Trapping cold molecular hydrogen

Abstract

Translationally cold H₂ molecules excited to non-penetrating |M_J| = 3 Rydberg states of principal quantum number in the range 21–37 have been decelerated and trapped using time-dependent inhomogeneous electric fields. The |M_J| = 3 Rydberg states were prepared from the X ¹Σ⁺_g(v = 0, J = 0) ground state using a resonant three-photon excitation sequence via the B ¹Σ⁺_u(v = 3, J = 1) and I ¹Π_g (v = 0, J = 2) intermediate states and circularly polarized laser radiation. The circular polarization of the vacuum ultraviolet radiation used for the B ← X transition was generated by resonance-enhanced four-wave mixing in xenon and the degree of circular polarization was determined to be 96%. To analyse the deceleration and trapping experiments, the Stark effect in Rydberg states of molecular hydrogen was calculated using a matrix diagonalization procedure similar to that presented by Yamakita et al., J. Chem. Phys., 2004, 121, 1419. Particular attention was given to the prediction of zero-field positions of low- states and of avoided crossings between Rydberg–Stark states with different values of |M_J|. The calculated Stark maps and probabilities for diabatic traversal of the avoided crossings were used as input to Monte-Carlo particle-trajectory simulations. These simulations provide a quantitatively satisfactory description of the experimental data and demonstrate that particle loss caused by adiabatic traversals of avoided crossings between adjacent |M_J| = 3 Stark states of H₂ is small at principal quantum numbers beyond n = 25. The main source of trap losses was found to be from collisional processes. Predissociation following the absorption of blackbody radiation is estimated to be the second most important trap-loss mechanism at room temperature, and trap loss by spontaneous emission is negligible under our experimental conditions.