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. 2014 Nov 7;113(19):193201.
doi: 10.1103/PhysRevLett.113.193201. Epub 2014 Nov 7.

High resolution multiphoton spectroscopy by a tunable free-electron-laser light

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High resolution multiphoton spectroscopy by a tunable free-electron-laser light

M Žitnik et al. Phys Rev Lett. .

Abstract

Seeded free electron lasers theoretically have the intensity, tunability, and resolution required for multiphoton spectroscopy of atomic and molecular species. Using the seeded free electron laser FERMI and a novel detection scheme, we have revealed the two-photon excitation spectra of dipole-forbidden doubly excited states in helium. The spectral profiles of the lowest (-1,0)(+1) (1)S(e) and (0,1)(0) (1)D(e) resonances display energy shifts in the meV range that depend on the pulse intensity. The results are explained by an effective two-level model based on calculated Rabi frequencies and decay rates.

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