Selector for Structural Isomers of Neutral Molecules

We have selected and spatially separated the two conformers of 3-aminophenol (${\mathrm{C}}_6{\mathrm{H}}_7\mathrm{NO}$) present in a molecular beam. Analogous to the separation of ions based on their mass-to-charge ratios in a quadrupole mass filter, the neutral conformers are separated based on their different mass-to-dipole-moment ratios in an ac electric quadrupole selector. For a given ac frequency, the individual conformers experience different focusing forces, resulting in different transmissions through the selector. These experiments demonstrate that conformer-selected samples of large molecules can be prepared, offering new possibilities for the study of gas-phase biomolecules.

Figure 1

(a) Scheme of the experimental setup. (b) Cut through the high-voltage electrodes and electric field configurations; gray (red) areas represent regions of strong electric field, black (blue) areas regions of weak electric field. The electric field strength at the center is $45\mathrm{kV}/\mathrm{cm}$ and contour lines are given every $4\mathrm{kV}/\mathrm{cm}$. White arrows indicate the direction of the force experienced by molecules in high-field seeking quantum states. (c) Two-color ionization detection scheme.

Figure 2

Molecular structures, dipole moments, and energies of the lowest rotational states of cis- and trans-3-aminophenol as a function of the electric field strength.

Figure 3

Experimental (solid lines) and simulated (dashed lines) transmission as a function of ac frequency for cis-3-aminophenol (black) and trans-3-aminophenol [gray (red)]. The inset shows an enlarged view of the transmission curve of trans-3-aminophenol. The black dotted line represents simulations for cis-3-aminophenol that include the details of the detection process.

Figure 4

($1+1^{\prime }$)-REMPI spectrum of 3-aminophenol. The spectrum is measured without high voltage (HV) applied to the selector, and shows the two origin transitions. In the inset, the rotational envelope of the cis-3-aminophenol origin transition is shown without high voltage (black) and with high voltage for ac frequencies of 2.95 kHz [dark gray (red)] and 1.6 kHz [light gray (blue)].