26 research outputs found
Coulomb crystal mass spectrometry in a digital ion trap
We present a mass spectrometric technique for identifying the masses and relative abundances of Coulomb-crystallized ions held in a linear Paul trap. A digital radio-frequency wave form is employed to generate the trapping potential, as this can be cleanly switched off, and static dipolar fields are subsequently applied to the trap electrodes for ion ejection. Close to 100% detection efficiency is demonstrated for Ca+ and CaF+ ions from bicomponent Ca+ â CaF+ Coulomb crystals prepared by the reaction of Ca+ with CH3F. A quantitative linear relationship is observed between ion number and the corresponding integrated time-of-flight (TOF) peak, independent of the ionic species. The technique is applicable to a diverse range of multicomponent Coulomb crystalsâdemonstrated here for Ca+ â NH3+ â NH4+ and Ca+ â CaOH+ â CaOD+ crystalsâand will facilitate the measurement of ion-molecule reaction rates and branching ratios in complicated reaction systems
Measurement of the orientation of buffer-gas-cooled, electrostatically-guided ammonia molecules
AbstractThe 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 â with respect to both the local field direction and a space-fixed axis â as the molecules pass through different electric field regions. The simulations indicate that the orientation of âŒ95% 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
Inverse kinetic isotope effects in the charge transfer reactions of ammonia with rare gas ions
In the absence of experimental data, models of complex chemical environments rely on predicted reaction properties. Astrochemistry models, for example, typically adopt variants of capture theory to estimate the reactivity...</p
A stand-alone magnetic guide for producing tuneable radical beams
Radicals are prevalent in gas-phase environments such as the atmosphere, combustion systems and the interstellar medium. To understand the properties of the processes occurring in these environments, it is helpful to study radical reaction systems in isolationâthereby avoiding competing reactions from impurities. There are very few methods for generating a pure beam of gas-phase radicals, and those that do exist involve complex set-ups. Here, we provide a straightforward and versatile solution. A magnetic radical filter (MRF), composed of four Halbach arrays and two skimming blades, can generate a beam of velocity selected low-field-seeking hydrogen atoms. As there is no line-of-sight through the device, all species that are unaffected by the magnetic fields are physically blocked; only the target radicals are successfully guided around the skimming blades. The positions of the arrays and blades can be adjusted, enabling the velocity distribution of the beam (and even the target radical species) to be modified. The MRF is employed as a stand-alone device-filtering radicals directly from the source. Our findings open up the prospect of studying a range of radical reaction systems with a high degree of control over the properties of the radical reactants
Using a direct simulation Monte Carlo approach to model collisions in a buffer gas cell
A direct simulation Monte Carlo (DSMC) method is applied to model collisions between
He buffer gas atoms and ammonia molecules within a buffer gas cell. State-tostate
cross sections, calculated as a function of collision energy, enable the inelastic
collisions between He and NH3 to be considered explicitly. The inclusion of rotationalstate-changing
collisions affects the translational temperature of the beam, indicating
that elastic and inelastic processes should not be considered in isolation. The properties
of the cold molecular beam exiting the cell are examined as a function of the cell
parameters and operating conditions; the rotational and translational energy distributions
and are in accord with experimental measurements. The DSMC calculations
show that thermalisation occurs well within the typical 10-20 mm length of many
buffer gas cells, suggesting that shorter cells could be employed in many instances â
yielding a higher flux of cold molecules
Zeeman deceleration beyond periodic phase space stability
In Zeeman deceleration, time-varying spatially-inhomogeneous magnetic fields are used to create
packets of translationally cold, quantum-state-selected paramagnetic particles with a tuneable
forward velocity, which are ideal for cold reaction dynamics studies. Here, the covariance matrix
adaptation evolutionary strategy (CMA-ES) is adopted in order to optimise deceleration switching
sequences for the operation of a Zeeman decelerator. Using the optimised sequences, a 40%
increase in the number of decelerated particles is observed compared to standard sequences for
the same final velocity, imposing the same experimental boundary conditions. Furthermore, we
demonstrate that it is possible to remove up to 98% of the initial kinetic energy of particles in the
incoming beam, compared to the removal of a maximum of 83% of kinetic energy with standard
sequences. Three-dimensional particle trajectory simulations are employed to reproduce the experimental
results and to investigate differences in the deceleration mechanism adopted by standard
and optimised sequences. It is experimentally verified that the optimal solution uncovered by the
evolutionary algorithm is not merely a local optimisation of the experimental parameters { it is a
novel mode of operation that goes beyond the standard periodic phase stability approach typically
adopted