LAUSR

The photoionization of Lithium (Li + ) via its doubly-excited state 2s2p 1 P in intense free electron laser (FEL) radiation is studied. A recently developed perturbative statistical description of the atomic dynamics is used to calculate the ionization yield. It is observed that the FEL temporal fluctuations affect the lineshape significantly, strongly dependent on the product of the pulse’s coherence time with its intensity, ∼ τ c I 0 , which is a measure of the effect of the field in one correlation time. The weak-field long-pulse asymmetric resonant Fano-profile is broadened to resemble a Voight profile. As the intensity increases, the subsequent ionization of Li 2 + takes over and causes further distortion of the lineshape for Li + .