PILATUS3 R CdTe FOR CHALLENGING CHARGE DENSITY ANALYSIS
Charge density analysis is one of the most challenging crystallographic applications where highest data quality is absolutely paramount. PILATUS3 R CdTe detectors excel because of the absence of detector noise. In addition to that, direct detection in the CdTe sensor achieves quantum efficiency greater than 90% and makes best use of the weak Ag and Mo sources used in this application. Fluorescence background suppression is another advantage only HPC detectors can provide in laboratory applications. In HPC detectors, X-rays with an energy below a threshold set by the user are not measured. This way, fluorescence background can be substantially reduced or even eliminated from the acquired data. When working with integrating detectors such as CCDs or so called CPADs, fluorescence suppression is not possible and data quality suffers from increased background.
Data for charge density analysis were collected with an Ag microfocus source on an integrating CCD and a single-photon counting PILATUS3 R CdTe 300K detector. Two samples were studied, dibenzyl diselenide and paracyclophane. Dibenzyl diselenide is a particularly challenging sample because it exhibits strong Se-fluorescence when exposed to Ag radiation. For each of the samples, data reach the same high resolution and have a completeness of 100%.
Fig. 1 illustrates the benefit of fluorescence suppression on the dibenzyl diselenide data. A diffraction image acquired at a threshold energy of 11 keV shows the background from the Se-fluorescence between 11.2 and 12.5 keV. Setting the threshold to 13.5 keV effectively suppresses fluorescence and substantially reduces background. The better data quality thus achieved leads to cleaner and flatter residual density maps of dibenzyl diselenide (Fig. 2). Fluorescence suppression is not possible with any charge-integrating detector available for the laboratory.
Even when fluorescence suppression is not necessary, data acquired with the PILATUS3 R is of superior quality. Fig. 3 shows residual density maps for paracyclophane calculated from data acquired with the PILATUS3 R detector and a CCD. Again detection with the PILATUS3 R CdTe gives a flatter map.
DECTRIS is grateful to Regine Herbst-Irmer, Christian Schürmann, Lennard Krause, Felix Engelhardt, and Dietmar Stalke (University Göttingen) for performing the experiments and sharing their results.
Figure 1: Fluorescence suppression decreases background. At a threshold energy (Eth) of 11 keV (left panel), Se fluorescence causes strong background. A diffraction image acquired with a charge integrating detector will suffer from the same fluorescence background in addition to its detector background. At Eth = 13.5 keV fluorescence is effectively suppressed (right panel), which dramatically improves the signal-to-noise ratio of the Bragg spots.
Figure 2: Residual density maps (isolevels at ± 0.11 e Å-3) of dibenzyl diselenide calculated from data acquired with a CCD (left panel) and a PILATUS3 R CdTe (right panel). Thanks to fluorescence suppression, the map obtained from PILATUS3 R CdTe data is much flatter and largely featureless.