Felician Muntean

Guided ion beam study of collision-induced dissociation dynamics: integral and differential cross sections

The low energy collision-induced dissociation (CID) of Cr(CO)6+ with Xe is investigated using a recently modified guided ion beam tandem mass spectrometer, in the energy range from 0 to 5 eV in the center-of-mass (CM) frame. The additions to the instrument, updated with a double octopole system, and the new experimental methods available are described in detail. Integral cross sections for product formation are presented and analyzed using our standard modeling procedure. A slightly revised value for the bond dissociation energy of (CO)5Cr+–CO of 1.43±0.09 eV is obtained, in very good agreement with literature values. Axial and radial velocity distributions for primary and product ions are measured at 1.3, 2.0, and 2.7 eV, in the threshold region for product formation. The resulting velocity scattering maps are presented and discussed. Evidence of efficient energy transfer is observed from angular scattering of CID products. Experimental distributions of residual kinetic energies are derived and extend to...

Femtosecond study of Cu(H2O) dynamics

The short-time nuclear dynamics of Cu(H(2)O) is investigated using femtosecond photodetachment-photoionization spectroscopy and time-dependent quantum wave packet calculations. The Cu(H(2)O) dynamics is initiated in the electronic ground state of the complex by electron photodetachment from the Cu(-)(H(2)O) complex, where hydrogen atoms are oriented toward Cu. Several time-resolved resonant multiphoton ionization schemes are used to probe the ensuing reorientation and dissociation. Immediately following photodetachment, the neutral complex is far from its minimum energy geometry and possesses an internal energy comparable to the Cu-H(2)O dissociation energy and undergoes both large-amplitude H(2)O motion and dissociation. Dissociation is observed to occur on three distinct time scales: 0.6, 8, and 100 ps. These results are compared to the results of time-dependent J=0 wave packet calculations, propagating the initial anion vibrational wave functions on the ground-state potential of the neutral complex. An excellent agreement is obtained between the experimental results and the ionization signals derived from the calculated probability amplitudes. Related experiments and calculations are carried out on the Cu(D(2)O) complex, with results very similar to those of Cu(H(2)O).

A theoretical and computational study of the anion, neutral, and cation Cu(H2O) complexes

An ab initio investigation of the potential energy surfaces and vibrational energies and wave functions of the anion, neutral, and cation Cu(H(2)O) complexes is presented. The equilibrium geometries and harmonic frequencies of the three charge states of Cu(H(2)O) are calculated at the MP2 level of theory. CCSD(T) calculations predict a vertical electron detachment energy for the anion complex of 1.65 eV and a vertical ionization potential for the neutral complex of 6.27 eV. Potential energy surfaces are calculated for the three charge states of the copper-water complexes. These potential energy surfaces are used in variational calculations of the vibrational wave functions and energies and from these, the dissociation energies D(0) of the anion, neutral, and cation charge states of Cu(H(2)O) are predicted to be 0.39, 0.16, and 1.74 eV, respectively. In addition, the vertical excitation energies, that correspond to the 4 (2)P<--4 (2)S transition of the copper atom, and ionization potentials of the neutral Cu(H(2)O) are calculated over a range of Cu(H(2)O) configurations. In hydrogen-bonded, Cu-HOH configurations, the vertical excitation and ionization energies are blueshifted with respect to the corresponding values for atomic copper, and in Cu-OH(2) configurations where the copper atom is located near the oxygen end of water, both quantities are redshifted.

Low energy collision-induced dissociation and photodissociation studies of the (N2O,H2O)+ cluster ion

Low energy collision-induced dissociation (CID) and photodissociation measurements of monohydrated nitrous oxide cluster ions are presented. The CID measurements have been conducted with ions produced in both thermal and supersonic jet sources, and with both Ne and Ar as collision gases. In all experiments, H2O+, N2O+, and N2OH+ fragments are observed, for which CID thresholds (0 K) of 1.04±0.06, 1.43±0.12 and 1.32±0.10 eV are determined, respectively. The thermal source experimental thresholds are consistent with all fragment ions originating from a single isomeric precursor ion, [N2O⋅H2O]+. Whereas both N2O+ and N2OH+ CID curves are comparable in the thermal source and supersonic jet source experiments, considerable differences are observed in the H2O+ CID measurements. The differences are attributed to loosely bound cluster-ion isomeric forms produced in the jet source experiment. In the photodissociation experiments, branching ratios measured with the present jet source are very similar to those obser...

Modeling kinetic shifts in threshold collision-induced dissociation. Case study: Dichlorobenzene cation dissociation

A threshold collision-induced dissociation (CID) study is performed on dichlorobenzene cation dissociation of both the ortho and para isomers. Ab initio calculations are performed on the system to investigate the details of the potential energy surface with respect to Cl atom loss and to provide the molecular parameters necessary for CID cross section modeling. The effects of kinetic shifts on the CID threshold determinations are investigated using a model that incorporates statistical unimolecular decay theory. The model is tested using unimolecular dissociation rate constants as a function of energy provided by earlier photoelectron–photoion-coincidence (PEPICO) experiments. The different possible sets of parameters involved in the CID model, their effect on the dissociation rates, and their effect on the final CID threshold determination are discussed. A tight transition state is observed to reproduce the experimental dissociation rates better than a phase-space limit loose transition state, a result a...

Femtosecond dynamics of Cu(H2O)2

The ultrafast relaxation dynamics of Cu(H(2)O)(2) is investigated using femtosecond photodetachment-photoionization spectroscopy. In addition, stationary points on the Cu(H(2)O)(2) anion, neutral, and cation potential energy surfaces are characterized by ab initio electronic structure calculations. Electron photodetachment from Cu(-)(H(2)O)(2) initiates the dynamics on the ground-state potential energy surface of neutral Cu(H(2)O)(2). The resulting Cu(H(2)O)(2) complexes experience large-amplitude H(2)O reorientation and dissociation. The time evolution of the Cu(H(2)O)(2) fragmentation products is monitored by time-resolved resonant multiphoton ionization. The parent ion, Cu(+)(H(2)O)(2), is not detected above background levels. The rise to a maximum of the Cu(+) signal from Cu(-)(H(2)O)(2), and the decay of the Cu(+)(H(2)O) signal from Cu(-)(H(2)O)(2) have similar tau approximately 10 ps time dependences to the corresponding signals from Cu(-)(H(2)O), but display clear differences at very short and long times. The experimental observations can be understood in terms of the following picture. Prompt dissociation of H(2)O from nascent Cu(H(2)O)(2) gives rise to a vibrationally excited Cu(H(2)O) complex, which dissociates to Cu+H(2)O due to coupling of H(2)O internal rotation to the dissociation coordinate. This prompt dissociation removes all intra-H(2)O vibrational excitation from the intermediate Cu(H(2)O) fragment, which quenches the long time vibrational predissociation to Cu+H(2)O previously observed in analogous experiments on Cu(-)(H(2)O).

Reaction of Cu+ with dimethoxyethane: competition between association and multiple dissociation channels.

The reaction of Cu+ with dimethoxyethane (DXE) is studied using kinetic-energy dependent guided ion beam mass spectrometry. The bimolecular reaction forms an associative Cu(+)(DXE) complex that is long-lived and dissociates into several competitive channels: C4H9O2(+)+CuH, Cu(+)(C3H6O)+CH3OH, back to reactants, and other minor channels. The kinetic-energy dependences of the cross sections for the three largest product channels are interpreted with several different models (including rigorous phase space theory) to yield 0 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, Doppler broadening, and dissociation lifetimes. These values are compared with bond energies obtained from collision-induced dissociation (CID) studies of the Cu(+)(DXE) complex and found to be self-consistent. Although all models provide reasonable thermochemistry, phase space theory reproduces the details of the cross sections most accurately. We also examine the dynamics of this reaction using time-of-flight methods and a retarding potential analysis. This provides additional insight into the unimolecular decay of the long-lived Cu(+)(DXE) association complex. Comparison of results from this study with those from the complementary CID study, thus forming the same energized Cu(+)(DXE) complex in two distinct ways, allows an assessment of the models used to interpret CID thresholds.

Collision-induced dissociation dynamics of the [OCS · C2H2]+ complex. A combined experimental and theoretical study

Collision-induced dissociation (CID) of the [OCS · C2H2]+ complex ion with both Xe and Ar over an energy range of 0 to 10 eV in the center of mass frame is studied using a guided ion beam tandem mass spectrometer. The cross sections of the ionic products observed (C2H2S+, OCS+, C2H2+, and S+) are analyzed by taking into account reactant energy distributions, multiple collisions, lifetime effects and competition. A recently devised statistical model for the simultaneous analysis of competitive product channels is used to analyze three channels for the first time, with good results. Thresholds for product formation at 0 Kelvin are 0.33±0.07 eV for C2H2S+, 0.95±0.07 eV for OCS+, 1.22±0.08 eV for C2H2+, and an upper limit of 4.26 eV for S+. These results are comparable to available literature thermochemical data within experimental errors. Competitive shifts are significant, about 0.3 eV for both OCS+ and C2H2+. Ab initio calculations at the QCISD/6-311+G**//MP2/6-311+G** and CCSD//6-31G*//CCD/6-31G* levels are performed on the system. The reaction coordinates of the potential energy surface of the system is quantitatively mapped using results from CID and ab initio calculations. The identity of the C2H2S+ product is suggested to be the cyclic ethylene sulfide cation on the basis of the results of calculations and previous kinetic energy release measurements. Product branching ratios as a function of energy are analyzed and compared to those determined in previous photodissociation and bimolecular reaction experiments.