Near-Ultraviolet Luminescence of ${\mathbf{N}}_2$ Irradiated by Short X-Ray Pulses

The Linac Coherent Light Source is an x-ray free-electron laser that recently demonstrated lasing in the 1.5–15 Å wavelength range. We report on luminescence measurements of a molecular nitrogen gas irradiated by $\sim 2\mathrm{mJ}$, 80 fs x-ray pulses at energies of 0.83, 2.7, and 8.3 keV. These results provide a direct test of our current understanding of photoabsorption, electron dynamics, and fluorescence processes for such intense, ultrashort x-ray pulses. At 0.83 keV, the duration of the fluorescence signal depends strongly on space-charge effects. At 8.3 keV, space-charge effects are weak, and the signal duration is determined by the Auger electron dynamics.

Figure 1

Experimental setup.

Figure 2

Temporal dependence of the fluorescence signal for different nitrogen pressures, magnetic fields, and photon energies, measured with the PMT located at the top of the chamber. For the lower and upper curves the magnetic field is 0 and 240 G, respectively. $N$ is the normalization constant.

Figure 3

Calculated travel distance of photoelectrons. For 8.3 keV, $r$ is much larger than 100 mm.

Figure 4

Time-integrated fluorescence signal as a function of the magnetic field for photon energies of (a) 0.83 keV, (b) 2.7 keV, and (c) 8.3 keV. For each photon energy, data for different nitrogen pressures (given in mTorr) are shown.

Figure 5

Temporal width of the fluorescence signal as a function of the nitrogen pressure for different photon energies at a magnetic field of 100 G.