The quadrupole velocity selector is a device which separates out the low-velocity tail of the Maxwell distribution from an effusive source of molecules. This tail constitutes a very small fraction of the total molecules as shown below, but is nevertheless sufficient for collisional experiments.
In our lab the velocity selector consists of a bent quadrupole as shown above – the molecules enter via a leak valve from the righthand side, and, as illustrated by the trajectory simulations below, only the slow molecules are guided around the bend. A quadrupole mass spectrometer is used to monitor the flux of molecules emerging from the guide.
Only the low field seeking states (those for which the energy of the molecule increases with increasing field) are guided, but otherwise the guide is only selective for the translational motion, not for the internal degrees of freedom – thus the output is translationally cold, but the distribution of vibration-rotation states is approximately room temperature.
In our guide we have worked with a range of polar molecules, as shown above. As the quadrupole voltage is increased, a greater proportion of the molecules are transmitted, corresponding to a higher average velocity (a greater fraction of the tail of the velocity distribution).
The figure above shows the energy distribution of CH3F molecules transmitted by the guide expressed in terms of the corresponding temperature in Kelvin.