Lithium-ion soft x-ray laser

Abstract

A two‐stage optical‐pumping scheme is considered for achieving laser action on the 199‐Å lithium‐ion resonance line. Incoherent radiation from a CO₂ laser‐produced plasma preferentially photoionizes the K shell of neutral lithium, producing highly energetic ions in the metastable 1s2s state. A detailed model of the system indicates that with an initial lithium vapor density ∼10¹⁴ cm⁻³, an effective metastability of several nsec can be obtained, allowing a drastic reduction in pumping requirements compared to schemes involving direct pumping of the upper laser level. After a sufficient buildup of metastable ions, a dye laser, tuned to the 9584‐Å 1s2s ¹S→1s2p ¹P transition, is switched on, and lasing at 199 Å occurs due to the stimulated‐resonant‐Raman anti‐Stokes process. At 10¹⁴ cm⁻³ initial vapor density, the resonant‐Raman gain is ∼10 cm⁻¹, so that a gain length ∼3 cm should be adequate to demonstrate lasing in a single pass. The dye laser acts as a traveling‐wave excitation source, providing a collinear well‐collimated x‐ray laser beam.