Stopping molecular rotation using coherent ultra-low-energy magnetic manipulations

Abstract

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. Existing schemes involve perturbing the molecules with photons or electrons which supply or remove energy comparable to the rotational level spacing. Here, we study the possibility of de-exciting the molecular rotation of a D₂ molecule, from J = 2 to the non-rotating J = 0 state, without using an energy-matched perturbation. 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. Calculations confirm that different rotational orientations have different de-excitation probabilities but underestimate rotational flips (∆m_J[Formula: see text]0), highlighting the importance of the results as a sensitive benchmark for further developing theoretical models of molecule-surface interactions.

Fig. 1. Schematic of the coherent magnetic manipulation apparatus.

Overview of the experimental set up showing the positions of the different magnetic field elements used for controlling and manipulating the rotational orientation of the D₂ molecules that collide with the Cu(111) surface. The $J$ = 2 molecules are shown in blue, and the $J$ = 0 in red.

Fig. 2. Experimental data for D₂ scattering from Cu(111).

a Incident angle scan, showing the positions of the specular scattering peaks for rotationally elastic scattering (∆J = 0, red) and rotationally inelastic scattering for J = 0 to J' = 2 (blue) and J = 2 to J' = 0 (black) transitions. b Normalised intensity of D₂ scattering from Cu(111) for the RID peak corresponding to the J = 2 to J' = 0 transition. The error bars represent standard errors from repeated B1 scans. c Comparison of the normalised intensity of D₂ scattering from Cu(111) for the RID peak (black) and the elastic specular peak (red). The error bars represent standard errors from repeated B1 scans. Source data are provided as a Source Data file.

Fig. 3. Manipulation of the m_I, m_J states of the J = 2 rotational level of D₂ in magnetic fields.

a Magnetic field dependence of the energy of the 30 m_I, m_J states for D₂ in the J = 2 state. The legend specifies the colour scheme used, where different types of lines were used to distinguish different I and m_I combinations, and the colours identify the m_J projection state. b Magnification of a showing the energy of some of the states in low magnetic field. c Calculated populations in the m_J states of the J = 2 D₂ molecules that collide with the surface. The quantisation axis is the surface normal, and the colour scheme for the m_J populations follows that of a and b. d The alignment parameter of the impinging molecules. Source data are provided as a Source Data file.

Fig. 4. Comparison of the experimental data with different models.

a Comparison of the experimentally measured signal for the J = 2 to J’ = 0 RID peak (black) and the signal calculated using the S-matrix obtained from DFT calculations (red). The error bars represent standard errors from repeated B1 scans. b Comparison of the experimental data (black) with the simulated signal using the best fit S-matrix (red). The error bars represent standard errors from repeated B1 scans. Source data are provided as a Source Data file.