Reading some random encyclopedia article, I came across a reference to the dissociation energy of I₂ being (the photon energy of light of wavelength) 578 nm, so that irradiating a mixture of H₂ and I₂ with this light (kind of yellow-green apparently) accelerates the formation of HI.
It occurred to me that monatomic halogens can react with a remarkably large number of things, so doping a thin layer of something with I₂ might be a straightforward way to make it photosensitive, so that you can pattern it with a yellow, green, blue, violet, or UV laser. Where the laser strikes, free iodine radicals are released, kicking off some kind of polymerization or depolymerization or something.
Dunno, I guess iodine is not a very convenient element.