(note: excerpt and image from istp.gfsc.nasa.gov)
Clouds of Barium Ions
An atom can be ionized by the adsorption of light. The atom of barium is particularly easy to ionize, because its outermost electron is very loosely bound.
If a mass of barium is vaporized in space, producing a barium cloud, much of the barium becomes ionized by sunlight within less than a minute. The cloud then moves in response to electric forces in space, and can be used to study the electrical field in space.
In practice the barium is packed into canisters with copper oxide, and these are released from rockets or satellites and ignited. The resulting chemical reaction produces great heat, but more barium is packed into the canister than can combine chemically, and some of the excess is vaporized to form a large spherical greenish cloud.
(left image is of barium release cloud)