A dye made up of two molecules joined together is in the frame for making film more sensitive to light. When combined into the film's emulsion, the dye can generate two 'hits' on the film for the price of a single photon of light.

Greater light sensitivity is particularly important in high-speed or night photography and microphotography -- it enables photographers to use shorter exposure times and so take sharper images.

Normally, the photographic image is produced when light falls on tiny crystals of a silver salt, such as silver iodide, dispersed in a layer of gelatin. Each photon of light kicks an electron out of the salt. Each electron then attaches itself to a positively charged silver ion and converts it to a neutral silver atom.

When four or so of these silver atoms cluster together, they form a metal particle, which can seed the growth of a larger grain of silver when the film is developed. The developing process converts the 'latent' image, formed from these silver clusters, into a visible image made up of grains of silver on the negative.

Dye molecules are generally added to the emulsion to broaden the film's colour sensitivity. These dyes absorb light at wavelengths that the silver salt alone will not. When the dye is hit by a photon of light it also spits out an electron, which can go on to create a silver atom from an ion.

But a dye that has lost an electron wants one back again, and will sometimes strip one from a nearby silver atom, reversing the process that creates the latent image and limiting the light sensitivity of the film.

Various ideas have been explored to reduce these wasteful 'recombination' events [see 'Through a lens, darkly']. But Ian Gould of Arizona State University and co-workers at Eastman Kodak in Rochester, New York, are the first group to turn the problem into the solution.

By combining two molecules, they have produced a system that is not only happy to give up one electron when hit by a photon, but gives up a second electron for good measure.

The new two molecules in the dye are linked by a chemical bond. When a photon is absorbed, an electron gets released from the bond and the bond breaks. This leaves one molecule with a positive charge, and the other with a 'spare' electron (which was originally paired up with the electron lost from the chemical bond).

The group chose the two molecules carefully. One likes to form positive ions and so has no inclination to get an electron back from a silver atom. The other is uncomfortable with its lone electron, which it shrugs off to form another stable positive ion. So from one photon, the dye releases two electrons (which help to generate a latent image).

Gould's team found a family of light-absorbing molecules that had a negative charge to begin with, in the form of a 'carboxylate group'. When a dye containing this group loses an electron, it becomes neutral. It can then break up by losing the carboxylate group, leaving behind another molecule that is apt to lose a second electron.

In the Journal of the American Chemical Society1 the researchers show that dyes of this sort would increase the light sensitivity of a standard silver-salt photographic emulsion, in some cases by almost twofold.