Dissociative Ionization Processes Research Articles

Femtosecond dynamics of multielectron dissociative ionization by use of a picosecond laser.

The multiple ionization of molecular nitrogen has been studied by use of an intense picosecond laser (0.6 ps: 3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$: wavelength 600 nm). By measurement of the energies of the various fragment atomic ions (${\mathrm{N}}^{+}$, ${\mathrm{N}}^{2+}$, and ${\mathrm{N}}^{3+}$) with a time-of-flight mass spectrometer it has proved possible to investigate the dynamics of the multielectron dissociative ionization process on a time scale of about 30 fs.

Molecular beam photoionization study of S2

Higher resolution photoionization efficiency data for the formation of S+2 and S+ from S2 in the wavelength region of 600–1350 Å have been obtained using the supersonic oven beam method. The ionization energy (IE) of S2 is determined to be 9.356±0.002 eV (1325.2±0.3 Å). The measured appearance energy of 14.732±0.005 eV (841.6±0.3 Å) for the dissociative ionization process S2+hν→S++S+e−, together with the IE of S2, yields a value of 5.376±0.005 eV for the bond dissociation energy of S+2. Two Rydberg series converging to the b̃ 4Σ−g state (13.20 eV) of S+2 are observed. Window resonances resolved in the wavelength region of 670–850 Å are assigned as members of the Rydberg series converging to the c̃ 4Σ−u (17.70 eV) and 2Πu (or 2Σ−u) (18.66 eV) states of S+2.

Unimolecular decay of CF 4+ and CCl 4+

The results of a search for metastable dissociations of CF 4 + and CCl 4 + are presented. These ions are produced by electron ionization of CF 4 and CCl 4, respectively, and metastable (and collision-induced) fragmentations are studied with a double focussing mass spectrometer by decoupling the acceleration and analyzer fields. It was possible to detect metastable transitions, but only with small intensities. No stable CF 4 + or CCl 4 + has been observed in accordance with earlier experiments. The present observation confirms (1) the theoretical study of the molecular structure and energies of CCl 4 + by Bews and Glidewell [17], predicting that CCl 4 + has several bound states accessible by electron ionization of CCl 4, but that these CCl 4 + states are separated by only a low potential barrier from fragment ions, and (2) the experimental evidence presented by Brehm et al. [15] indicating some sort of predissociation involved in the dissociative ionization process CF 4 + e → CF 3 + + F + e

Absolute dipole oscillator strengths for photoabsorption, photoionization and ionic photofragmentation processes in HBr

Abstract Absolute dipole oscillator strengths (cross section) have been obtained for valence shell photoabsorption (7–100eV) and a variety of partial photoionization (11–40 eV) processes in gaseous HBr. Partial dipole oscillator strengths are reported for the formation of the X 2 Π, A 2 Σ + and B 2 Σ electronic state of HBr + as well as the respective photoelectron branching ratios. The photoelectron binding energy spectra show clear evidence of many-body effects in photoionization to the B 2 Σ state of HBr + with the ionization oscillator strength divided over many bands as predicted by many-body Green's function calculations. Partial dipole oscillator strengths are also reported for molecular ion formation as well as for all dissociative ionization processes. The measurements have been made by the dipole (e,e) (e,2e) and (e,e + ion) methods, which respectively provide quantitative measurements of photoabsorption, photoelectron spectroscopy and photoionization mass spectrometry at continuously tuneable energies. The measurements of dipole oscillator strengths for production of electronic states of HBr + are combined with those for molecular and dissociative photoionization. These, considered together with the ionization and appearance potentials, provide a quantitative dipole breakdown picture for the ionic photofragmentation pathways of HBr in the energy region up to 40 eV.

Coincidence studies of fluorescence and dissociation processes in electronic excited states of I2+, Br2+, IBr+ and ICl+

The PIFCO technique in which mass-selected photoion—fluorescence photon coincidences are counted, was used to investigate whether I2+, IBr+ and ICl+ fluoresce. Measurements were made of lifetimes and fluorescence quantum yields of electronic excited states of these ions. Emission was discovered for I2+ and IBr+, but ICl+ apparently does not fluoresce. Information on the radiative properties of Br2+ was obtained as a by-product of the work on IBr+. Fragment ion kinetic energy releases were determined and provide information on dissociative ionization processes in the halogen and interhalogen ions studied.

Mesure experiementale de l'electroaffinite du germanium, de l'etain et du plomb par attachement electronique dissociatif sur les sulfures de ces elements

The process of dissociative ionization of GeS, SnS and PbS by electron impact has been investigated by systematic measurements of ion kinetic energies using an electrostatic energy-analyzer. The energetic balances of the processes responsible for the formation of Ge −, Sn −, Pb − by dissociative electronic attachment allowed calculation of the atomic electron affinities of Ge (1.2 ±0.1 eV), Sn (1.4 ±0.1 eV) and Pb (1.2 ±0.1 eV).

Mass-spectral behavior of substituted 2-alkylaminobenzoxazoles and 3-methyl-2-alkyliminobenzoxazolines

Schemes that reflect the peculiarities of each of the investigated groups of compounds and make it possible to distinguish the amino and imino forms are given on the basis of a study of the dissociative ionization processes of 18 2-alkyl(dialkyl)-aminobenzoxazoles and six 3-methyl-2-alkyliminobenzoxazolines. A qualitative relationship between the electronic properties of the substituents in the aromatic ring and the formation of the imino forms of the molecular ions in the gas phase was found.

Dissociative and double photoionization of CO from threshold to 90 Å

Partial cross sections for molecular photoionization (CO+), dissociative photoionization (C+ and O+), and dissociative double photoionization (C2+) in CO have been measured from their thresholds to 90 Å using techniques of mass spectrometry. The results are compared with data reported previously. Several peaks observed in the cross section curves for dissociated fragments are tentatively assigned by comparing with those in the photoelectron spectra reported for CO. It is concluded that the shoulder in the total absorption cross section curve between 400 and 90 Å results solely from the dissociative ionization processes.

Dissociative and double photoionization of CO2 from threshold to 90 Å

The molecular photoionization [Math], dissociative photoionization (CO + O + and C + ) and double photoionization (CO 2+ and C 2+ ) cross sections for CO 2 have been measured from their onsets down to 90 A by using various combinations of mass spectrometers (a coincidence time of-flight mass spectrometer and a magnetic mass spectrometer) and light sources (synchrotron radiation, and glow and spark discharge). It Is concluded that the one broad peak and the three shoulders In the total absorption cross section curve between 640 and 90 A are caused completely by dissociative ionization processes. Several peaks observed In the cross section curve for the total fragmentation (CO + , O + and C + ) are compared with those in the photoelectron spectrum reported for CO 2 . The accuracy of the present cross sections Is estimated to be about 10, 15 and 50 % for σ (CO 2 ) σ (CO + , O + and C + ) and σ (CO 2+ and C 2+ ), respectively.

Predicted electron transport coefficients at high E/N values. II. Nitrogen

Transport coefficients of electrons in nitrogen are calculated over the range 10−16⩽E/N⩽7.3×10−15 V cm2 by solving the Boltzmann equation using the classical two-term expansion in spherical harmonics. However, as in the case of H2 treated in a previous paper, the sources of new electrons produced by ionization has been accounted for in the Boltzmann equation. It is shown that the release of these new electrons has an effect on the distribution function starting from E/N≃2×10−15 V cm2, which increases rapidly as E/N increases. The recently available cross sections for the excitation of the electronic states of N2 have been used along with the total-momentum-transfer cross section derived by Cartwright. The predicted values of the ionization coefficient are in very good agreement with the experimentally measured values except at low E/N values, for which our predicted ionization coefficients are slightly less than the experimental ones. The agreement is also good for the drift velocities over the entire E/N range, while for the characteristic energies the agreement becomes less satisfactory for E/N≳2.5×10−15 V cm2. The electron excitation rates for the first eight vibrational levels, the A 3Σ+u, B 3Πg, W 3Δu, and C 3Πu electronic states are presented as a function of E/N. The ionization rate and the total dissociation rate (including the dissociative ionization process) are also given as a function of E/N. The percentage of the input power which is delivered to the elastic and the various inelastic processes are presented as a function of E/N.

1,1-Dimethyl-1-silaethylene : Heat of formation, ionization potential and the energy of the siliconcarbon π-bond

The thermodynamic cycle consisting of thermal decomposition and dissociative ionization processes for 1,1-dimethyl-1-silacyclobutane is calculated. The heat of formation and the ionization potential ( IP) for 1,1-dimethyl-1-sila-ethylene (DMSE) have been obtained: Δ H o f(DMSE) = 15.5 ± 5 kcal/mol; IP(DMSE) = 7.5 ± 0.3 eV. The siliconcarbon π-bond energy in DMSE is estimated: D π(SiC)  28 ± 8 kcal/mol.

Multiphoton excitation and dissociative ionization of SF 6

A simple model of multiphoton excitation and dissociative ionization process of SF 6 is proposed. The model is compared with experimental results of Brunner et al. It was found that a three-photon absorption law reproduced the experimental results with good agreement. In addition the relative enhancement ratios of dissociative ionization of excited SF 6 were evaluated.

Vacuum ultraviolet fluorescence from photodissociation fragments of CO and CO2

The vacuum ultraviolet fluorescence from photofragments of CO and CO2 has been studied and the cross sections for the production of fluorescence have been measured. The fragments were produced by interactions of the gases with synchrotron radiation in the wavelength region λλ175–800 Å. The dependence of the fluorescence intensity on gas pressure was studied and at high pressure additional fluorescence induced by photoelectrons was observed. Discrete features were observed in CO in the primary photon wavelength region λλ570–610 Å. Similar features were observed in CO2 in the λλ690–800 Å region corresponding to Rydberg states converging to the CO+2 (A 2Π) and CO+2 (B 2Σ+) states. The observed structure results from interactions of Rydberg states with dissociation continua. At shorter primary photon wavelengths the cross sections for the production of fluorescence from photofragments of CO decrease monotonically from 3.6 Mb at λ480 Å to 1.2 Mb at λ180 Å. In contrast with this, two broad bands were observed in the fluorescence from photofragments of CO2 with peak cross sections of 0.73 Mb at λ230 Å. The photodissociation and dissociative ionization processes that produce the observed results are discussed.

Dissociative ionization of H 2, N 2 and CO by electron impact — measurement of kinetic energy, angular distributions, and appearance potentials

For investigating dissociative ionization processes induced by electron impact, an apparatus was constructed which enables the kinetic energy, angular distributions and appearance potentials of the observed ions to be measured. Special ion optics were designed in order to avoid kinetic energy discrimination effects. Kinetic energy and angular distributions of H +(H 2), N +(N 2), O +(CO) and C +(CO) ions and appearance potentials of O +(CO) and C +(CO) ions of given kinetic energies were measured. Reaction models are proposed and discussed in terms of potential energy curves by correlating them with the kinetic energy distributions and appearance potentials. In contrast to H +(H 2) and N +(N 2), the O +(CO) and C +(CO) angular distributions measured here cannot be interpreted by existing theories.

Vacuum ultraviolet fluorescence from photodissociation fragments of N2O

The vacuum ultraviolet fluorescence from fragments of N2O has been studied and the fluorescence production cross sections have been measured. The fragments are produced by extreme ultraviolet excitation using synchrotron radiation in the wavelength region lambda lambda 175-750 AA. Structure appearing in the lambda lambda 620-680 AA region has been identified as the absorption Rydberg series VI and VII. Two broad fluorescence bands were observed with peak cross sections of 0.6 Mb at lambda 370 AA and 0.4 Mb at lambda 230 AA. The photodissociation and dissociative ionization processes that are probably responsible for the observed fluorescence are discussed.

Vacuum ultraviolet fluorescence from photodissociation fragments of O2 and N2

The vacuum ultraviolet fluorescence from the atomic photofragments of O2 and N2 produced by extreme ultraviolet excitation has been investigated and their production cross sections measured. The processes were studied using synchroton radiation, in the λλ175–780 Å region, to excite the gases. The nature of the fluorescence spectrum in the wavelength range λλ1050–1800 Å was studied by using a combination of optical and gaseous filters. The dependence of the intensity of the fluorescence radiation on the gas pressure was studied in order to determine the relative contributions from direct excitation and photoelectron excited emissions. Direct excitation processes were verified to be the primary contributor, particularly at low pressures. In the incident wavelength range λλ200–350 Å, the cross sections for the production of fluorescence for each gas shows a broad band, which is presumably produced by dissociative ionization processes. In the longer wavelength region, discrete structure was observed at the positions of known Rydberg series. These features are the result of the interaction of dissociation continuum states with bound states. Photodissociation continua were also observed in the λλ400–780 Å region.

Electron energy dependence of the kinetic energy and angular distributions of O+ from dissociative ionization of O2

The electron energy dependence of the kinetic energy and angular distributions of O+ have been measured for the process of dissociative ionization of O2. The kinetic energy distributions for energies above 0.5 eV are compared with other dissociative ionization measurements as well as results of dissociative photoionization and dissociative excitation by electrons leading to production of Rydberg excited neutral fragments. Angular distributions are interpreted in terms of symmetries of possible molecular ion states contributing to dissociation, and the results near threshold are compared with recent measurements of O− from dissociative ion pair formation in O2. Information about the states of excitation of the dissociation fragments as a function of ion energy is obtained from appearance potential measurements. The results indicate that predissociation may be important and that dissociative ionization of O2 is in general a complicated process involving many O2+ states.

Dissociative ionization by electron impact. I. Protons from H2

A new apparatus designed to investigate various aspects of the electron impact dissociative ionization process is described in detail. It is shown that the angular distributions obtained for protons from the 2 Sigma u+ state of H2+ are consistent with theoretical arguments and confirm the results of previous measurements. The first angular distribution of protons from the 2 Sigma g+ state of H2+ is also reported. Absolute cross sections for the production of protons from both the 2 Sigma g+ and 2 Sigma u+ state of H2+ at electron energies close to the respective thresholds are also presented. The ratio H+/H2+ is measured as a function of incident electron energy for ions originating from the 2 Sigma g+ state.

The mass spectra of some 1,3,2-oxazaphospholanes

The mass spectra of six 1,3,2-oxazaphospholanes have been obtained. The compounds studied give strong m/e=M lines; the relative stability of the molecular ion correlates with the structure of the molecule. During the process of dissociative ionization, cleavage of the C-C and the P-O bonds in the ring takes place, with the molecular ion losing a R'CHO molecule. It has been shown that the molecule of an olefin is formed from the molecular ion mainly through an alkoxy group. A probable process of dissociative ionization is the loss by the molecular ion of an alkoxy group or, in the case of the 2-chloro derivative, the loss of a Cl atom.

Dissociative ionization of methyl-substituted hydroxyquinolines

The mass spectra of the following methyl-substituted hydroxyquinolines have been studied in the range of energies of the ionizing electrons of 15–50 eV: 2-hydroxy-4-methylquinoline, 6-hydroxy-2-methylquinoline, 6-hydroxy-4-methylquinoline, 8-hydroxy-2-methylquinoline, 8-hydroxy-4-methylquinoline, 2,6-dihydroxy-4-methylquinoline, and 4,6-dihydroxy-2-methylquinoline. It has been shown that the processes of dissociative ionization for the series of compounds investigated take place exclusively from the keto forms of the molecular ions. The stability to electron impact is determined largely by the degree of enolization of the structures considered. The values of the selective decomposition are given and a scheme is proposed for the identification of the isomeric methyl-substituted hydroxyquinolines.