Inelastic scattering dynamics of ammonia with small molecules
Rusher, Cassandra Amelia
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Crossed molecular beam velocity map imaging (VMI) is employed to state-selectively record the NH3 inelastic scattering products from collisions with a series of colliders; atomic Ar, diatomic D2, and the polyatomics CH4, C2H6 and C(CH3)4. The differential cross sections (DCSs) presented for NH3-Ar expand on the existing literature. The angular distribution of the scattering is found to exhibit a dependence on the angular momentum projection quantum number (k) of the NH3 product state in addition to the known dependency on the total angular momentum quantum number (J). It is found that for increasing k the total scattering into sideways and backwards angles (θ ≥ 60°) decreases. For the smallest molecular colliders D2 and CH4 co-excitation is observed. DCSs are presented for each NH3 product state and evidence of separate D2 co-excitation channels is obtained by a new extraction method for acquiring multiple DCSs from a single VMI image. The promotion of D2 into the JD2 = 3 rotational level exhibits an unusual propensity for excitation at larger impact parameters. It is proposed that this is due to interactions at larger distances with attractive regions of the potential energy surface (PES), at smaller impact parameters the steeper gradient of the PES inhibits coexcitation. The DCSs of NH3 scattering with methane, ethane and neopentane exhibit a striking similarity across all three systems and when comparing individual NH3 product states from a single collider. The DCSs are all dominated by forwards scattering angles (θ ≤ 60°) for all NH3 collisional excitations. The dominant mechanism suggested is a “tug-of- war” type trajectory, however calculations are required for confirmation of this interaction.