A conceivable way could be to disrupt the nuclear force of the target atoms, maybe like an anti-Pion/Gluon ray that self-propagates the reaction through the released energy.

(As we might remember, splitting the atom yields a bunch of energy, and uncontrolled such reactions go Hiroshima)

It might be controlled by sub-particle lensing, probably some kind of magnetic field, to be active at a specific distance.

For the reaction to be contained, either there's a radially limiting component (air is not particle dense enough to propagate the reaction, or atoms not energy dense enough) or it's a cascade triggered by the beam which stops when the beam stops (or the reaction gets too far away from it)

As I believe Pions and Gluons are their own anti-particles, I don't know how we would go about doing this, but hey, that's for Science!™ to solve.