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

Area of Science:

• Atomic and Molecular Physics
• Quantum Optics
• Laser Spectroscopy

Background:

• Seeded free electron lasers (FELs) offer high intensity, tunability, and resolution for advanced spectroscopy.
• Multiphoton spectroscopy enables the study of electronic states not accessible by single-photon transitions.

Purpose of the Study:

• To investigate the two-photon excitation spectra of dipole-forbidden doubly excited states in helium.
• To explore the influence of pulse intensity on spectral profiles and resonance energies.

Main Methods:

• Utilizing the seeded free electron laser (FEL) facility, FERMI.
• Employing a novel detection scheme for high-resolution spectral analysis.
• Applying an effective two-level model incorporating Rabi frequencies and decay rates for theoretical explanation.

Main Results:

• Revealed the two-photon excitation spectra of specific doubly excited states in helium.
• Observed meV-range energy shifts in spectral profiles of (1)S(e) and (1)D(e) resonances.
• Demonstrated a dependence of these spectral shifts on the FEL pulse intensity.

Conclusions:

• The study successfully characterized dipole-forbidden states in helium using advanced FEL technology.
• Intensity-dependent spectral shifts provide insights into light-matter interactions in atomic systems.
• The effective two-level model accurately explains the observed phenomena, validating theoretical approaches.