X-Ray Diffraction from Isolated and Strongly Aligned Gas-Phase Molecules with a Free-Electron Laser

Abstract

We report experimental results on x-ray diffraction of quantum-state-selected and strongly aligned ensembles of the prototypical asymmetric rotor molecule 2,5-diiodobenzonitrile using the Linac Coherent Light Source. The experiments demonstrate first steps toward a new approach to diffractive imaging of distinct structures of individual, isolated gas-phase molecules. We confirm several key ingredients of single molecule diffraction experiments: the abilities to detect and count individual scattered x-ray photons in single shot diffraction data, to deliver state-selected, e.g., structural-isomer-selected, ensembles of molecules to the x-ray interaction volume, and to strongly align the scattering molecules. Our approach, using ultrashort x-ray pulses, is suitable to study ultrafast dynamics of isolated molecules.

Ultrafast isomerization initiated by X-ray core ionization

“…These light sources have been employed for molecular and material science, as well as structural biology [5][6][7][8][9][10][11][12][13][14][15] . Current and future X-ray FELs plan to deliver ultrashort X-ray pulses with sufficiently high intensity to collect diffraction images of selected targets in a single shot.…”

Electron gun for diffraction experiments off controlled molecules

“…Thus, radiation damage is not relevant in these experiments. Importantly, it is much smaller than in similar x-ray-diffraction experiments, where radiation damage was not necessarily negligible, but could be reduced by increasing the x-ray beam diameter [21].…”

“…Clean molecular samples can be generated by their spatial separation according to shape [13][14][15] and size [16]. Controlling the spatial orientation of the molecules leads to an enhancement of the information that can be retrieved from a diffraction pattern, as proposed theoretically [17][18][19][20] and demonstrated experimentally for x-ray [21,22] as well as for electron diffraction [23,24]. Strong alignment or orientation is generally necessary [11,20] for three-dimensional structure reconstruction [24] and can be provided in cold supersonic molecular beams by strong-field laser alignment and mixed-field orientation [25][26][27][28][29].…”

“…For the prototypical 2,5-diiodobenzonitrile molecule the effective electron-impact-ionization cross section is about 10 −16 cm 2 , whereas the effective elasticscattering cross section is 10 −15 cm 2 . For 10 6 electrons per pulse, a molecular density of 10 8 cm −3 , and an interaction volume of 5 × 10 −6 cm 3 the signal on the detector of electrons elastically scattered off an already destroyed molecule is S D ≈ 0.5·S D elastic ·S D ionized = 0.5·5×10 −3 ·5×10 −4 = 1.25 × 10 −6 per shot [21]. This corresponds to a fraction of P D = S D /S D elastic = 2.5 × 10 −4 electrons scattered off damaged molecules per elastically-scattered signal electron.…”

“…The use of ultra cold atoms as electron sources increases the coherence [36,37]. Other possible sources for time-resolved electron diffraction are low energy electron setups [38] or laser-induced electron diffraction [39,40].If the molecular samples are prepared in the necessary strongly-controlled fashion, their densities are typically on the order of some 10 8 molecules/cm 3 [21,41]. Assuming Rutherford scattering, for the prototypical molecule 2,5-diiodobenzonitrile an effective cross section on the order of 10 −15 cm 2 can be derived for our experimental geometry and a beam stop blocking a solid angle of 1.3 × 10 −3 sr. To align or orient the molecules, they are typically exposed to laser fields with intensities of 1 TW/cm 2 [25], which can be achieved by focusing the ps-duration mJ-pulse-energy laser beam to 100 µm [42].…”