From what I understand, there is a discharge along the length of the electrodes which would excite the medium (air) in the gap. Now photons are being emitted ra

From what I understand, there is a discharge along the length of the electrodes which would excite the medium (air) in the gap. Now photons are being emitted randomly in all directions. This means also in direction along the gap. This is important since these photons will interact with more of the excited medium and thereby stimulate emission of additional photons, leading to 'lasing'. As long as there are excited molecules (or atoms) left, this is a self-amplifying process. (Lasers are basically clever contraptions to keep these excited states populated) Notice that there is no resonator with mirrors at each end as you would see in other lasers. It is also a UV laser which means you can not see the beam without it interacting with a substance that can convert UV to visible light. That's dangerous because you could inadvertently look into the beam (via reflections from somewhere in the lab!) and take eye damage.