If atoms and molecules are cooled down to temperatures close to absolute zero (minus 273 degrees Celsius), they are subject to the laws of quantum mechanics. Under such conditions interactions no longer take place among atoms, molecules and light with any amount of energy on the part of the particles, but only specified levels. This also applies if the molecule begins rotating because of the collision with an atom.
In this case, the atom has to precisely transmit the quanta of energy to the molecule at the time of collision in which it was initially made to rotate from the state of being dormant. If the atom is too slow at the time of the collision, the molecule will not begin to rotate.
"This is the consequence of quantum mechanics. The angular momentum no longer takes place freely, and there are no longer all possible rotating speeds. Instead, only one, two or three quanta are in motion”, said Roland Wester of the Institute for Ion Physics and Applied Physics at the University of Innsbruck. The scientists have precisely measured and described these theoretical quantum-mechanical predictions.