A general method for controlling and resolving rotational orientation of molecules in molecule-surface collisions

Abstract

The outcome of molecule-surface collisions can be modified by pre-aligning the molecule; however, experiments accomplishing this are rare because of the difficulty of preparing molecules in aligned quantum states. Here we present a general solution to this problem based on magnetic manipulation of the rotational magnetic moment of the incident molecule. We apply the technique to the scattering of H₂ from flat and stepped copper surfaces. We demonstrate control of the molecule's initial quantum state, allowing a direct comparison of differences in the stereodynamic scattering from the two surfaces. Our results show that a stepped surface exhibits a much larger dependence of the corrugation of the interaction on the alignment of the molecule than the low-index surface. We also demonstrate an extension of the technique that transforms the set-up into an interferometer, which is sensitive to molecular quantum states both before and after the scattering event.

Figure 1. Experimental set-up.

(a) Dimensional drawing of the apparatus. (b) Illustration of the beam deflecting magnetic manipulation discussed in the text. The initial quantum state of the beam is defined by the magnetic lens (lens 1), which focuses and defocuses different quantum states. (c) Illustration of the manipulation of the rotation projection states within the B1 field. The effect of the field can be visualized as a precession of the axis of rotation of a molecule.

Figure 2. Flux detection and oscillation.

(a,b) Flux detection measurements and corresponding spectra for H₂ scattering from copper surfaces. The magenta curves show results for Cu(111), whereas the black and green curves are for Cu(115) at two different crystal azimuths (parallel/perpendicular to the atomic step direction, respectively). As we modify the field B1, we change the quantum state of the impinging molecule and the stereo-selective scattering process leads to clearly resolved flux oscillations. (c) Calculated flux oscillations for H₂ scattering from Cu(111) using the scattering probabilities mentioned in the text. (d) Spectrum of flux oscillation calculation shown in c. The horizontal axis of the spectra shown in b and d, was plotted using frequency per magnetic field units, labelled as γ.

Figure 3. Density functional theory calculations.

(a,b) DFT calculations comparing the effect of the alignment of the H₂ molecule on the interaction with Cu(115) and Cu(111). The polar angles of orientation with respect to the surface normal are given in the legend (see also Supplementary Fig. 1).

Figure 4. Full-interferometer mode.

(a) Measurements in full-interferometer mode close to the spin echo condition (B1=437 gauss). (b) Spectra of measurements close to the spin echo condition measured on Cu(111) and Cu(115) (blue crosses and red circle markers). (c) Field dependence of the Ramsey eigenenergies. (d) 2D map created from concatenating spectra measured at different magnetic fields and plotting the logarithm of the intensity. The frequency peaks appear as high-intensity (yellow) bands. All the high-intensity frequency bands we measured fit the position of the Ramsey transitions superimposed on the image (black dots) and labelled using the scheme in c.