Laser Pulsed Field Ionization-photoion Research Articles

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.

Quantum-vibrational-state-selected Integral Cross Sections and Product Branching Ratios for the Ion-molecule Reactions of N2 +(X2Σg +; v+ = 0–2) + H2O and H2O+(X2B1: v1 +v2 +v3 + = 000 and 100) + N2 in the Collision Energy Range of 0.04–10.00 eV

Abstract By combining the vacuum ultraviolet laser pulsed field ionization-photoion (VUV-PFI-PI) ion source with the double quadruple-double octopole (DQDO) ion-guided mass spectrometer, we have investigated the center-of-mass collision energy (E cm) and vibrational-state dependences of the ion-molecule reactions of and and 100) + N2 covering the E cm range of 0.04–10.00 eV. The absolute integral cross sections (σ’s) for the charge transfer (CT) [σ CT(v +)] channel to form H2O+ and the H-atom transfer (HT) [σ HT(v +)] channel to form N2H+ from the reactions have been determined, revealing the dominance of σ CT(v +) over σ HT(v +) at E cm = 0.04–8.00 eV. The E cm dependence of σ CT(v +) at low E cm &lt; 1.00 eV is consistent with the long-range ion-dipole and ion-induced dipole CT mechanism. Minor vibrational inhibition is observed for the σ CT(v +) at low E cm ≤ 0.30 eV, which can be rationalized by the near-resonance CT mechanism. While the σ HT(v +) values are consistent with previous measurements, the σ CT(v +) obtained here resolve a hump at E cm = 1.0–5.0 eV, which is not observed previously. This feature is attributed to the formation of excited H2O+(B 2 B 2) ions via the collision-assisted CT mechanism. The branching ratio for product H2O+[BR(H2O+)] is found to be constant (0.82 ± 0.05) at E cm = 0.04–1.00 eV, and is independent of v + vibrational state. As E cm is increased from 1.0 eV, the BR(H2O+) reaches a maximum of 0.93 at E cm ≈ 3.00 eV, followed by the decline to 0.20 at E cm ≥ 9.0 eV, where σ HT(v +) becomes dominant compared to σ CT(v +). The ) for the formation of N2H+ via the proton transfer (PT) channel of the H2O+(X 2 B 1: 000 and 100) + N2 reaction has also been measured. The comparison of the σ PT(000 and 100) values reveals significant (100) vibrational enhancement. Furthermore, the E cm thresholds determined here for σ PT(000 and 100) are in agreement with their thermochemical thresholds. The BR and σ values determined here are valuable for modeling the ion chemistry occurring in planetary atmospheres, in addition to serving as benchmarks for state-of-the-art quantum dynamics calculations.

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.

ABSOLUTE INTEGRAL CROSS SECTIONS FOR THE STATE-SELECTED ION–MOLECULE REACTION ; v+ = 0–2) + C2H2 IN THE COLLISION ENERGY RANGE OF 0.03–10.00 eV

ABSTRACT Using the vacuum ultraviolet laser pulsed field ionization-photoion source, together with the double-quadrupole–double-octopole mass spectrometer developed in our laboratory, we have investigated the state-selected ion–molecule reaction ; v + = 0–2, N+ = 0–9) + C2H2, achieving high internal-state selectivity and high kinetic energy resolution for reactant ions. The charge transfer (CT) and hydrogen-atom transfer (HT) channels, which lead to the respective formation of product and N2H+ ions, are observed. The vibrationally selected absolute integral cross sections for the CT [σ CT(v +)] and HT [[σ HT(v +)] channels obtained in the center-of-mass collision energy (E cm) range of 0.03–10.00 eV reveal opposite E cm dependences. The σ CT(v +) is found to increase as E cm is decreased, and is consistent with the long-range exothermic CT mechanism, whereas the E cm enhancement observed for the σ HT(v +) suggests effective coupling of kinetic energy to internal energy, enhancing the formation of N2H+. The σ HT(v +) curve exhibits a step at E cm = 0.70–1.00 eV, suggesting the involvement of the excited state in the HT reaction. Contrary to the strong E cm dependences for σ CT(v +) and σ HT(v +), the effect of vibrational excitation of on both the CT and HT channels is marginal. The branching ratios and cross sections for the CT and HT channels determined in the present study are useful for modeling the atmospheric compositions of Saturn's largest moon, Titan. These cross sections and branching ratios are also valuable for benchmarking theoretical calculations on chemical dynamics of the titled reaction.

The translational, rotational, and vibrational energy effects on the chemical reactivity of water cation H2O+(X 2B1) in the collision with deuterium molecule D2

By employing the newly established vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) double quadrupole-double octopole ion guide apparatus, we have examined the translational, rotational, and vibrational energy effects on the chemical reactivity of water cation H2O(+)(X(2)B1) in the collision with deuterium molecule D2. The application of a novel electric-field pulsing scheme to the VUV laser PFI-PI ion source has enabled the preparation of a rovibrationally selected H2O(+)(X(2)B1; v1 (+)v2 (+)v3 (+); N(+) K a+Kc+) ion beam with not only high internal-state selectivity and high intensity but also high translational energy resolution. Despite the unfavorable Franck-Condon factors, we are able to prepare the excited vibrational states (v1 (+)v2 (+)v3 (+))=(100) and (020) along with the (000) ground vibrational state, for collisional studies, where v1 (+), v2 (+), and v3 (+) represent the symmetric stretching, bending, and asymmetric stretching modes of H2O(+)(X(2)B1). We show that a range of rotational levels from N(+) K a+Kc+ = 000 to 322, covering a rotational energy range of 0-200 cm(-1) of these vibrational states, can also be generated for absolute integral cross section (σ) measurements at center-of-mass collision energies (Ecms) from thermal energies to 10.00 eV. The Ecm dependences of the σ values are consistent with the prediction of the orbiting model, indicating that translational energy significantly hinders the chemical reactivity of H2O(+)(X(2)B1). Rotational enhancements are observed at Ecm < 0.30 eV for all the three vibrational states, (000), (100), and (020). While the σ values for (100) are found to be only slightly below those for (000), the σ values for (020) are lower than those for (000) and (100) by up to 20% at Ecm ≤ 0.20 eV, indicative of vibrational inhibition at low Ecm by excitation of the (020) mode. Rationalizations are proposed for the observed rotational enhancements and the bending vibrational inhibition. Rigorous theoretical calculations are needed to interpret the wealth of rovibrationally selected cross sections obtained in the present study.

ABSOLUTE INTEGRAL CROSS SECTIONS AND PRODUCT BRANCHING RATIOS FOR THE VIBRATIONALLY SELECTED ION-MOLECULE REACTIONS: N$_{2}^{+}$(X2Σ$_{\rm g}^{+}$;v+= 0-2) + CH4

Absolute vibrationally selected integral cross sections (σ v +'s) for the ion-molecule reaction N(X 2Σ; v + = 0-2) + CH4 have been measured by using the newly developed vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) double-quadrupole-double-octopole ion guide apparatus. By employing a novel electric field pulsing scheme to the VUV laser PFI-PI source, we have been able to prepare reactant N ions in single-vibrational quantum states with not only high intensity and high purity but also high kinetic energy resolution, allowing integral cross section measurements to be conducted in the center-of-mass kinetic energies (E cm's) from 0.05 to 10.00 eV. Three primary product channels corresponding to the formations of CH, CH, and N2H+ were identified. After correcting for the secondary reactions involving CH and CH, we have determined the σ v + values of the formation of these primary product ions, σv+(CH), σv+(CH), and σv+(N2H+), and their branching ratios, [σv+(CH): σv+(CH): σv+(N2H+)]/σv+(CH + CH + N2H+), v + = 0-2, in the E cm range of 0.05-10.00 eV, where σv+(CH + CH + N2H+) = σv+(CH) + σv+(CH) + σv+(N2H+). The branching ratios are found to be nearly independent of the v + state and E cm. Complex v +-state and E cm dependences for σv+(CH), σv+(CH), and σv+(N2H+) along with vibrational inhibition for the formation of these product ions are observed. The vibrational effects on the σv+ values are sufficiently large to warrant the inclusion of the vibrationally excited reactions N(X 2Σ; v + ≥ 1) + CH4 for a more realistic modeling of the ion and neutral densities observed in the atmosphere of Titan. The cross-sectional data obtained in the present study are also useful for benchmarking theoretical calculations on ion-neutral collision dynamics.

Communication: Rovibrationally selected absolute total cross sections for the reaction H2O+(X2B1; v1+v2+v3+ = 000; N+Ka+Kc+) + D2: Observation of the rotational enhancement effect

By employing the newly established vacuum ultraviolet laser pulsed field ionization-photoion (PFI-PI) double quadrupole-double octopole ion guide apparatus, we have measured the rovibrationally selected absolute total cross sections of the ion-molecule reaction H(2)O(+)(X(2)B(1); v(1)(+)v(2)(+)v(3)(+) = 000; N(+)(Ka+Kc+)) + D(2) → H(2)DO(+) + D in the center-of-mass collision energy (E(cm)) range of 0.05-10.00 eV. The pulsing scheme used for the generation of PFI-PIs has made possible the preparation of reactant H(2)O(+)(X(2)B(1); v(1)(+)v(2)(+)v(3)(+) = 000) ions in single N(+)(Ka+Kc+) rotational levels with high kinetic energy resolutions. The absolute total cross sections observed in different N(+)(Ka+Kc+) levels with rotational energies in the range of 0-200 cm(-1) were found to exhibit a significant rotational enhancement on the reactivity for the titled reaction. In contrast, the measured cross sections reveal a decreasing trend with increasing E(cm), indicating that the rotational enhancement observed is not a total energy effect, but a dynamical effect. Furthermore, the rotational enhancement is found to be more pronounced as E(cm) is decreased. This experiment provided evidence that the coupling of the core rotational angular momentum with the orbital angular momentum could play a role in chemical reactivity, particularly at low E(cm).

Rovibrationally selected ion-molecule collision study using the molecular beam vacuum ultraviolet laser pulsed field ionization-photoion method: Charge transfer reaction of N2+(X 2Σg+; v+ = 0–2; N+ = 0–9) + Ar

We have developed an ion-molecule reaction apparatus for state-selected absolute total cross section measurements by implementing a high-resolution molecular beam vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) ion source to a double-quadrupole double-octopole ion-guide mass spectrometer. Using the total cross section measurement of the state-selected N(2)(+)(v(+), N(+)) + Ar charge transfer (CT) reaction as an example, we describe in detail the design of the VUV laser PFI-PI ion source used, which has made possible the preparation of reactant N(2)(+)(X (2)Σ(g)(+), v(+) = 0-2, N(+) = 0-9) PFI-PIs with high quantum state purity, high intensity, and high kinetic energy resolution. The PFI-PIs and prompt ions produced in the ion source are shown to have different kinetic energies, allowing the clean rejection of prompt ions from the PFI-PI beam by applying a retarding potential barrier upstream of the PFI-PI source. By optimizing the width and amplitude of the pulsed electric fields employed to the VUV-PFI-PI source, we show that the reactant N(2)(+) PFI-PI beam can be formed with a laboratory kinetic energy resolution of ΔE(lab) = ± 50 meV. As a result, the total cross section measurement can be conducted at center-of-mass kinetic energies (E(cm)'s) down to thermal energies. Absolute total rovibrationally selected cross sections σ(v(+) = 0-2, N(+) = 0-9) for the N(2)(+)(X (2)Σ(g)(+); v(+) = 0-2, N(+) = 0-9) + Ar CT reaction have been measured in the E(cm) range of 0.04-10.0 eV, revealing strong vibrational enhancements and E(cm)-dependencies of σ(v(+) = 0-2, N(+) = 0-9). The thermochemical threshold at E(cm) = 0.179 eV for the formation of Ar(+) from N(2)(+)(X; v(+) = 0, N(+)) + Ar was observed by the measured σ(v(+) = 0), confirming the narrow ΔE(cm) spread achieved in the present study. The σ(v(+) = 0-2; N(+)) values obtained here are compared with previous experimental and theoretical results. The theoretical predictions calculated based on the Landau-Zener-Stückelberg formulism are found to be in fair agreement with the present measured σ(v(+) = 1 or 2; N(+)). Taking into account of the experimental uncertainties, the measured σ(v(+) = 1 or 2, N(+)) for N(+) = 0-9 at E(cm) = 0.04-10.0 eV are found to be independent of N(+).

Communication: Rovibrationally selected study of the N2+(X; v+ = 1, N+ = 0−8) + Ar charge transfer reaction using the vacuum ultraviolet laser pulsed field ionization-photoion method

By employing an electric field pulsing scheme for vacuum ultraviolet laser pulsed field ionization-photoion (PFI-PI) measurements, we have been able to prepare a rovibrationally selected PFI-PI beam of N(2)(+)(v(+) = 1, N(+)) with not only high intensity and high quantum state purity, but also high kinetic energy resolution, allowing absolute total cross sections [σ(v(+) = 1, N(+))] for the N(2)(+)(X; v(+) = 1, N(+)) + Ar, N(+) = 0-8 charge transfer reaction to be measured at center-of-mass collision energies (E(cm)) down to thermal energies. The σ(v(+) = 1, N(+) = 0-8) values determined at E(cm) = 0.04-10.00 eV are in good agreement with the theoretical predictions based on the Landau-Zener-Stückelberg formulism. Taking into account the experimental uncertainties, the σ(v(+) = 1, N(+)), N(+) = 0-8, measured at E(cm) = 1.56 eV are found to be independent of N(+).