Single-Molecule Imaging with X-Ray Free-Electron Lasers: Dream or Reality?

X-ray free-electron lasers (XFEL) are revolutionary photon sources, whose ultrashort, brilliant pulses are expected to allow single-molecule diffraction experiments providing structural information on the atomic length scale of nonperiodic objects. This ultimate goal, however, is currently hampered by several challenging questions basically concerning sample damage, Coulomb explosion, and the role of nonlinearity. By employing an original ab initio approach, we address these issues showing that XFEL-based single-molecule imaging will be only possible with a few-hundred long attosecond pulses, due to significant radiation damage and the formation of preferred multisoliton clusters which reshape the overall electronic density of the molecular system at the femtosecond scale.

Figure 1

(Left) Ground-state configurations for H, ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{CH}}_4$, ${\mathrm{N}}_2$, and HNCO; (Right) Number of “bounded” electrons versus time for molecules illuminated by a 100 nm-waist Gaussian beam of cw XFEL radiation with power $P=150\mathrm{GW}$ and wavelength $\lambda =0.4\mathrm{nm}$.

Figure 2

Time evolution of electrons number [(a), solid lines] and nuclei distance (b) of HNCO irradiated by a random train of incoherent energy bursts of peak power $P=500\mathrm{GW}$, wavelength $\lambda =0.15\mathrm{nm}$ and overall time length $T=2\mathrm{fs}$ [(a), dashed line].

Figure 3

(a)–(d) Far-field scattered angular pattern (red to yellow color map), nuclei position and electron density (blu to yellow colormap) time evolution of an HNCO molecule irradiated by a short XFEL pulse of 2 fs length, wavelength $\lambda =0.15\mathrm{nm}$ and peak power $P=500\mathrm{GW}$ [Fig. 2a dashed line].

Figure 4

(a) Angular far-field, nuclei position/electron density, (b) photoionization and (c) nuclei distance evolution of an HNCO molecule irradiated by a 400 as XFEL pulse of wavelength $\lambda =0.15\mathrm{nm}$ and peak power $P=500\mathrm{GW}$ (b), dashed line); (d) Image difference between the far field of (a) and the one generated by the ground state of HNCO in the absence of any radiation damage.