Dissociation Lifetimes Research Articles

Collision-Induced Dissociation and Theoretical Studies of Na+−Acetonitrile Complexes

Collision-induced dissociation of Na+(CH3CN)x, x = 1−5, with xenon is studied as a function of kinetic energy using guided ion beam mass spectrometry. In all cases, the primary and lowest energy dissociation channel observed is endothermic loss of one acetonitrile molecule. The cross section thresholds are interpreted to yield 0 and 298 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the complexes, and dissociation lifetimes. Ab initio and density functional calculations at the MP2(full)/6-31G* and B3LYP/6-31G* levels of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical bond dissociation energies are determined from single point calculations at the MP2(full)/6-311+G(2d,2p) and B3LYP/6-311+G(2d,2p) levels using the MP2(full)/6-31G* and B3LYP/6-31G* optimized geometries, respectively. The experimental bond energies determined here are in good agreement with previous experimental measurements for all complexes. Good agreement between theory and experiment is also found for all clusters except x = 5, where it is apparent that more basis functions are necessary to adequately describe the very weak noncovalent interactions in this complex.

Solvation of Copper Ions by Acetonitrile. Structures and Sequential Binding Energies of Cu+(CH3CN)x, x = 1−5, from Collision-Induced Dissociation and Theoretical Studies

Collision-induced dissociation of Cu+(CH3CN)x, x = 1−5, with xenon is studied as a function of kinetic energy using guided ion beam mass spectrometry. In all cases, the primary and lowest energy dissociation channel observed is endothermic loss of one acetonitrile molecule. The cross section thresholds are interpreted to yield 0 and 298 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the complexes, and dissociation lifetimes. Density functional calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical bond dissociation energies are determined from single point calculations at the B3LYP/6-311+G(2d,2p) level and also with an extended basis on Cu+ of 6-311+G(3df) using the B3LYP/6-31G* optimized geometries. The experimental bond energies determined here are in excellent agreement with previous experimental measurements made in a high-pressure mass spectrometer for the sum of the first and second bond energy (i.e., Cu+(CH3CN)2 → Cu+ + 2CH3CN) when these results are properly anchored. Excellent agreement between theory and experiment is also found for the Cu+(CH3CN)x, x = 1 and 2 clusters. Theory systematically underestimates the binding energy in the larger clusters such that higher levels of theory are necessary to adequately describe the very weak noncovalent interactions in these systems.

Periodic Trends in the Binding of Metal Ions to Pyrimidine Studied by Threshold Collision-Induced Dissociation and Density Functional Theory

Threshold collision-induced dissociation of M+(pyridine) with xenon is studied using guided ion beam mass spectrometry. M+ include the following metal ions: Mg+, Al+, Sc+, Ti+, V+, Cr+, Mn+, Fe+, Co+, Ni+, Cu+, and Zn+. In all cases, the primary product corresponds to endothermic loss of the intact pyridine molecule, with minor production of MXe+ formed by ligand exchange. In the Cr+(pyridine) system, an additional minor reaction pathway involving a ring-opening reaction with small neutral molecule loss is also observed. The cross-section thresholds are interpreted to yield 0 and 298 K bond dissociation energies for M+−pyridine after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Density functional calculations at the B3LYP/6-31G(d,p) level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical bond dis...

Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones.

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI-MS). 1Cl-AQ and 2Br-AQ display dramatically steep positive temperature dependencies of Hal(-) ion abundance in the low electron energy region. Molecular NI intensity decreases rapidly with increasing temperature in the case of 1I-AQ. In the case of 2Br-AQ, a metastable NI peak (m/z 22.9) corresponding to the process BrAQ(-) --> Br(-) + AQ(0) was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br-AQ(-) is at least approximately 25 micros at the energy approximately 0.67 eV in the low-temperature spectrum (T approximately 80 degrees C), and at the energy approximately 0.13 eV in the hot spectrum (T approximately 290 degrees C). Together with the observed temperature dependence of the 2Br-AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl-AQ. There are three consecutive stages in the process of molecular NI dissociation of Cl- and Br-substituted AQ, namely, electron capture into the empty pi-orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic pi-state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the pi-term into the sigma-term, so-called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I-AQ, an effect of inversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so-called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI-MS, but also in other experimental investigations using low-energy electron-molecule and ion-molecule collisions.

Substituent Effects in the Binding of Alkali Metal Ions to Pyridines Studied by Threshold Collision-Induced Dissociation and ab Initio Theory: The Aminopyridines

Threshold collision-induced dissociation of M+(x-aminopyridine) with xenon is studied using guided ion beam mass spectrometry. M+ include the following alkali metal ions: Li+, Na+, and K+. All three structural isomers are examined, x = ortho, meta, and para. In all cases, the primary product corresponds to endothermic loss of the intact x-aminopyridine molecule, with minor production of MXe+ formed by ligand exchange. The cross section thresholds are interpreted to yield zero and 298 K bond dissociation energies for M+−x-aminopyridine after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at the MP2(full)/6-31G* level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical bond dissociation energies are determined from single point calculations at the MP2(full)/6-311+G(2d,2p) level u...

Short-wavelength photolysis of jet-cooled OClO(2A2 ν1>20)→ClO(X 2ΠΩ,v,J)+O(3PJ)

Highly inverted vibrational level populations are found for ClO fragments resulting from the UV photodissociation of OClO(2A2 ν1>20) into ClO(X 2ΠΩ,v,J) and O(3PJ) fragments. These distributions depend significantly on the spin–orbit J state of the oxygen O(3PJ) partner atom. In contrast, the ClO rotational excitation is modest. Distinct rotational structure is visible in the O (3PJ) photofragment yield spectrum recorded from the highly excited OClO (2A2 ν1=21) vibronic band, which is indicative for hitherto unforeseen long dissociation lifetimes of very highly excited OClO. The data point towards an unexpected nearly-linear and highly asymmetric dissociation geometry. Carrying out near-threshold fragmentation experiments of OClO (2A20,0,0)→ClO (X 2Π3/2 v=0, J)+O(3P2,1,0) the dissociation energy D0 of OClO has been accurately determined to 247.3±0.5 kJ/mol.

Collision-induced dissociation and theoretical studies of Cu +-dimethoxyethane complexes

Collision-induced dissociation of the dimethoxyethane (DXE) complexes with copper ions, Cu +(DXE) n, n = 1 and 2, is studied using kinetic energy dependent guided ion beam mass spectrometry. For Cu +(DXE) 2, the only product formed corresponds to endothermic loss of a neutral ligand, while the Cu +(DXE) complex dissociates by several competitive channels. The cross-section thresholds for single ligand loss are interpreted to yield 0 and 298 K bond energies for Cu +-DXE and (DXE)Cu +-DXE after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. We find absolute 0 K bond dissociation energies for these complexes of 2.74 ± 0.08 and 1.87 ± 0.06 eV, respectively. These values are compared with theoretical values obtained using density functional and second order Møller-Plesset perturbation, MP2, theories. We also compare our results with previously studied alkali cation-ether complexes. Although Cu + and all alkali cations have 1S electronic ground states, the comparison shows different trends for Cu + because of hybridization effects involving the valence d-electrons.

Observation of accurate ion dissociation thresholds in pulsed field ionization-photoelectron studies.

We report the first observation, together with a mechanism for such an observation, of a steplike feature in the pulsed field ionization photoelectron measurement of CH4(C2H2), marking the 0 K dissociation threshold for the formation of CH3(+) + H(C2H(+) + H) from CH4(C2H2). The nonexistence of a step in the spectrum for C 2H4 at its dissociation threshold for C2H2(+) formation provides strong support for the proposed mechanism. This experiment shows that, for a range of molecules, where the ion dissociation lifetimes near the dissociation thresholds are <10(-7) s, pulsed field ionization photoelectron measurements will yield not only highly accurate ionization energies, but also 0 K dissociation thresholds.

Substituent Effects in the Binding of Alkali Metal Ions to Pyridines, Studied by Threshold Collision-Induced Dissociation and ab Initio Theory: The Methylpyridines

Threshold collision-induced dissociation of M+(x-methylpyridine) with xenon is studied using a newly constructed guided ion beam mass spectrometer, described in detail here. M+ include the following alkali metal ions: Li+, Na+, and K+. All three structural isomers are examined, x = ortho, meta, and para. In all cases, the primary product corresponds to endothermic loss of the intact x-methylpyridine molecule, with minor production of MXe+ formed by ligand exchange. The cross section thresholds are interpreted to yield zero and 298 K bond dissociation energies for M+−x-methylpyridine after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at the MP2(full)/6-31G* level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical bond dissociation energies are determined from single point calculations at the MP2(full)/6-311+G(2d,2p) level using the MP2(full)/6-31G* geometries. Excellent agreement between theory and experiment is found for the Na+ and K+ systems, whereas the theoretical BDEs to Li+ systems are systematically low but still within experimental error. Good agreement is found for the single previously measured bond dissociation energy of Li+ to m-methylpyridine.

Absolute Binding Energies of Alkali-Metal Cation Complexes with Benzene Determined by Threshold Collision-Induced Dissociation Experiments and ab Initio Theory

The sequential bond dissociation energies (BDEs) of the mono- and bis-benzene complexes with alkali metal cations (Li+, Na+, K+, Rb+, and Cs+) are determined experimentally by collision-induced dissociation (CID) with Xe in a guided ion beam mass spectrometer and theoretically by ab initio calculations. The kinetic energy dependence of the CID cross sections are analyzed to yield 0 and 298 K bond energies for (C6H6)x-1M+−C6H6 (x = 1−2) after accounting for the effects of the internal energies of the reactant ions, the multiple collisions of the ions with xenon, and the dissociation lifetimes of the ionic complexes. Ab initio binding energies are calculated at the MP2(full)/6-311+G(2d,2p)//MP2(full)/6-31G* level and corrected for zero-point energies (ZPE) and basis set superposition errors (BSSE). The theoretical BDEs are in reasonably good agreement with the experimentally determined 0 K bond energies when full electron correlation is included (for Li+, Na+, and K+) but differ appreciably when effective c...

Femtosecond photodissociation dynamics of HNO3 after exitation of the S3-state at 200 nm

The photodissociation dynamics of HNO3 in the electronic S3 (2 1 A ′ ) state leading to the fragments OH and NO2 was investigated in real time. HNO3 was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO2 by three-photon ionization). A 2 1 A ′ lifetime of 60–80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO2 * leading to products NO(X 2 Π) and O(3 P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO2 from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO2.

Effect of temperature and ionic strength on the dissociation kinetics and lifetime of PNA-DNA triplexes.

Dissociation kinetics of triplexes formed by molecules of peptide nucleic acid (PNA) and DNA have been studied. The complexes consisted of oligomeric PNA containing 10 thymine bases and the dA(10) target incorporated in single-stranded (ssDNA) or double-stranded DNA (dsDNA). Their dissociation was followed by means of the gel mobility shift assay at various temperatures and sodium ion concentrations. In all experiments, the dissociation kinetics of triplexes were exponential; the effective lifetime of a triplex, tau, depended on temperature in accordance with the Arrhenius law. The tau values for T(10) PNA complexes with ss- and dsDNA were equal within the accuracy of experiments. The activation energy, U, value for T(10) PNA-DNA complexes did not change when the NaCl concentration was increased from 50 to 200 or 600 mM. Conversely, the tau values decreased with the increase in NaCl concentration. The equal lifetimes of the T(10) PNA-DNA triplexes containing ss- and dsDNA suggest that the loop formed in dsDNA does not noticeably affect the triplex structure. The decrease in the triplex lifetime tau with an increase in ionic strength was accounted for by the fact that the PNA backbone is neutral. The lack of relationship between the activation energy of dissociation and salt concentration suggests that the dissociation enthalpy does not depend on the ionic strength. Thus, the effect of ionic strength on the lifetime is entropic by its nature. Contrary to this, for complexes of ssDNA with bis-PNA 1743, which also consists of 10 thymine bases but contains 2 additional positive charges inside the sequence in 1 of the PNA arms, an increase of the dissociation enthalpy at low salt concentration was observed. We suggest that this effect is a result of a direct electrostatic interaction of the positive charges of the PNA with the DNA backbone. Finally, our results allow an estimate of the lifetime of a 10-mer triplex invasion complex in dsDNA at 37 degrees C in excess of several hundred days.

Noncovalent Interactions of Nucleic Acid Bases (Uracil, Thymine, and Adenine) with Alkali Metal Ions. Threshold Collision-Induced Dissociation and Theoretical Studies

Threshold collision-induced dissociation of M+L (M+ = Li+, Na+, and K+; L = uracil, thymine, and adenine) with xenon is studied using guided ion beam mass spectrometry. In all cases, the primary product formed corresponds to endothermic loss of the intact neutral molecule. The only other product observed is the result of ligand exchange processes to form MXe+. Cross-section thresholds are interpreted to yield 0 and 298 K bond dissociation energies for M+−L after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at the MP2(full)/6-311+G(2d,2p)//MP2(full)/6-31G* level of theory are used to determine the structures and relative energetics of several conformers of these complexes and to provide molecular constants necessary for the thermodynamic analysis of the experimental data. We find that all of the complexes are very nearly planar. Calculated M+−L bond dissociation energies compare favorably to the expe...

Temperature and doping level dependence of solar cell performance including excitons

Recent papers have indicated that the excitonic population in silicon can approach that of minority carriers and make their presence felt even at room and higher temperatures in generation–recombination processes and solar energy conversion. This paper presents an investigation of temperature and doping-level dependence of silicon solar cell perfomance for doping levels from 10 16 to 10 18 cm −3 and temperature interval 100–500 K. Our investigation was been performed using the three-particle theory developed by R.Corkish, D.S.-P.Chan, and M.A.Green (J. Appl. Phys. 79 (1996) 195). We have found that dissociation lifetime and diffusion length of excitons are 3–6 orders of magnitude less than that of free carriers. Inclusion of excitons results in a significant increase of dark current, decrease of short-circuit current, open-circuit voltage, fill factor, and efficiency. The results obtained, taking into account the effect of excitons, are in good agreement with experimental data. We have found accurate and simplified expressions for the dark saturation current and short-circuit current. Approximate value of the upper limit of photogeneration intensity of excitons φ ex is found that is at least eight times less than that of free carriers φ, i.e., φ ex⩽ φ. An optimal doping-level range of silicon solar cell base region was found that is less than 10 17 cm −3, and agrees with experimental data. The work includes a suggestion for experimental methods to confirm exciton involvement in photocurrent transport in silicon solar cells.

Double-resonance overtone photofragment spectroscopy of trans-HONO. I. Spectroscopy and intramolecular dynamics

Using the technique of double-resonance overtone photofragment spectroscopy (DROPS), we have measured rotationally resolved vibrational overtone transitions to the previously unobserved 5v1, 6v1, and 7v1 levels of gas-phase trans-nitrous acid (HONO) in its electronic ground state. Observing the onset of dissociation from different rovibrational states of 5v1 near threshold determines the HO–NO bond energy to be D0=16 772±14 cm−1. Observed spectral splittings and broadening of individual rovibrational transitions provide quantitative data on the rate and extent of collision free vibrational energy redistribution that would result after coherent ultrashort pulse excitation. In parallel with these frequency domain measurements, we determine the unimolecular dissociation rates directly in time for trans-HONO molecules excited to several rotational states near threshold. The combination of time- and frequency-resolved data allows us to estimate the linewidth contributions from the finite dissociation lifetime of the molecule. Our results reveal intramolecular dynamics that are clearly not a simple function of the vibrational energy but rather depend sensitively upon specific couplings and, in turn, on the vibrational character of the individual states excited.

Charmonium dissociation in hadronic matter

Production of J/$\psi$ from nucleus-nucleus reactions depends sensitively on the dissociation cross section with light hadrons. Effective lagrangian methods are used to describe the hadronic degrees of freedom, including strangeness and charm. Cross sections with pions, rho mesons, kaons and nucleons having magnitudes 4--8 mb are found, and with steep thresholds. This, folded with thermal momentum distributions for the scattering partners, suggests a mean dissociation lifetime $\simeq$ 20 fm/c. Therefore, the ``abnormal'' J/$\psi$ suppression seen in recent Pb+Pb experiments seems to owe to expected hadron kinetics.

Collision-Induced Dissociation and Theoretical Studies of Mg+ Complexes with CO, CO2, NH3, CH4, CH3OH, and C6H6

The sequential bond energies for complexes of Mg+ with CO, CO2, NH3, CH4, CH3OH, and C6H6 are determined by collision-induced dissociation (CID) with xenon or argon in a guided ion beam tandem mass spectrometer. The kinetic energy dependence of the CID and ligand exchange cross sections are analyzed to yield 0 and 298 K bond energies for Mg+−L after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Bond energies (in eV) to Mg+ at 0 K are determined for L = Ar (0.10 ± 0.07), Xe (0.32 ± 0.12), 1−2 CO (0.43 ± 0.06 and 0.40 ± 0.03), 1−3 CO2 (0.60 ± 0.06, 0.50 ± 0.03, and 0.46 ± 0.06), 1−5 NH3 (1.60 ± 0.12, 1.27 ± 0.07, 0.99 ± 0.09, 0.45 ± 0.11, and 0.58 ± 0.12), 1−2 CH4 (0.29 ± 0.07 and 0.15 ± 0.07), 1−3 CH3OH (1.51 ± 0.07, 1.25 ± 0.07, and 0.95 ± 0.09), and one C6H6 (1.39 ± 0.10 eV). As expected for largely electrostatic interactions, the sequential bond energies generally decrease monotonically with increasing number of ligands. These values are in good agreement with theoretical values in the literature and ab initio calculations performed here, but the agreement is mixed for comparison with results of photodissociation measurements. Qualitatively, geometries of these complexes are controlled by interactions of the ligands with the single polarized valence electron on Mg+.

Absolute alkali metal ion binding affinities of several azines determined by threshold collision-induced dissociation and ab initio theory

Threshold collision-induced dissociation of M +(azine) with xenon is studied using guided ion beam mass spectrometry. M + include the following alkali metal ions: Li +, Na +, and K +. The azines studied include pyridine, pyridazine, pyrimidine, pyrazine, and 1,3,5-triazine. In all cases, the primary product formed corresponds to endothermic loss of the intact azine and the only other product observed is the result of ligand exchange processes to form MXe +. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for M +–azine after accounting for the effects of multiple ion–molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at the MP2(full)/6-311+G(2 d,2 p)//MP2(full)/6-31G∗ level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. We find that all of the complexes are very nearly planar, the π complexes being significantly less stable. Calculated M +–azine bond dissociation energies compare favorably to the experimentally determined bond energies for these systems.

An Absolute Sodium Cation Affinity Scale: Threshold Collision-Induced Dissociation Experiments and ab Initio Theory

Threshold collision-induced dissociation of Na+(L) with xenon is studied using guided ion beam mass spectrometry. The ligand L includes ethene, benzene, phenol, ammonia, acetaldehyde, acetone, and N,N-dimethylformamide. In all cases, the primary product formed corresponds to endothermic loss of the neutral ligand and the only other product observed is the result of ligand exchange processes to form NaXe+. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for Na+−L after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at several levels of theory compare favorably to the experimentally determined bond energies for these and previously studied systems, L = Ar, CO, dimethyl ether, H2O, methylamine, imidazole, dimethoxyethane, and several alcohols. Combined, these ligands cover a very wide range in binding energies, and thereby help to establish an absolute scale for sodium cat...

Photodissociation of ClNO2 at 235 nm

The photodissociation of jet-cooled nitryl chloride ClNO2+hν→Cl+NO2 at 235 nm was investigated by resonance enhanced multiphoton ionisation/time-of-flight spectroscopy. In a one-colour experiment using nanosecond laser pulses, the Cl ground state (2P3/2) photofragment was detected and its speed distribution P(v) and recoil anisotropy β were obtained. Two contributions are observed in the P(v) which are assigned to a Cl 2P3/2 fragment emerging with a X̃2A1 ground state and electronically excited (most likely in the Ã2B2 state) NO2 counterfragment, respectively. The recoil anisotropy was found to be β=+1.1±0.1, yielding an upper limit of the dissociation lifetime of τ∼2 ps.