It is more a physical reaction than a chemical reaction. The americium in the smoke detector is emitting high-speed alpha particles (helium nuclei). The particles hit molecules in the air and knock off electrons to form ions. So, for example, you might say:
O2 + He(+2) --> O2(+1) + e(-1) + He(+2)
(Charge is indicated inside the parentheses.)
See How Smoke Detectors Work for complete details on the inner workings of smoke detectors.
Here are some interesting links: