Dissociative electron attachment to H2S probed by ion momentum imaging

The dissociation dynamics of negative ion resonance states in H(2)S formed upon electron attachment are studied using momentum imaging of the fragment H(-) and S(-) ions and compared with similar resonances in water. The H(-) momentum images show that dissociation dynamics at the 5.2 eV resonance are very similar to those of the 6.5 eV (B(1)) resonance in water. Unlike the 8.5 eV resonance in water, which has A(1) symmetry but is found to display considerable deviation from the axial recoil approximation in the momentum distribution of H(-) ions, the distribution from the corresponding resonance in H(2)S at 7.5 eV is found to follow the axial recoil approximation fairly well. The resonance state with B(2) symmetry at 10 eV is found to decay via four dissociation channels viz.-H(-) + H + S, H(-) + SH(A(2)Σ), H(-) + SH(X(2)Π) and S(-) + H + H channels, similar to those that were seen in the B(2) resonance in water at 12 eV, including sequential fragmentation in the multiple fragmentation channels. However, the angular distributions for the fragment ions from this resonance are found to be distinctly different from those in water, even while displaying considerable deviation from the axial recoil approximation similar to that in water.     

Functional group dependent dissociative electron attachment to simple organic molecules

Dissociative electron attachment (DEA) cross sections for simple organic molecules, namely, acetic acid, propanoic acid, methanol, ethanol, and n-propyl amine are measured in a crossed beam experiment. We find that the H(-) ion formation is the dominant channel of DEA for these molecules and takes place at relatively higher energies (>4 eV) through the core excited resonances. Comparison of the cross sections of the H(-) channel from these molecules with those from NH(3), H(2)O, and CH(4) shows the presence of functional group dependence in the DEA process. We analyze this new phenomenon in the context of the results reported on other organic molecules. This discovery of functional group dependence has important implications such as control in electron induced chemistry and understanding radiation induced damage in biological systems.     

Photoelectron trapping in N2O 7sigma-->ksigma resonant ionization

Vibrationally resolved photoelectron spectroscopy of the N2O+(A 2Sigma+) state is used to compare the dependence of the photoelectron dynamics on molecular geometry for two shape resonances in the same ionization channel. Spectra are acquired over the photon energy range of 18< or =hv< or =55 eV. There are three single-channel resonances in this range, two in the 7sigma-->ksigma channel and one in the 7sigma-->kpi channel. Vibrational branching ratio curves are determined by measuring vibrationally resolved photoelectron spectra as a function of photon energy, and theoretical branching ratio curves are generated via Schwinger variational scattering calculations. In the region 30< or =hv< or =40 eV, there are two shape resonances (ksigma and kpi). The ksigma ionization resonance is clearly visible in vibrationally resolved measurements at hv=35 eV, even though the total cross section in this channel is dwarfed by the cross section in the degenerate, more slowly varying 7sigma-->kpi channel. This ksigma resonance is manifested in non-Franck-Condon behavior in the approximately antisymmetric v3 stretching mode, but it is not visible in the branching ratio curve for the approximately symmetric v1 stretch. The behavior of the 35-eV ksigma resonance is compared to a previously studied N2O 7sigma-->ksigma shape resonance at lower energy. The mode sensitivity of the 35-eV ksigma resonance is the opposite of what was observed for the lower-energy resonance. The contrasting mode-specific behavior observed for the high- and low-energy 7sigma-->ksigma resonances can be explained on the basis of the "approximate" symmetry of the quasibound photoelectron resonant wave function, and the contrasting behavior reflects differences in the continuum electron trapping. An examination of the geometry dependence of the photoelectron dipole matrix elements shows that the ksigma resonances have qualitatively different dependences on the individual bond lengths. The low-energy resonance is influenced only by changes in the end-to-end length of the molecule, whereas the higher-energy resonance depends on the individual N-N and N-O bond lengths. Branching ratios are determined for several vibrational levels, including the symmetry-forbidden bending mode, and all of the observed behavior is explained in the context of an independent particle, Born-Oppenheimer framework.     

Dissociative electron attachment to gas-phase formamide

Dissociative electron attachment (DEA) to gaseous formamide, HCONH(2), has been investigated in the energy range between 0 eV and 18 eV using a crossed electron/molecule beam technique. The negative ion fragments have been comprehensively monitored and assigned to molecular structures by comparison with the results for two differently deuterated derivatives, namely 1D-formamide, DCONH(2), and N,N,D-formamide, HCOND(2). The following products were observed: HCONH(-), CONH(2)(-), HCON(-), OCN(-), HCNH(-), CN(-), NH(2)(-)/O(-), NH(-), and H(-). NH(2)(-) was also separated from O(-) by using high-resolution negative ion mass spectrometry. Four resonant dissociation channels can be resolved, the strongest ones being located between 2.0 and 2.7 eV and between 6.0 and 7.0 eV. CN(-) as the most abundant fragment and HCONH(-) are the dominant products of the first of these two resonances. The most important products of the latter resonance are NH(2)(-), CN(-), H(-), CONH(2)(-), and OCN(-). It is thus found that the loss of neutral H is a site-selective process, dissociation from the N site taking place between 2.0 and 2.7 eV while dissociation from the C site occurs between 6.0 and 7.0 eV. The suitability of these reactions and thus of formamide as an agent for electron-induced surface functionalisation is discussed.     

Dissociative electron attachment to dinitrogen pentoxide, N2O5

Electron attachment was studied in gaseous dinitrogen pentoxide, N(2)O(5), for incident electron energies between a few meV and 10 eV. No stable parent anion N(2)O(5) (-) was observed but several anionic fragments (NO(3) (-), NO(2) (-), NO(-), O(-), and O(2) (-)) were detected using quadrupole mass spectrometry. Many of these dissociative pathways were found to be coupled and provide detailed information on the dynamics of N(2)O(5) fragmentation. Estimates of the cross sections for production of each of the anionic fragments were made and suggest that electron attachment to N(2)O(5) is amongst the most efficient attachment reactions recorded for nonhalogenated polyatomic systems.     

Low-Energy Electron Attachment to Serine Conformers: Shape Resonances and Dissociation Dynamics

Shape resonances of electron-molecule system formed in the low-energy electron attachment to four low-lying conformers of serine (serine 1, serine 2, serine 3, and serine 4) in gas phase are investigated using the quantum scattering method with the non-empirical model potentials in single-center expansion. In the attachment energy range of 0-10 eV, three shape resonances for serine 1, serine 2, and serine 4 and four shape resonances for serine 3 are predicted. The one-dimensional potential energy curves of the temporary negative ions of electron-serine are calculated to explore the correlations between the shape resonance and the bond cleavage. The bond-cleavage selectivity of the different resonant states for a certain conformer is demonstrated, and the recent experimental results about the dissociative electron attachment to serine are interpreted on the basis of present calculations.     

Pressure and temperature dependence of dissociative and non-dissociative electron attachment to CF3: experiments and kinetic modeling

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.     

VO2+ ions in zinc lead borate glasses studied by EPR and optical absorption techniques

Electron paramagnetic resonance (EPR) and optical absorption spectra of vanadyl ions in zinc lead borate (ZnO-PbO-B2O3) glass system have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of VO2+ ions. The values of spin-Hamiltonian parameters indicate that the VO2+ ions in zinc lead borate glasses were present in octahedral sites with tetragonal compression and belong to C4V symmetry. The spin-Hamiltonian parameters g and A are found to be independent of V2O5 content and temperature but changing with ZnO content. The decrease in Deltag( parallel)/Deltag( perpendicular) value with increase in ZnO content indicates that the symmetry around VO2+ ions is more octahedral. The decrease in intensity of EPR signal above 10 mol% of V2O5 is attributed to a fall in the ratio of the number of V4+ ions (N4) to the number of V5+ ions (N5). The number of spins (N) participating in resonance was calculated as a function of temperature for VO2+ doped zinc lead borate glass sample and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility was calculated at various temperatures and the Curie constant was evaluated from the 1/chi-T graph. The optical absorption spectra show single absorption band due to VO2+ ions in tetragonally distorted octahedral sites.     

Electron attachment to chlorouracil: a comparison between 6-ClU and 5-ClU

Low energy electron impact to the isomers 6-chlorouracil (6-ClU) and 5-chlorouracil (5-ClU) yields a variety of negative ion fragments with surprisingly high cross sections. These ions are dominantly formed via sharply structured resonance features at energies below the threshold for electronic excitation and result from dissociative electron attachment (DEA). The most dominant DEA channel is formation of (M-HCl)-, i.e., ejection of a neutral HCl molecule with the negative charge remaining on the ring. The reaction cross section is 9 x 10(-18) m2 and 5 x 10(-18) m2 for 6-Cl and 5-ClU, respectively, and thus about two orders of magnitude higher than the geometrical cross section of the molecule. Further reactions also operative via low energy resonances (<2.5 eV) are Cl- abstraction, dehydrogenation [formation of (M-H)-, M=ClU], and DEA processes associated with a ring opening. Most of the ion yield curves exhibit remarkably sharp structures which have not been observed before in DEA to a polyatomic system. Although some possibilities on their origin are discussed, their interpretation remains a challenge for theory and further experiments. While electron attachment to both 6-ClU and 5-ClU generates fragments of the same stoichiometric composition, their ion yields and also their relative intensities show some very pronounced differences which can be explained by the different structure but also the different energetic situation in the two isomers.     

Dissociative electron attachment to Pt(PF3)4--a precursor for focused electron beam induced processing (FEBIP)

Experimental absolute cross sections for dissociative electron attachment (DEA) to Pt(PF(3))(4) are presented. Fragment anions resulting from the loss of one, two, three and four PF(3) ligands as well as the Pt(PF(3))F(-) and the F(-) ions were observed. The parent anion Pt(PF(3)) is too short-lived to be detected. The dominant process is loss of one ligand, with a very large cross section of 20?000 pm(2); the other processes are about 200× weaker, with cross sections around 100 pm(2), the naked Pt(-) anion is formed with a cross section of only 1.8 pm(2). The resonances responsible for the DEA bands were assigned based on comparison with electron energy-loss spectra and spectra of vibrational excitation by electron impact. Bands around 0.5 eV and 2 eV were assigned to shape resonances with single occupation of virtual orbitals. A DEA band at 5.9 eV was assigned to a core-excited resonance corresponding to an electron very weakly bound to the lowest excited state. An F(-) band at 12.1 eV is assigned to a core excited resonance with a vacancy in an orbital corresponding to the 2nd ionization energy of the PF(3) ligand. Implications of these findings for FEBIP are discussed.     

ESR and resonance Raman spectroscopic studies of peroxide intermediates derived from iron diazaoxacyclononane

A peroxide-Fe3+ intermediate generation during the Fenton reaction of iron chelate involving a ligating N,N'-di-2-picolyl-4, 7-diaza-1-oxacyclononane (DPC), H2O2/[Fe2+ DPC]2+, is reported. The identity of this peroxide complex is confirmed by resonance Raman (RR) and electron spin resonance (ESR) spectroscopies. The RR spectrum of [Fe2+ DPC]2+ treated with H2O2 shows a frequency at 854 cm(-1) ascribable to v(O-O) vibrational modes of the peroxide-Fe3+ complex with a side-on geometry. On the other hand, the ESR spectrum of H2O2/[Fe2+ DPC]2+ acquired at 77 K exhibits the resonance transition at g = 2.196 and 2.017 due to the peroxide-Fe3+ complex with an axial symmetry. It has been concluded that the H2O2/[Fe2+ DPC]2+ reaction proceeds by rapid bonding of H2O2 to an open coordination site on the central Fe2+ cation.     

Fe(3+) ions in alkali lead tetraborate glasses--an electron paramagnetic resonance and optical study

Glass systems of composition 90R(2)B(4)O(7)+9PbO+1Fe(2)O(3) (R=Li, Na and K) and 90Li(2)B(4)O(7)+(10-x)PbO+xFe(2)O(3) (x=0.5, 1, 3, 4, 5, 7 and 9 mol %) have been investigated by means of electron paramagnetic resonance (EPR) and optical absorption techniques. The EPR spectra exhibit three resonance signals at g approximately 6.0, 4.2 and 2.0. The resonances at g approximately 6.0 and 4.2 are attributed to Fe(3+) ions in rhombic and axial symmetry sites, respectively. The g approximately 2.0 resonance signal is due to two or more Fe(3+) ions coupled together with dipolar interaction. The EPR spectra of 1 mol % of Fe(2)O(3) doped in lithium lead tetraborate glass samples have been studied at different temperatures (123-433 K). The intensity of g approximately 4.2 resonance signal decreases and the intensity of g approximately 2.0 resonance signal increases with the increase of temperature. The line widths are found to be independent of temperature. The EPR spectra exhibit a marked concentration dependence on iron content. A decrease in intensity for the resonance signal at g approximately 4.2 with increase in iron content for more than 4 mol % has been observed in lithium lead tetraborate glass samples and this has been attributed to the formation of Fe(3+) ion clusters in the glass samples. The paramagnetic susceptibility (chi) is calculated from the EPR data at various temperatures and the Curie constant (C) has been evaluated from 1/chi versus T graph. The optical absorption spectrum of Fe(3+) ions in lithium lead tetraborate glasses exhibits three bands characteristic of Fe(3+) ions in an octahedral symmetry. The crystal field parameter D(q) and the Racah interelectronic repulsion parameters B and C have also been evaluated. The value of interelectronic repulsion parameter B (825 cm(-1)) obtained in the present work suggests that the bonding is moderately covalent.     

High resolution electron attachment to CO₂ clusters

Electron attachment to CO? clusters performed at high energy resolution (0.1 eV) is studied for the first time in the extended electron energy range from threshold (0 eV) to about 10 eV. Dissociative electron attachment (DEA) to single molecules yields O(-) as the only fragment ion arising from the well known (2)Π(u) shape resonance (ion yield centered at 4.4 eV) and a core excited resonance (at 8.2 eV). On proceeding to CO? clusters, non-dissociated complexes of the form (CO?)(n)(-) including the monomer CO?(-) are generated as well as solvated fragment ions of the form (CO?)(n)O(-). The non-decomposed complexes appear already within a resonant feature near threshold (0 eV) and also within a broad contribution between 1 and 4 eV which is composed of two resonances observed for example for (CO?)(4)(-) at 2.2 eV and 3.1 eV (peak maxima). While the complexes observed around 3.1 eV are generated via the (2)Π(u) resonance as precursor with subsequent intracluster relaxation, the contribution around 2.2 eV can be associated with a resonant scattering feature, recently discovered in single CO? in the selective excitation of the higher energy member of the well known Fermi dyad [M. Allan, Phys. Rev. Lett., 2001, 87, 0332012]. Formation of (CO?)(n)(-) in the threshold region involves vibrational Feshbach resonances (VFRs) as previously discovered via an ultrahigh resolution (1 meV) laser photoelectron attachment method [E. Leber, S. Barsotti, I. I. Fabrikant, J. M. Weber, M.-W. Ruf and H. Hotop, Eur. Phys. J. D, 2000, 12, 125]. The complexes (CO?)(n)O(-) clearly arise from DEA at an individual molecule within the cluster involving both the (2)Π(u) and the core excited resonance.     

The formation of NO+ from the reaction of N2(2+) with O2

We have studied the potentially ionospherically significant reaction between N(2)2+ with O2 using position-sensitive coincidence spectroscopy. We observe both nondissociative and dissociative electron transfer reactions as well as two channels involving the formation of NO+. The NO+ product is formed together with either N+ and O in one bond-forming channel or O+ and N in the other bond-forming channel. Using the scattering diagrams derived from the coincidence data, it seems clear that both bond-forming reactions proceed via a collision complex [N2O2]2+. This collision complex then decays by loss of a neutral atom to form a daughter dication (NO2(2+) or N2O2+), which then decays by charge separation to yield the observed products.     

固载型光化学传感器的形貌及结构对检测Hg2+性能的影响

采用氨丙基三乙氧基硅烷(APTES)、异氰酸酯基三乙氧基硅烷(Tri)及纳米金颗粒作为连接体,纯硅中孔分子筛HMS作为无机载体,以分子改造后的染料罗丹明B(RhB)作为有机分子,制得3种固载型光化学传感器,用于检测水中的Hg2+.采用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、N2物理吸附和傅里叶变换红外光谱(FTIR)对材料进行表征,并利用荧光光谱检测水溶液中的Hg2+.结果表明,3种连接体均成功地将有机分子RhB固载到HMS上,所制备的固载型光化学传感器均能够检测水溶液中的Hg2+.研究发现,固载后样品的表面形貌及孔道结构对其检测能力产生影响,具有完好表面形貌及孔道结构的样品RhB-Au-HMS对于Hg2+的检测能力最强,内部孔道坍塌的样品RhB-APTES-HMS对于Hg2+的检测能力最弱.     

Kinetics of electron attachment to OH and HNO3 and mutual neutralization of Ar+ with NO2(-) and NO3(-) at 300 and 500 K

The electron attachment rate constant to nitric acid (HNO(3)) has been measured in a flowing afterglow-Langmuir probe (FALP) apparatus at 300 and 500 K using three independent methods: the traditional FALP technique of monitoring electron depletion, "one-gas" VENDAMS (variable electron and neutral density attachment mass spectrometry), and "two-gas" VENDAMS. The three measurements are in agreement with a 300 K weighted average of 1.4 ± 0.3 × 10(-7) cm(3) s(-1), 2 to 10 times higher than previously reported values. Attachment is primarily dissociative yielding NO(2)(-) as previously reported, but for the first time a small endothermic channel to produce OH(-) was also observed at 500 K. From the one-gas VENDAMS data, associative attachment to the OH produced in the primary attachment was found to occur with an effective two body rate constant of 1.2±(0.7) (3)×10(-11) cm(3) s(-1) at 300 K, the first reported rate constant for this radical species. Finally, ion-ion neutralization rate constants of NO(2)(-) and NO(3)(-) with Ar(+) were determined to be 5.2±(2.5) (1.5) × 10(-8) and 4.5 ± 2.5 × 10(-8) cm(3) s(-1) at 300 K, respectively.     

氧原子与二硫化碳反应的机理

用从头计算法、内禀反应坐标和电子密度拓扑分析方法研究了3P态氧原子与二硫化碳的反应.找到了分别形成CS SO,S OCS和S2 CO三个反应通道上的极小点和过渡态.采用UHF/631G进行几何构型优化,并在UMP2/631G水平上进行能量校正.三个反应通道上的稳定点和过渡态都经内禀反应坐标(IRC)跟踪得以确认,并用电子密度拓扑分析方法考察了反应过程中化学键的变化.计算结果表明,反应过程中所有稳定点和过渡态都具有Cs对称性,即对每个反应通道而言在整个反应过程中分子始终保持在同一平面内.在三个反应通道中,第一个反应通道O CS2→CS SO由于具有较小的活化能而更容易发生,与实验结果相一致.文中对反应机理进行了较详细的讨论.     

二氧化三氮(N3O2)中性分子和离子的一些重要反应过程的理论研究

二重态的N3O2中性分子作为中间体, 在N3O2阴离子的光解离反应和NO+N2O←→N2+NO2反应中均起重要作用. 在CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p)+ZPE的水平上, 对这两个反应进行了理论计算. 结果表明, 在N3O2阴离子的光解离反应中, 该阴离子先在光照下解离为与其具有相同的W构型的中性分子和一个电子, 这个中性分子是一个过渡态, 它将打破C2v构型变成具有Cs对称性的W型中间异构体, 然后再经过一个过渡态, 裂解成N2O+NO两个小分子. 这个裂解过程的能垒非常低(5.96 kJ/mol), 因此在实验中很难检测到W型的中间异构体. 在另一个重要的[N3O2]体系的反应(NO+N2O←→N2+NO2)中, 找到了两条反应通道, 其中不经过中间异构体的一步转化通道更为可行.     

Electron attachment and detachment, and the electron affinities of C5F5N and C5HF4N

Rate constants have been measured for electron attachment to C5F5N (297-433 K) and to 2, 3, 5, 6-C5HF4N (303 K) using a flowing-afterglow Langmuir-probe apparatus (at a He gas pressure of 133 Pa). In both cases only the parent anion was formed in the attachment process. The attachment rate constants measured at room temperature are 1.8 +/- 0.5 X 10(-7) and 7 +/- 3 X 10(-10) cm(-3) s(-1), respectively. Rate constants were also measured for thermal electron detachment from the parent anions of these molecules. For C5F5N- detachment is negligible at room temperature, but increases to 2530 +/- 890 s(-1) at 433 K. For 2, 3, 5, 6-C5HF4N-, the detachment rate at 303 K was 520 +/- 180 s(-1). The attachment/detachment equilibrium yielded experimental electron affinities EA(C5F5N)=0.70 +/- 0.05 eV and EA(2, 3, 5, 6-C5HF4N)=0.40 +/- 0.08 eV. Electronic structure calculations were carried out for these molecules and related C5HxF5-xN using density-functional theory and the G3(MP2)//B3LYP compound method. The EAs are found to decrease by 0.25 eV, on average, with each F substitution by H. The calculated EAs are in good agreement with the present experimental results.     

Dissociative photoionization of N(2)O in the region of the N(2)O(+)(B (2)Pi) state studied by ion-electron velocity vector correlation

Dissociative direct photoionization of the N2O(X 1Sigma+) linear molecule via the N2O+(B 2Pi) ionic state induced by linearly polarized synchrotron radiation P in the 18-22 eV photon energy range is investigated using the (VA+,Ve,P) vector correlation method, where VA+ is the nascent velocity vector of the NO+, N2+, or O+ ionic fragment and Ve that of the photoelectron. The DPI processes are identified by the ion-electron kinetic energy correlation, and the IchiA+(thetae,phie) molecular frame photoelectron angular distributions (MFPADs) are reported for the dominant reaction leading to NO+ (X 1Sigma+,v) + N(2D)+ e. The measured MFPADs are found in satisfactory agreement with the reported multichannel Schwinger configuration interaction calculations, when bending of the N2O+(B 2Pi) molecular ion prior to dissociation is taken into account. A significant evolution of the electron scattering anisotropies is observed, in particular in the azimuthal dependence of the MFPADs, characteristic of a photoionization transition between a neutral state of Sigma symmetry and an ionic state of Pi symmetry. This interpretation is supported by a simple model describing the photoionization transition by the coherent superposition of two ssigma and ddelta partial waves and the associated Coulomb phases.