Coherent interaction of femtosecond extreme-UV light with He atoms

We show experimentally that spectral phase manipulation of ultra-short extreme-UV light pulses can induce and control coherent transient excitation of the He(1s3p) excited state by the nonresonant components of the broadband EUV light. The spectral phase manipulation of the 15th harmonic of an intense 805 nm, 80 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 He(1s3p) resonance enhances and modifies the transient excitation. The temporal evolution of the coherently-prepared transient He(1s3p) amplitude is probed by ionization to the continuum with a 400 nm, 80 fs pulse.