Modeling unimolecular reactions in photoelectron photoion coincidence experiments

A computer program has been developed to model and analyze the data from photoelectron photoion coincidence (PEPICO) spectroscopy experiments. This code has been used during the past 12 years to extract thermochemical and kinetics information for almost a hundred systems, and the results have been published in over forty papers. It models the dissociative photoionization process in the threshold PEPICO experiment by calculating the thermal energy distribution of the neutral molecule, the energy distribution of the molecular ion as a function of the photon energy, and the resolution of the experiment. Parallel or consecutive dissociation paths of the molecular ion and also of the resulting fragment ions are modeled to reproduce the experimental breakdown curves and time‐of‐flight distributions. The latter are used to extract the experimental dissociation rates. For slow dissociations, either the quasi‐exponential fragment peak shapes or, when the mass resolution is insufficient to model the peak shapes explicitly, the center of mass of the peaks can be used to obtain the rate constants. The internal energy distribution of the fragment ions is calculated from the densities of states using the microcanonical formalism to describe consecutive dissociations. Dissociation rates can be calculated by the RRKM, SSACM or VTST rate theories, and can include tunneling effects, as well. Isomerization of the dissociating ions can also be considered using analytical formulae for the dissociation rates either from the original or the isomer ions. The program can optimize the various input parameters to find a good fit to the experimental data, using the downhill simplex algorithm. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparison of resolving time values of different scintillation detectors in the coincidence experiments

The resolving time values of the coincidence spectrometers composed by NaI(Tl) inorganic, BC408 type and BC418 type organic scintillation detectors were obtained in this work. For this purpose, fast–fast coincidence measurements were carried out. Best timing performance was obtained by BC418 type plastic scintillator for various source-detector distances.     

Photoelectron-fragment ion correlations and fixed-molecule photoelectron angular distributions from velocity imaging coincidence experiments

A new technique, based on velocity imaging, has been developed to examine correlations in energy and angle between the several particles formed in dissociative single and double photoionisation at VUV wavelengths. Electrons and positive fragment ions are imaged in coincidence at position-sensitive detectors; the resulting multidimensional data sets contain separable energy distributions, angular distributions and correlations in energy and angle between the particles. In some cases internal or external evidence indicates that pure axial recoil occurs, without molecular rotation. In such cases fixed-molecule photoelectron angular distributions can be extracted and parametrised. Results for H2, N2, NO, CO and O2 are presented. Effects of shape resonances in the ionisation channels are apparent in several cases.     

Selection experiments with dynamic combinatorial libraries: the importance of the target concentration

Experimental data are presented which demonstrate that the target concentration has a strong influence on the outcome of selection experiments with a dynamic mixture of self-assembled receptors.     

Dissociation of energy-selected c-C2H4S+ in a region 10.6-11.8 eV: threshold photoelectron-photoion coincidence experiments and quantum-chemical calculations

Dissociation of energy-selected c-C2H4S+ was investigated in a region of 10.6-11.8 eV with a threshold photoelectron-photoion coincidence technique and a synchrotron as a source of vacuum ultraviolet radiation. Branching ratios and average releases of kinetic energy in channels of formation of c-C2H4S+, CH3CS+, and HCS+ were obtained from well-resolved time-of-flight peaks in coincidence mass spectra. Measured average releases of kinetic energy for channel CH3CS+ + H of least energy are substantial and much greater than calculated with quasiequilibrium theory; in contrast, small releases of kinetic energy near the appearance onset for channel HCS+ + CH3 agree satisfactorily with statistical calculations. Calculations of molecular electronic structures and energetics of c-C2H4S+ and C2H3S+ isomers and various fragments and transition states were also performed with Gaussian 3 method to establish dissociation mechanisms. A predicted dissociation energy of 11.05 eV for c-C2H4S --> HCS+ + CH3 agrees with a linearly extrapolated threshold at 10.99+/-0.04 eV and a predicted dissociation mechanism that c-C2H4S+ isomerizes to CH3CHS+ before dissociating to HCS+ + CH3 supports the experimental results. The large releases of kinetic energy for channel CH3CS+ + H might result from a dissociation mechanism according to which c-C2H4S+ isomerizes to a local minimum CH3CSH+ and then dissociates through a transition state to form CH3CS+ + H.     

INAA with gamma-gamma coincidence for selenium determination in food

Selenium detection limits of INAA are normally above its concentration in most biological materials. Gamma-gamma coincidence methodology can be used to improve the detection limits and uncertainties in the determination of selenium. Here, some edible parts of plants were measured using a HPGe detector equipped with a NaI(Tl) active shielding, producing spectra both in normal and coincidence modes. The results presented the reduction of the detection limits of selenium by a factor of 2 to 3 times and improvement in the uncertainty of up to 2 times.     

Formation of multi-charged Kr ions through photoionization of 2p electrons studied with a coincidence technique

Multi-charged Kr ions have been measured using monochromatized undulator radiation combined with a coincidence technique. The coincidence measurements between multi-charged ions and energy-selected Auger electrons have clarified decay processes, as follows. The Auger final states formed through L₃M₄₅M₄₅ decays turn significantly into Kr⁴⁺ and those through L₃M₂₃M₄₅ decays generate Kr⁵⁺ mainly. The Auger decays of L₃M₂₃M₂₃ types yield Kr⁶⁺ dominantly. These findings are consistent with the consideration on energy levels of Kr ions.     

Revealing defects hampering the formation of epoxy networks with extremely high thermal properties: Theory and experiments

We present a combined experimental and theoretical investigation of thermal properties of cycloaliphatic epoxy networks. The networks are prepared from 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate ERL-4221 as a monomer and 4-methylhexahydrophthalic anhydride as a curing agent and their glass transition temperature T_g is evaluated by dynamic mechanical and thermal mechanical analyses as well as by differential scanning calorimetry. It is found that the cured epoxy networks have high T_g values reaching 233–238 °C. The method of anharmonic oscillators is first proposed to simulate the effect of network structure on the thermal properties. It suggests that further increase of T_g values is not attained because of the formation of intramolecular cyclic structures. Studies of model reaction by mass-spectrometry confirm the formation of such structures at curing.     

Evaluation of thick target PIXE analyses with a minicomputer

Thick target particle-induced X-ray emission (TTPIXE) spectrometry has been increasingly used for multielement and trace element analyses. The theoretical background and a formalism are presented for their evaluation by means of a minicomputer. Numerical results are given and compared with the experimental ones. The advantage of the program is shown by a special experimental example.     

Radium determination in soil samples using a gamma-ray coincidence spectrometer

A technique for the assay of radium by detection of coincident γ-radiation is evaluated. The sensitivity of various counting modalities is compared. The influence of finite sample size and density is discussed. Interference from other natural radioactivities is investigated and it is shown that a two channel arrangement is sufficient to take this into account.     

ATR-FTIR experiments with chlorosilanes

OH groups on the surface of semiconductors (SC) such as Si, Ge, TiO₂, SnO₂ react with Cl-Si groups to give stable SC-O-Si bonds. ATR-FTIR is applied to study the infrared spectra of adsorbed H₈Si₈O₁₂ on ZnSe and Cl₈Si₈O₁₂ which has reacted with the surface-OH group of Ge. A comparison is made with the spectra of the free molecules.     

Accurate detection of on-state quantum dot and biomolecules in a microfluidic flow with single-molecule two-color coincidence detection

Due to their unique optical and electronic properties, quantum dots (QDs) have been widely used in a variety of biosensors for sensitive detection of biomarkers and small molecules. However, single QD exhibits dynamic fluctuation of fluorescence intensity (i.e., blinking) with the transition between on and off states, which adversely influences the development of QD-based optical biosensors. Therefore, the methods for efficient evaluation of on-state QD are especially important and highly desirable. In this paper, a novel and unique approach based on single-molecule two-color coincidence detection is developed to simply and accurately evaluate the on-state QDs in a microfluidic flow. Our results demonstrate that improved QDs in the on state are detected in a microfluidic flow in comparison with that in the Brownian motion state, thus paving the way to the development of single QD-based biosensors for sensitive detection of low-abundance biomolecules. This single-molecule two-color coincidence detection has been applied for the homegeneous detection of nucleic acids in a microfluidic flow with the detection sensitivity of 5.0 fM.     

Translocation of biomolecules through solid‐state nanopores: Theory meets experiments

The interest in polynucleotide translocation through nanopores has moved from purely biological to the need of realizing nanobiotechnological applications related to personalized genome sequencing. Polynucleotide translocation is a process in which biomolecules, like DNA or RNA, are electrophoretically driven through a narrow pore and their passage can be monitored by the change in the ionic current through the pore. Such a translocation process, which will be described here offers a very promising technology aiming at ultra‐fast low‐cost sequencing of DNA, though its realization is still confronted with challenges and drawbacks. In this review, we present the main aspects involved in the polynucleotide translocation through solid‐state nanopores by discussing the most relevant experimental, theoretical, and computational approaches and the way these can supplement each other. The discussion will expose the goals that have been reached so far, the open questions, and contains an outlook to the future of nanopore sequencing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 985–1011, 2011     

Theory of electrode polarization: application to parallel plate cell dielectric spectroscopy experiments

An extension of the model for electrode polarization of Cirkel et al. [Physica A 235 (1997) 269] is given. The problem is solved using both classical boundary conditions and the new boundary conditions using excess densities presented in a previous paper [J. Phys. Chem. B 105 (2001) 11743]. In the present paper, the electrodes are supposed to be ideal, meaning that charge transfer or adsorption are not considered. The advantage of the new boundary conditions lies in the possibility to extend to more complicated situations including for instance specific ion adsorption. We prove that the new boundary conditions and classical ones give the same results. A comparison of the model predictions, involving no adjustable parameters, experimental dielectric spectroscopy data is performed and fairly good agreement is found.     

Theory of error for target factor analysis with applications to mass spectrometry and nuclear magnetic resonance spectrometry

Based on the theory of error for abstract factor analysis described earlier, a theory of error for target factor analysis is developed. The theory shows how the error in the data matrix mixes with the error in the target test vector. The apparent error in a target test is found to be a vector sum of the real error in the target vector and the real error in the predicted vector. The theory predicts the magnitudes of these errors without requiring any a priori knowledge of the error in the data matrix or the target vector. A reliability function and a spoil function are developed for the purpose of assessing the validity and the worthi-ness of a target vector. Examples from model data, mass spectrometry and nuclear magnetic -resonance spectrometry are presented.     

Extra-column dead volume in simulated moving bed separations: Theory and experiments

In small-scale SMB units typically set up by a number of HPLC columns connected in series, the volume of the connecting tubing parts and valves may become comparable to the column volume. Therefore, to guarantee proper and satisfying separation results, the introduced extra-column dead volume needs to be considered in the calculations of the operating parameters. In this work, the impact of extra-column dead volume on the separation performance is studied, with the objective to introduce guidelines and rather simple rules to account for it. It is shown, how these results can be used in the frame of the triangle theory to determine operating conditions that allow to achieve the desired separation performance. For the experiments the separation of a racemic mixture of (±$\pm$)-3,5-bis[1-(4-methoxyphenyl)-1-methyl]hepta-3,4-diene-1,6-diyne was carried out. The numerical model used for the simulation describes explicitly the geometric configuration of the HPLC–SMB laboratory unit to take into account the effect of extra-column dead volume.     

Theory of the time-dependent-thermal-lensing (TDTL) technique as used in energy-transfer experiments

A unified theory of the TDTL technique that incorporates energy-transfer rates, diffusion, and thermal conductivity is presented. In the appropriate limits, the unified approach reduces to simplified theories that have appeared in the literature. Example calculations are given and the physics of the effect is discussed.     

Nuclide analysis of a tantalum target irradiated with high-energy protons

First results of γ-spectrometric measurements of radionuclides produced in a tantalum target irradiated for 500 days with 800 MeV protons are presented. The activities of the γ-ray-emitting nuclides measured after a cooling period of 2–4 years differ by more than four orders of magnitude. Within this cooling period about 95% of the activity of the target is determined by¹⁷²Lu,¹⁷³Lu,¹⁷⁴Lu,¹⁷²Hf (daughter nuclide of¹⁷²Lu) and¹⁸²Ta nuclides. These highly active nuclides, together with some other medium-active nuclides, were measured with good precision by γ-ray spectrometry directly in the sample solution. Weakly active nuclides could not be measured in this way because of the high Compton background in the γ-ray spectrum. For the sensitive measurement of many other nuclides present in weaker activities a simple chemical separation procedure was developed to separate Lu, Hf and Ta. With the separation of these elements and simultaneously of the high-active nuclides, the Compton background in the γ-ray spectra could be drastically reduced and many weakly active nuclides additionally measured.     

Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH(3)Cl(+) ions was investigated in the excitation energy range of 11.0-18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH(3)(+) dissociated from CH(3)Cl(+)(A(2)A(1) and B(2)E) ions were recorded. CH(3)(+) was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH(2)Cl(+) fragment was very low. For dissociation of CH(3)Cl(+)(A(2)A(1)) ions, a series of homocentric rings was clearly observed in the CH(3)(+) image, which was assigned as the excitation of umbrella vibration of CH(3)(+) ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH(3)(+)(1(1)A') provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH(3)Cl(+)(B(2)E) ions, total kinetic energy released distribution for CH(3)(+) fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B(2)E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH(3)Cl(+), CH(3)(+) formation from CH(3)Cl(+)(A(2)A(1)) ions was a rapid direct fragmentation, while CH(3)Cl(+)(B(2)E) ions statistically dissociated to CH(3)(+) + Cl via internal conversion to the high vibrational states of X(2)E.