Quantum electronic control on chemical activation of methane by collision with spin-orbit state selected vanadium cation.

By coupling a newly developed quantum-electronic-state-selected supersonically cooled vanadium cation (V+) beam source with a double quadrupole-double octopole (DQDO) ion-molecule reaction apparatus, we have investigated detailed absolute integral cross sections (σ's) for the reactions, V+[a5DJ (J = 0, 2), a5FJ (J = 1, 2), and a3FJ (J = 2, 3)] + CH4, covering the center-of-mass collision energy range of Ecm = 0.1-10.0 eV. Three product channels, VH+ + CH3, VCH2+ + H2, and VCH3+ + H, are unambiguously identified based on Ecm-threshold measurements. No J-dependences for the σ curves (σ versus Ecm plots) of individual electronic states are discernible, which may indicate that the spin-orbit coupling is weak and has little effect on chemical reactivity. For all three product channels, the maximum σ values for the triplet a3FJ state [σ(a3FJ)] are found to be more than ten times larger than those for the quintet σ(a5DJ) and σ(a5FJ) states, showing that a reaction mechanism favoring the conservation of total electron spin. Without performing a detailed theoretical study, we have tentatively interpreted that a weak quintet-to-triplet spin crossing is operative for the activation reaction. The σ(a5D0, a5F1, and a3F2) measurements for the VH+, VCH2+, and VCH3+ product ion channels along with accounting of the kinetic energy distribution due to the thermal broadening effect for CH4 have allowed the determination of the 0 K bond dissociation energies: D0(V+-H) = 2.02 (0.05) eV, D0(V+-CH2) = 3.40 (0.07) eV, and D0(V+-CH3) = 2.07 (0.09) eV. Detailed branching ratios of product ion channels for the titled reaction have also been reported. Excellent simulations of the σ curves obtained previously for V+ generated by surface ionization at 1800-2200 K can be achieved by the linear combination of the σ(a5DJ, a5FJ, and a3FJ) curves weighted by the corresponding Boltzmann populations of the electronic states. In addition to serving as a strong validation of the thermal equilibrium assumption for the populations of the V+ electronic states in the hot filament ionization source, the agreement between these results also confirmed that the V+(a5DJ, a5FJ, and a3FJ) states prepared in this experiment are in single spin-orbit states with 100% purity.

Chemical Activation of Water Molecule by Collision with Spin-Orbit-State-Selected Vanadium Cation: Quantum-Electronic-State Control of Chemical Reactivity.

We have obtained absolute integral cross sections (σ's) for the reactions of spin-orbit-state-selected vanadium cations, V+[a5DJ(J = 0, 2), a5FJ(J = 1, 2), and a3FJ(J = 2, 3)], with a water molecule (H2O) in the center-of-mass collision energy range Ecm = 0.1-10.0 eV. On the basis of these state-selected σ curves (σ versus Ecm plots) observed, three reaction product channels, VO+ + H2, VH+ + OH, and VOH+ + H, from the V+ + H2O reaction are unambiguously identified. Contrary to the previous guided ion beam study of the V+(a5DJ) + D2O reaction, we have observed the formation of the VO+ + H2 channel from the V+(a5DJ) + H2O ground reactant state at low Ecm's (<3.0 eV). No spin-orbit J-state dependences for the σ curves of individual electronic states are discernible, indicating that spin-orbit interactions are weak with little effect on chemical reactivity of the titled reaction. For the three product channels identified, the triplet σ(a3FJ) values are overwhelmingly higher than the quintet σ(a5DJ) and σ(a5FJ) values, showing that the reaction is governed by a "weak quintet-triplet spin crossing" mechanism, favoring the conservation of total electron spins. The σ curves for exothermic product channels are found to exhibit a rapid decreasing profile as Ecm is increased, an observation consistent with the prediction of the charge-dipole and induced-dipole orbiting model. This experiment shows that the V+ + H2O reaction can be controlled effectively to produce predominantly the VO+ + H2 channel via the V+(a3FJ) + H2O reaction at low Ecm's (≤0.1 eV) and that the ion-molecule reaction dynamics can be altered readily by selecting the electronic state of V+ cation. On the basis of the measured Ecm thresholds for the σ(a5DJ, a5FJ, and a3FJ: VH+) and σ(a5DJ, a5FJ, and a3FJ: VOH+) curves, we have deduced upper bound values of 2.6 ± 0.2 and 4.3 ± 0.3 eV for the 0 K bond dissociation energies, D0(V+-H) and D0(V+-OH), respectively. After correcting for the kinetic energy distribution resulting from the Doppler broadening effect of the H2O molecule, we obtain D0(V+-H) = 2.2 ± 0.2 eV and D0(V+-OH) = 4.0 ± 0.3 eV, which are in agreement with D0 determinations obtained by σ curve simulations.

What is the Bond Dissociation Energy of the Vanadium Hydride Cation?

Recent electronic state-selected measurements of the reactions of atomic vanadium cations with D2 and CO2 are reanalyzed to properly account for the kinetic energy distribution of the reactant neutrals. The need for this is demonstrated in the present work by comparing the D2 data to that obtained previously in earlier experiments but unpublished. It is shown that the earlier data, which utilized a surface ionization source of V+, and the state-selected data for V+(a5D2) are essentially identical in the threshold regions where they overlap. Differences in the electronic state energies and kinetic energy distributions of V+ in the two experiments are very small and much smaller than the kinetic energy distribution of the neutral reactant, which is identical in both experiments. It is shown that properly accounting for the latter distribution alters the conclusions regarding the threshold energy for the endothermic formation of VD+ such that recent conclusions regarding the bond energy of VD+ are substantially altered and found to reproduce the original bond energy determination. Accounting for all experiments, a revised best value for D0(VH+) is 2.07 ± 0.09 eV [or D0(VD+) = 2.10 ± 0.09 eV]. This conclusion is validated by high-level ab initio calculations. Differences in the new and older data sets for the V+ + D2 reaction at higher energies (above the onset for dissociation of the product ion) are also discussed. The same methodology is then applied to recent studies on the state-selected V+ + CO2 reaction.

Quantum state control on the chemical reactivity of a transition metal vanadium cation in carbon dioxide activation.

By combining a newly developed two-color laser pulsed field ionization-photoion (PFI-PI) source and a double-quadrupole-double-octopole (DQDO) mass spectrometer, we investigated the integral cross sections (σs) of the vanadium cation (V+) toward the activation of CO2 in the center-of-mass kinetic energy (Ecm) range from 0.1 to 10.0 eV. Here, V+ was prepared in single spin-orbit levels of its lowest electronic states, a5DJ (J = 0-4), a5FJ (J = 1-5), and a3FJ (J = 2-4), with well-defined kinetic energies. For both product channels VO+ + CO and VCO+ + O identified, V+(a3F2,3) is found to be greatly more reactive than V+(a5D0,2) and V+(a5F1,2), suggesting that the V+ + CO2 reaction system mainly proceeds via a "weak quintet-to-triplet spin-crossing" mechanism favoring the conservation of total electron spins. In addition, no J-state dependence was observed. The distinctive structures of the quantum electronic state selected integral cross sections observed as a function of Ecm and the electronic state of the V+ ion indicate that the difference in the chemical reactivity of the title reaction originated from the quantum-state instead of energy effects. Furthermore, this work suggests that the selection of the quantum electronic states a3FJ (J = 2-4) of the transition metal V+ ion can greatly enhance the efficiency of CO2 activation.

Quantum Spin-Orbit Electronic State Selection of Atomic Transition Metal Vanadium Cation for Chemical Reactivity Studies.

By combining a pulsed laser ablation vanadium atom (V) beam source with the two-color laser sequential electric field pulse scheme for pulse field ionization-photoion (PFI-PI) detection, we have developed a quantum spin-orbit state selected transition metal ion source for ion-molecule reaction studies. As a demonstration, we show that the V+ ion can be prepared in the single spin-orbit levels of its three lowest quantum electronic states, V+[a5DJ ( J = 0-4), a5FJ ( J = 1-5), and a3FJ ( J = 2-4)], achieving laboratory kinetic energy ( Elab) resolutions of ≤0.2 eV. The precursor V atom beam is first excited to high- n Rydberg states by resonance-enhanced visible-ultraviolet laser photoexcitation via the V*[3d3(4F) 4s4p (3P°)] neutral intermediate state. The total photon energy is tuned in the regions from 54 380 to 63 520 cm-1 to cover the photoionization energies for the formation of these spin-orbit states. Sharp Rydberg transitions converging to the V+[a5DJ ( J = 1 and 2)] spin-orbit levels are identified in the respective PFI-PI spectra for the V+[a5DJ ( J = 0 and 1)] states. The analysis of these Rydberg members observed yields an ionization energy of 54 412.65 ± 0.15 cm-1 for V atom, which is in excellent accord with the literature value of 54 413 ± 1 cm-1 eV. In order to understand the profile for the PFI-PI spectrum of V+ ion observed and thus obtain reliable Stark shift corrections by using the sequential PFI-PI detection scheme, we have also examined the PFI-PI spectrum for Ar+(2P3/2) in detail by varying the retarding as well as the PFI electric field pulses.

Chemical Activation of a Deuterium Molecule by Collision with a Quantum Electronic State-Selected Vanadium Cation.

By combining a newly developed spin-orbit electronic state-selected ion source for vanadium cations (V+) with a double quadrupole-double octopole mass spectrometer, we have investigated in detail the chemical reactivity or integral cross sections (σ's) for the reactions of V+[a5DJ (J = 0, 1), a5FJ (J = 1, 2), and a3FJ (J = 2, 3)] ion with a deuterium molecule (D2). The vanadium deuteride ion (VD+) is identified to be the only product ion formed in the center-of-mass collision energies of Ecm = 0.1-10.0 V. No J dependence for the σ's is discernible for individual electronic states, indicating that the spin-orbit coupling is weak and has little effect on the chemical reactivity of the titled reaction. The maximum σ value for the V+(a3FJ) state [σ(a3FJ)] is about 7 and 70 times larger than those for σ(a5DJ) and σ(a5FJ), respectively, showing that the triplet V+(a3FJ) state is dominantly more reactive than the quintet states. Although the V+(a5FJ) state is 0.3 eV higher than the V+(a5DJ) ground state, the chemical reactivity of the V+(a5FJ) state is significantly lower than that of the V+(a5DJ) state, clearly indicating that the differences in chemical activity observed are due to quantum electronic states rather than energy effects. The Ecm thresholds determined for σ(a5DJ), σ(a5FJ), and σ(a3FJ) are consistent with the respective energetics for the formation of VD+ from the V+(a5DJ, a5FJ, and a3FJ) + D2 reactions. The analysis of Ecm threshold measurements yields a bond energy of D0(V+-D) = 2.5 ± 0.2 eV, suggesting that the previously reported values are too low by up to 0.4 eV. The large differences for σ(a5DJ, a5FJ, and a3FJ) observed here indicate that the activation of D2 by a V+ ion can be efficiently controlled by selecting the V+ electronic state as well as the Ecm. The quantum state-selected σ values presented here can also serve as experimental benchmarks for first-principles theoretical reaction dynamics calculations.

Branching Ratios of the N(2D03/2) and N(2D05/2) Spin-Orbit States Produced in the State-Selected Photodissociation of N2 Determined Using Time-Sliced Velocity-Mapped-Imaging Photoionization Mass Spectrometry (TS-VMI-PI-MS).

Branching ratios for N(2D03/2) and N(2D05/2) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck-Condon region from the ground electronic, 1Σg+, vibrational, v″, and rotational, J″ state to an excited Eu', v', J' state with a tunable vacuum ultraviolet, VUV1, laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV2 laser. Measurement of the relative populations of these two atoms formed in their spin-orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV2 are mass analyzed with a time-of-flight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin-orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV2 in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm-1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm-1 prove that the lines observed in this region originate from the N(2D03/2) and N(2D05/2) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm-1 originate from the nitrogen N(4S03/2) that is photoionized in a 1+ 1 VUV-UV resonant multiphoton ionization process. The spin-orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm-1, and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck-Condon region of the excited states. In the energy region below 117 153.8 cm-1 the pathways at long internuclear distances appear to determine [N(2D03/2)]/[N(2D05/2)] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N(2D03/2) and N(2D05/2) atoms produced in the spin-allowed channels that produce two N(2D0J) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

Quantum-State-Selected Integral Cross Sections and Branching Ratios for the Ion-Molecule Reaction of N2+(X2Σg+: ν+ = 0-2) + C2H4 in the Collision Energy Range of 0.05-10.00 eV.

By implementing a vacuum ultraviolet laser-pulsed field ionization-photoion ion source with a double quadrupole-double octopole ion guide mass filter, we have obtained detailed quantum-vibrational-state-selected integral cross sections σν+, ν+ = 0-2, for the ion-molecule reaction of N2+(X2Σg+: ν+ = 0-2) + C2H4 in the center-of-mass kinetic energy range of Ecm = 0.05-10.00 eV. Three primary product channels corresponding to the formation of C2H3+, C2H2+, and N2H+ ions are identified with their σν+ values in the order of σν+(C2H3+) > σν+(C2H2+) > σν+(N2H+). The minor σν+(N2H+) channel is strongly inhibited by Ecm and observed only at Ecm < 0.70 eV. The high σν+(C2H3+) and σν+(C2H2+) values indicate that C2H3+ and C2H2+ product ions are formed by prompt dissociation of internally excited C2H4+ (C2H4+*) intermediates produced via the near-energy-resonance charge-transfer mechanism. The σν+(C2H3+) and σν+(C2H2+) are found to drop only mildly or stay nearly constant as a function of Ecm in the range of 0.05-6.00 eV. This observation is contrary to the expectation of a steep decline for the σν+ value commonly observed for an exothermic reaction pathway as Ecm is increased. Significant vibrational enhancement is observed for the σν+(C2H3+) and σν+(C2H2+) at ν+ = 2 and in the Ecm range of ∼0.20-7.00 eV. The branching ratios σν+(C2H3+):σν+(C2H2+):σν+(N2H+) are also determined with high precision by measuring the intensities of product C2H3+, C2H2+, and N2H+ ions simultaneously at fixed Ecm values. The σν+ and branching ratio values reported here are useful contributions to the database needed for realistic modeling of the chemical compositions and evolutions of planetary atmospheres, such as the ionosphere of Titian. The quantum-state-selective results can also serve as experimental benchmarks for theoretical calculations on fundamental chemical reaction dynamics.

Branching Ratio Measurements of the Predissociation of 12C16O by Time-Slice Velocity-Map Ion Imaging in the Energy Region from 106 250 to 107 800 cm-1.

Photodissociation of CO is a fundamental chemical mechanism for mass-independent oxygen isotope fractionation in the early Solar System. Branching ratios of photodissociation channels for individual bands quantitatively yield the trapping efficiencies of atomic oxygen resulting into oxides. We measured the branching ratios for the spin-forbidden and spin-allowed photodissociation channels of 12C16O in the vacuum ultraviolet (VUV) photon energy region from 106 250 to 107 800 cm-1 using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to four 1Π bands and three 1Σ+ bands of 12C16O were 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 strongly depend on the electronic and vibrational states of CO being excited. By plotting the branching ratio of the spin-forbidden dissociation channels versus the excitation energy from 102 500 to 110 500 cm-1 that has been measured so far, the global pattern of the 1Π-3Π interaction that plays a key role in the predissociation of CO is revealed and discussed.

Quantum-state-selected integral cross sections for the charge transfer collision of O2+(a4Πu5/2,3/2,1/2,-1/2: v+ = 1-2; J+) [O2+(X2Πg3/2,1/2: v+ = 22-23; J+)] + Ar at center-of-mass collision energies of 0.05-10.00 eV.

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.

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 (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: 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.

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 (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 (vvv) = (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.

A vacuum ultraviolet laser pulsed field ionization-photoion study of methane (CH4): determination of the appearance energy of methylium from methane with unprecedented precision and the resulting impact on the bond dissociation energies of CH4 and CH4.

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.

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 (Ecm) 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 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+).

High-Level ab Initio Predictions for the Ionization Energies, Bond Dissociation Energies, and Heats of Formation of Titanium Oxides and Their Cations (TiOn/TiOn+, n = 1 and 2).

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.

Comparison of experimental and theoretical quantum-state-selected integral cross-sections for the H2O(+) + H2 (D2) reactions in the collision energy range of 0.04-10.00 eV.

To understand the dynamics of H3O(+) formation, we report a combined experimental-theoretical study of the rovibrationally state-selected ion-molecule reactions H2O(+)(X(2)B1; 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.

State-to-state vacuum ultraviolet photodissociation study of CO2 on the formation of state-correlated CO(X(1)Σ(+); v) with O((1)D) and O((1)S) photoproducts at 11.95-12.22 eV.

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(X(1)Σ(+); v = 0-18) + O((1)D) and CO(X(1)Σ(+); v = 0-9) + O((1)S) photoproduct channels are directly observed from the measurement of time-sliced VMI-PI images of O((1)D) and O((1)S). The total kinetic energy release (TKER) spectra obtained based on these VMI-PI images shows that the observed energetic thresholds for both the O((1)D) and O((1)S) channels are consistent with the thermochemical thresholds. Furthermore, the nascent vibrational distributions of CO(X(1)Σ(+); v) photoproducts formed in correlation with O((1)D) differ significantly from that produced in correlation with O((1)S), indicating that the dissociation pathways for the O((1)D) and O((1)S) channels are distinctly different. For the O((1)S) channel, CO(X(1)Σ(+); v) photoproducts are formed mostly in low vibrational states (v = 0-2), whereas for the O((1)D) channel, CO(X(1)Σ(+); v) photoproducts are found to have significant populations in high vibrationally excited states (v = 10-16). The anisotropy ß parameters for the O((1)D) + CO(X(1)Σ(+); v = 0-18) and O((1)S) + CO(X(1)Σ(+); v = 0-9) channels have also been determined from the VMI-PI measurements, indicating that CO2 dissociation to form the O((1)D) and O((1)S) 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(X(1)Σ(+)) + O((1)S) photoproducts occurs nearly exclusively on the 4(1)A' PES, which is generally repulsive with minor potential energy ripples along the OC-O stretching coordinate. The formation of CO(X(1)Σ(+)) + O((1)D) photofragments can proceed by non-adiabatic transitions from the 4(1)A' PES to the lower 3(1)A' PES of CO2via the seam of conical intersections at a near linear OCO configuration, followed by the direct dissociation on the 3(1)A' PES. The theoretical PES calculations are consistent with the experimental observation of prompt CO2 dissociation and high rotational and vibrational excitations for CO(X(1)Σ(+)) photoproducts.

Rotationally resolved state-to-state photoionization and the photoelectron study of vanadium monocarbide and its cations (VC/VC(+)).

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(+)(X(3)Δ1; v(+) = 0; J(+) = 1) ← VC(X(2)Δ3/2; v'' = 0; J'' = 3/2) photoionization transition. The spectroscopic constants for VC(+)(X(3)Δ1) 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(+)(X(3)Δ1,2,3). 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: ΔH°(f0)(VC) = 835.2, ΔH°(f298)(VC) = 840.4, ΔH°(f0)(VC(+)) = 1522.8, and ΔH°(f298)(VC(+)) = 1528.0 kJ mol(-1).

Rotationally Selected and Resolved State-to-State Photoelectron Study of Vanadium Monoxide Cation VO(+)(X(3)Σ(-); v(+) = 0-3).

Vanadium monoxide cation VO(+)(X(3)Σ(-)) 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 X(3)Σ(-). 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(+)(X(3)Σ(-)), from which the equilibrium bond distance of VO(+)(X(3)Σ(-)) is determined to be re(+) = 1.557 ± 0.002 Å. This PFI-PE study covers the vibrational bands, VO(+)(X(3)Σ(-); v(+) = 0, 1, 2, and 3) ← VO(X(4)Σ(-); v″ = 0), which has made possible the determination of the vibrational constants for VO(+)(X(3)Σ(-)) 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(+)(X(3)Σ(-)) and that of VO(X(4)Σ(-)) to be D0(V(+)-O) - D0(V-O) = IE(V) - IE(VO) = -3967 ± 1 cm(-1).

Photodissociation of CO2 between 13.540 eV and 13.678 eV.

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 (1)Σ(+)) + O((1)D), CO(X (1)Σ(+)) + O((1)S), CO(a (3)Π) + O((3)P), CO(a (3)Π) + O((1)D), CO(a' (3)Σ(+)) + O((3)P), CO(d (3)Δ) + O((3)P) and CO(e (3)Σ(-)) + O((3)P), whereas no CO(X (1)Σ(+)) + O((3)P) 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.