Dissociative Excitation Cross Sections Research Articles

Dissociative recombination of NO+

Cross sections for dissociative recombination and dissociative excitation of ${\mathrm{NO}}^{+}$ in the vibrational ground state have been measured at a heavy-ion storage ring. Branching ratios for dissociative recombination into different electronic states of the atomic fragments have also been determined by an imaging technique. More than 80% of the dissociative recombination events at E = 0 eV produced $\mathrm{N}{(}^{2}D)$ + O${(}^{3}P)$. The rotational temperature of the ${\mathrm{NO}}^{+}$ ion beam was changed by a factor of 2 by employing different types of ion sources. The branching ratios and the dissociative recombination cross section as a function of energy were independent of the rotational temperature. The dissociative recombination cross section was proportional to ${E}^{\ensuremath{-}1}$ at low energies and had peaks in the energy regions 3--8 eV and 10--16 eV. We obtain a total dissociative recombination rate $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{3}/\mathrm{s}$ at $T=300$ K for ${\mathrm{NO}}^{+}$ in the vibrational ground state.

Non-Franck-Condon electron-impact dissociative-excitation cross sections of molecular hydrogen producingH(1s)+H(2l)throughX1Σg+(v=0)→{B1Σu+,B′1Σu+,C1Πu}

Dissociation cross sections of H${}_{2}$ for high-energy electron impact (100--1000 eV) producing $H(1s)$, $H(2s)$, and $H(2p)$ for excitation from the ground vibrational state $(v=0)$ to the continuum of the ${B}^{1}{\ensuremath{\Sigma}}_{u}^{+}$, ${B}^{\ensuremath{'}}{}^{1}{\ensuremath{\Sigma}}_{u}^{+}$, and $C{}^{1}{\ensuremath{\Pi}}_{u}$ states were computed in the first Born approximation. Configuration-interaction electronic wave functions were used and vibrational degrees of freedom taken in account. The dissociative excitation cross sections as a function of the continuum energy for each final state were presented, and the accuracy of the wave function, including the importance of relaxation effects and the validity of the Franck-Condon approximation, is analyzed in comparison to available previous theoretical results. The computed dissociation cross sections were compared to experimental results making use of the separation of the various breakup channels proposed by Ajello, Shemansky, and James [Astrophys. J. 371, 422 (1991)]. The obtained cross sections to produce $\mathrm{H}(2p)+\mathrm{H}(1s)$ fragments via dissociative excitation to the $B$ and $C$ states have agreed well with the decomposed experimental results within the error bars. The dissociation cross sections to produce $\mathrm{H}(2s)+\mathrm{H}(1s)$ through the ${B}^{\ensuremath{'}}$ state were in most cases somewhat larger than the reported experimental error bars. In the most favorable case our theoretical ${B}^{\ensuremath{'}}$ dissociation cross section was 3.1% within the reported error bar at 300 eV electron impact energy. A possible experimental reason for this discrepancy was raised.

Dissociative recombination and excitation of N2+: Cross sections and product branching ratios

The absolute dissociative recombination and absolute dissociative excitation rate coefficients and cross sections have been determined for N2+ and electrons for collision energies between 10 meV and 30 eV. The ion storage ring CRYRING has been used in combination with an imaging technique with a position-and-time-sensitive detector. Information is retrieved on the ion beam vibrational state populations and on the product branching in the dissociative recombination process at 0 eV collisions. A hollow cathode ion source has been used to lower the vibrational excitation in the ion beam; a more traditional hot-cathode ion source was used as well. The most important findings are the following. The rate coefficient for an N2+ ion beam (46%, v=0, 27% v=1) versus electron temperature (K) is α(Te)=1.75(±0.09)×10−7(Te/300)−0.30 cm3 s−1. The dissociative recombination rate is found to be weakly dependent on the N2+ vibrational level. At 0 eV collision energy, the v=0 product branching is found to be 0.37(8):0.11(6):0.52(4) for N(4S)+N(2D):N(2P)+N(4S):N(2D)+N(2D) fragments. The dissociative recombination cross section does not have a high-energy peak as was found in a number of lighter molecular systems. The dissociative excitation signal starts only slightly above the energy threshold for dissociation, and peaks near 25 eV. From the dissociative excitation data and literature data, information is retrieved on the dissociative ionization of N2+. The comparison of these results with earlier DR measurements is extensively discussed.

Dissociative-recombination and excitation measurements with <emb><base>H</base><sup>n+></sup><sub>2</sub></emb> and <emb><base>HD</base><sup>+</sup></emb>s

The cross sections for dissociative recombination (DR) and dissociative excitation (DE) have been measured for the Hn+>2 and HD+ molecular ions in the energy range from 0 to \ensuremath{\sim}30 eV. An imaging technique that provides a measure of the kinetic energy release in the DR process was used to yield information about the initial vibrational distribution of the molecular ions and the final atomic states of the DR reaction. The results for DR with HD+ in the vibrational ground level, \ensuremath{\nu}=0, are compared with other measurements and recent multichannel quantum defect theory calculations. Significant deviations that appear at high energy \ensuremath{\sim}5--10 eV are discussed. The present work reports on an absolute DE measurement with HD+ . The vibrational distribution of Hn+>2 was manipulated by photodissociation of ions in vibrationally excited levels. We obtained DE and DR cross sections for Hn+>2 ions populating primarily the two lowest vibrational levels \ensuremath{\nu}=0,1 and Hn+>2 ions in a broad vibrational distribution. It is found that DR of Hn+>2 with zero energy electrons results in the H(n=1)+H(n=2) final channel, open for all vibrational levels, and in the H(n=1)+H(n=3) channel, which is energetically open for initial vibrational levels \ensuremath{\nu}\ensuremath{\geqslant}5.

Dissociative recombination and dissociative excitation of 4HeH+: Absolute cross sections and mechanisms.

Absolute cross sections have been determined for the dissociative recombination and dissociative excitation of $^{4}\mathrm{HeH}^{+}$ for electron energies below 40 eV. The dissociative recombination cross section is in semiquantitative agreement with recent theoretical results by Sarpal, Tennyson, and Morgan [J. Phys. B 27, 5943 (1994)] and Guberman [Phys. Rev. A 49, R4277 (1994); in XIXth International Conference on the Physics of Electronic and Atomic Collisions, Whistler, Canada, AIP Conf. Proc. No. 360, edited by L. J. Dube, J. B. A. Mitchell, J. W. McConkey, and C. E. Brion (AIP, New York, 1995), p. 307]. The calculated resonant structure below a collision energy of 1 eV was not fully reproduced by the experiment. The quantum states of the dissociative recombination products at 0 eV collision energy have been determined; ground-state helium and excited hydrogen atoms (n=2) are dominantly formed, in agreement with recent predictions by Guberman. The dissociative excitation has an onset around 10 eV and follows the shape of the dissociative recombination cross section, illustrating that both processes start with the formation of doubly excited neutral states that lie in the ionization continuum as well as in the dissociation continuum. The dissociative excitation cross section is in quite good agreement with recent calculations by Orel and Kulander. \textcopyright{} 1996 The American Physical Society.

Photodissociation of CO: partial cross sections for neutral dissociative excitation

Dispersed vacuum-ultraviolet fluorescence from neutral dissociation fragments in the wavelength range from 100 - 200 nm has been measured after photodissociation of CO as a function of the exciting photon energy between 18.5 and 29 eV. The decay channels of neutral dissociative excitation of the formed CO Rydberg states converging to the D and C states have been analysed. The vibrationally excited levels of the and Rydberg states preferentially dissociate into the fragments . Relative cross sections for the observed lines of and have been obtained. For the lines at 193, 167.5 and 156 nm estimations for absolute cross sections are presented. Evidence for superexcited states has been found in the exciting photon energy range between 24 and 27.5 eV.

Cross sections of dissociative excitation of Li I in collisions of electrons with lithium iodide molecules

Cross sections of dissociative excitation of Li I in collisions of electrons with lithium iodide molecules

Effective potential methods in variational treatments of electron-molecule collisions. II. Application to HBr

We report the results of variational calculations on low energy e−+HBr collisions using the complex Kohn method. We compare the results of all-electron numerical calculations with those in which effective core potentials are used. We present total, differential, and momentum transfer cross sections for electronically elastic scattering, as well as dissociative excitation cross sections for the low-lying electronic states that dissociate to ground-state neutral atoms. We find excellent agreement between the all-electron and core-potential results for all processes considered.

Electron-impact dissociative excitation of H 2 + : low energy studies

Electron impact dissociative excitation cross sections for vibrationally excited H2+ ions have been measured over the energy range from 0.01–35 eV. They are found to blend smoothly with previous crossed beam measurements and to match theory based upon the Born Approximation.

Low-energy electron collision processes in NF3.

The results of close-coupling calculations are reported for low-energy electron collision processes involving NF{sub 3}. We have used the complex Kohn variational method to calculate the elastic differential and momentum-transfer cross sections, as well as cross sections for (dissociative) excitation of the lowest singlet and triplet electronically excited states. Comparison is made between the results of this study and the limited body of experimental results available for this system.

Dissociative excitation of K and K+ emission by electron impact on KI molecules

Abstract Inelastic collisions of the slow monokinetic electrons with the KI molecules have been investigated using the method of extended crossing beams with recording an optical signal from the intersection region. The dissociative excitation cross sections for K and K + lines are measured at the 100 eV electron energy. It is found that for K the dependence of the dissociative excitation cross sections from the principal quantum number is described by the power law. The optical excitation functions are recorded in a range of electron energies 0 to 100 eV. Probable reaction mechanisms at the low electron energies are briefly discussed.

Dissociative excitation of potassium in collisions of electrons with KI molecules

An experimental study of dissociative collisions of monokinetic electrons with simple molecules is of interest for developing a theory of the structure of molecules and electron-molecular interactions. At the same time, the information obtained on the cross sections of dissociative excitation can be used as reference values for solving problems of theoretical and applied plasmachemistry, laser technology, etc. Among simple molecules (with not more than five atoms) ones of halogenides of alkali metals are of particular interest. A characteristic feature of these molecules is the presence of a comparatively strong bond in the electron ground state with a very small binding energy in electron excited states: calculations performed for some of these molecules give binding energy values in electron excited states equal to about 0.0t eV [1, 2 I. Therefore, inelastic collisions of electrons with molecules of halogenides of alkali metals lead first of all to their dissociation with the possible resulting formation of excited atoms. Investigation of the dissociative excitation of molecules of the class considered began only in recent years using the method of extended crossing beams. Up to now inelastic collisions of monoenergetic electrons with the KCI I3 l, LiCI 14 l, and LiF 151 molecules have been investigated. In the present work the method of extended crossing beams, whose use for thc study of electron-molecular collisions is discussed in detail in [3, 4 I, is used for measuring the dissociative excitation cross section of potassium in collisions of monoenergetic electrons with potassium iodide molecules. A beam of KI molecules was produced by vaporizing potassium iodide from a tantalum crucible and subsequently limiting it to transverse dimensions of 26 x 200 mm by water-cooled diaphragms. The crucible was heated by an electron beam with an electron energy of 10 keV. To obtain more uniform heating, the beam was defocused to a spot diameter of 30-40 mm. At the temperature of the upper surface of the crucible 850 K the concentration of molecules in the region of the crossing of beams at a distance of 280 mm from the cruicible was 2.3.1011 cm -3. Since at such a concentration the mean free path of the molecules attains several dozens of meters, collisions of molecules in the beam play a negligibly small role. Moreover, reabsorption of atomic lines, including resonance ones, is also negligibly small, since atoms in the molecular beam have a concentration of only 107 cm -3 or less. A ribbon beam of monoenergetic electrons with transverse dimensions of 13x200 mm and a drift space of 30 mm was formed by a special low-voltage gun, the design and characteristics of which are discussed in detail in [6, 7 ]. The current density in the electronic beam did not exceed 0.7 mA/cm 2 in the entire range of electron energies 0-100 eV. The width of the energy distribution of electrons constituted 0.9 eV for 90Yo of the electrons at an energy of 100 eV and 1.0 eV at 20 eV. To whom correspondence should be addressed.

Dissociative excitation and recombination of H3+

Measurements of the cross sections for the electron-impact dissociative excitation of ${\mathrm{H}}_{3}^{+}$ as a function of electron energy are reported. Sharp structures are observed close to the excitation thresholds for the upper states of the ion, and these are interpreted as Feshbach resonances. Direct excitation appears to play a subordinate role at the energies studied. The implication of these findings applied to previous measurements of the dissociative recombination of ${\mathrm{H}}_{3}^{+}$ is discussed.

Estimation of the integral cross section for dissociative excitation of F 2 by electron impact

Integral cross sections for electron impact dissociation of F 2 as a function of incident electron energy are very important for predicting the performance of electron-beam-pumped KrF lasers. Approximate scaling relationships from similar transitions in N 2 are combined with the few available electron energy-loss spectra in F 2 to obtain an estimate of the direct dissociation cross section of F 2 by electron impact from threshold to 100 eV. A conservative estimate of the direct dissociation cross section obtained by this process is very large (1.6 Å 2 at 25 ev) and appears to have a large contribution from excitation of perturbed Rydberg states at 12.87 eV. This estimated magnitude and the prediction of F + and F − fragment production below the F + ionization threshold (15.7 eV) should be checked experimentally because of the important role these processes play in rare-gas fluoride lasers.

Kinetics of HgX (B-X) formation due to collisions of Ar+2 ions with CH3HgX (X=Cl, Br, I) molecules related to blue-green laser emission

Collisional behavior of HgX (B state) (X=Cl, Br, I) has been studied during collisions of Ar+2 ions with CH3HgX (X=Cl, Br, I) molecules in the kinetic energy (lab.) range 100–1000 eV. By using the integrated intensity of the most intense band of the (B, v′=0−X, v″=22) transitions in HgCl, HgBr, and HgI at 558, 502, and 443 nm, respectively, emission cross sections were measured at various kinetic energies of the projectile ions. The maximum values of these cross sections for dissociative excitation were found to be 3.8×10−19, 5.7×10−19, and 6.1×10−19 cm2 for HgCl (B), HgBr (B), and HgI (B), respectively.

Kinetic energy distribution of excited H atoms produced through dissociative photoionization excitation of H2

The kinetic energy distributions of excited hydrogen atoms produced in the dissociative photoionization excitation of H2 by He II 304 Å photons have been obtained. A fluorescence photon–photoion coincidence technique has been utilized in the present study. A coincidence spectrum showing a broad peak with a weak shoulder corresponding to a time delay of 5.6–7.5 μs is obtained. The kinetic energy distribution of the peak is between 1.2 and 7.5 eV with a maximum at 4.2 eV indicating that this peak is attributable to the lowest energy dissociative photoionization excitation processes which correlate with H(2l)+H+. The relative partial cross sections for direct dissociative photoionization excitation through the 2pπu and 2sσg states at 304 Å are determined to be 1: 0.25.

Dissociative excitation cross sections of quartet states of O(II) by electron impact

Dissociative excitation cross sections of quartet states of O(II) by electron impact

Reactions of metastable argon with photodissociation aligned carbon disulfide: A study of the steric dependence of two competing reaction channels

CS2 molecules in a beam were aligned using laser photodissociation at 193 nm. Two competing reactions with metastable Ar were studied as a function of CS2 alignment. The Penning ionization reaction cross section for formation of CS+2 (X 2Πg) was found to be largest when the CS2 bond is parallel to the relative velocity vector, while the cross section for dissociative excitation giving CS*(A 1Π) is largest when the CS2 bond is perpendicular to the relative velocity vector.

Vacuum ultraviolet studies of electron impact of helium Excitation of He n1P0 Rydberg series and ionization-excitation of He(+) nl Rydberg series

The cross sections for the He I 1s2 1S-1snp 1P0 series have been measured using a relative flow method, with the absolute scale fixed by the H Ly-alpha dissociative excitation cross section standard. The results are compared with those obtained using a relative cross section data analysis by modified Born approximation, and good agreement is found. Cross sections for the ionization-excitation of the He II 121.51 nm and He II 164.04 nm transmissions have been measured, and the results strongly suggest that theoretical calculations of the reactions differ fundamentally from physical reality. The failure of the theory to describe experimental results stems from the neglect in the theory of electron correlation effects between the two orbital electrons.

Studies of extreme-ultraviolet emission from Rydberg series ofH2by electron impact

Electron excitation cross sections have been measured for the following two Rydberg series of H_2: ^1Σ_u^+ 1sσnpσ (B, B′, and B, states with principal quantum numbers n=2, 3, and 4, respectively) and ^1Π_u 1sσnpπ (C, D, and D′ states with principal quantum numbers n=2, 3, and 4, respectively) over the energy range from threshold to 350 eV. The cross sections for these six states account for all (>99%) of the vacuum-ultraviolet (78-170 nm) of the singlet states of H_2. The estimated total direct-excitation cross sections for these six states at 100 eV in decreasing value are (4.02±0.60)×10^(−17) cm^2 for (3.86±0.60)×10^(−17) cm^2 for C^1Π_u, (0.76±0.11)×10^(−17) cm^2 for D^1Π_u, (0.76±0.11)×10^(−17) cm^2 for B' ^1Σ_u^+, (0.30±0.06)×10^(−17) cm^2 for D′^1Π_u, and (0.23±0.05)×10^(−17) cm^2 for B^1Σ_u^+ and, additionally, (0.43±0.10)×10^(−17) cm^2 for E,F^1Σ_g^+ which populates the state through radiative cascade transitions. We estimate the predissociation (autoionization is weak) and yields of the vibrational levels of the D, D′, and B states whose band systems exhibit strong breaking off in emission for wavelengths below 85 nm. Furthermore, we report the first direct measurement of the dissociative excitation cross section for production of Lyman-β of (8.9±3.0)×10^(−19) cm^2 at 100 eV. In particular, it is shown that the high-lying Rydberg states (n=3 and 4) make a substantial contribution to the observed below 110 nm while above 110 nm the Lyman bands (B^1Σ_u^+→X^1Σ_g^+) and Werner bands (C^1Π_u→X^1Σ_g^+), the first members of the Rydberg series, dominate the spectrum. As a result of these measurements and spectroscopic models the ultraviolet (UV) spectrum from H_2 by electron impact can serve as an intensity calibration standard from 80 to 170 nm.