Ion-molecule Collisions Research Articles

Numerical Analysis of the Breakdown Process of CF3I at Low Pressure

The breakdown of CF3I gas at low pressure is of significant importance for applications in fields such as aerospace and microelectronics. However, the DC low-pressure breakdown characteristics of CF3I remain underexplored. In this work, we utilize a one-dimensional implicit particle-in-cell/Monte Carlo collision (PIC/MCC) algorithm to investigate the complete DC breakdown process of low-pressure CF3I. Our model accounts for ion–molecule collisions, recombination reactions, and external circuit influences. The breakdown process is delineated into three stages: before breakdown, breakdown, and after breakdown. In the before-breakdown stage, both the density and energy of particles are low. In the breakdown stage, the rapid increase in electron density and energy accelerates ionization reactions, leading to successful breakdown. The circuit behavior transitions from capacitive to resistive, sharing voltage with the external resistance. In the after-breakdown stage, continued positive ion growth leads to the formation of a thin anode sheath and a negative plasma potential. Energy production, including heating power and secondary electron emission (SEE) power, balances with energy loss through collision and boundary absorption. Specifically, 62% of the total heating power comes from positive ions, 1.5% from negative ions, and approximately 85% of electron energy is lost via boundary absorption. Finally, we compare the Paschen curves of CF3I with those of SF6, providing insights that are beneficial for the application of CF3I as an SF6 alternative.

An innovative method to identify structural change through ion-molecule collision, making use of Time-Of-Flight measurements and SIMION simulations.

Structural change of ions induced by collision with a neutral has been studied in a guided ion beam tandem mass spectrometer, using Time-Of-Flight measurements and SIMION simulation. The exothermic catalytic isomerization of HOC+ to HCO+ is used to explore the new methodology. Isomerization is catalyzed via a proton transport mechanism through the interplay of a neutral molecule, the catalyst. Four different potential catalysts, Ne, D2, CH4, and C18O, were studied at different collision energies. SIMION simulation of the ion path and collision in the instrument leads to the highlight of a specific signature related to the catalytic isomerization in the time-of-flight spectra. This signature is used to identify the experimental conditions where isomerization takes place. Only C18O, at low collision energies, gives a clear signature of catalytic isomerization, and a quantitative estimate of the catalyzed isomerization cross-section and rate constant is derived. This new methodology is sensitive to clear presence of catalyzed isomerization and can be used in instruments designed for cross-section measurements, provided low collision energy is used and ion bunching is available.

Gas-phase formation of fullerene/9-hydroxyfluorene cluster cations

ABSTRACT In interstellar environment, fullerene species readily react with large molecules (e.g. polycyclic aromatic hydrocarbons, PAHs and their derivatives) in the gas phase, which may be the formation route of carbon dust grains in space. In this work, the gas-phase ion–molecule collision reaction between fullerene cations (${\rm C}_{n}\, ^+$, n = 32, 34,…, 60) and functionalized PAH molecules (9-hydroxyfluorene, C13H10O) are investigated both experimentally and theoretically. The experimental results show that fullerene/9-hydroxyfluorene cluster cations are efficiently formed, leading to a series of large fullerene/9-hydroxyfluorene cluster cations (e.g. [(C13H10O)C60]+, [(C13H10O)3C58]+, and [(C26H18O)(C13H10O)2C48]+). The binding energies and optimized structures of typical fullerene/9-hydroxyfluorene cluster cations were calculated. The bonding ability plays a decisive role in the cluster formation processes. The reaction surfaces, modes, and combination reaction sites can result in different binding energies, which represent the relative chemical reactivity. Therefore, the geometry and composition of fullerene/9-hydroxyfluorene cluster cations are complicated. In addition, there is an enhanced chemical reactivity for smaller fullerene cations, which is mainly attributed to the newly formed deformed carbon rings (e.g. 7 C-ring). As part of the co-evolution network of interstellar fullerene chemistry, our results suggest that ion–molecule collision reactions contribute to the formation of various fullerene/9-hydroxyfluorene cluster cations in the interstellar medium, providing insights into different chemical reactivity caused by oxygenated functional groups (e.g. hydroxyl, OH, or ether, C-O-C) on the cluster formations.

Contribution of different electron-capture mechanisms in the fragmentation of CO23+ upon slow ion-molecule collision

Abstract We present recoil ion - projectile ion coincidence study on the three body fragmentation of CO23+ ion into C+ + O+ + O+ fragments upon impact with
Ar3+, Ar6+ and Ar8+ projectile ions having the same velocity of vp = 0.49 a.u. (atomic units) using recoil ion momentum imaging technique. The kinetic energy
release (KER) distributions are presented and effect of different electron-capture associated processes such as pure capture, electron capture followed by target ionization or projectile auto-ionization is discussed based on the analysis of KER corresponding to different final projectile charge state post collision. As the capture stabilization increased the relative yields in the KER distribution are found to decrease in the high KER side for low charged Ar3+ impact, while an increase is observed for Ar6+ and Ar8+ impacts. We discussed the interplay between electron-capture, projectile auto-ionization and target ionization for the dominance of particular process. We also observed a low-KER feature around 15 eV which was observed for electron
and proton impacts, which was not yet observed for highly charged ion impacts

Ion-molecule collision cross-section calculations using trajectory parallelization in distributed systems

Ion Mobility coupled with Mass Spectrometry (IM-MS) stands as a strong analytical method for structurally characterizing complex molecules. In IM-MS, the sample under investigation is ionized and propelled by an electric field into a drift tube, which collides against a buffer gas. The separation of the ion gas phase is then measured through the differences in their rotationally averaged Collision Cross-Section (CCS) values. The effectiveness of the measured Collision Cross-Section (CCS) for structural characterization critically depends on the validation against theoretical calculations. This validation process relies on intensive molecular mechanics simulations, which can be computationally demanding, especially for large systems such as molecular assemblies and viruses. Therefore, reliable and fast CCS calculations are needed to help interpret IM-MS experimental data. This work presents the MassCCS software, which considerably increases the CCS simulation performance by implementing a linked-cell-based algorithm, incorporating High-Performance Computing (HPC) techniques. We performed extensive tests regarding the system size, shape, and number of CPU cores. Experimental results reveal speedups up to 3 orders of magnitude faster than Collision Simulator for Ion Mobility Spectrometry (CoSIMS) and High-Performance Collision Cross Section (HPCCS), optimized solutions for CCS simulations, for a single node execution. In addition, we extended MassCCS at the inter-node level by employing OpenMP Cluster (OMPC). OMPC is an innovative programming model designed for the development of HPC applications. It streamlines the development process and simplifies software maintenance using only OpenMP directives. Notably, OMPC delivers a performance level comparable to a pure MPI implementation. This enhancement enabled expensive CCS calculations using nitrogen buffer gas for large systems such as human adenovirus with ∼11 million atoms in just ∼4 min, making MassCCS the most performant software nowadays, to the best of our knowledge. MassCCS is available as free software for Academic use at https://github.com/cces-cepid/massccs.

Gas-phase hydrogenation of large, astronomically relevant PAH cations

ABSTRACT To investigate the gas-phase hydrogenation processes of large, astronomically relevant cationic polycyclic aromatic hydrocarbon (PAH) molecules under the interstellar environments, the ion–molecule collision reaction between six PAH cations and H-atoms is studied. The experimental results show that the hydrogenated PAH cations are efficiently formed, and no even–odd hydrogenated mass patterns are observed in the hydrogenation processes. The structure of newly formed hydrogenated PAH cations and the bonding energy for the hydrogenation reaction pathways are investigated with quantum theoretical calculations. The exothermic energy for each reaction pathway is relatively high, and the competition between hydrogenation and dehydrogenation is confirmed. From the theoretical calculation, the bonding ability plays an important role in the gas-phase hydrogenation processes. The factors that affect the hydrogenation chemical reactivity are discussed, including the effect of carbon skeleton structure, the side-edged structure, the molecular size, the five- and six-membered C-ring structure, the bay region structure, and the neighbouring hydrogenation. The infrared spectra of hydrogenated PAH cations are also calculated. These results we obtain once again validate the complexity of hydrogenated PAH molecules, and provide the direction for the simulations and observations under the co-evolution interstellar chemistry network. We infer that if we do not consider other chemical evolution processes (e.g. photoevolution), then the hydrogenation states and forms of PAH compounds are intricate and complex in the interstellar medium.

Single Electron Capture in Collisions of Fast Ions with Molecular Hydrogen in the Impact Parameter Representation

A theoretical method for evaluating the single-electron capture (SEC) cross sections in collisions of fast ions with a ground-state H2molecule is presented. The scattering problem for ion-molecule collisions is formulated in the impact parameter representation using the relation between the quantum-mechanical amplitude and quasi-classical impact parameter one. The capture amplitudes and corresponding probabilities of capture to (nlm) states of an incident ion are derived within the framework of the Brinkman–Kramers approximation. The general expressions for the SEC probability amplitudes ton-states, summed overlandmquantum numbers, are deduced, from which the corresponding SEC probabilities can be then calculated using a procedure of multichannel normalization. The dependence of the differential cross sections, integrated over projectile impact parameters, on the molecular orientation for charge exchange in H++ H2collisions is considered and compared with measurements and other calculations. Total SEC cross sections, integrated over the molecular orientations and summed overn-states for several bare and dressed ions, are calculated and compared with available experimental data and results of calculations by means of other theoretical methods.

Multicenter continuum distorted wave with eikonal initial state model for single ionization in ion-molecule collisions: Differential electron emission from water under energetic ion impact

Multicenter continuum distorted wave with eikonal initial state model for single ionization in ion-molecule collisions: Differential electron emission from water under energetic ion impact

Ultrahigh Sensitivity PTR-MS Instrument with a Well-Defined Ion Chemistry.

Proton-transfer-reaction mass spectrometry (PTR-MS) is widely used for measuring organic trace gases in air. In traditional PTR-MS, both nonpolar and polar analytes are ionized with unit efficiency, as predicted from ion-molecule collision theories. This well-defined ion chemistry allows for direct quantification of analytes without prior calibration and therefore is an important characteristic of PTR-MS. In an effort to further increase the sensitivity, recently developed ultrahigh sensitivity chemical ionization mass spectrometry (CIMS) analyzers have, however, been reported to have sacrificed unit ionization efficiency for selected analytes or classes of analytes. We herein report on the development of a novel ultrasensitive PTR-MS instrument, the FUSION PTR-TOF 10k, which exhibits the same universal unit response as conventional PTR-MS analyzers. The core component of this analyzer is the newly designed FUSION ion-molecule reactor, which is a stack of concentric ring electrodes generating a static longitudinal electric field superimposed by a focusing transversal radiofrequency (RF) field. The FUSION PTR-TOF 10k instrument is equipped with an improved ion source, capable of switching between different reagent ions (H3O+, O2+, NO+, NH4+) in less than one second. The improved time-of-flight mass spectrometer analyzes m/z signals with a mass resolution in the 10000-15000 range. FUSION PTR-TOF 10k achieves sensitivities up to 80000 cps ppbV-1 and detection limits down to 0.5 pptV for a 1 s measurement time. We show time-series of naphthalene and 13C-napthalene as measured in ambient air in Innsbruck for demonstrating the sub-pptV detection capability of this novel FUSION PTR-TOF 10k.

Towards quantum state-to-state understanding of ion-molecule collisions.

Towards quantum state-to-state understanding of ion-molecule collisions.

The Role of Ion Rotation in Ion Mobility: Ultrahigh-Precision Prediction of Ion Mobility Dependence on Ion Mass Distribution and Translational to Rotational Energy Transfer.

The role of ion rotation in determining ion mobilities is explored using the subtle gas phase ion mobility shifts based on differences in ion mass distributions between isotopomer ions that have been observed with ion mobility spectrometry (IMS) measurements. These mobility shifts become apparent for IMS resolving powers on the order of ∼1500 where relative mobilities (or alternatively momentum transfer collision cross sections; Ω) can be measured with a precision of ∼10 ppm. The isotopomer ions have identical structures and masses, differing only in their internal mass distributions, and their Ω differences cannot be predicted by widely used computational approaches, which ignore the dependence of Ω on the ion's rotational properties. Here, we investigate the rotational dependence of Ω, which includes changes to its collision frequency due to thermal rotation as well as the coupling of translational to rotational energy transfer. We show that differences in rotational energy transfer during ion-molecule collisions provide the major contribution to isotopomer ion separations, with only a minor contribution due to an increase in collision frequency due to ion rotation. Modeling including these factors allowed for differences in Ω to be calculated that precisely mirror the experimental separations. These findings also highlight the promise of pairing high-resolution IMS measurements with theory and computation for improved elucidation of subtle structural differences between ions.

Coevolution of the interstellar chemistry: gas-phase laboratory formation of hydrogenated fullerene-PAH clusters

ABSTRACT Fullerene molecules are affected and constrained by different interstellar environmental factors, such as UV radiation, atoms, and other coexisting molecules. To understand the coevolution of the interstellar fullerene chemistry, by tracking the accretion processes on fullerene cations, we present an investigation of the chemical reactivity of fullerene (C60) cations and smaller fullerene (C54/56/58) cations with hydrogen and C14H10 in the gas phase. Experiments are performed using a quadrupole ion trap in combination with time-of-flight mass spectrometry. The experimental results show hydrogenated fullerene-C14H10 cluster cations (i.e. [Hn C60(C14H10)m ]+ and [Hn C54/56/58(C14H10)m ]+) are efficiently formed through ion-molecule collision reaction. H-atoms are more likely to accumulate on the surface of fullerenes than C14H10; not only does hydrogen more easily form a covalent bond, the later accreted hydrogen will also expel the already accreted C14H10. Through theoretical calculations, we obtain the structure of newly formed clusters (e.g. [HC60(C14H10)]+ and [HC58(C14H10)]+) and the binding energies of their reaction pathways, together with IR spectra. The bonding ability plays a decisive role in the ternary cluster formation processes, and the existence of occupation and expulsion competitive reaction channels in the accretion processes on fullerene surfaces is confirmed. As part of the coevolution of the interstellar chemistry, the occupation and expulsion reaction modes should be considered when fullerenes further react with H-atoms and PAHs. As a result, the molecular structures of hydrogen/fullerene/PAH clusters are diverse, and hydrogenated-fullerene-related clusters (e.g. hydrogenated fullerenes or hydrogenated fullerenes-PAHs) have a higher distribution than non-hydrogenated-fullerene-related clusters (e.g. fullerenes or fullerenes-PAHs) in the interstellar environment.

Electron Capture by Proton Beam in Collisions with Water Vapor

In low energy ion-molecule collisions, electron capture is one of the most important channels. A new experimental setup was developed to study the electron capture process using low-energy ion beams extracted from an electron cyclotron resonance (ECR) plasma-based ion accelerator. Experiments were carried out with the proton beam colliding with water vapor in the energy range of 70–300 keV. Capture events were detected using a position-sensitive detection system comprising micro channel plates (MCPs) and a delay line detector (DLD). These e-capture events can be a result of pure capture reactions as well as transfer ionization. The capture cross section was found to decrease sharply with the beam energy and agreed well with previous measurements. The setup was also used to detect the events that gave rise to the single and multiple e-capture (integrated over all recoil-ion charge states) of C4+ ions. The capture cross-sections for one, two, three, and four electrons were measured for 100 keV C4+ ions. The ratio of multielectron capture yield to that for single e-capture decreased with the number of captured electrons.

Comprehensive ion-molecule reactive collision model for processing plasmas

The ion-molecule collision model for endothermic reactions created by Denpoh and Nanbu, which is the so-called “Denpoh–Nanbu theory (DNT),” has been extended to exothermic reactions. In addition to short-range charge exchanges between ions and molecules, a long-range charge exchange has been incorporated into the extended theory named “DNT+” in this work. Although, even today, there still is a lack of ion-molecule collision data required as fundamental input to plasma simulations for processing plasmas, DNT+ can provide a comprehensive cross section data set of ion-molecule collisions, including elastic, both endothermic and exothermic reactions, and short- and long-range charge exchanges, as well as other inelastic collisions. The cross sections for Ar+-CF4 and H2+-H2 collisions obtained using DNT+ agree well with data from the literature. Therefore, DNT+ could be widely used, not only for plasma simulation but also as a tool to easily generate preliminary data prior to experiments, especially when ion-molecule cross sections are not available.

Post collision analyzer to study charge-exchange processes in ion-molecule collisions.

We have designed an electrostatic charge state analyzer for ion beams having energies in the range of 5-20 keV/q. It is primarily built to investigate the different ionization processes involved in the slow (v < 1 a.u.) impact of highly charged ions on molecules. The analyzer is a cylindrical deflector analyzer (CDA) based on a pair of concentric cylindrical sectors of radii 110.2 and 95mm, subtending an angle of 127° at its center. Additionally, an Einzel lens and a quadrupole deflector are deployed to focus and steer the ion beam. The compact design of the analyzer permits easy integration with an ion momentum spectrometer used for studying the fragmentation of the target molecules. The characterization of the CDA including its calibration and its transmission function is carried out using an ion beam delivered from an electron beam ion source. To check the performance of the setup, we have carried out experiments comprising the impact of Ar16+ projectiles on CO2 target molecules at an energy of 18 keV/q. With the help of the CDA, different charge exchange ionization processes, such as single capture, double capture, and triple capture of electrons by the projectile ion have been separated efficiently. The possibility of a modification in the geometry of CDA is discussed to further improve its performance.

Gas-phase Hydrogen/Deuterium Exchange on Large, Astronomically Relevant Cationic PAHs

To examine the gas-phase hydrogen/deuterium exchange on large, astronomically relevant cationic polycyclic aromatic hydrocarbons (PAHs), the ion-molecule collision reaction between C42H18 + (hexa-peri-hexabenzocoronene cations, HBC+) and D atoms is studied. The experimental results show that the deuterated HBC cations ([C42H m D n ]+, m+2 ∗ n up to ∼54) are efficiently formed, and an effective hydrogen/deuterium exchange is determined. The structure of newly formed deuterated HBC cations and the bonding energy for these reaction pathways are investigated with quantum theoretical calculations. The exothermic energy for each reaction pathway is relatively high, and the existence of competition between deuteration and dedeuteration and of hydrogen/deuterium exchange is confirmed. A kinetic model is constructed to simulate the deuteration and hydrogenation processes and the hydrogen/deuterium exchange on HBC+ as a function of the reaction time over the experimental and typical astrophysical conditions. We infer that if we do not consider other chemical evolution processes (e.g., photoevolution), then cationic PAHs will reach the final equilibrium state (reaction with H/D atoms) very quickly regardless of the initial state of PAHs, and deuterated cationic PAHs are scarce in the interstellar medium.

Theoretical study of the formation of large, astronomically relevant PAH-organic molecule clusters

Context. Polycyclic aromatic hydrocarbon (PAH) molecules play an essential role in the prebiotic compound evolution network in the interstellar medium (ISM). A recent experimental study revealed that large, astronomically relevant PAH-organic molecule clusters are gradually formed through the ion-molecule collision reaction pathway in the presence of a strong radiation field. Aims. We present a theoretical survey for the formation processes of PAH-organic molecule clusters (e.g., such as the graphene carbon cluster (C48) organic molecule (Pyroglutaminol, pgn, C5H9NO2) cluster cations, (pgn)nC48+, n = [1,6]), to illustrate the building block mechanism for the formation of large prebiotic compounds. Methods. To investigate the stability and the building block formation mechanisms of PAH-organic molecule clusters in the ion-molecule collision reaction process, we carried out theoretical calculations with DFT, including the hybrid density functional B3LYP, as implemented in the Gaussian 16 program. The basis set of the 6-311++G** and 6-31+G** was selected and used for different cluster systems. Results. We investigated the structure of newly formed species and the energy for these reaction pathways. The ion-molecule reaction between ((C5H9NO2)nC48+, n = [0,5]) with C5H9NO2 readily occur, resulting in a very large number of reaction pathways and very complex newly formed molecular clusters. An expanded tree (in building block pathways) shows the trunk and branches of these various formation pathways. These clusters (e.g., the graphene carbon cluster and its organic molecules) provide a possible formation and chemical-evolution route for the large complex prebiotic compounds in bottom-up and energy allowed processes in the ISM. Conclusions. The gas-phase reactions between large PAH species and organic molecules occur relatively easily, resulting in a very large number of reaction pathways and very complex newly formed molecular clusters. These PAH-organic molecule clusters will lead to large organic molecules, which may contain some of the critical molecular configurations that can characterize living material.

Gas-phase reaction of fullerene monocations with 2,3-benzofluorene indicates the importance of charge exchanges

Fullerene and polycyclic aromatic hydrocarbon (PAH) molecules, as well as their cations and clusters, are of great interest in astrochemistry. In this work, the ion-molecule collision reaction between fullerene (e.g. a C54/56/58 and C60 system or a C64/66/68 and C70 system) monocations and neutral PAHs (e.g. 2,3-benzofluorene, C17H12) is studied in the gas phase to determine the importance of charge exchanges and to illustrate the competition between charge transfer and molecular adduct formation channels. The experimental results show that the charge transfer channel is the dominant channel (i.e. charge exchange) in the reaction between fullerene (C60 and C70) monocations and 2,3-benzofluorene, while the molecular adduct formation channels are the dominant channels in the reaction between fullerene (C54/56/58 and C64/66/68) monocations and 2,3-benzofluorene. The observed reaction behaviours are investigated with quantum calculations, and the CH2 unit binding effect of 2,3-benzofluorene is determined to be the main reason for the results. Our findings on the ion-molecule collision reaction between fullerene monocations and 2,3-benzofluorene provide a good model for understanding the physical-chemical processes of the charge transfer channel and the cluster adduct formation channels. Neutral fullerenes (C60 and C70) increase the abundance of their monocations through collision reactions with coexisting neutral molecules in the interstellar medium.

Potassium Binding Interactions with Aliphatic Amino Acids: Thermodynamic and Entropic Effects Analyzed via a Guided Ion Beam and Computational Study.

Noncovalent interactions between alkali metals and amino acids are critical for many biological processes, especially for proper function of protein ion channels; however, many precise binding affinities between alkali metals and amino acids still need to be measured. This study addresses this need by using threshold collision-induced dissociation with a guided ion beam tandem mass spectrometer to measure binding affinities between potassium cations and the aliphatic amino acids: Gly, Ala, hAla, Val, Leu, and Ile. These measurements are supplemented by theoretical calculations and include commentary on effects of enthalpy, entropy, and structural preference. Notably, all levels of theory indicate that the lowest-lying isomers at 298 K have K+ binding to the carbonyl oxygen in either a monodentate ([CO]) or bidentate ([CO,OH]) fashion, isomers that are linked in a double-well potential. This complicates the analysis of the data, although does not greatly influence the final results. Analysis of the resulting cross sections includes accounting for multiple ion-molecule collisions, internal energy of reactant ions, and unimolecular decay rates. The resulting experimental bond dissociation energies generally increase as the polarizability of the amino acid increases, results that agree well with quantum chemical calculations done at the B3LYP, B3P86, and MP2(full) levels of theory, with B3LYP-GD3BJ predicting systematically larger values.

Fragmentation inside proton-transfer-reaction-based mass spectrometers limits the detection of ROOR and ROOH peroxides

Abstract. Proton transfer reaction (PTR) is a commonly applied ionization technique for mass spectrometers, in which hydronium ions (H3O+) transfer a proton to analytes with higher proton affinities than the water molecule. This method has most commonly been used to quantify volatile hydrocarbons, but later-generation PTR instruments have been designed for better throughput of less volatile species, allowing detection of more functionalized molecules as well. For example, the recently developed Vocus PTR time-of-flight mass spectrometer (PTR-TOF) has been shown to agree well with an iodide-adduct-based chemical ionization mass spectrometer (CIMS) for products with 3–5 O atoms from oxidation of monoterpenes (C10H16). However, while several different types of CIMS instruments (including those using iodide) detect abundant signals also at “dimeric” species, believed to be primarily ROOR peroxides, no such signals have been observed in the Vocus PTR even though these compounds fulfil the condition of having higher proton affinity than water. More traditional PTR instruments have been limited to volatile molecules as the inlets have not been designed for transmission of easily condensable species. Some newer instruments, like the Vocus PTR, have overcome this limitation but are still not able to detect the full range of functionalized products, suggesting that other limitations need to be considered. One such limitation, well-documented in PTR literature, is the tendency of protonation to lead to fragmentation of some analytes. In this work, we evaluate the potential for PTR to detect dimers and the most oxygenated compounds as these have been shown to be crucial for forming atmospheric aerosol particles. We studied the detection of dimers using a Vocus PTR-TOF in laboratory experiments, as well as through quantum chemical calculations. Only noisy signals of potential dimers were observed during experiments on the ozonolysis of the monoterpene α-pinene, while a few small signals of dimeric compounds were detected during the ozonolysis of cyclohexene. During the latter experiments, we also tested varying the pressures and electric fields in the ionization region of the Vocus PTR-TOF, finding that only small improvements were possible in the relative dimer contributions. Calculations for model ROOR and ROOH systems showed that most of these peroxides should fragment partially following protonation. With the inclusion of additional energy from the ion–molecule collisions driven by the electric fields in the ionization source, computational results suggest substantial or nearly complete fragmentation of dimers. Our study thus suggests that while the improved versions of PTR-based mass spectrometers are very powerful tools for measuring hydrocarbons and their moderately oxidized products, other types of CIMS are likely more suitable for the detection of ROOR and ROOH species.