Studies of state-selected and spatially oriented molecules (1995)
AuthorsBlunt, David Alanshow all
Two novel investigations of rotational quantum state-selected and of spatially oriented symmetric top molecules are reported here. In the first instance, upper Stark state relaxation cross-sections for molecular beams of CH₃Cl and for beams of 20% CCl₃H seeded in Ar are measured for a range of polar and non-polar quencher species. The variation of cross-section with quencher species is explained in terms of the strength of the long range van der Waals interaction potential between colliding species. Statespecific upper Stark state relaxation cross-sections are measured for molecular beams of CH₃X (X= F, Cl and Br) species with an Ar quencher. In the case of CH₃Cl, cross-sections are also measured for N₂ as the quencher. Assignment of the cross-sections to individual |JKM〉 rotational states or a small range of |JKM〉 rotational states is facilitated by the computational simulation of hexapole filter upper Stark state transmission characteristics. In the second investigation, it is demonstrated, for the first time, that the efficiency and outcome of electron impact ionisation is influenced by molecular orientation. The ionisation efficiencies for the molecules CH₃Cl and CCl₃H are lowest for electron impact on the negative end of the dipoles. Preliminary results for CH₃Br and CF₃Br also show this orientation dependence. In addition, the mass spectrum is orientation dependent, for example, in the ionisation of CH₃Cl the ratio CH₃Cl⁺:CH₃⁺ depends on molecular orientation.