- Lam, Royce K;
- Raj, Sumana L;
- Pascal, Tod A;
- Pemmaraju, CD;
- Foglia, Laura;
- Simoncig, Alberto;
- Fabris, Nicola;
- Miotti, Paolo;
- Hull, Christopher J;
- Rizzuto, Anthony M;
- Smith, Jacob W;
- Mincigrucci, Riccardo;
- Masciovecchio, Claudio;
- Gessini, Alessandro;
- De Ninno, Giovanni;
- Diviacco, Bruno;
- Roussel, Eleonore;
- Spampinati, Simone;
- Penco, Giuseppe;
- Di Mitri, Simone;
- Trovò, Mauro;
- Danailov, Miltcho B;
- Christensen, Steven T;
- Sokaras, Dimosthenis;
- Weng, Tsu-Chien;
- Coreno, Marcello;
- Poletto, Luca;
- Drisdell, Walter S;
- Prendergast, David;
- Giannessi, Luca;
- Principi, Emiliano;
- Nordlund, Dennis;
- Saykally, Richard J;
- Schwartz, Craig P
We have examined the transmission of soft X-ray pulses from the FERMI free electron laser through carbon films of varying thickness, quantifying nonlinear effects of pulses above and below the carbon K-edge. At typical of soft X-ray free electron laser intensities, pulses exhibit linear absorption at photon energies above and below the K-edge, ∼308 and ∼260 eV, respectively; whereas two-photon absorption becomes significant slightly below the K-edge, ∼284.2 eV. The measured two-photon absorption cross section at 284.18 eV (∼6 × 10−48 cm4 s) is 7 orders of magnitude above what is expected from a simple theory based on hydrogen-like atoms - a result of resonance effects.