Time-Resolved Electron Diffraction from Selectively Aligned Molecules

We experimentally demonstrate ultrafast electron diffraction from transiently aligned molecules in the absence of external (aligning) fields. A sample of aligned molecules is generated through photodissociation with femtosecond laser pulses, and the diffraction pattern is captured by probing the sample with picosecond electron pulses shortly after dissociation—before molecular rotation causes the alignment to vanish. In our experiments the alignment decays with a time constant of $2.6\pm 1.2\mathrm{ps}$.

Figure 1

Experimental geometry: the electron beam, laser beam, and gas jet are mutually orthogonal. The polarization of the laser beam is orthogonal to the direction of propagation of the electron beam. $\theta$ is the scattering angle and $\varphi$ is the azimuthal detector angle with respect to the laser polarization. $\overrightarrow{\epsilon }$: laser polarization (blue arrow), ${\mathrm{C}}_2{\mathrm{F}}_4{\mathrm{I}}_2$ molecule (schematic); $\alpha$: angle between the C-I bond (dissociation axis) and laser polarization. The constituent atoms are color coded, with yellow for iodine, light blue for carbon, and red for fluorine.

Figure 2

Ground-state results for ${\mathrm{C}}_2{\mathrm{F}}_4{\mathrm{I}}_2$. (a) Azimuthally averaged $sM$ functions, from experiment (blue or gray line) and theory (dotted black line). (b) Corresponding radial distribution functions $f(r)$. The dominant contributions to the peaks are labeled.

Figure 3

2D time-resolved diffraction patterns. (a) $\Delta [sM(s,\varphi )]$ close to zero delay of probe: the pattern is the result of averaging patterns recorded within the delay range of $t=0\mathrm{ps}$ and $t=1.7\mathrm{ps}$. The white arrow indicates the direction of the laser polarization. The upper left half of the image displays the experimental data, while the lower right half shows the theoretical prediction. The dark region splitting the image is the shadow of our beam stop. (b) $\Delta [sM](s,\varphi )$ averaged between $t=3.3\mathrm{ps}$ and $t=5\mathrm{ps}$. (c) $\Delta [sM](s,\varphi )$ averaged between $t=8.3\mathrm{ps}$ and $t=28.3\mathrm{ps}$.

Figure 4

(a) Experiment: $\Delta [s{M}_{||}(s)]$ and $\Delta [s{M}_{\perp }(s)]$ before time zero (averaged between $t=-5\mathrm{ps}$ and $t=-1.7\mathrm{ps}$), $\Delta [s{M}_{||}(s)]$ and $\Delta [s{M}_{\perp }(s)]$ close to time zero (averaged between $t=0\mathrm{ps}$ and $t=1.7\mathrm{ps}$) and for late time steps (averaged between $t=8.3\mathrm{ps}$ and $t=28.3\mathrm{ps}$). For visual clarity the curves for $t=-5\mathrm{ps}$, $-1.7\mathrm{ps}$, and for $t=0\mathrm{ps}$, 1.7 ps are multiplied by a factor of 2.8. Theory: corresponding theory curves. (b) $A_{||}$ and $A_{\perp }$ measured parallel and perpendicular to the laser polarization as functions of delay time $t$; (c) Anisotropy parameter $\delta$ as a function of delay time $t$; the black line shows an exponential fit to the data.