Ion-molecule reaction rate coefficients of
importance to our understanding of both
fundamental reaction mechanisms and naturally
complex chemical environments will be studied
over a wide temperature range (4-800 K). A next
generation variable temperature multipole trap
mass spectrometer employing neutral molecular
beams will be constructed allowing for: a)
multiple reaction probes of trapped ions
including the capability to interact trapped
ions to low velocity, dense molecular beams of
condensable species and to chemically probe
reaction products, b) in situ synthesis of
complex molecular ions using pulsed reagent
gas/buffer gas collisional cooling for
subsequent beam species rate measurements, and
c) future studies using other techniques such as
state-selected laser pumping of ions and
velocity selection of neutral beams. Reactions
of cold cationic species relevant to the
interstellar medium will be investigated at
their natural temperatures. These studies will
equally provide insight into the effects of
nonequilibrium degrees of freedom in driving
endothermic processes and dynamical processes
requiring extremely long lived collision
complexes or proceeding through a limited set of
low lying reactant states.
program will progressively make use of new
trapping technology to investigate variable
temperature reactions of selected hydrocarbon
ions with H2 followed by polar
molecule studies, the kinetics of reactions
leading to HCO+/HOC+
product isomers and a variety of key radiative
association processes of modest complexity.
Using chemical probing, product branching will
help clarify open questions regarding detailed
mechanisms in association reactions relevant for
synthesizing complex molecules in cold rarefied
environments. For example, in the well studied
reaction of CH3+ + HCN, which leads
to several isomers, new experiments will be
directed towards the so far undetected
protonated 2H-azirine product which is an
oxidation away from the interstellar synthesis
proposed will greatly improve the foundation of
low temperature reaction dynamics as well as
astrochemical laboratory studies, enhancing
international collaboration. The program will
help develop some of the next generation of
scientists to move into this emerging area. The
particular growth of astrochemistry at the
University of Arizona regarding the development
of new institutes, the formal bridging of
faculty across Chemistry, Astronomy and Biology
as well as the specific hiring activities in
astrochemistry will have a tremendous symbiotic
relationship to the proposed program.
Ion-Molecule Reactions at Variable Temperatures Using a Molecular Beam-Cold Ion Trap: Radiative association, isomerization and reactions of polar
We have progressed significantly on our multidisciplinary efforts to build an entirely new program around a next generation variable temperature radiofrequency ion trap mass spectrometer. The results are taking us along in our understanding of the unique characteristics of very low temperature collisions. In particular, we are beginning to reveal how some of these aspects impact upon our attempts to understand natural low temperature environments such as occur in the terrestrial atmosphere, outer planetary atmospheres and the interstellar medium (ISM). Our group continues to spend a certain portion of its efforts on the further development of kinetic technique towards generation of laboratory environments uniquely suited to the study of low temperature collisions. In this regard, we continue to develop instrumentation capable of generating well understood low temperature collisional environments employing traps, supersonic flows, molecular beams, and fluid dynamic models capable of describing certain aspects of these flows. We have presently set aside our efforts on the use of supersonic flows as collisional environments to focus on the unique capabilities of traps and trap/beam interactions.
Trap studies of HOC+ and HCO+ at interstellar temperatures
In an example which illustrates trap capabilities and motivated the new instrument in Tucson, we have investigated the chemistry of HCO+ and its energetic isomer HOC+ relevant to current problems in the interstellar medium (ISM). Recent detection of the metastable HOC+ and DCO+ isomers toward a wide range of interstellar environments has demonstrated abundances which are difficult to explain within current chemical models. The abundance of HOC+ in these models relies heavily upon the rate of isomerization of HOC+ by H2 to the lowest energy isomer, HCO+. Again, using the Chemnitz 22-pole ion trap apparatus, we have studied the reactions
of HOC+ and HCO+ with H2 and its isotopomers D2 and HD [smi02]. In particular, the rate of isomerization of HOC+ by H2 is observed to be two orders of magnitude faster at 25 K than has been expected from computational models [jar86] [her96]. The observed 25 K rate coefficient for isomerization is (3.8 ± 0.5) × 10-10 cm3 s-1, indicating that there is probably no temperature dependence in this reaction below 300 K and also suggesting the absence of any significant energetic barrier on the lowest adiabatic H3CO+ potential surface. This result suggests that in regions where abundant HOC+ is observed, the production of this isomer must be strongly favored or that there is a concomitant rapid physicochemical loss for the isomer HCO+.
In these preliminary studies we have also investigated the reactions of both isomers with D2. Efficient deuteration of HOC+ is observed, occurring through an isotope exchange isomerization process, producing DCO+. However, no deuteration is observed in direct collisions between HCO+ with either D2 or HD at 25 K, indicating the presence of a significant barrier along this channel. These studies leave open many questions regarding the origins of the observed HOC+/HCO+ and DOC+/DCO+ ratios in a wide variety of interstellar objects which will be addressed later in this proposal.
Design, construction and characterization of a next generation variable temperature Rf ion trap/molecular beam mass spectrometer
The past three years of effort in our group have been devoted to the design and full construction of a new instrument in Tucson for the study of variable temperature (10-800 K) ion collisions with molecules and radicals. The instrument is schematically shown in Figure 1.
The ions are produced by electron bombardment in a radiofrequency trap ion source to maximize thermalizing collisions prior to extraction. The source projects at right angles to the mass selecting quadrupole/condensing-trap with a quadrupole bender guiding the ion beam into the selection/condensing region. This design, with its large differential pumping minimizes gas loading on the reaction trap chamber from ion precursor species. This contamination of the reaction trap zone has been the dominant determiner of reactive ion storage time in complex ion studies, since most of these ions are reactive towards their neutral precursors. Preliminary studies of the current instrument demonstrate cross contamination of the trap region well below 3 x 10-10 torr, allowing for unperturbed trapping times in excess of 100 s. The mass selected beam or packet of ions is then turned 90° and injected at low energy into the Rf trap. The trap axis lies along the path of the molecular beam which can enter the trap through the open quadrupole bender. Once stored, the ions are further cooled to equilibrium with the trap walls through buffer gas collisions and allowed to react either with static reactant gas injected into the trap, or neutrals from the molecular beam source traversing the cell. Following trap interactions, the ions are emptied from the trap into a quadrupole and analyzed with respect to mass and intensity.
The molecular beam is pre-cooled using a cryostat thermally coupled to the effusive nozzle. Typical nozzle flows allow for trap densities of beam neutrals in excess of 1012 cm-3. Coupled with long storage times this allows for the determination of reaction rate coefficients from the collision limit down to the range of 10-15 cm3 s-1. Use of static gas reactant increases this range several orders of magnitude. The planned installation of an Rf discharge atom beam source to our instrument will greatly aid in the generation of reactive atom and radical beams and is discussed in the proposed work.
Following a long development and construction period which included moving our laboratory into a new building, we have observed our first trapping in Spring 2008. Installed initially was an Rf driven ring electrode trap (RET) for high temperature studies (300 – 800 K) while we continue to assemble our low temperature 22 pole trap obtained from J. Maier at the University of Basel. Using the RET at temperatures between 100 and 350 K we have measured a broad range of charge transfer reactions in order to characterize the instrument and trap density measurements. We have particularly focused upon the reversible charge transfer reaction between ions of Ar and N2, owing both to its significant complexity as well as its study in multiple laboratories. [ani03]
The reaction at 300 K must account for internal energy in both ions and reversibility in multiple channels as the reaction system approaches equilibrium in the trap, as outlined in Scheme I for the most active channels at 300 K. In the low density limit of our ion source, we can store a mixture of excited and ground state ions and at various densities of Ar and N2 observe effects from all of the channels in Scheme I in the trap
We have recently submitted a paper describing the charge transfer reaction of H2O with N2+, at water molecular beam temperatures of 300-450K and trap temperature of 100K. This study of mode nonequilibrium temperature effects demonstrates statistical behavior in this presumably complex io-dipole reaction with very clear T-1/2 temperature dependence simply in the water (and corresponding changes in center of mass collision energy) [yua10].
More recently we have studied the radiative association reaction H3O+ + C2H2 and C2H4 at temperatures between 25 and 80K. These reactions show surprisingly large radiative association rates over those predicted using 3 body association data. The study clearly indicates the importance of direct rate measurement under interstellar temperatures to obtain new reaction class rate behavior applicable to the ISM. The results are currently being written up.