Induced Dissociation Processes Research Articles

Electron-ion coincidence measurements of molecular dynamics with intense X-ray pulses

Molecules can sequentially absorb multiple photons when irradiated by an intense X-ray pulse from a free-electron laser. If the time delay between two photoabsorption events can be determined, this enables pump-probe experiments with a single X-ray pulse, where the absorption of the first photon induces electronic and nuclear dynamics that are probed by the absorption of the second photon. Here we show a realization of such a single-pulse X-ray pump-probe scheme on N_2 molecules, using the X-ray induced dissociation process as an internal clock that is read out via coincident detection of photoelectrons and fragment ions. By coincidence analysis of the kinetic energies of the ionic fragments and photoelectrons, the transition from a bound molecular dication to two isolated atomic ions is observed through the energy shift of the inner-shell electrons. Via ab-initio simulations, we are able to map characteristic features in the kinetic energy release and photoelectron spectrum to specific delay times between photoabsorptions. In contrast to previous studies where nuclear motions were typically revealed by measuring ion kinetics, our work shows that inner-shell photoelectron energies can also be sensitive probes of nuclear dynamics, which adds one more dimension to the study of light-matter interactions with X-ray pulses.

The Study of Titanium and Zirconium Ions in Water by MPT-LTQ Mass Spectrometry in Negative Mode.

Microwave plasma torches (MPTs) can be used as simple and low power-consumption ambient ion sources. When MPT-mass spectrometry (MPT-MS) is applied in the detection of some metal elements, the metallic ions exhibit some novel features which are significantly different with those obtained by the traditional inductively coupled plasma (ICP)-mass spectrometry (ICP-MS) and may be helpful for metal element analysis. As the representative elements of group IVA, titanium and zirconium are both of importance and value in modern industry, and they have impacts on human health. Here, we first provide a study on the complex anions of titanium and zirconium in water by using the MPT as ion source and a linear ion trap mass spectrometer (LTQ-MS). These complex anions were produced in the plasma flame by an aqueous solution flowing through the central tube of the MPT, and were introduced into the inlet of the mass spectrometry working in negative ion mode to get the feature mass spectrometric signals. Moreover, the feature fragment patterns of these ions in multi-step collision- induced dissociation processes have been explained. Under the optimized conditions, the limit of detection (LOD) using the MS2 (the second tandem mass spectrometry) procedure was estimated to be at the level of 10 μg/L for titanium and 20 μg/L for zirconium with linear dynamics ranges that cover at least two orders of magnitude, i.e., between 0–500 μg/L and 20–200 μg/L, respectively. These experimental data demonstrated that the MPT-MS is a promising and useful tool in field analysis of titanium and zirconium ions in water, and can be applied in many fields, such as environmental control, hydrogeology, and water quality inspection. In addition, MPT-MS could also be used as a supplement of ICP-MS for the rapid and on-site analysis of metal ions.

The dynamics of the Hg + Br2 reaction: elucidation of the reaction mechanism for the Br exchange reaction.

In spite of its importance in the Hg atmospheric chemistry, the dynamics of the Hg + Br2 → HgBr + Br reaction is poorly understood. In this article, we have carried out a comprehensive study of the reaction mechanism of this reaction by means of quasiclassical trajectories (QCTs) on an existing ab initio potential energy surface (PES). The reaction has a non trivial dynamics, as a consequence of its large endothermicity, the presence of a deep potential well, and the competition between the Br exchange and the collision induced dissociation processes. Our calculations demonstrate that insertion is only relevant at energies just above the reaction threshold and that, at energies above 2.3 eV, HgBr formation typically takes place via a sort of frustrated dissociation. In order to compare directly with the results obtained in extensive cross molecular beam experiments for the homologous reaction with I2, angular distributions in the laboratory frame for Hg + Br2 have been simulated under similar experimental conditions. The lack of agreement at the highest energies considered suggests that either the two reactions have substantially different mechanisms or that calculations on a single PES cannot account for the dynamics at those energies.

Electron capture induced dissociation of water embedded nucleotide anions

We have studied fragmentation of the hydrated nucleotide anion adenosine 5'-monophosphate after collisions with neutral sodium atoms. Regardless of the initial number (≤ 16) of water molecules attached to the ion, dianion formation always leads to water evaporation as well as loss of a single hydrogen atom, possibly from the adenine part. This damaging effect becomes more important as the number of initially attached water molecules increases, which is just the opposite to the protective behavior observed for collision induced dissociation processes without electron transfer.

Reactive and charge transfer processes in electronically inelastic collisions between ground state cadmium(II) diiodide molecules and lithium ions in the 0.10–4.00 keV energy range by crossed molecular beams

Charge transfer processes leading to different excited states of lithium atoms in the collision system CdI 2 ( 1 Σ g + ) + Li + ( 1 S g ) have been studied by crossed molecular beams techniques. Absolute emission cross-sections for the different exit channels have been measured as a function of the collision energy in the 0.10–4.00 keV energy range. Also the excitation function for the collision induced dissociation process leading to Cd (5 3P 1) has been obtained. Ab initio structure calculations performed at the second order Möller–Plesset level furnish a picture of the non-adiabatic effects for some of the electron transfers experimentally observed.

Adduct formation of steroids in APCI and its relation to structure identification

adducts could well be explained with the dissociation energies (DE) of the complexes [14]. The DE is related to the proton affinity (PA) of the constituent base by DE = a-b ΔPA where a and b are constants and ΔPA is the absolute PA difference [10]. Therefore it can be argued that the DE hypothesis used for the thermospray data can also be used for the explanation of the adduct formation in APCI. As the PA, and consequently the DE, of a molecule is dependent on its structure, the tendency of a molecule to form adduct ions can be used for structural identification purposes. A major problem with this theory is the presence of the collisionally induced dissociation processes occurring in the sampling region of the atmospheric pressure source (somewhat like the effect of the repeller in the thermospray interface) which are, ironically enough, meant to de-cluster the adduct ions. The following, limited study, discusses the data from the acetate adduct ions with steroid molecules (see Fig. 1 for a general structure and the numbering of the C atoms) in the negative ion mode. Abundant adduct ions for these type of molecules have been reported both with the thermospray [7] as with the HN [8,9] interface. These types of compounds are chosen because the MSMS data are considered hard to interpret [15]. That is hydrogen transfer

Ion-trap multiple mass spectrometry in pesticide analysis

Liquid chromatography pultiple mass spectrometry with atmospheric pressure ionisation and ion-trap instruments is a potentially useful technique for structure elucidation of polar pestcides and their degradation products. The major drawback today is the lack of spectral libraries and our limited knowledge on collision induced dissociation processes. The present paper gives an introduction to this area of research and presents interesting examples of fragmentation pathaways for protonated and deprotonated ions obtained by atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI).

Competitive charge transfer reactions in small [Mg(H2O)N]2+ clusters

Production of stable hydrated magnesium complexes of the general form [Mg(H2O)N]2+ (where 2⩽N⩽24) has been possible using the pick-up technique. Observations of ion intensities as a function of N together with data from collision induced dissociation processes (for ions in the range 3⩽N⩽10), indicates the existence of a closed solvation shell for N=6 to which additional water molecules are strongly bound. Collision-induced charge transfer in ions of all sizes yields solvated magnesium hydroxide ions Mg+OH(H2O)N−M−2 accompanied by the loss of a hydronium ion, H3O+, and M water molecules. For N=3, 4, and 5, the above process is seen to be in competition with charge transfer to unprotonated water, and clusters of the general form Mg(H2O)N−M+ are detected, where M now represents the total number of water molecules lost. These two separate loss channels are interpreted as being due to the presence of different structural (or transient) forms of those cluster ions where N⩽6. One structure corresponds to a highly symmetrical arrangement of the water molecules bonded directly to the magnesium dication, and is responsible for the formation of Mg(H2O)N−M+ ions by charge transfer. In the second type of structure, at least one water molecule moves to an outer solvation shell, but remains hydrogen bonded to a molecule in the first shell. In this latter configuration, it is suggested that the formation of a salt-bridge structure may lower the barrier to proton transfer and lead to the loss of a hydronium ion.

Resonant vibrational excitation and electron induced dissociation processes in HREELS studies of trimethylindium chemisorbed on GaAs(100)

Vibrational high resolution electron energy loss spectroscopy (HREELS) has been used to monitor the wide variety of electron induced processes involving excited electronic states for trimethylindium ((CH 3 ) 3 In) chemisorbed on GaAs(100). The vibrational modes of the adsorbed CH 3 groups undergo strong resonant vibrational excitation at 10 eV. HREEL spectra of the adsorbed layer, recorded with beam energies greater than 8 eV, indicate the formation of a surface CH 2 species characterised by a single deformation mode (δ(CH 2 )) at 1340 cm −1 . The conversion of CH 3 to CH 2 rises dramatically at the resonance energy of 10 eV and reaches a broad maximum between 20 and 30 eV. The implication is that the incident HREELS beam leads to decomposition of adsorbed CH 3 groups. It is suggested that resonant dissociative electron attachment and dipolar dissociation processes are responsible for the conversion of CH 3 to CH 2 at the surface.

Resonant vibrational excitation and electron induced dissociation processes in HREELS studies of trimethylindium chemisorbed on GaAs(100)

Vibrational high resolution electron energy loss spectroscopy (HREELS) has been used to monitor the wide variety of electron induced processes involving excited electronic states for trimethylindium ((CH 3) 3In) chemisorbed on GaAs(100). The vibrational modes of the adsorbed CH 3 groups undergo strong resonant vibrational excitation at 10 eV. HREEL spectra of the adsorbed layer, recorded with beam energies greater than 8 eV, indicate the formation of a surface CH 2 species characterised by a single deformation mode (δ(CH 2)) at 1340 cm −1. The conversion of CH 3 to CH 2 rises dramatically at the resonance energy of 10 eV and reaches a broad maximum between 20 and 30 eV. The implication is that the incident HREELS beam leads to decomposition of adsorbed CH 3 groups. It is suggested that resonant dissociative electron attachment and dipolar dissociation processes are responsible for the conversion of CH 3 to CH 2 at the surface.

Sputtered protons during surface-induced dissociation (SID) tandem mass spectrometry (MS/MS)

A new tandem mass spectrometer has been constructed for the study of ion/surface interactions and the study of surface induced dissociation processes. During the development and initial testing of this new instrument, experiments have been carried out using a stainless steel surface under high vacuum (10 −7-10 −8 Torr) conditions where hydrocarbons (pump oil) would be expected to be adsorbed onto the surface. A surprising result of these experiments has been the observation of sputtered protons. The yield of sputtered protons remains relatively constant with varying incident ion kinetic energy (over the 20–100 eV range) and appears to have little or no dependence on momentum transfer from the incident ions. These findings provide direct evidence (via observation of an excess of the charged intermediate protons) for a charge exchange mechanism for associative ion/surface reactions involving protonation. However, the yield of protonated products (protonated pyridazine ions) due to associative ion/surface reactions is dependent on incident ion kinetic energy. These results suggest that the protonation of pyridazine may depend on interaction times at the surface which, in turn, depend on the depth of penetration of the incident species into the hydrocarbon overlayer. In addition, the observation of sputtered protons as the likely intermediates to associative ion/surface protonation reactions may have implications for the proposed mechanisms of ion/surface reactions where alkylation of incident ions occurs.

Lowest ionization potentials of aluminum dimer

Potential curves for the lowest two electronic states, X 2Sigma(+)g and A 2Pi(u), of Al2(+) were computed using complete active space SCF/multireference CI wave functions and large Gaussian basis sets. The lowest observable vertical ionization potential, to Al2(+) X 2Sigma(+)g, of the Al2 X 3 Pi(u) ground state is calculated to occur around 6.1 eV, in excellent agreement with the experimental range of 6.0 to 6.42 eV obtained in recent cluster ionization studies by Cox and co-workers. The second vertical ionization potential, to Al2(+) A 2Pi(u), occurs near 6.4 eV, also within the experimental range. The adiabatic IP of 5.90 eV is in good agreement with the value of 5.8 to 6.1 eV deduced by Hanley and co-workers from the difference in thresholds between collision induced dissociation processes of Al3(+). The computed IP values are somewhat larger than those deduced from branching ratios in cluster fragmentation experiments by Jarrold and co-workers. The observation of an ionization threshold below 6.42 eV is shown to be incompatible with an Al2 ground electronic state assignment of 3Sigma(-)g, but the separation between the two lowest states of Al2 is so small that it is likely that both are populated in the experiments, so that this does not provide unambiguous support for the recent theoretical assignment of the ground state as 3Pi(u).

Vibrational nonequilibrium dissociation of Br2 in collisions with Ar and Br atoms

The collision induced dissociation processes in dilute mixtures of Br2–Ar and Br2–Br are investigated. Vibrational transition rates and dissociation rates from individual vibrational levels are derived from quasiclassical Monte Carlo trajectory calculations where only vibrational nonequilibrium effects are considered. The full transition rate matrix is completed by a moment analysis of the trajectory results. From the steady state solution of vibrational-level master equations, it is found that steady state populations in excited vibrational states are largely depleted but the nonequilibrium dissociation rates are about 30%–75% less than the equilibrium rates. The low activation energy observed in the previous experiments in the dissociation process, Br2+Br→Br+Br+Br cannot be explained by the present analysis.

Ion kinetic energy spectroscopy of the doubly charged ion of carbon monoxide

Spontaneous and collision-induced dissociation processes of CO2+ ions, formed by electron impact, have been studied in a double-focusing mass spectrometer using techniques of ion kinetic energy spectroscopy. The predissociation process, responsible for unimolecular dissociation of CO2+ on the microsecond time scale, is almost certainly electronically adiabatic tunneling through a potential barrier, though predissociation via electronic curve crossing cannot be entirely ruled out. Semiempirical potential curves for states of CO2+ were revised in order to better accommodate all of the available data, including Auger spectra, appearance energies, and kinetic energy release. Collision induced dissociation processes with Ar, N2, and H2 proceed via charge exchange, and involve predissociation of the D 2Π state by the C 2Δ state of CO+. When He is used as collision gas, the dissociation processes involving charge exchange are different, and require an energetic contribution from the relative kinetic energy (kinetic energy loss). In addition, He is quite different in inducing dissociation of CO2+ without prior charge exchange, from states of CO2+ up to 13 eV above the dissociation limit.

On the use of collision induced dissociation spectra in the determination of the structural composition of ions

AbstractThe application of the superposition principle of McLafferty and co‐workers to collision induced dissociation or collisional activation spectra to obtain quantitative estimates of the composition of isomeric mixtures of ions is discussed in detail. A set of explicitly stated assumptions is used to derive expressions for the percent composition based on normalized peak heights or peak height ratios in the collision induced dissociation spectra. The simple linear interpolation based on peak ratios which has been used in the past is shown to be invalid. It is demonstrated that the common practice of ignoring those collision induced dissociation peaks where there is interference from non‐collision induced dissociation processes may lead to substantial uncertainty in the final results. It is also demonstrated that the superposition principle may lead to results which are not internally consistent with the model. The final conclusion is that the present method of quantitative structural analysis based on the superposition principle cannot generally be expected to yield reliable results. In specific cases the results may be assumed to be reliable if it is demonstrated that the model is self‐consistent and the inherent experimental uncertainty is reasonably small. Further work may be useful in developing a generally valid method of analysis.

Excitation in ion–molecule collisions. A study of protonated aromatic molecules by ion kinetic energy spectrometry

AbstractCollision induced dissociation of protonated aromatic ions formed in a chemical ionization source has been studied using the technique of mass analyzed ion kinetic energy spectrometry. The observed fragmentations are concordant with those seen in the photodissociation spectra of the same ions with one significant exception. Loss of H is always a major collision induced dissociation process but is not observed in photodissociation spectra. This differences is not due to differences in the total amount of energy transferred in ionic excitation but to differences in the form in which the energy is supplied. Collision induced dissociation can lead to processes forbidden in photoexcitation, particularly those associated with electron unpairing in violation of the even→even+even rule for fragmentations.