Significance of Harmonics of [lambda] in CCD Data Sets. A. Alan Pinkerton, Kristin Kirschbaum, Anthony Martin, Department of Chemistry, University of Toledo, Toledo, Ohio 43606

A method is proposed to estimate the amount of [lambda]/2 contamination inherent in area detector data using Mo radiation.

The intensity increase due to [lambda]/2 contribution to I(hkl) has been determined by measuring the intensity for reflections where h, k or l is half integral, i.e. reflections where there is no contribution from [lambda], for three crystals using two different CCD diffractometers. This information is present in all data sets obtained with area detectors but is usually ignored.

The correction thus determined has been applied to nine data sets. Improvement to the data, as measured from the least-squares refinement is shown to be insignificant for routine data sets, even for the weak low angle reflections. Some reduction in the number of "observed" systematic absences is noted thus improving space group assignments. In addition, for strongly diffracting crystals, incorrect unit cells may be obtained due to the presence of strong [lambda]/2 reflections at the reciprocal lattice nodes.