We report velocity–flux contour maps for H–D abstraction in selected Cl + alkane reactions measured by means of crossed beam scattering combined with universal DC slice imaging. The studied hydrocarbons are propane and its two selectively deuterated isotopologues, namely 1,1,1,3,3,3-propane-d6 and 2,2-propane-d2, n-butane and isobutane (2-methyl-propane), with detection of the hydrocarbon radical product by 157 nm single photon ionization. Data are obtained at collision energies of 12–13 kcal mol−1 using a high-density atomic chlorine radical source combining Cl2 photolysis with ablation. All presented scattering distributions involving secondary and tertiary abstractions show distinct differences. Their comparisons allow for revisiting the dynamical picture of these reactions in terms of the nature of the abstraction sites, radical product energy disposal, and H vs. D abstraction. Results are discussed in the light of previous work and ab initio thermochemical calculations, along with proposals to future directions for investigation.

We present a crossed-beam imaging study of the reaction of chlorine atoms with several butene isomers. A high-intensity pulsed ablation Cl source is used with DC slice imaging and single-photon ionization detection at 157 nm to record the velocity-flux contour maps for these reactions. The target unsaturated hydrocarbons are 1-butene, trans-2-butene, cis-2-butene, and isobutene (2-methylpropene). Data are obtained at collision energies of ∼13.0 kcal·mol–1. Distinct differences in the scattering distributions and in particular the coupling of angular and translational energy release provide insight into the dynamics of this little-studied class of reactions. We find that these distributions reflect the energetics for competition between addition/elimination and direct abstraction in line with ab initio thermochemical data. A possible role for Cl atom roaming mediating the addition/elimination pathway is suggested.

Time-of-flight mass spectra obtained for strong-field ionization using simply a transform-limited femtosecond laser pulse allows for quantitative characterization of the composition of an unknown mixture, including determination of isomeric composition. The approach is described and example applications presented.

In this work, the primary product branching ratio (BR) for the reaction of state-prepared nitrogen cation (N2+) with acetonitrile (CH3CN), a possible minor constituent of Titan’s upper atmosphere, is reported. The ion–molecule reaction occurs in the collision region of the supersonic nozzle expansion that is characterized by a rotational temperature of 45 ± 5 K. A BR of 0.86 ± 0.01/0.14 ± 0.01 is obtained for the formation CH2CN+ and the CH3CN+ product ions, respectively. The reported BR overwhelmingly favors the formation of CH2CN+ product channel and is consistent with a simple capture process that is accompanied by a nonresonant dissociative charge transfer reaction. The BRs are independent of the N2 rotational levels excited. Apart from providing insights onto the dynamics of the title ion–molecule reaction, the reported BR represents the most accurate available low-temperature experimental measurement for the reaction useful to aid in the accurate modeling of Titan’s nitrile chemistry.

We report the primary (D-atom) and secondary (H-atom) abstraction dynamics of chlorine atom reaction with butane-1,1,1,4,4,4-d6. The H- and D-atom abstraction channels were studied over a range of collision energies: 10.4 kcal mol−1 and 12.9 kcal mol−1; 5.2 kcal mol−1 to 12.8 kcal mol−1, respectively, using crossed molecular beam dc slice ion imaging techniques. Single photon ionization at 157 nm was used to probe the butyl radical products resulting from the H- and D-atom abstraction reactions. These two channels manifest distinct dynamics principally in the translational energy distributions, while the angular distributions are remarkably similar. The reduced translational energy distribution for the primary abstraction showed marked variation with collision energy in the backward direction, while the secondary abstraction showed this variation in the forward direction.

Product branching ratios (BRs) are reported for ion−molecule reactions of state-prepared nitrogen cation (N2+) with methane (CH4), acetylene (C2H2). and ethylene (C2H4) at low temperature using a modified ion imaging apparatus. These reactions are performed in a supersonic nozzle expansion characterized by a rotational temperature of 40 ± 5K. For the N2+ + CH4 reaction, a BR of 0.83:0.17 is obtained for the dissociative charge-transfer (CT) reaction that gives rise to the formation of CH3+ and CH2+ product ions, respectively. The N2+ + C2H2 ion−molecule reaction proceeds through a nondissociative CT process that results in the sole formation of C2H2+ product ions. The reaction of N2+ with C2H4 leads to the formation of C2H3+ and C2H2+ product ions with a BR of 0.74:0.26, respectively. The reported BR for the N2+ + C2H4 reaction is supportive of a nonresonant dissociative CT mechanism similar to the one that accompanies the N2+ + CH4 reaction. No dependence of the branching ratios on N2+ rotational level was observed. In addition to providing direct insight into the dynamics of the state-prepared N2+ ion−molecule reactions with the target neutral hydrocarbon molecules, the reported low-temperature BRs are also important for accurate modeling of the nitrogen-dominated upper atmosphere of Saturn’s moon, Titan.

High-resolution ion and electron imaging techniques have been used to explore a series of problems in the reaction dynamics of gas phase ions. These have applications ranging from fundamental dynamical studies to atmospheric chemistry and astrochemistry. In this minireview we illustrate these approaches with several examples from our recent work. We examine the conformationally- and vibrationally-mediated photodissociation dynamics of propanal and ethylene cations, and show how these can reveal reaction dynamics across multiple electronic potential surfaces of these molecules. Recent results for methylamine cation photodissociation provide insight into the rich chemistry of the ionosphere of Saturn's great moon, Titan. The combination of high-resolution ion and photoelectron imaging, REMPI spectroscopy and state-of-the-art ab initio calculations yields a powerful multifaceted approach to studying photoionization and photofragmentation dynamics in ions.

Direct current (DC) slice imaging of state-selected ions is combined with high-level ab initio calculations to give insight into reaction pathways, dynamics, and energetics for ethylamine cation photodissociation at 233 nm. These reaction pathways are of interest for understanding the rich chemistry of Titan’s ionosphere recently revealed by the Cassini mission. The result for the H-loss product has a bimodal translational energy distribution, indicating two distinct H-loss pathways: these are assigned to triplet CH3CH2NH+ product ions and the singlet CH3CHNH2+ species. The distribution shows a modest fraction of energy available in translation and is consistent with barrierless dissociation from the ground state. HCNH+ formation is observed as the dominant channel and exhibits a bimodal translational energy distribution with the faster component depicting a significant angular anisotropy. This suggests a direct excited-state decay pathway for this portion of the distribution. We have also observed the H + H2 loss product as a minor secondary dissociation channel, which correlates well with the formation of CH2CNH2+ ion with an exit barrier.

(2 + 1) Resonance enhanced multiphoton ionization (REMPI) spectra were recorded for 2-butanone to study its photoionization dynamics. Two-photon excitation (53 200−55 000 cm−1 and 57 000−59 500 cm−1) was used to prepare the molecule in the 3s and 3p Rydberg states, respectively. Vibrational transitions in the spectrum were assigned with the aid of ab initio calculations as well as photoelectron imaging results. Photoelectron imaging data in the 3s Rydberg region exhibits a vibrational progression in the CCOC deformation mode in the ionic state superimposed on an otherwise diagonal (delta v = 0) ionization. Photoelectron imaging data in the 3p Rydberg region shows 3p−3s Rydberg−Rydberg mixing. The ionization energy obtained directly from the 3p photoelectron imaging is 9.541 eV.

The reaction of the cyano radical with 1-pentene has been studied in crossed molecular beams experiments at a range of collision energies from 5 to 9 kcal/mol. In these studies, the pentenyl radical product is probed using single-photon ionization at 157 nm, and the product flux contour map is obtained directly using DC slice imaging. The results clearly demonstrate the presence of the hydrogen abstraction channel in this reaction, which has largely been neglected in studies of analogous systems. The reaction likely gives rise to the resonantly stabilized C5H9 radical. The yield of this channel is estimated by comparison to the reaction of the cyano radical with n-pentane. The results have implications for hydrocarbon growth and nitrile incorporation in formation of haze particles on Saturn’s moon, Titan.Keywords (keywords): astrochemistry; interstellar chemistry; nitriles; planetary atmospheres; reaction dynamics; resonance-stabilized radicals; Titan;

We report an ion imaging and time-of-flight mass spectroscopy study of the photodissociation of a variety of heptane isomers using 157 nm dissociation and ionization. Time-of-flight mass spectra show that C3H7 + C4H9 is the dominant detected product channel following one-color 157 nm dissociation/ionization of heptanes. The results further allow determination of the relative ionization efficiencies of 1- and 2-butyl and propyl radicals at 157 nm. Momentum matching for the two radical products indicates that, for the C3–C4 products, neutral dissociation followed by ionization is the main source of the detected signals. The images show isotropic angular distributions and the translational energy distributions peak at very low energy, with only ∼0.3 eV or 8% of the available energy appearing in translation. This is consistent with dissociation from the ground state or low-lying triplet states following non-radiative electronic relaxation.

We present a detailed photoion and photoelectron imaging study of isobutanal cation dynamics. The 2 + 1 REMPI spectrum via the (n,3s) Rydberg transition was recorded and analyzed under both cold and warm beam conditions in an effort to identify the predicted trans conformer. Photoelectron imaging was used to establish the ion energetics accurately and to aid in the assignment of the REMPI spectra. On the basis of the photoelectron spectra and ab initio calculations, a peak at 54336.8 cm−1 is assigned to the trans conformer. We have also assigned the photoelectron spectra and identified some hot band transitions not reported in previous work. We determined the adiabatic ionization energy to be 9.738 eV. We also studied the photodissociation dynamics of isobutanal cations leading to several product channels, but no evidence of vibrational mode or conformational isomer dependence on either the product branching or dynamics was seen for this system.

A novel spectroscopic technique has been developed which makes it possible to record Doppler-free resonance-enhanced multiphoton ionization (REMPI) spectra with just one laser beam. The approach simply involves masking the outer side of the phosphor screen under velocity map imaging conditions so that only those species having no velocity component parallel to the laser beam propagation direction are detected. The benefits of this method are demonstrated in spectroscopic characterization of highly translationally and rotationally excited CO fragments resulted from the 230 nm photolysis of OCS and acetone, yielding substantially improved values of the rotational constants for the B state (v′′ = 0) of the CO molecule. The resolving power and the state distribution analysis of reaction products are also demonstrated for room-temperature H atoms generated by dissociation of background hydrogen molecules and oxygen atom detected from the 225.6 nm photolysis of ozone.

The photodissociation of cyanoacetylene, one of the key minor constituents in Titan’s atmosphere, was studied in a molecular beam under collisionless conditions using direct current slice ion imaging at 121.6, 193.3, and 243.2 nm. The experimental results were augmented by high-level theoretical calculations of stationary points on the ground-state and second excited singlet potential surfaces, and by statistical calculations of the dissociation rates and product branching on the ground-state surface. Results at 121.6 and 243.2 nm are nearly identical, suggesting that the 243.2 nm photodissociation is the result of a two-photon process. The translational energy distributions show only a modest fraction of the available energy in translation and are consistent with barrierless dissociation from the ground state. The results at 193.3 nm are quite distinct, showing up to half of the available energy in translation, implying dissociation with an exit barrier. The 193 nm result is ascribed to dissociation on the S1 potential energy surface. The theoretical calculations show significant rates for H loss on the ground state at 193 nm and significant branching to CN + CCH at 157 nm and higher.

State-resolved photodissociation dynamics of formaldehyde-d2, i.e., D2CO, at energies slightly above the deuterium atom elimination channel have been studied both experimentally and theoretically. The results showed a clear bimodal distribution of energy into molecular photofragments. Substantial translational excitation of products at high rotational levels of CO was observed together with the D2 cofragment in moderately excited vibrational levels, whereas rather small translational energy release of CO in low rotational levels was matched by a large degree of vibrational excitation in the D2 molecule. An analogous distribution of energy in two distinct channels has been recently observed under similar conditions in H2CO photolysis and attributed to two different dissociation pathways, namely, a pathway via the conventional transition state geometry and the previously unobserved pathway, deemed “roaming”. Our experimental and theoretical data indicated that the same two dissociation pathways were responsible for the bimodal energy distribution into the molecular fragments resulting from the photolysis of D2CO. Energy partitioning into molecular products was compared between photolysis of H2CO and D2CO at energies slightly above the H/D atom abstraction threshold.

We present an experimental investigation of the UV photochemistry of diacetylene under collisionless conditions. The H loss channel is studied using DC slice ion imaging with two-color reduced-Doppler detection at 243 nm and 212 nm. The photochemistry is further studied deep in the vacuum UV, that is, at Lyman-alpha (121.6 nm). Translational energy distributions for the H + C₄H product arising from dissociation of C₄H₂ after excitation at 243, 212, and 121.6 nm show an isotropic angular distribution and characteristic translational energy profile suggesting statistical dissociation from the ground state or possibly from a low-lying triplet state. From these distributions, a two-photon dissociation process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a one-photon dissociation process prevails. The results are interpreted with the aid of ab initio calculations on the reaction pathways and statistical calculations of the dissociation rates and product branching. In a second series of experiments, nanosecond time-resolved phototionization measurements yield a direct determination of the lifetime of metastable triplet diacetylene under collisionless conditions, as well as its dependence on excitation energy. The observed submicrosecond lifetimes suggest that reactions of metastable diacetylene are likely to be less important in Titan's atmosphere than previously believed.

DC slice imaging has been employed to study the photodissociation dynamics of acetone at 230 nm, with detection of the CO photoproduct via the B (v′ = 0) 1Σ+ ← X (v′′ = 0) 1Σ+ transition. A bimodal translational energy distribution observed in the CO fragments points to two distinct dissociation pathways in the 230 nm photolysis of acetone. One pathway results in substantial translational energy release (Eave ≈ 0.3 eV) along with rather high rotational excitation (up to J′′ = 50) of CO, and is attributed to the thoroughly investigated stepwise mechanism of bond cleavage in acetone. The other dissociation pathway leads to rotationally cold CO (J′′ = 0−20) with very little energy partitioned into translation (Eave ≈ 0.04 eV) and in this way it is dynamically similar to the recently reported roaming mechanism found in formaldehyde and acetaldehyde dissociation. We ascribe the second dissociation pathway to an analogous roaming dissociation mechanism taking place on the ground electronic state following internal conversion. For acetone, this would imply highly vibrationally excited ethane as a coproduct of rotationally cold CO, with the ethane formed above the threshold for secondary decomposition. We estimate that about 15% of the total CO fragments are produced through the roaming pathway. Rotational populations were obtained using a new Doppler-free method that simply relies on externally masking the phosphor screen under velocity map conditions in such a way that only the products with no velocity component along the laser propagation direction are detected.

We demonstrate a hybrid Doppler-free/Doppler-sliced ion imaging approach that is well-suited for detection of H or D atoms. The method relies on 2 + 1 resonant ionization with identical, nearly counterpropagating beams that are coplanar but directed at a small angle relative to the detector face. This results in Doppler selection of the velocity component along the time of flight axis but Doppler-free detection in the plane perpendicular to this axis. The results show high signal level and excellent slicing (∼5%), yielding velocity resolution completely determined by cation recoil in the ionization step.

We report the conformationally- and vibrationally-selected photoelectron spectroscopy of propanal obtained by resonance-enhanced multiphoton ionization (REMPI) using photoelectron imaging. These photoelectron spectra, employing (2 + 1) ionization via the (n, 3s) Rydberg transitions in the range from 365 to 371 nm, confirm that there are two stable conformer origins in the lowest ionic state, the cis conformer with a co-planar CCCO geometry and a gauche conformer with a ∼119° CCCO dihedral angle. From ab initio calculations at the B3LYP/6-311++G** level, we find the gauche conformer is slightly more stable, with the energy difference between two conformers determined to be only 65 cm−1. In our photoelectron spectra, the vertical ionization potential (IP) for the cis conformer of propanal was then determined to be 9.999 (±0.003) eV, while that of the gauche conformer of propanal was estimated to be 9.944 eV. A long vibrational progression in the in-plane CCCO deformation vibrational mode, v+15, for the cis conformer is systematically observed in all photoelectron spectra in which this mode is excited, suggesting that the geometry of the ground ionic state is significantly different from that of the 3s Rydberg state, particularly along the v15 coordinates.

High resolution kinetic energy release spectra were obtained for C+ and O+ from CO multiphoton ionization followed by dissociation of CO+. The excitation was through the CO (B 1Σ+) state via resonant two-photon excitation around 230 nm. A total of 5 and 6 photons are found to contribute to the production of carbon and oxygen cations. DC slice and Megapixel ion imaging techniques were used to acquire high quality images. Major features in both O+ and C+ spectra are assigned to the dissociation of some specific vibrational levels of CO+(X 2Σ+). The angular distributions of C+ and O+ are very distinct and those of various features of C+ are also different. A dramatic change of the angular distribution of C+ from dissociation of CO+(X 2Σ+, ν+ = 1) is attributed to an accidental one-photon resonance between CO+(X 2Σ+, ν+ = 1) and CO+(B 2Σ+, ν+ = 0) and explained well by a theoretical model. Both kinetic energy release and angular distributions were used to reveal the underlying dynamics.

We demonstrate an alternative mass spectrometric technique, Velocity map imaging mass spectrometry, which combines transverse deflection by short, pulsed electric fields with velocity map imaging in a single stage reflectron to achieve spatially resolved mass dispersion. In this simple configuration, ions are clearly separated according to their mass-to-charge ratio through the transverse deflection of the ions perpendicular to the flight direction. In this communication, we demonstrate the technique with the simultaneous detection of Xe isotopes spatially dispersed on a position sensitive detector, showing a mass resolution greater than 600 and dynamic range >103. The approach has potential applications in tandem mass spectrometry and a variety of dynamics studies.

We present the absolute velocity-dependent orbital orientation for O(1D2) atoms produced from the photodissociation of ozone in the 248–285 nm region obtained using the DC slice imaging method. The results are analyzed in terms of laboratory frame anisotropy parameters describing distinct excitation and dissociation mechanisms possessing characteristic angular distributions. The results show negligible orbital orientation produced in dissociation by circularly polarized light, but strong recoil speed-dependent orientation following photolysis by linearly polarized light at all wavelengths studied. The origin of this polarization is ascribed to nonadiabatic transitions at avoided crossings and at long range.

We report velocity–flux contour maps for H–D abstraction in selected Cl + alkane reactions measured by means of crossed beam scattering combined with universal DC slice imaging. The studied hydrocarbons are propane and its two selectively deuterated isotopologues, namely 1,1,1,3,3,3-propane-d6 and 2,2-propane-d2, n-butane and isobutane (2-methyl-propane), with detection of the hydrocarbon radical product by 157 nm single photon ionization. Data are obtained at collision energies of 12–13 kcal mol−1 using a high-density atomic chlorine radical source combining Cl2 photolysis with ablation. All presented scattering distributions involving secondary and tertiary abstractions show distinct differences. Their comparisons allow for revisiting the dynamical picture of these reactions in terms of the nature of the abstraction sites, radical product energy disposal, and H vs. D abstraction. Results are discussed in the light of previous work and ab initio thermochemical calculations, along with proposals to future directions for investigation.

High-resolution ion and electron imaging techniques have been used to explore a series of problems in the reaction dynamics of gas phase ions. These have applications ranging from fundamental dynamical studies to atmospheric chemistry and astrochemistry. In this minireview we illustrate these approaches with several examples from our recent work. We examine the conformationally- and vibrationally-mediated photodissociation dynamics of propanal and ethylene cations, and show how these can reveal reaction dynamics across multiple electronic potential surfaces of these molecules. Recent results for methylamine cation photodissociation provide insight into the rich chemistry of the ionosphere of Saturn's great moon, Titan. The combination of high-resolution ion and photoelectron imaging, REMPI spectroscopy and state-of-the-art ab initio calculations yields a powerful multifaceted approach to studying photoionization and photofragmentation dynamics in ions.

We report the primary (D-atom) and secondary (H-atom) abstraction dynamics of chlorine atom reaction with butane-1,1,1,4,4,4-d6. The H- and D-atom abstraction channels were studied over a range of collision energies: 10.4 kcal mol−1 and 12.9 kcal mol−1; 5.2 kcal mol−1 to 12.8 kcal mol−1, respectively, using crossed molecular beam dc slice ion imaging techniques. Single photon ionization at 157 nm was used to probe the butyl radical products resulting from the H- and D-atom abstraction reactions. These two channels manifest distinct dynamics principally in the translational energy distributions, while the angular distributions are remarkably similar. The reduced translational energy distribution for the primary abstraction showed marked variation with collision energy in the backward direction, while the secondary abstraction showed this variation in the forward direction.

We report an ion imaging and time-of-flight mass spectroscopy study of the photodissociation of a variety of heptane isomers using 157 nm dissociation and ionization. Time-of-flight mass spectra show that C3H7 + C4H9 is the dominant detected product channel following one-color 157 nm dissociation/ionization of heptanes. The results further allow determination of the relative ionization efficiencies of 1- and 2-butyl and propyl radicals at 157 nm. Momentum matching for the two radical products indicates that, for the C3–C4 products, neutral dissociation followed by ionization is the main source of the detected signals. The images show isotropic angular distributions and the translational energy distributions peak at very low energy, with only ∼0.3 eV or 8% of the available energy appearing in translation. This is consistent with dissociation from the ground state or low-lying triplet states following non-radiative electronic relaxation.