Measurement of the orientation of buffer-gas-cooled, electrostatically-guided ammonia molecules

Edward W. Steer, Lorenzo S. Petralia, Colin M. Western, Brianna R. Heazlewood, Timothy P. Softley

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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 J K = 1 1 could be achieved with the application of a small bias voltage (17 V) 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.
Original languageEnglish
Pages (from-to)94-102
Number of pages9
JournalJournal of Molecular Spectroscopy
Volume332
Early online date11 Nov 2016
DOIs
Publication statusPublished - Feb 2017

Keywords

  • Cold molecules
  • Quadrupole guide
  • Alignment
  • Ammonia
  • Polarisation

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