Conformer Selection by Matter-Wave Interference

We establish that matter-wave diffraction at near-resonant ultraviolet optical gratings can be used to spatially separate individual conformers of complex molecules. Our calculations show that the conformational purity of the prepared beam can be close to 100% and that all molecules remain in their electronic ground state. The proposed technique is independent of the dipole moment and the spin of the molecule and thus paves the way for structure-sensitive experiments with hydrocarbons and biomolecules, such as neurotransmitters and hormones, which have evaded conformer-pure isolation so far.

Figure 1

Setup for the conformer-specific diffraction of molecules. After the first slit skimmer, the adiabatically expanded molecular beam is further reduced in width by the source skimmer (1). Here, the transverse coherence of the molecular beam is prepared to illuminate several antinodes of the standing light wave (2). Matter-wave interference at the optical grating is determined by the dispersive and absorptive interaction between the molecule and the spatially periodic laser field. The resulting interference pattern (3) is spatially filtered by two movable slits to isolate the relevant diffraction peaks. This results in a pure beam of the desired conformer in the science region (4). The molecular beam is collimated by the skimmer $S_3$ to $20\mu \mathrm{rad}$ to prevent the diffraction orders from overlapping.

Figure 2

Conformer selection by matter-wave interference is illustrated for the example of 2-phenylethylamine (PEA). In a supersonic expansion it reveals four spectroscopically separated conformers, which differ in the orientation of the $\mathrm{C}─\mathrm{N}$ bond with respect to the ${\mathrm{C}}_6─{\mathrm{C}}_7$ bond (atomic numbering on the left) and in the lone pair of the amino group relative to the chromophore. All conformers can be isolated in matter-wave diffraction, even though their dipole moments are virtually identical. Below each conformer we note the electronic transition energy ${\lambda }_0$ and the relative population $P$ [86] in molecular beams.

Figure 3

(a) Molecular interference patterns of the four conformers of PEA for a surface energy density $E_L/A_L$ of $0.67\mathrm{mJ}/{\mathrm{mm}}^2$ at ${\lambda }_L=265.8\mathrm{nm}$. Here the first diffraction orders are preferentially populated by the gauche(out) conformer (red) with a conformer-selection efficiency $\eta$ of $>93\%$ (grey stripes). (b) Conformer-selection efficiency $\eta$ in the first diffraction order for the (1) gauche(out), (2) anti(up), (3) anti(out), and (4) gauche(up) conformer. Areas with a selectivity of less than 50% are left blank. The position of the electronic resonance is indicated by the broken line. For all conformers a selectivity of a least 80% and up to 99% is predicted, corresponding to an up to ninefold increase in the relative population. The respective areas in parameter space are easily accessible in an experiment even for conformers of low abundance, as shown in the inset for the anti(out) conformer.