Resonant energy transfer between atomic levels of equal parity in low-temperature collisions of Rydberg atoms with polar molecules

The large spatial separation between the excited electron and parent ion core in a Rydberg atom, means that interactions with polar ground-state molecules display distinctly different characteristics at difference distance scales. At long range, i.e., outside the Rydberg electron charge distribution, van der Waals or resonant dipole-dipole interactions dominate [1]. At the intermediate length scale, around the maximum of the Rydberg electron probability density distribution, the charge-dipole interaction of the electron with the polar molecule plays a pronounced role [2] and, under appropriate conditions, can give rise to weakly bound ultra-long-range molecular states [3]. At short range, the charge-dipole interaction of the ion core with the polar molecule takes over, and can result in the occurrence of the barrier-free exothermic ion-molecule chemical reaction [4].

Here, we report the observation of a resonant energy transfer process that occurs in low-temperature collisions of Rydberg helium (He) atoms with polar ground state ammonia (NH₃) molecules, but cannot proceed at long range. Using an intra-beam collision apparatus with NH₃ seeded in a pulsed supersonic beam of He, and a relative translational temperature in the moving frame of reference <100 mK, we observe the resonant transfer of the energy associated with the electric dipole allowed parity-changing transition between the inversion sublevels in the ground electronic state of NH₃, to excite Rydberg He atoms from the 1s65s ³S₁ level to the equal parity 1s66s ³S₁ level. We attribute the occurrence of this process to a combination of the mixing of Rydberg states of opposite parity that occurs when the polar NH3 molecule enters inside the Rydberg electron charge distribution, and a change of angular momentum in the centre-of-mass frame of reference that occurs upon energy transfer. We will describe the experiments in which these observations were made, and the theoretical work carried out to aid in their interpretation.

[1] K. Gawlas and S. D. Hogan, J. Phys. Chem. Lett. 11, 83 (2020)

[2] A. Guttridge, et al. Phys. Rev. Lett. 131, 013401 (2023)

[3] R. González-Férez, J. Shertzer and H. R. Sadeghpour, Phys. Rev. Lett. 126, 043401 (2021)

[4] V. Zhelyazkova et al. Phys. Chem. Chem. Phys. 23, 21606 (2021)