Unlike convention or more commonly used Bragg diffraction which utilises a single monochromatic wavelength of X-ray radiation Laue diffraction utilises a large spectral bandwidth of X-ray radiation. It is classical coined a white beam or pink beam experiment because of that. As like a laser is monochromatic and coloured whereas a lightbulb is polychromatic providing white light.

The use of a white beam of radiation means that each of the diffraction conditions are simultaneously achieved in one or two images. An image being the data recorded to the detector just like an image is recorded to your smartphone or camera. This produces several problems.

The problem

Firstly if the structure is completely unknown then it is hard to deconvolute the diffraction image. This is because is spot on the image contains information but which is not by itself extractable towards the solution. In short the white beam gives lots of data very fast but the data is produced is made many times more difficult to analyse than the monochromatic experiment. You can understand it in that you do not know what contribution to any given spot comes from what proportion of which specific wavelength. You also do not know how the detector scales for each wavelength but even if you did you still need to deconvolute that information from a peak with data from multiple harmonics.

A simple way to get around this problem is to place filters which absorb or only allow the emission of certain wavelengths during the experiment. This helps narrow down specific interactions between wavelength and spot position. But at the expense of reducing data signal and increasing experiment length.

With that in mind what is the comparison between experiment lengths, why would you use Laue diffraction? Well simply speaking if you can get all your possible diffraction data, a full sphere, in two exposures of the sample at seconds or less timescales then that is theoretically your Laue experiment. Whereas for the monochromatic experiment you would need many hundred exposures at many different crystal positions to achieve the same result. Which results in significantly longer experiment times.

So is there a way to bridge the gap? I think there is and have been discussing the use of dynamic Laue for several years. When I first thought of the technique there were not detectors out there which would be suitable but now I think there are.

What is dynamic Laue diffraction?

So rather than moving the crystal to lots of positions like you do with a normal Bragg style experiment (by move I really mean rotate within the diffractometer circles) you keep your crystal in one position whilst simultaneously scanning the wavelength of the X-ray source. It really is that simple, You can pre-calibrate your detector and build in a timing signal from the X-ray source (monochromator) so that any calibration required for the wavelength can be automatically achieved.  If you utilise a multi-bounce monochromator you can control the exit position of the beam fair more reliablly than say from a single monochromator (like the one I used to use). The rate limiting step of the experiment becomes the speed at which you can scan the optics. I say this because modern CCD detectors can now readout in the 1/16th of a second, there are new direct X-ray imaging detectors which can readout in ms and thus you can run effectively in a shutterless operational mode as long as you can keep up with the monochromator/wavelength selection.

Once you have done that you have a pseudo Laue diffraction pattern which you can then either reconstitute or treat like a multiple wavelength Bragg experiment (which it is really).

Possible Monochromators

X-ray optic technology is constantly changing and with these changes and advancements access to different characteristics of the X-ray experiment become possible. I’m sure you have over the years seen those strange little “mirrors” or parking aids in the back of cars? They are called Fresnel optics. Interestingly you can build a tunable X-ray monochromator using Fresnel optics and even more interestingly you can do it using old records or LPs.