# Gravitative Velocity Selection

The de Broglie wavelenght $$\lambda=\frac{h}{m \cdot v}$$ depends on the molecules’ velocity. For interferograms with good visibility we need a molecular beam with sufficient temporal (spectral) coherence, that is, a narrow velocity distribution. We use gravity to achieve this. We consider only those molecules whose trajectories reach from the source to the detector. Since the molecules move in vacuum and due to Earth’s gravitational field they fly in an ideal parabola. For any possible trajectory there is a unique velocity between source and detector. Below, you can follow the trajectories and select for different velocities.

 Velocity - + Keep trajectories Slit Slit-position - + Slit-size - +

By constraining the molecular beam from above and below, only particle with a confined velocity range reach the detector. The vertical position of the delimiter can be varied with a vacuum feedthrough. Its opening can be adjusted by applying electrical voltage to the piezo elements. The higher the voltage, the smaller the slit.

## Velocity-selection

Check that again, how the velocity-selection works!

## Position a slit in the beam

The countrate is the only information we need to do that. Move the slit slowly into the beam. The countrate will disappear, the first blocking of the slit prevents the beam from reaching the detector. If you turn further molecules will pass the slit opening and the countrate will rise. If you turn further the countrate will vanish to come back again when you move the slit out of the beam. Try it:

 Slit-position