Multiply charged ions induced by multiphoton absorption processes in rare-gas atoms at 1.064μm

Multiply charged ions are easily formed in rare gases by multiphoton absorption processes. For Kr and Xe up to quadruply charged ions are formed. They are induced by a bandwidth-limited 50 ps laser pulse at 1.064 W cm ^-2 intensity range. Doubly charged ions are formed through the absorption of a very large number of photons (29 for Xe) in a direct transition from the ground state of the atom to the second ionisation limit. The percentage of Xe²⁺ ions relative to Xe⁺ ions is 1% at 10 ¹³ W cm^-2, and reaches 20% at 10¹⁴ W cm ^-2. At 1.06 mu m, the removal of two electrons from the ground state of rare-gas atoms in a single step is responsible for the creation of doubly charged ions. This could be a general rule for atoms which have two or more electrons in the outer shell.