A swing only gets going if you push it in rhythm. Miss the beat, and your effort is wasted. Light interacts with an atomic nucleus in much the same way: a photon is usually absorbed only when its energy exactly matches the gap between nuclear levels. But what if the photon's energy doesn't fit? The authors of a new study have found a workaround.
They considered an atom of platinum-193. Its nucleus has a transition with an energy of 14.2 keV: that's hard X-rays. No one has observed this transition yet. The idea is to hit the atom with a powerful pulse from an X-ray free-electron laser. The pulse contains a huge number of photons, but their energy doesn't match the nuclear transition. The atom's electron shell comes to the rescue. When one of the electrons recombines with the atom, it carries off the excess energy, allowing the nucleus to accept the "wrong" photon. This is reminiscent of frequency up-conversion in nonlinear crystals, except the nonlinear medium here is the nucleus itself.
The probability of such an event is extremely low — the cross-section is tiny. But the laser pulse contains an astronomical number of photons, which compensates for the rarity. If experiments confirm the calculations, we will have a new way to control nuclear transitions with X-rays. And then "wrong" pushes will start to swing the swing that once seemed out of reach.
The energy of 14.2 keV is about 3000 times greater than the energy of visible light. A single such photon can knock an electron out of the deepest shell of a lead atom.
In Stanislaw Lem's novel "Solaris," neutrinos interacted with the ocean's substance in an unusual way — here, too, a particle that usually flies right through suddenly gets into the game thanks to a mediator.