Simulated longitudinal velocity distribution of the ND<sub>3</sub> molecular beam before (solid curve) and after the bend filter (dots)

<p><strong>Figure 4.</strong> Simulated longitudinal velocity distribution of the ND<sub>3</sub> molecular beam before (solid curve) and after the bend filter (dots). The velocity spread and the most probable velocity of the guided molecular packet are both greatly reduced with respect to the original beam. The final beam has a most probable velocity of 49 m s<sup>−1</sup> with a velocity spread of 75 m s<sup>−1</sup> after the surface filter. The dashed line is a fitted one with the formula \frac{{2v_z }}{{\alpha ^2 }}\exp ( - v_z^2 /\alpha ^2 ). An initial molecular number of <em>N</em> ~ 10<sup>6</sup> is used for the simulation.</p> <p><strong>Abstract</strong></p> <p>We propose a scheme of a surface electrostatic velocity filter capable of preparing cold polar molecules on the surface of a substrate by selecting a low-velocity component of an effusive beam from a thermal gas reservoir. Using ND<sub>3</sub> as a molecular sample, the dependence of the performance of the filter on the parameters of both the filter setup and the incident molecular beam is investigated by using a theoretical model and Monte Carlo simulations. A detailed study of the guiding process of molecules, including the evolution of phase space density of the packet in the filter, is carried out and shows that the beam selection process is mainly completed in the front part of the filter.</p>