Coherent interaction of femtosecond extreme-uv light with He atoms

Abstract

We show experimentally that spectral phase manipulation of ultrashort extreme-uv light pulses can induce and control coherent transient excitation of the $\mathrm{He}(1s3p)$ excited state by the nonresonant components of the broadband extreme-ultraviolet light. The spectral phase manipulation of the 15th harmonic of an intense $805\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulse is achieved by propagation of the euv light through a variable optical density of He gas. The acquired spectral phase due to the dispersive interaction of the off-resonance components in the euv pulse with the $\mathrm{He}(1s3p)$ resonance enhances and modifies the transient excitation. The temporal evolution of the coherently prepared transient $\mathrm{He}(1s3p)$ amplitude is probed by ionization to the continuum with a $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulse.