Collisions of FeO⁺ with H₂ and He in a Cryogenic Ion Trap

Abstract

The nominal temperature range of cryogenic radio-frequency ion traps has recently been extended down to T=2.3 K. Whereas in situ He tagging of mass-selected ions embedded in dense helium buffer gas is becoming common for recording IR spectra through photofragmentation of small and large ions, much less activity is devoted to the field of cold chemistry, which in this contribution means the two orders of magnitude extending from 300 to below 3 K. The importance of this temperature range for understanding the dynamics of bi- and termolecular reactions is illustrated with new results for the time-honored reaction of FeO with H obtained with the cryogenic ion trap ISORI in Prague. The rate coefficient for forming Fe +H O increases steeply with decreasing temperature. In addition more product channels open up, such as the stabilized reaction-intermediate complexes H FeO and He -FeO formed by ternary association with He. For the FeOH +H channel only a minor signal is observed. The rate coefficients provide deep insight into lifetimes, bottlenecks, and barriers impeding almost completely the exothermic, but spin-forbidden, reaction at room temperature. For some of the He-tagged ions, IR predissociation spectra are recorded. A breakthrough is obtaining the first spectrum of [(H )FeO] , synthesized and tagged in situ with He. These results pave the way to study the structures of reaction intermediates stabilized in the gas phase by means of collisions with helium.

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

“…[122] While this process, Equation (1), has been studied in greatest detail both experimentally and theoretically, [122] it was only recently that by reacting[ FeO] + with H 2 in ac ryogenic ion trap at 2.3 Kt he He-tagged intermediate [(H 2 )FeO] + has been characterized spectroscopically. [123] All other intermediates,g iven in Figure 10, have to await ap roper structural characterization yet. Clearly,there is plenty of room left for future spectroscopic work even for systemshaving only ah andful of atoms!…”

Infrared spectroscopy of cold trapped molecular ions using He‐tagging

“…The details and simulations of the kinetics can be found in ref. . Although ultra‐pure helium was used, traces of hydrogen in the trap lead to the formation of a few stabilized reaction intermediates H 2 FeO + (pink)…”

“…To prepare spectroscopy experiments and to understand the spin‐forbidden reaction of FeO + with H 2 molecules, the addition of He to Fe + , FeO + , and FeOH + has been studied in detail . Figure shows a stationary mass distribution of He n –FeO + complexes, reached at a nominal temperature of 3.5 K after a storage time of 200 ms and at a helium density of 1.7 × 10 15 cm −3 .…”

“…The ternary rate coefficients, reported in ref. , increase from 1.7 × 10 −29 cm 6 /s for attaching the first He to 1.2 × 10 −27 cm 6 /s for growing from n = 4 to 5. It is important to note that not only the ternary rate coefficients but also the rate coefficients k CID are also surprisingly large, with some 10 −13 cm 3 /s.…”

“…As explained in detail in ref. , we succeeded in stabilizing the reaction intermediate H 2 *FeO + and in recording its IR spectrum, tagging it with He. The missing OH stretch revealed that the stabilized complex does not have the structure of [Fe–H 2 O] + but of [H 2 –FeO] + .…”