Spectroscopy of Size-Selected Neutral Clusters: Femtosecond Evolution of Neutral Silver Trimers

Negatively charged, mass-selected clusters of silver atoms are caught in a linear quadrupole trap. There their excess electrons are photodetached with a nearly 100 fs pulse of $\sim$400 nm radiation. The neutrals so prepared would remain in the trap for ns or even ms. However, a second ultrashort delayed pulse of radiation, intense enough to induce two-photon ionization of the clusters, is put into the trap, allowing the neutral to carry out internal motion. The positive ions generated are then mass analyzed and collected. The transient ion signal reveals the internal motions of the atoms of the neutral, monodisperse clusters, particularly in cases in which detachment, ionization, or both involve a significant change of geometry.