Dale J. Levandier

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(+).

Determination of photofragment ion translational energy and angular distributions in an octopole ion guide: A case study of the Ar2+ and (N2O⋅H2O)+ cluster ions

The first measurement of ion photodissociation product recoil velocity and angular anisotropy in an octopole ion guide are presented. The experimental and numerical procedures required to obtain photofragment ion translational energy and angular distributions are discussed. Cluster ions are photodissociated in an octopole ion guide, and photofragment ion velocity distributions are measured using time-of-flight (TOF). The instrumental discrimination function is determined using guiding field variation (VAR). A validation study using the 2Σg+←2Σu+ transition of Ar2+ probed at 300 nm and a photodissociation dynamics study of (N2O⋅H2O)+ to form N2OH++OH, N2O++H2O, and H2O++N2O in the 458–657 nm range are presented. The H2O+ and N2O+ photofragment translational energy and angular distributions are derived, and new information regarding the photodissociation of the (N2O⋅H2O)+ cluster ion is obtained.

Collision-induced dissociation dynamics of Ar2+ at high levels of vibrational excitation

The collision-induced dissociation (CID) dynamics of the Ar2++Ar collision system are investigated at different Ar2+ internal energy distributions in a guided-ion beam (GIB) apparatus. The internal energy of reactant ions, assumed vibrational in a first approximation, is controlled by varying the position of ionization in a supersonic jet, electron impact ion source. Three conditions are investigated: cold, in which the ions are produced as vibrationally relaxed as possible; intermediate, in which a substantial shift in the CID onset is observed; hot, in which the apparent CID threshold is at near thermal collision energies. The vibrational distribution of the Ar2+ ions is probed at the same conditions by measuring the kinetic energy release of photofragment Ar+ following 2Σg+←2Σu+ photodissociation. The derived internal energy distributions are then used to model the observed CID cross sections with a modified line-of-centers approach to assess vibrational effects in the single-collision cross sections. ...

Reactions of O+ with CnH2n+2, n=2–4: A guided-ion beam study

We have measured absolute reaction cross sections for the interaction of O(+) with ethane, propane, and n-butane at collision energies in the range from near thermal to approximately 20 eV, using the guided-ion beam (GIB) technique. We have also measured product recoil velocity distributions using the GIB time-of-flight (TOF) technique for several product ions at a series of collision energies. The total cross sections for each alkane are in excess of 100 A(2) at energies below approximately 2 eV, and in each case several ionic products arise. The large cross sections suggest reactions that are dominated by large impact parameter collisions, as is consistent with a scenario in which the many products derive from a near-resonant, dissociative charge-transfer process that leads to several fragmentation pathways. The recoil velocities, which indicate product ions with largely thermal velocity distributions, support this picture. Several product ions, most notably the C(2)H(3) (+) fragment for each of the alkanes, exhibit enhanced reaction efficiency as collision energy increases, which can be largely attributed to endothermic channels within the dissociative charge-transfer mechanism.

Deuterium isotope effects in collision-induced dissociation and photodissociation of the (N2O,H2O)+ cluster ion

We present low energy collision-induced dissociation (CID) and visible photodissociation (PD) data of jet-cooled (N2O,H2O/D2O)+ cluster ions. The same three fragment ions are observed in both CID and PD, namely, H2O+/D2O+, N2O+, and N2OH+/N2OD+. The H2O+ and N2O+ product channels exhibit a small deuterium isotope effect in both CID (10%) and PD (15%). The N2OH+ product channel, on the other hand, exhibits a much larger deuterium isotope effect for CID (30%) and PD (230%). The large difference in deuterium isotope effects observed in the N2OH+ product channel for CID and PD suggests that this product is formed via collision-induced and photoinduced proton-transfer reactions. These results are consistent with the interpretation that a single isomeric form, namely a (N2O⋅H2O)+ association complex, predominates. No evidence is found supporting the existence of a proton-transfer complex, i.e., N2OH+⋅OH. Evidence for the formation of minor amounts of a weakly bound isomeric form with the charge localized on the...

An ab initio and experimental study of vibrational effects in low energy O++C2H2 charge-transfer collisions

Theoretical and experimental studies are performed to elucidate the low energy charge-transfer dynamics of the reaction, O+(4S)+C2H2(X 1Σg+)→O+C2H2+. In particular, the role of the low-frequency acetylene bending modes (612 and 730 cm−1) in promoting charge transfer was examined. High-temperature guided-ion beam measurements are carried out over the energy range from near-thermal to 3 eV at 310 and 610 K. The charge-transfer cross sections are found to decrease up to 0.5 eV, to have a constant value at intermediate energies between 0.5 and 1.5 eV, and then to dramatically increase above a threshold of a spin-allowed process determined to be at 1.7 eV. A bending vibrational enhancement of ∼8 is observed at intermediate energies. Thermal energy rate co-efficients are measured in a variable temperature-selected ion flow drift tube apparatus from 193 to 500 K. At each temperature, a negative energy dependence is observed. In order to elucidate the reaction mechanism in detail, high level ab initio calculation...

Charge transfer and chemical reaction dynamics in hyperthermal O++NO collisions

The O+(4S)+NO(X 2Πr) collision system has been investigated in a guided-ion beam experiment. Absolute cross sections for production of NO+ and O2+ have been measured at collision energies from near thermal to ∼12 eV. Time-of-flight measurements of the NO+ product recoil velocities, at collision energies of 1.3, 3.3, 5.2, and 9.1 eV confirm two hyperthermal charge transfer channels with thresholds at 0.2 and 2.0 eV. A kinematic analysis indicates mechanisms that give rise to spin-forbidden NO+(X 1Σ+)+O(1D) and endothermic NO+(a 3Σ+)+O(3P) products, respectively. The O2++N channel is observed to have a threshold of 0.7±0.1 eV. Time-of-flight studies of the O2+ product at collision energies of 3.3 and 5.9 eV indicate a complex-mediated mechanism.

GUIDED-ION BEAM STUDY OF THE O2++C2H2 CHARGE-TRANSFER AND CHEMICAL REACTION CHANNELS

Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O2+(2Πg)+C2H2 reaction over the center-of-mass collision energy range of 0.05–12 eV. The reaction exhibits a strong charge-transfer (CT) channel and several weak chemical reaction channels leading to the following product ions: C2H2O+, CH2+, COH+/HCO+, CO+, CH+, and C+. The magnitude of the charge-transfer cross section is comparable to the capture cross section at low collision energies. The charge-transfer products are characterized predominantly by a direct, long-range mechanism that is accompanied by smaller impact parameter collisions involving longer-lived complexes, whose lifetimes are several times shorter than the complex rotational period. The TOF analysis indicates that the complexes also decay to chemical reaction products at higher energies. A density functional survey of intermediates is conducted. The chemical reaction products are postulated to be formed via an endothermic transition state...

Collision energy dependence and product recoil velocity analysis of O+(4S)+C2H2 charge-transfer and chemical reaction channels

Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O+(4S)+C2H2 reaction over the center-of-mass collision energy range of 0.05–18 eV. Despite a large number of exothermic channels, the total reaction cross section at low energies is more than two orders of magnitude smaller than the capture cross section. A common energy onset for charge-transfer, CH+, and COH+/HCO+ products is observed at 1.70±0.10 eV, above which the total cross section for these channels rapidly rises with energy, eventually exceeding 5 A2. Above 4 eV, the C2H+ and CO+ products also become significant, and weaker channels producing C+, C2+, and CH2O+ are also identified. The C2H+ fragment is interpreted as being primarily a dissociative charge transfer (CT) channel at collision energies above ∼3.79 eV, while the threshold for forming CO+ can be associated with a CO++H+CH channel. The TOF measurements demonstrate that efficient C2H2+ A state formation occurs above ∼2.7 eV. The onset at ...