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Above-threshold ionization
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<p>In <a href="/facts/Atomic%2c_molecular%2c_and_optical_physics/6HeJIr7P">atomic, molecular, and optical physics</a>, above-threshold ionization (ATI) is a <a href="/facts/Photoelectrochemical_processes/0N7JCeIM">multi-photon</a> effect where an atom is ionized with more than the energetically required number of <a href="/facts/Photon/mFNhAEzA">photons</a>. It was first observed in 1979 by <a href="/facts/Pierre_Agostini/Y8w12T82">Pierre Agostini</a> and colleagues in <a href="/facts/Xenon/9E3XR69R">xenon</a> gas. </p>