State-to-state resolved differential cross sections for rotationally inelastic scattering of ND3 with He

Abstract

State-to-state differential cross sections are reported for rotationally inelastic scattering of fully state-selected ND₃ (j_k^± = 1₁⁻) with He. Experimental measurements are compared with full close-coupling quantum-mechanical scattering calculations that used an ab initio potential energy surface. Results are presented for final states up to j′_k′ ^± = 7₇⁻ at a mean collision energy of 430 cm⁻¹. For selected final quantum states, the effect of collision energy on the differential cross sections is also explored in the range 230–720 cm⁻¹. For the experimental studies, a hexapole electrostatic lens was used for the j_k^± state-selection of ND₃ molecules in their electronic and vibrational ground states in a molecular beam. This state-selected molecular beam was then crossed with a beam of He atoms. The velocities of inelastically scattered ND₃ molecules in single j′_k′ ^± states were obtained by velocity map imaging, and converted to differential cross sections in the centre-of-mass frame by density-to-flux transformation. The close-coupling calculations reproduce well the measured angular distributions. For small changes in the rotational angular momentum quantum number (j), the ND₃ is predominantly forward scattered, but the scattering shifts to the sideways and backward directions as Δj increases. For scattering into a given j′_k′ ^± state, cross-sections for collisions that conserve the ± symmetry associated with the ND₃ inversion vibration are larger and generally more forward scattered than the corresponding symmetry-changing processes.