If a given molecular species survives it, a thermal beam is the simplest and most stable source we have. For thermally labile molecules, however, it is necessary to minimize the heating time. This can be done in nanosecond pulsed laser sources but also in a tightly focused laser beam, scanned across a molecular sample surface.
When the laser is focused to 1µm width the heat load to the surrounding sample is negligible and the focal waist realizes a tightly constrained effective source that emits individual molecules with a large momentum uncertainty.
This is how we prepare transverse coherence of the molecular beam in many molecular far-field diffraction experiments.