Lynmarie A. Posey

Pulsed photoelectron spectroscopy of negative cluster ions: Isolation of three distinguishable forms of N2O−2

Three different ionic species with stoichiometry N2O−2 are generated by varying the neutral precursors in an electron beam ionized free jet expansion. In each case, the ion is isolated by mass spectrometry and then probed using pulsed photoelectron spectroscopy (PES) at 532 and 355 nm (2.330 and 3.495 eV, respectively). The neutral starting materials used in the three preparations are (I) O2 seeded 5% in N2, (II) pure N2O, and (III) NO seeded 10% in Ar. Based on their PES and photofragmentation properties, the three species appear to be best described as (I) O−2 ⋅N2, (II) either O−⋅N2O or more likely a chemically bound species, and (III) NO−⋅NO. It is likely that two of these species are trapped intermediates in the O−+N2O→NO−+NO reaction, suggesting a double minimum potential energy surface. The formation mechanisms of these ions in our source are discussed in the context of previous preparation schemes.

Partial control of an ion-molecule reaction by selection of the internal motion of the polyatomic reagent ion.

The ion-molecule reaction NH3+ + ND3 has been studied at various collision energies (1 to 5 electron volts in the center of mass) with preparation of the NH3+ reagent in two nearly isoenergetic vibrational states. One state corresponds to pure out-of-plane bending of the planar NH3+ ion (0.60 electron volts), whereas the other state is a combination of in-plane and out-of-plane motion (0.63 electron volts). The product branching ratios differ markedly for these two vibrational-state preparations. The differences in reactivity suggest that the in-plane totally symmetric stretching mode is essentially inactive in controlling the branching ratio of this reaction.

Controlling the internal energy content of size‐selected cluster ions: An experimental comparison of the metastable decay rate and photofragmentation methods of quantifying the internal excitation of (H2O)−n

The metastable decay rates of (H2O)−n clusters are found to be strongly dependent on source conditions and are correlated to the distribution of photofragments. We relate these variations to changes in the internal energy content of the clusters and analyze the effects to provide two independent determinations of the internal energy variation. The two methods are found to be in surprisingly good agreement. Measurements on larger clusters are carried out using photofragmentation, and changes on the order of 0.30 eV are easily affected by changing the pressure of argon backing the expansion. An interesting result of this work is that the cooling efficiency increases with cluster size.

On the origin of the competition between photofragmentation and photodetachment in hydrated electron clusters, (H2O)n -

Photoexcitation of size‐selected hydrated electron clusters, (H2O)−n , in the near IR results in a competition between photofragmentation and electron photodetachment. To investigate the origin of this competition, the decay probability into ionic fragments for the n=25 cluster was measured as a function of photon energy from 0.91≤hν≤3.49 eV. The photofragmentation probability increases rapidly with decreasing excitation energy in the general vicinity of the vertical detachment energy of this cluster (1.4 eV) determined via photoelectron spectroscopy. This result suggests that fragmentation accompanies photoexcitation of the excess electron with near zero kinetic energy. Thus, photofragmentation appears to proceed through an optically prepared intermediate similar to that reached in electron scattering from neutral clusters, which displays an enhanced dissociative attachment pathway with near zero kinetic energy electrons.

Influence of vibrational excitation and collision energy on the ion‐molecule reaction NH+3(ν2)+ND3

The influence of vibrational excitation and collision energy on the ion‐molecule reaction NH+3(ν2)+ND3 has been investigated using a recently constructed quadrupole‐octopole‐quadrupole mass spectrometer. The NH+3 reagent ions are prepared state selectively with 0–7 quanta in the ν2 umbrella bending mode by (2+1) resonance enhanced multiphoton ionization through the B or C’ Rydberg states of ammonia. Reactive collisions between the mass‐filtered ion beam and a thermal distribution of neutral reagent molecules occur with controlled collision energies (0.5–10.0 eV center of mass) within the octopole ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NH+3 with ND3 has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. Each of these channels exhibits a strong dependence on ion vibrational excitation and collision energy. Product branching ratios and relative cross sections are reported and compared with prev...

Electrospray ionization of divalent transition metal ion bipyridine complexes: spectroscopic evidence for preparation of solution analogs in the gas phase

Abstract Laser photofragmentation mass spectrometry provides the first spectroscopic evidence that electrospray ionization (ESI) produces gas-phase clusters containing transition metal ion complexes analogous to those found in solution. In solution, the tris(2,2′-bipyridyl)iron(II) and ruthenium(II) coordination complexes exhibit strong metal-to-ligand charge transfer (MLCT) absorption bands in the visible. The characteristic MLCT absorption bands are intact in gas-phase methanolic clusters containing these complexes, indicating that the metal oxidation state is preserved during ESI. Comparison of the ionic species produced by ESI and other ionization/volatilization techniques offers some insight on stabilization of the +2 charge in these gas-phase systems.

The angular distribution of photoelectrons ejected from the hydrated electron cluster (H2O)−18

The angular distribution of the photoelectrons ejected from the hydrated electron cluster (H2 O)−18 was measured to determine the spatial character of the orbital in which the excess electron resides. The asymmetry parameter β was determined to be 0.92±0.1, consistent with a roughly spherical orbital, similar to that of the hydrated electron in solution.

Guided ion beam measurement of the product branching ratios for the ion‐molecule reaction N++O2 as a function of collision energy

A quadrupole‐octopole‐quadrupole mass spectrometer has been constructed for comparing ion‐molecule reaction product intensities as both the internal excitation and the kinetic energy of the reactant ion are varied. Such comparisons require an ion beam with a known kinetic energy distribution and, most importantly, they require product intensity measurements made without significant bias in detection of the different product channels. To assess the characteristics of our instrument, we have studied the ion‐molecule reaction N++O2 that is known to yield three different product channels: N+O+2, NO++O, and NO+O+. Ion beam trajectory simulations combined with experimental measurements show that the spread in the kinetic energy of the reagent ions has a fixed value in the range of 0.6 to 1.1 eV full width at half maximum in the center of mass (c.m.). Relative cross sections for the three different product channels are reported as a function of c.m. collision energy. A comparison of the observed product branchin...

Ligand-field photochemistry of the [CuII(bpy)(serine–H)]+· complex in the gas phase

Abstract Photochemistry resulting from excitation of a d–d transition at 575 nm between ligand-field states in the gas-phase [CuII(bpy)(serine–H)]+ · complex generated by electrospray ionization is reported. These results are discussed in terms of a reaction scheme proposed by Turecek and co-workers [C.L. Gatlin, F. Turecek, T. Vaisar, J. Mass Spectrom. 30 (1995) 1617] for collisionally activated dissociation of this complex at low center-of-mass collision energies under multiple-collision conditions. Three photofragment ions are observed, corresponding to elimination of formaldehyde, stepwise loss of formaldehyde followed by ·OC(OH)CHNH2, and transfer of a hydroxyl group to the CuII ion accompanied by loss of H2CCHNH2CO2. A fourth fragmentation channel reported by Turecek and co-workers corresponding to decarboxylation and associated dehydration is also observed at moderate center-of-mass collision energies under single-collision conditions but is absent in the laser photofragmentation difference mass spectrum resulting from excitation of [CuII(bpy)(serine–H)]+ · at 575 nm.

Investigation of the roles of vibrational excitation and collision energy in the ion-molecule reaction NH3+(v2) + ND3

The influence of vibrational excitation and collision energy on the reaction NH3+((nu) 2) + ND3 has been investigated using a quadrupole-octopole- quadrupole mass spectrometer. The NH3+ reagent ions are prepared state- selectively with 0 - 7 quanta in the (nu) 2 umbrella bending mode by (2 + 1) resonance enhanced multiphoton ionization. The mass-filtered reagent ion beam interacts with a thermal distribution of neutral ND3 molecules at controlled center-of-mass collision energies (0.5 - 10.0 eV) within the octopole ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NH3+ with ND3 has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. The product branching ratios and relative cross sections for each of these channels exhibit strong dependences on ion vibrational excitation and collision energy. Briefly, both deuterium abstraction and charge transfer are enhanced by vibrational excitation, whereas proton transfer is suppressed. As the collision energy is increased, the branching fraction for charge transfer increases sharply while proton transfer decreases. The branching ratio for deuterium abstraction does not exhibit a significant dependence on collision energy. The influence of ion vibrational excitation is discussed in terms of its relationship to the reaction coordinates for the three product channels. The behavior of this reaction points to a short-lived collision complex in which vibration and translation play inequivalent roles.