High Resolution Multiphoton Spectroscopy by a Tunable Free-Electron-Laser Light

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.

Figure 1

(a) Transition diagram for the two-photon excitation to the resonance state and its decay: $g$, the ground state; $r$, the resonance state; $m$, the metastable states (see main text for description). (b) Experimental scheme.

Figure 2

The metastable atom yield measured in the wavelength region of the $3b$ ${{}^{1}D}^e$ resonance at different pulse energies $W$ (circles). The solid (red) lines show the least-squares (Lorentzian) fit. The individual spectra are shifted vertically for clarity.

Figure 3

(a) The measured MS yield (full circles) at saturation compared to the model result [gray (red) line]. Empty circles correspond to the data obtained by a more restrictive pulse filtering [46]. The histogram of FEL pulses after the filtering. Inset: The measured MS yield of the $2b$ ${{}^{1}S}^e$ state collected at $30\mu \mathrm{J}\le W\le 60\mu \mathrm{J}$ compared to the typical EUV pulse spectrum $S_{\lambda }$ [gray (cyan) line], to its autoconvolution $(S*S{)}_{2\lambda }$ [dashed red (gray) line], and to the two model curves: with (${\gamma }_L=16\mathrm{meV}$, black line) and without an extra line broadening (${\gamma }_L=0$, dotted black line). (b) Theoretical two-photon ionization cross section from Ref. [25], broadened by the experimental resolution (full line) and the calculated total fluorescence yield spectrum (dotted line).

Figure 4

(a) Integrated metastable yield versus pulse energy: experimental data (circles), simulated data (curves) [46], and pulse energy distribution (histogram). (b) Resonance energy shift versus pulse intensity: data for $3b$ ${{}^{1}D}^e$ (full black circles) and $2b$ ${{}^{1}S}^e$ resonance [empty gray (red) circles], linear fit to the experimental data (dashed lines), calculated energy shift $S_{rg}/2$ (thick solid curve), and the ponderomotive shift divided by 2 (thin solid lines).