Spatial separation of state- and size-selected neutral clusters

We demonstrate the spatial separation of the prototypical indole(H${}_2$O) clusters from the various species present in the supersonic expansion of mixtures of indole and water. The major molecular constituents of the resulting molecular beam are H${}_2$O, indole, indole(H${}_2$O), and indole(H${}_2$O)${}_2$. It is a priori not clear whether such floppy systems are amenable to strong manipulation using electric fields. Here, we have exploited the cold supersonic molecular beam and the electrostatic deflector to separate indole(H${}_2$O) from the other molecular species as well as the helium seed gas. The experimental results are quantitatively explained by trajectory simulations, which also demonstrate that the quantum-state selectivity of the process leads to samples of indole(H${}_2$O) in low-lying rotational states. The prepared clean samples of indole(H${}_2$O) are ideally suited for investigations of the stereodynamics of this complex system, including time-resolved half-collision and diffraction experiments of fixed-in-space clusters. Our findings clearly demonstrate that the hydrogen bonded indole(H${}_2$O) complex behaves as a rigid molecule under our experimental conditions and that it can be strongly deflected.

Figure 1

The experimental setup consists of a pulsed molecular beam source, a deflector, a time-of-flight mass spectrometer, and a pulsed dye laser. The inset shows a cut through the deflector and a contour plot of the electric field strength for an applied voltage of 26 kV.

Figure 2

Structures, molecular constants, and Stark energies of (a) H${}_2$O, (b) indole, (c) indole(H${}_2$O), and (d) indole(H${}_2$O)${}_2$; see text for details.

Figure 3

Deflection curves of (a) indole and (b) indole(H${}_2$O) for deflector voltages of 0 (green diamonds), 10 kV (yellow circles), 20 kV (blue triangles), and 26 kV (red squares). The lines show the outcome of the respective trajectory calculations; see text for details.

Figure 4

The torsion potential and energy level scheme of indole(H${}_2$O) in its vibronic ground state (excluding torsion).

Figure 5

Normalized experimental (symbols) and simulated (lines) molecular beam profiles for water (dotted line), indole (blue circles–dashed line), indole(H${}_2$O) (red squares–solid line), and indole(H${}_2$O)${}_2$ (green triangles–dash-dotted line) at a deflector voltage of 26 kV. The inset shows the corresponding undeflected normalized beam profiles.