Previously, we talked just a little bit about halides, and how they fit into the formation of silver gelatin-based images. Today, we’re going to take a closer look at part of the “how.”

Let’s begin with a brief overview of the darkroom process. As a reminder, the mechanics of creating a black and white print generally include the following steps:

1. A paper is pre-coated with a halide salt and silver nitrate that are mixed in a binder such as gelatin.
2. An image is first generated by projecting a source of illumination (like the sun or a lamp) through film onto a coated piece of paper. 
3. Next, the paper or film is developed in a bath of chemicals.  This is the part of the process the image seems to “magically” appear!
4. The image developing chemical reaction is stopped in a “stop bath.”
5. The paper or film is moved to a second bath to “fix” the image in a fixative bath.
6. The photograph is rinsed and hung to dry – ready to safely see the light of day.

Now, let’s delve a little deeper into the chemical reaction described in step two, beginning with a bit of a thought experiment. If I say to you, “semiconductor,” what springs to mind? Something involving electronics, perhaps? Maybe chips, lots of little circuits and tiny wires? Maybe, if you’re more photography-minded, a digital camera? All excellent things to think of! With this in mind, what if I told you that our silver gelatin emulsion is also a semiconductor, one that converts light into latent images instead of electricity?

Well, that is what we’re going to explore today, the basics of the mechanism by which light gets our emulsion ready to record latent images. The light shines on an atom (in this case, our silver halide ion), energy is transferred to an electron, and the electron moves to an excited state and is ready to make chemical magic.

If you refer back to your basic chemistry, atoms are composed of three parts: protons, neutrons, and electrons. Protons are positively charged and reside in the nucleus (center) of an atom with chargeless neutrons. Negatively charged electrons orbit the nucleus in bands. When all things are equal, an atom has the same number of protons as electrons, leaving the atom neutral. There aren’t a lot of atoms that are naturally like this, however; an atom will often have too many or too few electrons in orbit. This is a good thing, as it not only makes them stable, it makes them available for bonding with other atoms and creating chemical reactions.

Now let’s apply some of that to our silver halide. The electrons near the nucleus of our ion are in the “ground” state. They’re unexcited. The nucleus has them firmly gripped in its gravitational pull and they’re uninterested in going anywhere. This area of grounded electrons is known as the valence band. In order for them to be available for any sort of exchange, they’ll need to get farther away from that nucleus, and out into an outer band of the ion, conveniently known as the conductance band.

How does this all relate to the parking lots I mentioned in the title? I’m glad you asked. The parking lot analogy is a fantastic illustration of the process by which light interacts with matter. Let’s think of it this way: the nucleus of the silver halide ion is your typical Big Box Store. Directly outside of the store is the valence band parking lot, full of electron cars, all off, all waiting in the ground state. Beyond the parking lot is a strip of grass, which we’ll get to shortly, and beyond that is the conductance band (i.e, the road), where the cars are all in motion, on their way to any number of places.

In order for the cars to get out of the valence band, they’re going to need some energy. For our electrons, the needed energy is light. Once they get light to get the engines going, they can pull out of the parking lot and onto the road and drive off to chemical reactions.

Now let’s detour briefly to that grassy strip that I mentioned earlier. It’s known as the forbidden gap. Ideally, this area is empty. However, due to defects such as insufficient energy, an electron may not be able to completely cross to the conductance band, and may be temporarily stuck in this gap. Even here they can be useful as stepping stones for other electrons that need to cross over. Stuck electrons will either receive more energy to get them to the outer bands, or they’ll lose energy and be pulled back to the valence band.

I’ll note here that this structure is characteristic of all semiconductors, including digital camera sensors. In silver halide grains, this excitement of electrons will always happen when it comes into contact with light, as silver halide has a light sensitivity of 100%. No matter what, when a grain of silver halide is exposed to light, it will always liberate an electron. You also needn’t think of it as just one electron at a time being excited in this fashion. The grain can have so much energy that its valence band is completely empty, and vice versa.

What happens after this? Well, that’s an exploration for next time.

Hyacinth Tucker (UCL) — Bindery and Conservation Technician