The overall goal of this x-ray free electron laser powder diffraction experiment is to probe the electron dynamics induced in nanocrystals of C60, via interaction with intense femtosecond x-ray pulses. This method can help answer the question of where the x-rays delivered in extremely short pulses in very intense bursts, affect samples in ways that deviate from conventional x-ray crystallography. Though this method has been applied to the translational electronic structure of C60, it could equally well be applied to a range of other samples including protein crystals.
Generally, individuals new to this method will struggle, because x-ray science is relatively new. The protocols are still currently being developed and tested. The experiment depends on the correct preparation of the C60 sample.
Have the necessary materials ready in a bio-safety cabinet. These materials include a source of five grams of C60 powder and mortar and pestle. In addition, prepare a sample holder for the experiment.
This aluminum frame sample holder has 48 approximately two millimeter by 12 millimeter cells and has a thickness of one micrometer. Complete the sample holder by adhering a 10 micron thick adhesive polyamide film to cover one side of the frame. With the polyamide film in place, the cells now form wells where the C60 powder.
Once the sample holder is ready, begin working with the C60. Transfer a small batch of about 100 micrograms to the mortar, the powder level should not exceed the height of the rounded edge of the pestle. Crush the powder to produce fine nanoparticles.
Crystals that are too large diffract too many x-rays during the x-ray experiment. Check that the crystals do not show any visible light reflections, as this is a sign that they have not been crushed finely enough. Use a small spatula to remove the crushed C60 powder from the mortar.
Then spread the powder as thinly as possible across the cells of the sample holder and on to the adhesive backing of the polyamide film. Repeat crushing the powder and applying it to the cells until they are all filled. Next, get a second adhesive backed polyamide film that can cover the entire sample holder.
Apply the adhesive side directly onto the powder in the sample holder. Then pull the film off to create a uniform monolayer in the sample holder. Apply new polyamide sheets in the same manner until no more powder comes off.
When done, the C60 monolayer should appear evenly spread out across individual cells. Seal the sample holder in a plastic container for transport. This drawing provides an overview of the experiment at the linac coherent light source at Stanford.
The sample is mounted 79 millimeters in front of and parallel to the corneal SLAC pixel array detector in an experiment chamber at 10 to the minus seven Torr. A 32 femtosecond 10 kiloelectron volt pulse with the smallest practical focal spot size, is directed perpendicular to the sample. Viewing the sample holder on the beam side, program the rasteur scan procedure to start at the top left corner of a sample cell window.
Then scan horizontally in steps of 600 micrometers until reaching the cell boundary. Move down 600 micrometers and scan left to right again. After one last scan and at the lower right corner of the sample cell window.
Perform a scan with 90%of the incident x-rays attenuated at a one hertz pulse frequency. View the images to check for possible detector saturation as in this case, saturation indicates the need for more attenuation before the detector. Address attenuation issues so there is no detector saturation.
The experiment can then proceed, using another sample holder cell. For peak analysis, start with a two dimensional powder diffraction image file. Open the fit2D software and agree to the terms of use.
On the next screen, enter the diffraction image dimensions. Click on X dimension and set the value, 1, 800 pixels in this experiment, then, click on Y dimension and set its value, also 1, 800 pixels. Continue by moving to and clicking on okay.
On the next screen, select powder diffraction 2D, move on to select input to load the file. Choose the diffraction image file from the list. Select okay on the next screen and the screen after that.
Then, move the cursor to choose beam center from the menu. Select the circle coordinates option and within the input concentric coordinates bar, select the two click option. Now, on the diffraction image, work on the innermost diffraction ring.
Click one point on the ring, and then click the center of this ring portion seen in the spyglass. Repeat for three more points on the innermost diffraction ring, ensuring the four point selected are approximately equal intervals on the ring. When done, press, click here to finish to determine the center of the diffraction pattern.
Click integrate to perform integration of the image. At this point, the physical geometry parameters from the experiment are required. Pixel size, sample detector distance and x-ray wavelength.
When these values are entered, press continue to generate a one dimensional powder diffraction pattern. The software will generate a plot of intensity versus to data scattering angle. Go to the menu items and select output from the output options, choose chip plot.
Enter a name for the file before saving it. After saving the file, close the program. Here are one dimensional data from x-ray free electron data sets.
A 10%flux data set is in green and a 100%flux data set is in red. For comparison, there is a synchrotron data set in purple. When viewed closely, the synchrotron and 10%flux x-ray free electron laser datasets show similar structure and largely coincide.
However, the 100%x-ray free electron laser data shows additional peaks. The data motivates a model of the change to the electron structure of the C60 molecules due to interaction with the pulse. In this image, blue spheres represent C60 molecules and the red tips indicate the direction of order dipoles.
Induce electric dipole moments lead to an alignment of neighboring dipoles and ultimately, additional face contributions to the scattering amplitude. Here are data and output from models over a range of scattering angles where the 100%flux x=ray free electron laser data, show additional peaks. Again, the synchrotron data and the 10%flux data agree and are consistent with the FCC model.
The 100%flux x-ray free electron laser data differs from these, but it's accurately reproduced by the model created to explain it. Once mastered, this technique can take approximately 12 hours in a single LCIF shift. This includes everything from sample preparation to testing detector attenuation and sample positioning for two different x-ray fluxes.
While attempting this procedure, it is important to check detector saturations throughout all experimental runs. Be sure to view real time detected images and halt the experiment if saturation on the detector is seen. Following this procedure, other methods like mass spectroscopy can be carried out on the irradiated samples.
However, the samples are generally obliterated during the x-ray runs. After its development, this technique has paved the way for x-ray science. Looking at and studying the effects of intense x-ray light sources on crystal samples and then, using the principles of x-ray crystallography to understand this.
Don't forget that working with nanomaterials can be extremely hazardous, when preparing your C60 samples, be sure to use fumigated hoods and a bio-safety cabinet.
