Abstract
The extent to which the spatial orientation of internally and translationally cold ammonia molecules can be controlled as molecules pass out of a quadrupole guide and through different electric field regions is examined. Ammonia molecules are collisionally cooled in a buffer gas cell, and are subsequently guided by a three-bend electrostatic quadrupole into a detection chamber. The orientation of ammonia molecules is probed using (2+1) resonance-enhanced multiphoton ionisation (REMPI), with the laser polarisation axis aligned both parallel and perpendicular to the time-of-flight axis. Even with the presence of a near-zero field region, the ammonia REMPI spectra indicate some retention of orientation. Monte Carlo simulations propagating the time-dependent Schrödinger equation in a full basis set including the hyperfine interaction enable the orientation of ammonia molecules to be calculated – with respect to both the local field direction and a space-fixed axis – as the molecules pass through different electric field regions. The simulations indicate that the orientation of ∼95% of ammonia molecules in JK=11 could be achieved with the application of a small bias voltage (17V) to the mesh separating the quadrupole and detection regions. Following the recent combination of the buffer gas cell and quadrupole guide apparatus with a linear Paul ion trap, this result could enable one to examine the influence of molecular orientation on ion-molecule reaction dynamics and kinetics.
Highlights
A prevailing goal in the study of reaction dynamics is to develop a complete understanding of the reaction process
The laser polarisation axis is the only parameter that is changed between the two spectra, and no other known factor is believed to affect the intensities in this manner
Numerous repeat scans have been recorded over a shorter spectral range; the 63,008–63,028 cmÀ1 region of the ND3 resonance-enhanced multiphoton ionisation (REMPI) spectrum is chosen for examination as it exhibits several isolated transitions and the peak intensities display a strong dependence on the laser polarisation direction
Summary
A prevailing goal in the study of reaction dynamics is to develop a complete understanding of the reaction process. The long-range intermolecular forces experienced by slow-moving molecules can affect the orientation of reactants during the collision process – and influence the properties of the resulting products [1,2]. Over the past half century, the development of methodologies to control the spatial orientation of reactants has seen the investigation of steric effects [3] as well as the direct measurement of ‘‘molecular-frame” photofragment distributions [4,5]. Orienting molecules can allow one to control the outcome of reactive collisions. This was demonstrated in 1976, with the introduction of molecular beam scattering experiments: CH3I
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