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Photoelectron emission from excited states of laser-dressed atomic helium is analyzed with respect to laser intensity-dependent excitation energy shifts and angular distributions. In the two-color XUV (exteme ultra-violet) -- IR (infrared) measurement, the XUV photon energy is scanned between SI{20.4}{electronvolt} and the ionization threshold at SI{24.6}{electronvolt}, revealing electric dipole-forbidden transitions for a temporally overlapping IR pulse ($sim!SI{e12}{wattper centimetersquared}$). The interpretation of the experimental results is supported by numerically solving the time-dependent Schrodinger equation in a single-active-electron approximation.
We describe a numerical method that simulates the interaction of the helium atom with sequences of femtosecond and attosecond light pulses. The method, which is based on the close-coupling expansion of the electronic configuration space in a B-spline
The derivation of approximate wave functions for an electron submitted to both a coulomb and a time-dependent laser electric fields, the so-called Coulomb-Volkov (CV) state, is addressed. Despite its derivation for continuum states does not exhibit a
In dense atomic gases the interaction between transition dipoles and photons leads to the formation of many-body states with collective dissipation and long-ranged forces. Despite decades of research, a full understanding of this paradigmatic many-bo
We develop the theory of propagation of laser wave in a gas of two-level atoms (with an optical transition frequency $omega^{}_0$) under the condition of inhomogeneous Doppler broadening, considering the self-consistent solution of the Maxwell-Bloch
We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral