any difference between Anomalous Dispersion and > Anomalous Scattering? > > 2.Sometimes I found that while searching for anomalous > scatterer with SHELXD which give the coordinates of the > scatterer in PDB format with occupancy more than 1. Why > anomalous occupancy may be more than 1. > > 3.I found that there is one plot (FPH - FP)/FP Vs Resolution > indicate the isomorphism but how can I get this plot does > SHELX has any option to print it. > > > Thank you all for your time and reply to a beginners query in > advance. > > Sincerely > Debajyoti Dutta > > > > <http://sigads.rediff.com/RealMedia/ads/click_nx.ads/www.redif > fmail.com/signatureline....@middle?> > Disclaimer This communication is confidential and may contain privileged information intended solely for the named addressee(s). It may not be used or disclosed except for the purpose for which it has been sent. If you are not the intended recipient you must not review, use, disclose, copy, distribute or take any action in reliance upon it. If you have received this communication in error, please notify Astex Therapeutics Ltd by emailing [email protected] and destroy all copies of the message and any attached documents. Astex Therapeutics Ltd monitors, controls and protects all its messaging traffic in compliance with its corporate email policy. The Company accepts no liability or responsibility for any onward transmission or use of emails and attachments having left the Astex Therapeutics domain. Unless expressly stated, opinions in this message are those of the individual sender and not of Astex Therapeutics Ltd. The recipient should check this email and any attachments for the presence of computer viruses. Astex Therapeutics Ltd accepts no liability for damage caused by any virus transmitted by this email. E-mail is susceptible to data corruption, interception, unauthorized amendment, and tampering, Astex Therapeutics Ltd only send and receive e-mails on the basis that the Company is not liable for any such alteration or any consequences thereof. Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park, Cambridge CB4 0QA under number 3751674
