![]() ![]() ![]() We then alter the probability of this rotational de-excitation in a coherently controlled fashion, using a small magnetic field which splits the nearly degenerate m J states of the incoming molecules by about a pico-eV, and changes the stereodynamics of the impinging molecules both in terms of rotational projection populations and in terms of the relative phases of the superposition state. Here, we make use of RID to monitor the probability that a rotating D 2 molecule, which approaches a copper surface in the J = 2 rotational level, will scatter from the surface in a non-rotating state ( J = 0). ![]() Similarly to collisions in the gas phase, which can change the rotational state of the molecule 8, such events can occur when colliding with a surface and can either involve energy exchange with the surface 9, or an internal conversion between the kinetic and rotational energy of the scattered molecule, leading to the phenomena of rotationally inelastic diffraction (RID), see for example ref. ![]() Our control methodology makes use of the possibility of exciting or de-exciting rotations during a collision with a surface 2, 6, 7. The experimental results are compared with density functional theory (DFT) based calculations, which relate the observations to the rotational projection states of the impinging D 2 but underestimate rotational de-excitation events of helicopter-like molecules. We show that magnetic manipulations involving pico-eV energy splitting of rotational projection ( m J) levels, performed before a collision with a surface, can significantly alter the probability that a deuterium molecule will stop rotating after the collision, converting 22.7 meV rotational energy into translational energy. In this work, we present a different approach to controlling rotations, where the probability that a molecule will stop rotating is altered without exposing it to energetic photons or electrons. A common perception, manifested in existing experimental methods, is that to excite, control the directionality and even to de-excite rotational transitions, the external perturbations need to supply or remove energy from the molecule on the same order of magnitude as the spacing between the two rotational energy levels 3, 4, 5. Molecular rotations play a role in a huge range of chemistry related research fields and applications, stimulating the development of experimental techniques to control rotational state populations 1, 2. Calculations confirm that different rotational orientations have different de-excitation probabilities but underestimate rotational flips (∆ m J \(\ne\)0), highlighting the importance of the results as a sensitive benchmark for further developing theoretical models of molecule-surface interactions. We show that passing the beam through a 1 m long magnetic field, which splits the rotational projection states by only 10 −12 eV, can change the probability that a molecule-surface collision will stop a molecule from rotating and lose rotational energy which is 9 orders larger than that of the magnetic manipulation. Here, we study the possibility of de-exciting the molecular rotation of a D 2 molecule, from J = 2 to the non-rotating J = 0 state, without using an energy-matched perturbation. Existing schemes involve perturbing the molecules with photons or electrons which supply or remove energy comparable to the rotational level spacing. Rotational motion lies at the heart of intermolecular, molecule-surface chemistry and cold molecule science, motivating the development of methods to excite and de-excite rotations. ![]()
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