The study of state-selected ion-molecule reactions using the vacuum ultraviolet pulsed field ionization-photoion technique

Quantum and quasiclassical dynamical simulations for the Ar2H+ on a new global analytical potential energy surface

A new analytical potential energy surface (PES) has been constructed for the Ar₂H⁺ system from a dataset consisting of a large number of ab initio energies computed using the coupled-cluster singles, doubles and perturbative triples method and aug-cc-pVQZ basis set. The long-range interaction is added to the diatomic potentials using a standard long range expansion form to better describe the asymptotic regions. The vibrational states for the most stable structures of the Ar₂H⁺ system have been calculated, and few low lying states are assigned to quantum numbers. Reactive scattering studies have been performed for the Ar + Ar'H⁺ → Ar' + ArH⁺ proton exchange reaction on the newly generated PES. Reaction probability, cross sections, and rate constants are calculated for the Ar + Ar'H⁺(v = 0, j = 0) collisions within 0.01 eV-0.6 eV of relative translational energy using exact quantum dynamical simulations as well as quasiclassical trajectory (QCT) calculations. The effect of vibrational excitation of the reactants is also explored for the reaction. State averaged rate constants are calculated for the proton exchange reaction at different temperatures using the QCT method. The mechanistic pathways for the reaction are understood by analyzing the quasiclassical trajectories.

Experimental study of the proton-transfer reaction C + H2+ → CH+ + H and its isotopic variant (D2+)

We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of Er ∼ 0.01-10 eV. We used photodetachment of C- to produce a pure beam of atomic C in the ground electronic 3P term, with statistically populated fine-structure levels. The H2+ and D2+ were formed in an electron impact ionization source, with well known vibrational and rotational distributions. The experimental work is complemented by a theoretical study of the CH2+ electronic system in the reactant and product channels, which helps to clarify the possible reaction mechanisms underlying the ICS measurements. Our measurements provide evidence that the reactions are barrierless and exoergic. They also indicate the apparent absence of an intermolecular isotope effect, to within the total experimental uncertainties. Capture models, taking into account either the charge-induced dipole interaction potential or the combined charge-quadrupole and charge-induced dipole interaction potentials, produce reaction cross sections that lie a factor of ∼4 above the experimental results. Based on our theoretical study, we hypothesize that the reaction is most likely to proceed adiabatically through the 14A' and 14A'' states of CH2+via the reaction C(3P) + H2+(2Σ+g) → CH+(3Π) + H(2S). We also hypothesize that at low collision energies only H2+(v ≤ 2) and D2+(v ≤ 3) contribute to the titular reactions, due to the onset of dissociative charge transfer for higher vibrational v levels. Incorporating these assumptions into the capture models brings them into better agreement with the experimental results. Still, for energies ⪅0.1 eV where capture models are most relevant, the modified charge-induced dipole model yields reaction cross sections with an incorrect energy dependence and lying ∼10% below the experimental results. The capture cross section obtained from the combined charge-quadrupole and charge-induced dipole model better matches the measured energy dependence but lies ∼30-50% above the experimental results. These findings provide important guidance for future quasiclassical trajectory and quantum mechanical treatments of this reaction.

A quantum-rovibrational-state-selected study of the proton-transfer reaction H2+(X2Σ: v+ = 1-3; N+ = 0-3) + Ne → NeH+ + H using the pulsed field ionization-photoion method: observation of the rotational effect near the reaction threshold

Using the sequential electric field pulsing scheme for vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection, we have successfully prepared H₂⁺(X²Σ: v⁺ = 1-3; N⁺ = 0-5) ions in the form of an ion beam in single quantum-rovibrational-states with high purity, high intensity, and narrow laboratory kinetic energy spread (ΔE_lab ≈ 0.05 eV). This VUV-PFI-PI ion source, when coupled with the double-quadrupole double-octupole ion-molecule reaction apparatus, has made possible a systematic examination of the vibrational- as well as rotational-state effects on the proton transfer reaction of H₂⁺(X²Σ: v⁺; N⁺) + Ne. Here, we present the integral cross sections [σ(v⁺; N⁺)'s] for the H₂⁺(v⁺ = 1-3; N⁺ = 0-3) + Ne → NeH⁺ + H reaction observed in the center-of-mass kinetic energy (E_cm) range of 0.05-2.00 eV. The σ(v⁺ = 1, N⁺ = 1) exhibits a distinct E_cm onset, which is found to agree with the endothermicity of 0.27 eV for the proton transfer process after taking into account of experimental uncertainties. Strong v⁺-vibrational enhancements are observed for σ(v⁺ = 1-3, N⁺) in the E_cm range of 0.05-2.00 eV. While rotational excitations appear to have little effect on σ(v⁺ = 3, N⁺), a careful search leads to the observation of moderate N⁺-rotational enhancements at v⁺ = 2: σ(v⁺ = 2; N⁺ = 0) < σ(v⁺ = 2; N⁺ = 1) < σ(v⁺ = 2; N⁺ = 2) < σ(v⁺ = 2; N⁺ = 3), where the formation of NeH⁺ is near thermal-neutral. The σ(v⁺ = 1-3, N⁺ = 0-3) values obtained here are compared with previous experimental results and the most recent state-of-the-art quantum dynamics predictions. We hope that these new experimental results would further motivate more rigorous theoretical calculations on the dynamics of this prototypical ion-molecule reaction.

A quantum-rovibrational-state-selected study of the reaction in the collision energy range of 0.05-10.00 eV: translational, rotational, and vibrational energy effects

We report detailed absolute integral cross sections (σ's) for the quantum-rovibrational-state-selected ion-molecule reaction in the center-of-mass collision energy (E_cm) range of 0.05-10.00 eV, where (vvv) = (000), (100), and (020), and . Three product channels, HCO⁺ + OH, HOCO⁺ + H, and CO⁺ + H₂O, are identified. The measured σ(HCO⁺) curve [σ(HCO⁺) versus E_cm plot] supports the hypothesis that the formation of the HCO⁺ + OH channel follows an exothermic pathway with no potential energy barriers. Although the HOCO⁺ + H channel is the most exothermic, the σ(HOCO⁺) is found to be significantly lower than the σ(HCO⁺). The σ(HOCO⁺) curve is bimodal, indicating two distinct mechanisms for the formation of HOCO⁺. The σ(HOCO⁺) is strongly inhibited at E_cm < 0.4 eV, but is enhanced at E_cm > 0.4 eV by (100) vibrational excitation. The E_cm onsets of σ(CO⁺) determined for the (000) and (100) vibrational states are in excellent agreement with the known thermochemical thresholds. This observation, along with the comparison of the σ(CO⁺) curves for the (100) and (000) states, shows that kinetic and vibrational energies are equally effective in promoting the CO⁺ channel. We have also performed high-level ab initio quantum calculations on the potential energy surface, intermediates, and transition state structures for the titled reaction. The calculations reveal potential barriers of ≈0.5-0.6 eV for the formation of HOCO⁺, and thus account for the low σ(HOCO⁺) and its bimodal profile observed. The E_cm enhancement for σ(HOCO⁺) at E_cm ≈ 0.5-5.0 eV can be attributed to the direct collision mechanism, whereas the formation of HOCO⁺ at low E_cm < 0.4 eV may involve a complex mechanism, which is mediated by the formation of a loosely sticking complex between HCO⁺ and OH. The direct collision and complex mechanisms proposed also allow the rationalization of the vibrational inhibition at low E_cm and the vibrational enhancement at high E_cm observed for the σ(HOCO⁺).

Isotopic and quantum-rovibrational-state effects for the ion-molecule reaction in the collision energy range of 0.03-10.00 eV

We report detailed quantum-rovibrational-state-selected integral cross sections for the formation of H₃O⁺via H-transfer (σ_HT) and H₂DO⁺via D-transfer (σ_DT) from the reaction in the center-of-mass collision energy (E_cm) range of 0.03-10.00 eV, where (vvv) = (000), (100), and (020) and . The E_cm inhibition and rotational enhancement observed for these reactions at E_cm < 0.5 eV are generally consistent with those reported previously for H₂O⁺ + H₂(D₂) reactions. However, in contrast to the vibrational inhibition observed for the latter reactions at low E_cm < 0.5 eV, both the σ_HT and σ_DT for the H₂O⁺ + HD reaction are found to be enhanced by (100) vibrational excitation, which is not predicted by the current state-of-the-art theoretical dynamics calculations. Furthermore, the (100) vibrational enhancement for the H₂O⁺ + HD reaction is observed in the full E_cm range of 0.03-10.00 eV. The fact that vibrational enhancement is only observed for the reaction of H₂O⁺ + HD, and not for H₂O⁺ + H₂(D₂) reactions suggests that the asymmetry of HD may play a role in the reaction dynamics. In addition to the strong isotopic effect favoring the σ_HT channel of the H₂O⁺ + HD reaction at low E_cm < 0.5 eV, competition between the σ_HT and σ_DT of the H₂O⁺ + HD reaction is also observed at E_cm = 0.3-10.0 eV. The present state-selected study of the H₂O⁺ + HD reaction, along with the previous studies of the H₂O⁺ + H₂(D₂) reactions, clearly shows that the chemical reactivity of H₂O⁺ toward H₂ (HD, D₂) depends not only on E_cm, but also on the rotational and vibrational states of H₂O⁺(X²B₁). The detailed σ_HT and σ_DT values obtained here with single rovibrational-state selections of the reactant H₂O⁺ are expected to be valuable benchmarks for state-of-the-art theoretical calculations on the chemical dynamics of the title reaction.

State-resolved differential and integral cross sections for the Ne + H2 (+) (v = 0-2, j = 0) → NeH(+) + H reaction

State-to-state quantum dynamic calculations for the proton transfer reaction Ne + H2 (+) (v = 0-2, j = 0) are performed on the most accurate LZHH potential energy surface, with the product Jacobi coordinate based time-dependent wave packet method including the Coriolis coupling. The J = 0 reaction probabilities for the title reaction agree well with previous results in a wide range of collision energy of 0.2-1.2 eV. Total integral cross sections are in reasonable agreement with the available experiment data. Vibrational excitation of the reactant is much more efficient in enhancing the reaction cross sections than translational and rotational excitation. Total differential cross sections are found to be forward-backward peaked with strong oscillations, which is the indication of the complex-forming mechanism. As the collision energy increases, state-resolved differential cross section changes from forward-backward symmetric peaked to forward scattering biased. This forward bias can be attributed to the larger J partial waves, which makes the reaction like an abstraction process. Differential cross sections summed over two different sets of J partial waves for the v = 0 reaction at the collision energy of 1.2 eV are plotted to illustrate the importance of large J partial waves in the forward bias of the differential cross sections.

Wave packet and statistical quantum calculations for the He + NeH⁺ → HeH⁺ + Ne reaction on the ground electronic state

A real wave packet based time-dependent method and a statistical quantum method have been used to study the He + NeH(+) (v, j) reaction with the reactant in various ro-vibrational states, on a recently calculated ab initio ground state potential energy surface. Both the wave packet and statistical quantum calculations were carried out within the centrifugal sudden approximation as well as using the exact Hamiltonian. Quantum reaction probabilities exhibit dense oscillatory pattern for smaller total angular momentum values, which is a signature of resonances in a complex forming mechanism for the title reaction. Significant differences, found between exact and approximate quantum reaction cross sections, highlight the importance of inclusion of Coriolis coupling in the calculations. Statistical results are in fairly good agreement with the exact quantum results, for ground ro-vibrational states of the reactant. Vibrational excitation greatly enhances the reaction cross sections, whereas rotational excitation has relatively small effect on the reaction. The nature of the reaction cross section curves is dependent on the initial vibrational state of the reactant and is typical of a late barrier type potential energy profile.

Time-dependent wave-packet quantum dynamics study of the Ne + D2(+) (v0 = 0-2, j0 = 0) → NeD(+) + D reaction: including the coriolis coupling

The dynamics of the Ne + D2(+) (v0 = 0-2, j0 = 0) → NeD(+) + D reaction has been investigated in detail by using an accurate time-dependent wave-packet method on the ground 1(2)A' potential energy surface. Comparisons between the Coriolis coupling results and the centrifugal-sudden ones reveal that Coriolis coupling effect can influence reaction dynamics of the NeD2(+) system. Integral cross sections have been evaluated for the Ne + D2(+) reaction and its isotopic variant Ne + H2(+), and a considerable intermolecular isotopic effect has been found. Also obvious is the great enhancement of the reactivity due to the reagent vibrational excitation. Besides, a comparison with previous theoretical results is also presented and discussed.

Understanding the effect of vibrational excitation in reaction dynamics: the Ne + H2(+)(v = 0-17, j = 1) → NeH(+) + H, Ne + H(+) + H proton transfer and dissociation cross sections

The dependence of the cross section (σ) of the Ne + H2(+)→ NeH(+) + H proton transfer reaction on the vibrational excitation of H2(+), v = 0-17 and j = 1, was analyzed in detail at the collision energies (Ecol) of 0.7 and 1.7 eV, using the quasi-classical trajectory (QCT) method and the PHHJ3 and LZHH potential energy surfaces (PESs), taking advantage of the rich experimental data available for this reaction as a function of H2(+)(v). The efficiency of vibrational excitation to promote the reaction was investigated from the analysis of the σ(QCT) vs. v dependence in terms of the average reaction probability, maximum impact parameter, regions of the (late barrier) PES explored, and taking into account the Ne + H2(+)→ Ne + H(+) + H dissociative channel, which plays a dominant role at high enough total energies. Although the earlier PHHJ3 PES performs rather well, the LZHH PES QCT results show a better agreement with the experiment. On the other hand, some artifacts were found in recently reported QCT calculations (unphysical oscillations in σ(QCT) as a function of v), and the present study shows that special care is needed when carrying out QCT calculations involving highly excited vibrational states.

Quantum dynamical study of the He + NeH+ reaction on a new analytical potential energy surface

An analytical potential energy surface (PES) for the ground state of the [HeHNe](+) system has been constructed from a set of 19,605 ab initio data points, obtained from coupled cluster singles and doubles with perturbative triples correction calculations and the aug-cc-pVQZ basis set. The PES is based on the many-body expansion form proposed by Aguado and Paniagua (J. Chem. Phys. 1992, 96, 1265), and it has a root-mean-square error of 0.03 kcal/mol. The minimum energy pathways (MEPs) for different Ne-H-He angles are calculated, and it is found that the MEP for 180° (linear) goes through the deepest potential energy well. Preliminary quantum dynamical studies are performed for the He + NeH(+) (v = 0-2, j = 0-3) → HeH(+) + Ne reaction in the 0.0-0.5 eV collision energy range. Quantum calculations are carried out using a time-dependent wave packet method within the centrifugal sudden approximation. Reaction probabilities exhibit strong oscillatory behavior arising because of the metastable [HeHNe](+). Vibrational excitation has been found to enhance the reaction cross sections.

Time-dependent quantum wave packet study of the Ar+H2+→ArH(+)+H reaction on a new ab initio potential energy surface for the ground electronic state (1(2)A')

A new global potential energy surface for the ground electronic state (1(2)A') of the Ar+H2(+)→ArH(+)+H reaction has been constructed by multi-reference configuration interaction method with Davidson correction and a basis set of aug-cc-pVQZ. Using 6080 ab initio single-point energies of all the regions for the dynamics, a many-body expansion function form has been used to fit these points. The quantum reactive scattering dynamics calculations taking into account the Coriolis coupling (CC) were carried out on the new potential energy surface over a range of collision energies (0.03-1.0 eV). The reaction probabilities and integral cross sections for the title reaction were calculated. The significance of including the CC quantum scattering calculation has been revealed by the comparison between the CC and the centrifugal sudden approximation calculation. The calculated cross section is in agreement with the experimental result at collision energy 1.0 eV.

Resonances in the Ne + H2(+) → NeH(+) + H proton-transfer reaction

We investigated the oscillations found in the integral cross section of the title reaction, which are particularly evident for Ne + H2(+)(v0 = 2,j0 = 1) [essentially isoenergetic with NeH(+)(v' = 0,j' = 0) + H] at low collision energy (Ecol < 0.30 eV). We employed mainly an exact time-independent (TI) quantum dynamics method and used the best potential energy surface available. From analysis of TI initial state selected to all integral cross sections, state-to-state integral cross sections, and the corresponding differential cross sections (DCSs), we showed that the oscillations correspond to resonances. They arise from the influence of the global [Ne-H-H](+) (collinear) minimum on dynamics and probably correspond to Feshbach resonances. Besides, the forward-backward peaking DCS (which oscillates with Ecol) behavior observed could be a signature for this type of resonances. Finally, as most data on resonances in bimolecular reactions correspond to neutral systems, we hope that the present results will encourage experimentalists to re-examine this benchmark system.

Time dependent quantum dynamics study of the Ne + H2(+)(v0 = 0-4, j0 = 1) → NeH(+) + H proton transfer reaction, including the Coriolis coupling. A system with oscillatory cross sections

The Ne + H(2)(+)(v(0) = 0-4, j(0) = 1) proton transfer reaction has been studied in a wide collision energy (E(col)) interval, using the time dependent real wave packet method and taking into account the Coriolis coupling (CC-RWP method) and employing a recent ab initio potential energy surface, widely extending the reaction conditions previously explored at the CC level. The reaction probability shows a strong oscillatory behavior vs E(col) and the presence of sharp resonances, arising from metastable NeH(2)(+) states. The behavior of the reaction cross section σ vs E(col) depends on the vibrational level and can in general be interpreted in terms of the late barrier character of the potential energy surface and the existence (or not) of threshold energy. The situation is particularly complex for v(0) = 2, as σ(v0=2, j0=1) presents significant oscillations with E(col) up to ≈0.33 eV, which probably reflect the resonances found in the reaction probability. Hence, it would be particularly interesting to investigate the Ne + H(2)(+)(v(0) = 2, j(0) = 1) reaction experimentally, as some resonances survive the partial wave summation. The state selected cross sections compare well with previous CC quantum and experimental results, and although the previous centrifugal sudden RWP cross sections are reasonable, the inclusion of the Coriolis coupling is important to achieve a quantitative description of this and similar systems.

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.

Exact quantum scattering study of the Ne+H(2) (+) reaction on a new ab initio potential energy surface

We present a new potential energy surface (PES) for the ground state (1(2)A(')) of the chemical reaction Ne+H(2) (+) from a set of accurate ab initio data, which were computed using highly correlated complete active space self-consistent field and multireference configuration interaction wave functions with a basis set of aug-cc-pV5Z. The quantum reactive scattering dynamics calculation was carried out over the collision energy (E(col)) range of 0.5-1.5 eV based on the new PES. In this work we have taken the Coriolis coupling (CC) effect into account. The importance of including the CC quantum scattering calculation has been revealed by the comparison between the CC and the centrifugal sudden approximation calculation. The magnitude and profile of the CC total cross sections for v=0 and j=1 over the collision energy range of 0.5-1.5 eV are found to be in good agreement with the available experimental measurements obtained recently by Zhang et al. [J. Chem. Phys. 119, 10175 (2003)] after taking into account the experimental uncertainties.

Data acquisition schemes for continuous two-particle time-of-flight coincidence experiments

Three data acquisition schemes for two-particle coincidence experiments with a continuous source are discussed. The single-start/single-stop technique, implemented with a time-to-pulse-height converter, results in a complicated spectrum and breaks down severely at high count rates. The single-start/multiple-stop setup, based on a time-to-digital converter and the first choice in today's similar coincidence experiments, performs significantly better at high count rates, but its performance is still hampered if the time-of-flight range is large, and the false coincidence background is variable if the event frequency and the collection efficiency of the starts are both high. A straightforward, multistart/multistop setup is proposed for coincidence experiments. By collecting all detector data, it ensures the highest signal-to-noise ratio, constant background, and fast data acquisition and can now be easily constructed with commercially available time-to-digital converters. Analytical and numerically evaluated formulas are derived to characterize the performance of each setup in a variety of environments. Computer simulated spectra are presented to illustrate the analytically predicted features of the various raw time-of-flight distributions obtained with each technique.