Štefan Matejčík

High resolution dissociative electron attachment to gas phase adenine.

The dissociative electron attachment to the gas phase nucleobase adenine is studied using two different experiments. A double focusing sector field mass spectrometer is utilized for measurements requiring high mass resolution, high sensitivity, and relative ion yields for all the fragment anions and a hemispherical electron monochromator instrument for high electron energy resolution. The negative ion mass spectra are discussed at two different electron energies of 2 and 6 eV. In contrast to previous gas phase studies a number of new negative ions are discovered in the mass spectra. The ion efficiency curves for the negative ions of adenine are measured for the electron energy range from about 0 to 15 eV with an electron energy resolution of about 100 meV. The total anion yield derived via the summation of all measured fragment anions is compared with the total cross section for negative ion formation measured recently without mass spectrometry. For adenine the shape of the two cross section curves agrees well, taking into account the different electron energy resolutions; however, for thymine some peculiar differences are observed.

Formation of C60- and C70- by free electron capture. Activation energy and effect of the internal energy on lifetime

Abstract Electron attachment to C 60 − and C 70 − yields long-lived (i.e. mass spectrometrically observable) anions C 60 − and C 70 − within a remarkably broad energy region (up to 13–14 eV) as already established in a previous experiment. Inspection of the low energy region reveals, however, that electron capture is characterized by an activation barrier of 240 meV for C 60 close to the results of the previous FALP study and 200 meV for C 70 . Flight time measurements of the anions indicate that the lifetimes of C 60 − and C 70 − decrease exponentially with increasing internal energy in the 8⩽ E * ⩽12 eV. For example, at 10 eV the lifetime is 90 μs for C 60 − and 300 μs for C 70 − . It is also established that the lifetime of both anions is controlled by electron emission (detachment).

Dissociative electron attachment to gas phase valine: a combined experimental and theoretical study.

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH(3))(2)CHCH(NH(2))COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the pi* orbital of the carboxyl group. The dominant negative ion (M-H)(-) (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying pi* and through higher lying resonances at about 5.5 and 8.0-9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies.

Electron impact ionization of CH4: ionization energies and temperature effects

The threshold energy and the threshold behaviour for electron impact ionization of CH4 were investigated at two temperatures, 293 and 693 K. The study was performed with a crossed electron–molecule beam apparatus with an electron energy resolution of 120 meV full width at half maximum. The values of the ionization energies (IEs) and the threshold behaviour were determined using a fitting procedure involving a convolution of the cross section and the electron energy distribution function. At 293 K the following IEs were obtained: IE(CH4+/CH 4) = 12.65 ± 0.4 eV, IE1(CH3+/CH 4) = 13.58 ± 0.1 eV (ion pair) and IE2(CH3+/CH 4) = 14.34 ± 0.1 eV. At 693 K a red shift in these IEs of about 0.14 eV was observed for both CH4+/CH 4 and CH3+/CH 4 processes, which reflects the change in the internal energy of CH4 with increasing temperature. In addition, at 293 K IEs were determined also for the small fragment ions, i.e. IE(CH2+/CH 4) = 15.1 ± 0.1 eV, IE(CH+/CH 4) = 19.8 ± 0.1 eV and IE(C+/CH 4) = 20.5 ± 0.2 eV.

Formation and decay of C−60 following free electron capture by C60

The results of a detailed crossed electron/molecular beam study of electron attachment to C60 molecules and electron detachment from C−60 over the range of electron energies from near zero to about 15 eV are described. It is shown by comparing the experimental data for the attachment cross sections (normalized to the absolute thermal cross sections determined using the flowing afterglow/Langmuir probe apparatus) with quantum calculations that attachment occurs at low energies in the p‐wave channel, and in the d‐ and f‐wave channels (and probably higher‐order partial waves) at the higher electron energies. At electron energies above 7 eV, thermal detachment of electrons from the hot C−60 negative ions is seen to occur, and the unimolecular rate coefficients for detachment, kd, have been determined as a function of the energy of the attaching electron. Hence, by relating kd to the derived temperature of the hot C−60 ions, the electron detachment energy, Ed, has been determined as 2.6 eV, which is close to t...

Experimental study of negative corona discharge in pure carbon dioxide and its mixtures with oxygen

The products of a negative corona discharge in both pure CO2 and mixtures of CO2 + O2 have been studied using a coaxial cylindrical electrode geometry with particular emphasis on the production of ozone. The discharge current in pure CO2 was found to be highly sensitive to the presence of trace concentrations of molecular oxygen and to changes in the flow speed through the discharge. The effect of dissociative electron attachment to ozone on the discharge current was studied by measurements of ozone and CO production. The ozone concentration increases monotonically with increasing content of oxygen in the mixture with carbon dioxide, whereas the CO concentration exhibits a flat maximum for oxygen concentrations of around 4%. A simple kinetic model of the dominant chemical processes is described and compared with the experimental results.

Electron attachment to 5-chloro uracil

Electron attachment (EA) and dissociative electron attachment (DEA) to 5-chloro uracil (5-ClU) was studied in the gas phase using a crossed electron/molecule beams technique. Besides production of a parent anion via a zero energy resonance, ion yields of nine different negative ions were observed in the electron energy range from about 0 to 14 eV. In the electron energy range from about zero to 5 eV, the formation of a transient negative ion was induced by electron attachment to the π* resonances located at about 0.24, 1.5, and 3.6 eV leading subsequently by unimolecular decay to various negative fragment ions. Absolute partial cross sections for EA and DEA to 5-ClU were obtained from the measured ion yields using a simple calibrating method. The dominant negative ion observed in the present experiment was (C4H2N2O2)− (corresponding to 5-ClU minus HCl) with a mass to charge ratio of 110, followed by Cl− ion (mass to charge ratios 35 and 37), the partial cross sections being σ(0.23 eV)=5×10−18 m2 and σ(0.2...

Dissociative electron attachment study to nitromethane

Dissociative electron attachment (DEA) to CH3NO2 in the gas phase was studied in the electron energy range from zero up to 10 eV with an energy resolution of 140 meV. For the most intense negative fragments NO2−, O−, OH−, CN−, CNO− estimates for the absolute partial cross sections were obtained for the first time [σ(NO2)≈10−21 m2 at 0.62 eV, σ(O−)≈10−23 m2 at 5.4 eV, σ(OH−)≈10−24 m2 at 4 eV, σ(CN−)≈10−24 m2 at 1.7 eV, and σ(CNO−)≈10−25 m2 at 4 eV]. In the case of OH−, CN−, and CNO−, ion formation at very low electron energies (≈0 eV) has been observed in contrast to previous studies. The formation of OH− and CNO− at these low electron energies is explained in terms of DEA to vibrationally excited molecules. Analyzing measured partial cross sections, the standard enthalpy of formation of the CH3NO (nitroso-methane) and the CNO radical has been estimated, as ΔfHg∘(CH3NO)=129±30 kJ/mol and ΔfHg∘(CNO)=323±30 kJ/mol, respectively.

Low energy electron driven reactions in single formic acid molecules (HCOOH) and their homogeneous clusters

Low energy (0-3 eV) electron attachment to single formic acid (FA) and FA clusters is studied in crossed electron/molecular beam experiments. Single FA molecules undergo hydrogen abstraction via dissociative electron attachment (DEA) thereby forming HCOO(-) within a low energy resonance peaking at 1.25 eV. Experiments on the isotopomers HCOOD and DCOOH demonstrate that H/D abstraction occurs at the O-H/O-D site. In clusters, electron attachment is strongly enhanced leading to a variety of negatively charged complexes with the dimer M2(-) (M[triple bond]HCOOH) and its dehydrogenated form M (M-H)(-) as the most abundant ones. Apart from the homologous series containing the non-dissociated (Mn(-)) and dehydrogenated complexes (M(n-1) (M-H)(-), n > or = 1) further products are observed indicating that electron attachment at sub-excitation energies (approximately 1 eV) can trigger a variety of chemical reactions. Among these we detect the complex H2O (M-H)(-) which is interpreted to arise from a reaction initiated in the cyclic hydrogen bonded dimer target. In competition to hydrogen abstraction yielding the dehydrogenated complex M (M-H)(-) the abstracted hydrogen atom can react with the opposite FA molecule forming H2O and HCO with the polar water molecule attached to the closed shell HCOO(-) ion. The FA dimer can thus be used as a model system to study the response of a hydrogen bridge towards dehydrogenation in DEA.

Corona Discharge Ion Mobility Spectrometry with Orthogonal Acceleration Time of Flight Mass Spectrometry for Monitoring of Volatile Organic Compounds

We demonstrate the application of corona discharge ion mobility spectrometry with orthogonal acceleration time of flight mass spectrometry (CD IMS-oaTOF) for volatile organic compounds (VOCs) monitoring. Two-dimensional (2D) IMS-oaTOF spectra of VOCs were recorded in nearly real time. The corona discharge atmospheric pressure chemical ionization (APCI) source was operated in positive mode in nitrogen and air. The CD ion source generates in air H(3)O(+)(H(2)O)(n) and NO(+). The NO(+) offers additional possibility for selective ionization and for an increase of the sensitivity of monoaromatic compounds. In addition to H(3)O(+)(H(2)O)(n) and NO(+), we have carried out ionization of VOCs using acetone as dopant gas ((CH(3))(2)COH(+)). Sixteen model VOCs (tetrahydrofuran, butanol, n-propanol, iso-propano, acetone, methanol, ethanol, toluene, benzene, amomnia, dioxan, triethylamine, acetonitrile, formaldehyde, m-xylene, 2,2,2-trifluoroethylamine) were tested using these ionization techniques.