The ionization energies (IEs) of TiO and TiO2 and the 0 K bond dissociation energies (D0) and the heats of formation at 0 K (ΔH°f0) and 298 K (ΔH°f298) for TiO/TiO+ and TiO2/TiO2+ are predicted by the wave-function-based CCSDTQ/CBS approach. The CCSDTQ/CBS calculations involve the approximation to the complete basis set (CBS) limit at the coupled cluster level up to full quadruple excitations along with the zero-point vibrational energy (ZPVE), high-order correlation (HOC), core–valence (CV) electronic, spin–orbit (SO) coupling, and scalar relativistic (SR) effect corrections. The present calculations yield IE(TiO) = 6.815 eV and are in good agreement with the experimental IE value of 6.819 80 ± 0.000 10 eV determined in a two-color laser-pulsed field ionization-photoelectron (PFI-PE) study. The CCSDT and MRCI+Q methods give the best predictions to the harmonic frequencies: ωe (ωe+) = 1013 (1069) and 1027 (1059) cm–1 and the bond lengths re (re+) = 1.625 (1.587) and 1.621 (1.588) Å, for TiO (TiO+) compared with the experimental values. Two nearly degenerate, stable structures are found for TiO2 cation: TiO2+(C2v) structure has two equivalent TiO bonds, while the TiO2+(Cs) structure features a long and a short TiO bond. The IEs for the TiO2+(C2v)←TiO2 and TiO2+(Cs)←TiO2 ionization transitions are calculated to be 9.515 and 9.525 eV, respectively, giving the theoretical adiabatic IE value in good agreement with the experiment IE(TiO2) = 9.573 55 ± 0.000 15 eV obtained in the previous vacuum ultraviolet (VUV)–PFI-PE study of TiO2. The potential energy surface of TiO2+ along the normal vibrational coordinates of asymmetric stretching mode (ω3+) is nearly flat and exhibits a double-well potential with the well of TiO2+ (Cs) situated around the central well of TiO2+(C2v). This makes the theoretical calculation of ω3+ infeasible. For the symmetric stretching (ω1+), the current theoretical predictions overestimate the experimental value of 829.1 ± 2.0 cm–1 by more than 100 cm–1. This work together with the previous experimental and theoretical investigations supports the conclusion that the CCSDTQ/CBS approach is capable of providing reliable IE and D0 predictions for TiO/TiO+ and TiO2/TiO2+ with error limits less than or equal to 60 meV. The CCSDTQ/CBS calculations give the predictions of D0(Ti+–O) – D0(Ti–O) = 0.004 eV and D0(O–TiO) – D0(O–TiO+) = 2.699 eV, which are also consistent with the respective experimental determination of 0.008 32 ± 0.000 10 and 2.753 75 ± 0.000 18 eV.

We report detailed quantum-rovibrational-state-selected integral cross sections for the formation of H3O+via H-transfer (σHT) and H2DO+via D-transfer (σDT) from the reaction in the center-of-mass collision energy (Ecm) range of 0.03–10.00 eV, where (v+1v+2v+3) = (000), (100), and (020) and . The Ecm inhibition and rotational enhancement observed for these reactions at Ecm < 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 Ecm < 0.5 eV, both the σHT and σDT 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 Ecm 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 σHT channel of the H2O+ + HD reaction at low Ecm < 0.5 eV, competition between the σHT and σDT of the H2O+ + HD reaction is also observed at Ecm = 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 Ecm, but also on the rotational and vibrational states of H2O+(X2B1). The detailed σHT and σDT 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.

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 (Ecm) range of 0.05–10.00 eV, where (v+1v+2v+3) = (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 Ecm 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 Ecm < 0.4 eV, but is enhanced at Ecm > 0.4 eV by (100) vibrational excitation. The Ecm 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 Ecm enhancement for σ(HOCO+) at Ecm ≈ 0.5–5.0 eV can be attributed to the direct collision mechanism, whereas the formation of HOCO+ at low Ecm < 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 Ecm and the vibrational enhancement at high Ecm observed for the σ(HOCO+).

We report on the successful implementation of a high-resolution vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection method for the study of unimolecular dissociation of quantum-state- or energy-selected molecular ions. As a test case, we have determined the 0 K appearance energy (AE0) for the formation of methylium, CH3+, from methane, CH4, as AE0(CH3+/CH4) = 14.32271 ± 0.00013 eV. This value has a significantly smaller error limit, but is otherwise consistent with previous laboratory and/or synchrotron-based studies of this dissociative photoionization onset. Furthermore, the sum of the VUV laser PFI-PI spectra obtained for the parent CH4+ ion and the fragment CH3+ ions of methane is found to agree with the earlier VUV pulsed field ionization-photoelectron (VUV-PFI-PE) spectrum of methane, providing unambiguous validation of the previous interpretation that the sharp VUV-PFI-PE step observed at the AE0(CH3+/CH4) threshold ensues because of higher PFI detection efficiency for fragment CH3+ than for parent CH4+. This, in turn, is a consequence of the underlying high-n Rydberg dissociation mechanism for the dissociative photoionization of CH4, which was proposed in previous synchrotron-based VUV-PFI-PE and VUV-PFI-PEPICO studies of CH4. The present highly accurate 0 K dissociative ionization threshold for CH4 can be utilized to derive accurate values for the bond dissociation energies of methane and methane cation. For methane, the straightforward application of sequential thermochemistry via the positive ion cycle leads to some ambiguity because of two competing VUV-PFI-PE literature values for the ionization energy of methyl radical. The ambiguity is successfully resolved by applying the Active Thermochemical Tables (ATcT) approach, resulting in D0(H–CH3) = 432.463 ± 0.027 kJ mol−1 and D0(H–CH3+) = 164.701 ± 0.038 kJ mol−1.

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+g: 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 (ΔElab ≈ 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+g: 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 (Ecm) range of 0.05–2.00 eV. The σ(v+ = 1, N+ = 1) exhibits a distinct Ecm 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 Ecm 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.

By employing the sequential electric field pulsing scheme for vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection, we have successfully recorded the spin–orbit and rovibronic state resolved VUV-PFI-PI spectra for O2+(a4Πu5/2,3/2,1/2,−1/2: ν+ = 0–2; J+) and O2+(X2Πg3/2,1/2: ν+ = 21–23; J+), indicating that O2+(a4Πu) and O2+(X2Πg) ions in these spin–orbit and rovibronic states can be prepared for ion–molecule collision studies. The present experiment is concerned with the measurement of absolute integral cross sections (σ's) of the charge transfer reactions, O2+(a4Πu5/2,3/2,1/2,−1/2: ν+ = 1, 2; J+) [O2+(X2Πg1/2,3/2: ν+ = 22, 23)] + Ar → Ar+ + O2. The fact that the O2+(a4Πu5/2,3/2,1/2,−1/2: ν+ = 1) and O2+(X2Πg3/2,1/2: ν+ = 22) [O2+(a4Πu5/2,3/2,1/2,−1/2: ν+ = 2) and O2+(X2Πg3/2,1/2: ν+ = 23)] states are in close energy resonance, makes these reactions ideal model systems for investigating the energy resonance and Franck–Condon factor (FCF) effects on the charge transfer reactivity of O2+. The σ(a4Πu5/2,3/2,1/2,−1/2: ν+ = 1, 2) values are found to be about ten-fold higher than the σ(X2Πg3/2,1/2: ν+ = 22, 23) values at Ecm = 0.05–10.00 eV, indicating that the FCFs play a predominant role in promoting these charge transfer reactions. The present ion–molecule reaction study also shows that σ(a4Πu) depends strongly on the spin–orbit as well as the vibrational states with the order: σ(a4Πu: v+ = 2) > σ(a4Πu: v+ = 1), and σ(a4Πu5/2: v+) > σ(a4Πu3/2: v+) > σ(a4Πu1/2: v+) > σ(a4Πu−1/2: v+), where v+ = 1 and 2. The high σ(a4Πu5/2,3/2,1/2,−1/2: v+ = 1, 2) values, along with their decreasing trend with increasing Ecm, are consistent with those expected for a long range charge transfer mechanism. However, the low σ(X2Πg3/2,1/2: ν+ = 22, 23) values and the lack of Ecm-dependence observed in the Ecm range of 0.05–10.00 eV point to the involvement of short-range collision dynamics.

To understand the dynamics of H3O+ formation, we report a combined experimental–theoretical study of the rovibrationally state-selected ion–molecule reactions H2O+(X2B1; v1+v2+v3+; NKa+Kc++) + H2 (D2) → H3O+ (H2DO+) + H (D), where (v1+v2+v3+) = (000), (020), and (100) and NKa+Kc++ = 000, 111, and 211. Both quantum dynamics and quasi-classical trajectory calculations were carried out on an accurate full-dimensional ab initio global potential energy surface, which involves nine degrees of freedom. The theoretical results are in good agreement with experimental measurements of the initial state specific integral cross-sections for the formation of H3O+ (H2DO+) and thus provide valuable insights into the surprising rotational enhancement and vibrational inhibition effects in these prototypical ion–molecule reactions that play a key role in the interstellar generation of OH and H2O species.

The state-to-state photodissociation of CO2 is investigated in the VUV range of 11.94–12.20 eV by using two independently tunable vacuum ultraviolet (VUV) lasers and the time-sliced velocity-map-imaging-photoion (VMI-PI) method. The spin-allowed CO(X1Σ+; v = 0–18) + O(1D) and CO(X1Σ+; v = 0–9) + O(1S) photoproduct channels are directly observed from the measurement of time-sliced VMI-PI images of O(1D) and O(1S). The total kinetic energy release (TKER) spectra obtained based on these VMI-PI images shows that the observed energetic thresholds for both the O(1D) and O(1S) channels are consistent with the thermochemical thresholds. Furthermore, the nascent vibrational distributions of CO(X1Σ+; v) photoproducts formed in correlation with O(1D) differ significantly from that produced in correlation with O(1S), indicating that the dissociation pathways for the O(1D) and O(1S) channels are distinctly different. For the O(1S) channel, CO(X1Σ+; v) photoproducts are formed mostly in low vibrational states (v = 0–2), whereas for the O(1D) channel, CO(X1Σ+; v) photoproducts are found to have significant populations in high vibrationally excited states (v = 10–16). The anisotropy β parameters for the O(1D) + CO(X1Σ+; v = 0–18) and O(1S) + CO(X1Σ+; v = 0–9) channels have also been determined from the VMI-PI measurements, indicating that CO2 dissociation to form the O(1D) and O(1S) channels is faster than the rotational periods of the VUV excited CO2 molecules. We have also calculated the excited singlet potential energy surfaces (PESs) of CO2, which are directly accessible by VUV excitation, at the ab initio quantum multi-reference configuration interaction level of theory. These calculated PESs suggest that the formation of CO(X1Σ+) + O(1S) photoproducts occurs nearly exclusively on the 41A′ PES, which is generally repulsive with minor potential energy ripples along the OC–O stretching coordinate. The formation of CO(X1Σ+) + O(1D) photofragments can proceed by non-adiabatic transitions from the 41A′ PES to the lower 31A′ PES of CO2via the seam of conical intersections at a near linear OCO configuration, followed by the direct dissociation on the 31A′ PES. The theoretical PES calculations are consistent with the experimental observation of prompt CO2 dissociation and high rotational and vibrational excitations for CO(X1Σ+) photoproducts.

By employing two-color visible (VIS)-ultraviolet (UV) laser photoionization and pulsed field ionization-photoelectron (PFI-PE) techniques, we have obtained highly rotationally resolved photoelectron spectra for vanadium monocarbide cations (VC+). The state-to-state VIS-UV-PFI-PE spectra thus obtained allow unambiguous assignments for the photoionization rotational transitions, resulting in a highly precise value for the adiabatic ionization energy (IE) of vanadium monocarbide (VC), IE(VC) = 57512.0 ± 0.8 cm−1 (7.13058 ± 0.00010 eV), which is defined as the energy of the VC+(X3Δ1; v+ = 0; J+ = 1) ← VC(X2Δ3/2; v′′ = 0; J′′ = 3/2) photoionization transition. The spectroscopic constants for VC+(X3Δ1) determined in the present study include the harmonic vibrational frequency ωe+ = 896.4 ± 0.8 cm−1, the anharmonicity constant ωe+xe+ = 5.7 ± 0.8 cm−1, the rotational constants Be+ = 0.6338 ± 0.0025 cm−1 and αe+ = 0.0033 ± 0.0007 cm−1, the equilibrium bond length re+ = 1.6549 ± 0.0003 Å, and the spin–orbit coupling constant A = 75.2 ± 0.8 cm−1 for VC+(X3Δ1,2,3). These highly precise energetic and spectroscopic data are used to benchmark state-of-the-art CCSDTQ/CBS calculations. In general, good agreement is found between the theoretical predictions and experimental results. The theoretical calculations yield the values, IE(VC) = 7.126 eV; the 0 K bond dissociation energies: D0(V–C) = 4.023 eV and D0(V+–C) = 3.663 eV; and heats of formation: , , , and kJ mol−1.

Vanadium monoxide cation VO+(X3Σ–) has been investigated by two-color visible (VIS)–ultraviolet (UV) pulsed field ionization–photoelectron (PFI–PE) methods. The unambiguous rotational assignment of rotationally selected and resolved VIS–UV–PFI–PE spectra thus obtained confirms the ground state term symmetry of VO+ to be X3Σ–. The rotational analysis also yields the rotational constants Be+ = 0.5716 ± 0.0012 cm–1 and αe+ = 0.0027 ± 0.0005 cm–1 for VO+(X3Σ–), from which the equilibrium bond distance of VO+(X3Σ–) is determined to be re+ = 1.557 ± 0.002 Å. This PFI–PE study covers the vibrational bands, VO+(X3Σ–; v+ = 0, 1, 2, and 3) ← VO(X4Σ–; v″ = 0), which has made possible the determination of the vibrational constants for VO+(X3Σ–) to be ωe+ = 1068.0 ± 0.7 cm–1 and ωe+xe+ = 5.5 ± 0.7 cm–1. The present state-to-state measurement also yields a more precise value (58 380.0 ± 0.7 cm–1 or 7.238 20 ± 0.000 09 eV) for the ionization energy of VO [IE(VO)]. This value along with the known IE(V) has allowed the determination of the difference between the 0 K bond dissociation energy (D0) of VO+(X3Σ–) and that of VO(X4Σ–) to be D0(V+–O) – D0(V–O) = IE(V) – IE(VO) = −3967 ± 1 cm–1.

Photodissociation of CO2 is investigated between 13.540 eV and 13.678 eV using the time-sliced velocity-mapped ion imaging (TSVMI) apparatus that is combined with one-color and two-color pump–probe VUV + VUV and VUV + UV detection schemes by probing oxygen fragments at different levels. Several CO2 dissociation channels are directly observed from the ion images, namely CO(X 1Σ+) + O(1D), CO(X 1Σ+) + O(1S), CO(a 3Π) + O(3P), CO(a 3Π) + O(1D), CO(a′ 3Σ+) + O(3P), CO(d 3Δ) + O(3P) and CO(e 3Σ−) + O(3P), whereas no CO(X 1Σ+) + O(3P) production has been found. The product kinetic energy distributions of these channels are reported for the first time. Possible dissociation mechanisms have been discussed based upon the product vibrational and rotational distributions.

The branching ratios for the spin-forbidden photodissociation channels of 12C16O in the vacuum ultraviolet (VUV) photon energy region from 102 500 (12.709 eV) to 106 300 cm–1 (13.180 eV) have been investigated using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to three 1Σ+ and six 1Π Rydberg-type states, including the progression of W(3sσ) 1Π(v′ = 0, 1, and 2) vibrational levels of CO, have been identified and investigated. The branching ratios for the product channels C(3P) + O(3P), C(1D) + O(3P), and C(3P) + O(1D) of these predissociative states are found to depend on the electronic, vibrational, and rotational states of CO being excited. Rotation and e/f-symmetry dependences of the branching ratios into the spin-forbidden channels have been confirmed for several of the 1Π states, which can be explained using the heterogeneous interaction with the repulsive D′1Σ+ state. The percentage of the photodissociation into the spin-forbidden channels is found to increase with increasing the rotational quantum number for the K(4pσ) 1Σ+ (v′ = 0) state. This has been rationalized using a 1Σ+ to 1Π to 3Π coupling scheme, where the final 3Π state is a repulsive valence state correlating to the spin-forbidden channel.

To understand the dynamics of H3O+ formation, we report a combined experimental–theoretical study of the rovibrationally state-selected ion–molecule reactions H2O+(X2B1; v1+v2+v3+; NKa+Kc++) + H2 (D2) → H3O+ (H2DO+) + H (D), where (v1+v2+v3+) = (000), (020), and (100) and NKa+Kc++ = 000, 111, and 211. Both quantum dynamics and quasi-classical trajectory calculations were carried out on an accurate full-dimensional ab initio global potential energy surface, which involves nine degrees of freedom. The theoretical results are in good agreement with experimental measurements of the initial state specific integral cross-sections for the formation of H3O+ (H2DO+) and thus provide valuable insights into the surprising rotational enhancement and vibrational inhibition effects in these prototypical ion–molecule reactions that play a key role in the interstellar generation of OH and H2O species.

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+g: 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 (ΔElab ≈ 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+g: 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 (Ecm) range of 0.05–2.00 eV. The σ(v+ = 1, N+ = 1) exhibits a distinct Ecm 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 Ecm 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.

We report detailed quantum-rovibrational-state-selected integral cross sections for the formation of H3O+via H-transfer (σHT) and H2DO+via D-transfer (σDT) from the reaction in the center-of-mass collision energy (Ecm) range of 0.03–10.00 eV, where (v+1v+2v+3) = (000), (100), and (020) and . The Ecm inhibition and rotational enhancement observed for these reactions at Ecm < 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 Ecm < 0.5 eV, both the σHT and σDT 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 Ecm 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 σHT channel of the H2O+ + HD reaction at low Ecm < 0.5 eV, competition between the σHT and σDT of the H2O+ + HD reaction is also observed at Ecm = 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 Ecm, but also on the rotational and vibrational states of H2O+(X2B1). The detailed σHT and σDT 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.

Photodissociation of CO2 is investigated between 13.540 eV and 13.678 eV using the time-sliced velocity-mapped ion imaging (TSVMI) apparatus that is combined with one-color and two-color pump–probe VUV + VUV and VUV + UV detection schemes by probing oxygen fragments at different levels. Several CO2 dissociation channels are directly observed from the ion images, namely CO(X 1Σ+) + O(1D), CO(X 1Σ+) + O(1S), CO(a 3Π) + O(3P), CO(a 3Π) + O(1D), CO(a′ 3Σ+) + O(3P), CO(d 3Δ) + O(3P) and CO(e 3Σ−) + O(3P), whereas no CO(X 1Σ+) + O(3P) production has been found. The product kinetic energy distributions of these channels are reported for the first time. Possible dissociation mechanisms have been discussed based upon the product vibrational and rotational distributions.

By employing two-color visible (VIS)-ultraviolet (UV) laser photoionization and pulsed field ionization-photoelectron (PFI-PE) techniques, we have obtained highly rotationally resolved photoelectron spectra for vanadium monocarbide cations (VC+). The state-to-state VIS-UV-PFI-PE spectra thus obtained allow unambiguous assignments for the photoionization rotational transitions, resulting in a highly precise value for the adiabatic ionization energy (IE) of vanadium monocarbide (VC), IE(VC) = 57512.0 ± 0.8 cm−1 (7.13058 ± 0.00010 eV), which is defined as the energy of the VC+(X3Δ1; v+ = 0; J+ = 1) ← VC(X2Δ3/2; v′′ = 0; J′′ = 3/2) photoionization transition. The spectroscopic constants for VC+(X3Δ1) determined in the present study include the harmonic vibrational frequency ωe+ = 896.4 ± 0.8 cm−1, the anharmonicity constant ωe+xe+ = 5.7 ± 0.8 cm−1, the rotational constants Be+ = 0.6338 ± 0.0025 cm−1 and αe+ = 0.0033 ± 0.0007 cm−1, the equilibrium bond length re+ = 1.6549 ± 0.0003 Å, and the spin–orbit coupling constant A = 75.2 ± 0.8 cm−1 for VC+(X3Δ1,2,3). These highly precise energetic and spectroscopic data are used to benchmark state-of-the-art CCSDTQ/CBS calculations. In general, good agreement is found between the theoretical predictions and experimental results. The theoretical calculations yield the values, IE(VC) = 7.126 eV; the 0 K bond dissociation energies: D0(V–C) = 4.023 eV and D0(V+–C) = 3.663 eV; and heats of formation: , , , and kJ mol−1.

The state-to-state photodissociation of CO2 is investigated in the VUV range of 11.94–12.20 eV by using two independently tunable vacuum ultraviolet (VUV) lasers and the time-sliced velocity-map-imaging-photoion (VMI-PI) method. The spin-allowed CO(X1Σ+; v = 0–18) + O(1D) and CO(X1Σ+; v = 0–9) + O(1S) photoproduct channels are directly observed from the measurement of time-sliced VMI-PI images of O(1D) and O(1S). The total kinetic energy release (TKER) spectra obtained based on these VMI-PI images shows that the observed energetic thresholds for both the O(1D) and O(1S) channels are consistent with the thermochemical thresholds. Furthermore, the nascent vibrational distributions of CO(X1Σ+; v) photoproducts formed in correlation with O(1D) differ significantly from that produced in correlation with O(1S), indicating that the dissociation pathways for the O(1D) and O(1S) channels are distinctly different. For the O(1S) channel, CO(X1Σ+; v) photoproducts are formed mostly in low vibrational states (v = 0–2), whereas for the O(1D) channel, CO(X1Σ+; v) photoproducts are found to have significant populations in high vibrationally excited states (v = 10–16). The anisotropy β parameters for the O(1D) + CO(X1Σ+; v = 0–18) and O(1S) + CO(X1Σ+; v = 0–9) channels have also been determined from the VMI-PI measurements, indicating that CO2 dissociation to form the O(1D) and O(1S) channels is faster than the rotational periods of the VUV excited CO2 molecules. We have also calculated the excited singlet potential energy surfaces (PESs) of CO2, which are directly accessible by VUV excitation, at the ab initio quantum multi-reference configuration interaction level of theory. These calculated PESs suggest that the formation of CO(X1Σ+) + O(1S) photoproducts occurs nearly exclusively on the 41A′ PES, which is generally repulsive with minor potential energy ripples along the OC–O stretching coordinate. The formation of CO(X1Σ+) + O(1D) photofragments can proceed by non-adiabatic transitions from the 41A′ PES to the lower 31A′ PES of CO2via the seam of conical intersections at a near linear OCO configuration, followed by the direct dissociation on the 31A′ PES. The theoretical PES calculations are consistent with the experimental observation of prompt CO2 dissociation and high rotational and vibrational excitations for CO(X1Σ+) photoproducts.

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 (Ecm) range of 0.05–10.00 eV, where (v+1v+2v+3) = (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 Ecm 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 Ecm < 0.4 eV, but is enhanced at Ecm > 0.4 eV by (100) vibrational excitation. The Ecm 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 Ecm enhancement for σ(HOCO+) at Ecm ≈ 0.5–5.0 eV can be attributed to the direct collision mechanism, whereas the formation of HOCO+ at low Ecm < 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 Ecm and the vibrational enhancement at high Ecm observed for the σ(HOCO+).

We report on the successful implementation of a high-resolution vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection method for the study of unimolecular dissociation of quantum-state- or energy-selected molecular ions. As a test case, we have determined the 0 K appearance energy (AE0) for the formation of methylium, CH3+, from methane, CH4, as AE0(CH3+/CH4) = 14.32271 ± 0.00013 eV. This value has a significantly smaller error limit, but is otherwise consistent with previous laboratory and/or synchrotron-based studies of this dissociative photoionization onset. Furthermore, the sum of the VUV laser PFI-PI spectra obtained for the parent CH4+ ion and the fragment CH3+ ions of methane is found to agree with the earlier VUV pulsed field ionization-photoelectron (VUV-PFI-PE) spectrum of methane, providing unambiguous validation of the previous interpretation that the sharp VUV-PFI-PE step observed at the AE0(CH3+/CH4) threshold ensues because of higher PFI detection efficiency for fragment CH3+ than for parent CH4+. This, in turn, is a consequence of the underlying high-n Rydberg dissociation mechanism for the dissociative photoionization of CH4, which was proposed in previous synchrotron-based VUV-PFI-PE and VUV-PFI-PEPICO studies of CH4. The present highly accurate 0 K dissociative ionization threshold for CH4 can be utilized to derive accurate values for the bond dissociation energies of methane and methane cation. For methane, the straightforward application of sequential thermochemistry via the positive ion cycle leads to some ambiguity because of two competing VUV-PFI-PE literature values for the ionization energy of methyl radical. The ambiguity is successfully resolved by applying the Active Thermochemical Tables (ATcT) approach, resulting in D0(H–CH3) = 432.463 ± 0.027 kJ mol−1 and D0(H–CH3+) = 164.701 ± 0.038 kJ mol−1.