X-ray flashes film how a photograph is made

Researchers observe live the decay of the photosensitive emulsion on a photo paper

The X-ray scattering image contains detailed information about the nanostructure of the photosensitive film. © Jeff (Zhifeng) Huang / UCLA
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Filmed film: What happens when exposing a photographic film at the nano-level, researchers have for the first time made visible with X-rays. The X-ray flashes reveal how the photosensitive grains in the photographic emulsion deform, rotate and eventually decay. Only this process makes analogue photography possible, but it also takes place in other photochemical reactions.

When Louis Daguerre presented the first photographs to the public in 1839, this was a sensation: for the first time, images of reality could be produced and preserved. This milestone is based on a photochemical reaction: tiny silver bromide grains are embedded in the photo film. When exposed, a photochemical reaction occurs and the compound breaks down. Because it produces silver, the photo paper turns dark.

X-ray flashes on photo paper

What happens in detail is now made visible for the first time by Zhifeng Huang from the University of California in Los Angeles and his colleagues using the X-ray source PETRA III at the German Electron Synchrotron / (DESY) in Hamburg. They succeeded in observing the exposure of a photographic paper at the level of single nanocrystals.

Unexposed photo paper under the scanning electron microscope - the silver bromide grains are still intact © Jeff (Zhifeng) Huang / UCLA

The photographic paper used had silver bromide granules of about 700 nanometers in size. When X-rays fall on them, they are diffracted by these molecules in a characteristic way. From the resulting diffraction pattern on the detector, properties such as the grating width of the crystal, the chemical composition and the orientation can be read off. This allowed the researchers to follow the changes in the photographic paper like in a movie.

Molecular decay live

Because the X-ray exposed the silver bromide and started the photochemical decay. The researchers saw virtually live how the silver bromide granules deformed, began to spin, and finally disintegrated. "We were surprised how fast some of these individual grains rotate, " says Michael Sprung from DESY. "Some turn almost once in two seconds." At the same time, pure silver nanospheres grew up. display

After exposure: close-up of a crumbled silver bromide particle Jeff (Zhifeng) Huang / UCLA

Similar photo-induced chemical reactions also play a role in many other processes and technologies, from energy conversion in nature to microfabrication using photolithography. To be able to break down this reaction helps to better understand and possibly improve these processes. (Nature Materials, 2015; doi: 10.1038 / NMAT4311)

(German electron synchrotron (DESY), 10.06.2015 - NPO)