Laser Photoelectron Attachment Research Articles

High resolution low-energy electron attachment to molecular clusters of sulfur dioxide

Using the laser photoelectron attachment method, we have investigated the formation of cluster anions in low-energy free electron attachment (E=1–200meV) as well as in Rydberg electron transfer (at principal quantum numbers 20–260) to molecular clusters of sulfurdioxide (SO2) in a collimated supersonic beam at high electron energy resolution (energy width ≤2meV). At these low energies (almost) only homogeneous cluster anions (SO2)q− (q≥1) are found with energy dependences which show – superimposed on a monotonically decreasing continuum cross section – characteristic peaks which are interpreted as vibrational Feshbach resonances (VFR), associated with excitation of the three fundamental vibrational modes in the SO2 molecule, namely bending (010), symmetric stretch (100), and asymmetric stretch (001). The VFRs are clear for q=2–12, but only weak for q=1. The energy positions of the VFRs are progressively red-shifted with increasing cluster anion size q by about 1.7meV per added molecular unit, and the peaks broaden towards larger q. The cross section enhancement in the VFRs – compared to the size of the continuum – is substantial, indicating that the VFRs act as important doorway states for electron capture. Model R-matrix calculations are presented which recover the main features of the experimental attachment spectra for cluster anion formation and suggest that VFRs occur for neutral (SO2)N clusters with sizes N≥4. With growing N, the electron binding energy increases and the capture cross section decreases. In contrast, the Vogt–Wannier model for electron capture into a polarization well predicts a growth of the cross section with the cluster size.

High-resolution studies of (SF6)q− (q = 1–3) cluster anion formation in low-energy electron collisions with (SF6)N clusters (N ≥ 2)

Using two variants of the laser photoelectron attachment method, we have investigated the formation of cluster anions in low-energy electron attachment (E = 2–700 meV) to molecular clusters of sulfurhexafluoride (SF6) at high electron energy resolution (energy width ≤20 meV). Only homogeneous cluster anions (SF6)q− (q ≥ 1) are found with energy dependences which monotonically decrease towards higher electron energies E in a way which somewhat varies with q. Photodissociation of neutral clusters (SF6)N (N ≥ 2), transversely excited by lines of a continuous CO2 laser (920–980 cm−1) at intensities up to about 250 W cm−2, yields information on the size N of the neutral clusters from which the detected (SF6)q− (q = 1–4) anions are formed by electron capture and evaporative processes. For each q, the effective precursor size 〈N〉q of the neutral cluster is larger than q by at least 2. By comparing the experimental yields for anion formation with Vogt–Wannier cross sections for the primary electron capture step, information is obtained on the energy-dependent probability for the evaporative stabilization of the initial temporary anion (SF6)N−*, thus yielding the detected cluster anion (SF6)q− (q < N). We also calculated electron attachment cross sections for (SF6)N (N = 1–4) clusters, using a multi-centre zero-range potential method (E = 1–200 meV) and adopting simple cluster structures.

High resolution electron attachment to CO2 clusters

Electron attachment to CO₂ 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 CO₂ clusters, non-dissociated complexes of the form (CO₂)(n)(-) including the monomer CO₂(-) are generated as well as solvated fragment ions of the form (CO₂)(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 (CO₂)(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 CO₂ 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 (CO₂)(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 (CO₂)(n)O(-) clearly arise from DEA at an individual molecule within the cluster involving both the (2)Π(u) and the core excited resonance.

Low-Energy Electron Attachment to the Dichlorodifluoromethane (CCl2F2) Molecule

Results from a joint experimental study of electron attachment to dichlorodifluoromethane (CCl(2)F(2)) molecules in the gas phase are reported. In a high resolution electron beam experiment involving two versions of the laser photoelectron attachment method, the relative cross section for formation of the dominant anion Cl(-) was measured over the energy range 0.001-1.8 eV at the gas temperature T(G) = 300 K. It exhibits cusp structure at thresholds for vibrational excitation of the nu(3)(a(1)) mode due to interaction with the attachment channels. With reference to the thermal attachment rate coefficient k(T = 300 K) = 2.2(8) x 10(-9) cm(3) s(-1) (fitted average from several data), a new highly resolved absolute attachment cross section for T(G) = 300 K was determined. Partial cross sections for formation of the anions Cl(-), Cl(2)(-), F(-), ClF(-), and CCl(2)F(-) were measured over the range 0-12 eV, using three different electron beam experiments of medium energy resolution. The dependence of the attachment rate coefficient k(T(e);T(G) = 300 K) on electron temperature T(e) was calculated over the range 50-15 000 K, based on a newly constructed total cross section for anion formation at T(G) = 300 K. R-matrix calculations for Cl(-) production have been carried out for comparison with the experimental data. The R-matrix results are in line with the main experimental observations and predict the dependence of the DEA cross section on the initial vibrational level nu(3)() and on the vibrational temperature. Furthermore, the cross section for vibrational excitation of the nu(3) mode has been computed.

High-resolution electron attachment to the molecules CCl4 and SF6 over extended energy ranges with the (EX)LPA method

Using a variant of the laser photoelectron attachment (LPA) method with an extended energy range (EXLPA), we have studied low-energy electron attachment to the molecules CCl4 (Cl− and Cl−2 formation) and SF6 (SF−6 and SF−5 formation) in a diffuse gas target (TG = 300 K) from 0 eV up to 2 eV at energy widths down to 14 meV. In the EXLPA method, pulses of near-zero energy photoelectrons are produced in a guiding magnetic field, accelerated by a weak electric field, brought to the energy of interest prior to their traversal through the target region and subsequently accelerated and deflected onto a detecting plate. Anions due to electron attachment are extracted by a pulsed electric field, during which the photoelectron current is interrupted, and detected by a quadrupole mass spectrometer. The EXLPA anion yields are combined with absolute cross sections, obtained at very high resolution (≈1 meV) with the LPA method over the range 0–0.17 eV, to yield new recommended absolute partial and total attachment cross sections over the range 0–2 eV at the well-defined gas temperature TG = 300 K. Our cross sections show characteristic deviations from previously reported results. At least in part, these differences can be attributed to the fact that in the earlier electron beam experiments the gas temperature was higher than 300 K. For SF6, the branching fractions for SF−5 formation at electron energies 0.002–0.43 eV and for different initial rovibrational distributions are compared with those recently predicted from kinetic modelling within the framework of statistical unimolecular rate theory. Satisfactory agreement is observed, but our data provide evidence that an additional path for producing SF−6 and SF−5 ions is available at electron energies above about 0.3 eV.

Low-energy electron attachment to chloroform (CHCl3) molecules: A joint experimental and theoretical study

Results from a joint experimental and theoretical study of electron attachment to chloroform (CHCl3) molecules in the gas phase are reported. In an electron swarm study involving a pulsed Townsend technique with equal gas and electron temperatures, accurate attachment rate coefficients were determined over the temperature range 295–373K; they show an Arrhenius-type rise with increasing temperature, corresponding to an activation energy of 0.11(1)eV. In a high resolution electron beam experiment involving two versions of the laser photoelectron attachment method, the relative cross section for Cl− formation from CHCl3 over the energy range 0.001–1.25eV at the gas temperature TG=300K was measured. It exhibits clear downward cusp structure at the threshold for excitation of one quantum of the vibrational symmetric deformation mode ν3, indicating that this mode is active in the primary attachment process. With reference to our thermal attachment rate coefficient k(T=300K)=3.9(2)×10−9cm3s−1, a new highly resolved absolute attachment cross section for TG=300K was determined. This cross section is extended to higher energies by measurements, carried out with a pulsed electron beam apparatus which also provided new data for the distinctly weaker fragment anions HCl2− and CCl2−. The resulting total absolute cross section for anion formation is used to calculate the dependence of the attachment rate coefficient k(Te;TG) on electron temperature Te over the range 50–15000K at the fixed gas temperature TG=300K. In addition, we report the dependence of the relative cross section for Cl− formation on gas temperature (TG=310–435K). For comparison with the experimental data, R-matrix calculations have been carried out for the dominant anion channel Cl−. The results recover the main experimental observations and predict the dependence of the DEA cross section on the initial vibrational level ν3 and on the vibrational temperature. Our results are compared with those of previous electron beam and electron swarm experiments.

Highly resolved absolute cross-sections for dissociative electron attachment to SF5CF3

Using two complementary experimental methods, we have measured partial (mass-resolved) cross-sections for dissociative electron attachment to the molecule trifluoromethyl sulfurpentafluoride (SF5CF3) at the gas temperature TG=300K over a broad range of electron energies (E=0.001–12eV). The absolute scale for these cross-sections was obtained with reference to the thermal (T=300K) rate coefficient for anion formation (8.0(3)×10−8cm3s−1). Below 1eV, SF5− is the dominant product anion and formed through the lowest anion state which cuts the neutral SF5CF3 potential close to the S–C equilibrium distance. The highly resolved laser photoelectron attachment data exhibit a downward Wigner cusp at 86meV, indicating that the ν4(a1) vibrational mode is important for the primary attachment dynamics. Both SF5− and F− anions are formed with similar yields through the first excited resonance located near 3.6eV. Towards higher energies, the anions CF3−, SF4−, and SF3− are also produced. Summation of the partial cross-sections yields a total absolute cross-section for anion formation over the energy range 0.001–12eV. This is used to calculate the dependence of the rate coefficient for dissociative electron attachment over a broad range of electron temperatures for the fixed gas temperature TG=300K; good agreement is found between the calculated values and those obtained in a drift tube experiment. In addition to the experimental work, semiempirical R-matrix calculations have been carried out for the energy dependence of the cross-section for SF5− formation. The experimental findings are semi-quantitatively recovered.

Observation ofp-Wave Threshold Behavior in Electron Attachment toF2Molecules

Using the high resolution laser photoelectron attachment method, we demonstrate that the cross section for F- formation due to electron capture by F2(X{1}Sigma{g}{+}) molecules at very low energies exhibits p-wave threshold behavior. This finding confirms the theoretical expectation that low-energy attachment to F2 proceeds through the F2{-}(2Sigma{u}{+}) p-wave shape resonance in contrast with previous experimental claims for s-wave threshold behavior.

High resolution studies of dissociative electron attachment to molecules: dependence on electron and vibrational energy

For several molecules which are important for plasma processing and gaseous dielectrics (CF3I, CF3Br, CH3Br, and SF6), we have studied the dependence of dissociative electron attachment (DEA) on both the electron energy and on the initial vibrational energy. With reference to electron swarm data, we determine highly-resolved absolute DEA cross sections over a broad energy range, using the Laser Photoelectron Attachment (LPA) method (E = 0–0.2 eV, δE ≈ 1–3 meV) and the EXtended Laser Photoelectron Attachment (EXLPA) method (0–2 eV, δE ≈ 15–30 meV). The experimental data are compared with the results of R-matrix calculations, involving ab initioinformation on the potential energy curves and semiempirical autodetachment widths. For CF3I and CF3Br, previous DEA cross sections are found to be substantially too high. For CH3Br, the measurements confirm a predicted vibrational Feshbach resonance, associated with the v3 = 4 vibrational threshold, and the value of the activation energy (due to an intermediate barrier) for this exothermic DEA process. For SF6, we report absolute cross sections for SF6− as well as SF5− formation for vibrational temperatures ranging from 200 to 500 K. Moreover, the first absoluteDEA cross sections (SF5− formation) for CO2-laser excited SF6 molecules have been obtained at different initial vibrational temperatures. The results indicate that the effect of the mode-selective energy input into the v3-mode (predominantly v3 = 1) on the enhancement of SF5− formation is very similar to that of a corresponding rise of the average vibrational energy by thermal heating; at E = 2 meV electron energy, the results indicate an activation energy of about 0.38 eV.

Low-energy electron collisions with CH3Br: the dependence of elastic scattering, vibrational excitation, and dissociative attachment on the initial vibrational energy

Using the laser photoelectron attachment (LPA) method at an energy width of 1–2 meV, the Br− yield due to dissociative electron attachment to the molecule CH3Br has been measured over the energy range 1–180 meV at a gas temperature of 600 K. The data clearly exhibit the vibrational Feshbach resonance predicted by Wilde et al (2000 J. Phys. B: At. Mol. Opt. Phys. 33 5479) and associated with the v3 = 4 vibrational level of the C–Br stretch mode in the neutral molecule. We also report the dependence of the rate coefficient for Rydberg electron attachment (measured at high principal quantum numbers n ≈ 150) on gas temperature and find an Arrhenius-type behaviour compatible with the results of R-matrix calculations and with earlier findings of electron swarm experiments. With the aim to stimulate experiments, we report absolute cross sections for elastic scattering and vibrationally inelastic scattering involving the C–Br stretch mode v3 of CH3Br for several initial vibrational levels in the range v3 = 0–4; they were obtained with R-matrix calculations based on the same potential input as that involved in the computations of the cross sections for dissociative attachment.

IR photon enhanced dissociative electron attachment to SF 6: Dependence on photon, vibrational, and electron energy

The influence of IR photon excitation of the ν 3 vibrational mode in SF 6 on dissociative electron attachment ( SF 5 - formation ) is investigated at high electron energy resolution (down to 1 meV) over the energy range E = 0–0.5 eV with the laser photoelectron attachment method. The molecules are contained in a collimated seeded supersonic beam (nozzle temperatures T 0 = 300–600 K, corresponding to vibrational temperatures T V ≈ T 0 − 100 K) and transversely excited by the 10PX lines (X = 10–40) in the 10.6 μm band of a continuous CO 2 laser at intensities up to about 400 W cm −2. The IR excitation and the attachment regions are separated by 5 cm. The IR photon induced enhancement of the SF 5 - yield is found to be optimal on the 10P28 line (936.8 cm −1) at all nozzle temperatures (with the maximum reached for T 0 ≈ 390 K) and monotonically decreasing with rising electron energy from 0 eV over a range of about 0.3 eV. For a fixed spatial profile of the exciting IR beam, the enhancement at E ≈ 0 eV follows a near-square-root dependence on laser power. With reference to previous work on the excitation of supersonic SF 6 beams by CO 2 laser light, the fraction of laser-excited SF 6( ν 3 ⩾ 1) molecules is estimated, and the absolute cross sections σ L( E) for SF 5 - formation involving the IR-excited molecules are determined; they exceed the cross sections σ 0( E) for thermal molecules in a way which strongly depends on electron energy and initial vibrational energy. In contrast, the cross section for SF 6 - formation is found to be almost independent of laser-excitation and temperature. The experimental findings are discussed with regard to the multiphoton character of the IR excitation, and comparisons are made with the effects of thermal excitation. The mechanisms for SF 6 - and SF 5 - formation and the responsible potential energy surfaces are discussed in the light of the available experimental data.

High resolution studies of low-energy electron attachment to SF 5Cl: Product anions and absolute cross sections

Low-energy electron attachment to SF 5Cl was studied at high energy resolution by mass spectrometric detection of the product anions. Two variants of the laser photoelectron attachment (LPA) technique (Kaiserslautern) were used for determining the threshold behaviour of the yield for SF 5 − formation at about 1 meV resolution, and to investigate the relative cross sections for Cl −, FCl −, and SF 5 − formation towards higher energies (up to 1 eV) at about 20 meV resolution. Thermal swarm measurements (Birmingham) were used to place the relative LPA cross sections on an absolute scale. A trochoidal electron monochromator (Innsbruck) was used for survey measurements of the relative cross sections for the different product anions over the energy range of 0–14 eV with a resolution of 0.30 eV. Combined with earlier beam data (taken at Berlin) [M. Fenzlaff, R. Gerhard, E. Illenberger, J. Chem. Phys. 88 (1988) 149], the present experimental results provide a detailed set of partial cross sections for anion formation in low-energy electron collisions with SF 5Cl.

The dependence of low-energy electron attachment to CF3Br on electron and vibrational energy

In a joint experimental and theoretical effort, we have studied dissociative electron attachment (DEA) to the CF3Br molecule at electron energies below 2 eV. Using two variants of the laser photoelectron attachment method with a thermal gas target (T(G) = 300 K), we measured the energy dependent yield for Br- formation over the range E = 3-1200 meV with resolutions of about 3 meV (E < 200 meV) and 35 meV. At the onsets for excitation of one and two quanta for the C-Br stretching mode nu3, downward cusps are detected. With reference to the recommended thermal (300 K) attachment rate coefficient k(A)(CF3Br) = 1.4 x 10(-8) cm3 s(-1), absolute cross sections have been determined for Br- formation. In addition, we studied Br- and (CF3Br)Br- formations with a seeded supersonic target beam (10% CF3Br in helium carrier gas, with a stagnation pressure of 1-4 bars and nozzle temperatures of 300 and 600 K) and found prominent structure in the anion yields due to cluster formation. Using the microwave pulse radiolysis swarm technique, allowing for controlled variation of the electron temperature by microwave heating, we studied the dependence of the absolute DEA rate coefficient on the mean electron energy E over the range of 0.04-2 eV at gas temperatures T(G) ranging from 173 to 600 K. For comparison with the experimental results, semiempirical resonance R-matrix calculations have been carried out. The input for the theory includes the known energetic and structural parameters of the neutral molecule and its anion; the parameters of the resonant anion curves are chosen with reference to the known thermal rate coefficient for the DEA process. For the gas temperature T(G) = 300 K, good overall agreement of the theoretical DEA cross section with the experimental results is observed; moreover, rate coefficients for Br- formation due to Rydberg electron transfer, calculated with both the experimental and the theoretical DEA cross sections, are found to agree with the previously reported absolute experimental values. At T(G) = 300 K, satisfactory agreement is also found between the calculated and experimental attachment rate coefficients for mean electron energies E = 0.04-2 eV. The strong increase of the measured rate coefficients with rising gas temperature, however, could be only partially recovered by the R-matrix results. The differences may result from the influence of thermal excitations of other vibrational modes not included in the theory.

High resolution low-energy electron attachment to CF3I

Using several variants of the laser photoelectron attachment method, we have measured the energy-dependent yield for I− formation resulting from dissociative electron attachment (DEA) to CF3I molecules over the energy range 0.5–500 meV. One approach involved a static target gas (TG = 300 K) and pulsed electron production/anion extraction. In another approach, a collimated target was provided by a differentially pumped, seeded supersonic beam (10% CF3I in helium carrier gas, stagnation pressure 1 bar, nozzle temperature 300 K and 600 K). At the onsets for excitation of one and two quanta for the C–I stretching mode ν3, clear downward cusps are detected. With reference to the recommended thermal DEA rate coefficient kA(Te = TG = 300 K) = 1.9 × 10−7 cm3 s−1, a new highly resolved absolute cross section for I− formation has been determined. Our experimental results are well reproduced by a cross section calculated in the framework of the resonance R-matrix theory. The input for the theory includes the known energetic and structural parameters of the neutral molecule and its anion and adopts a revised vertical attachment energy and a surface amplitude chosen to reproduce the thermal DEA rate coefficient. The theory is also applied to predict absolute cross sections for vibrational excitation of the C–I stretching mode ν3. Using our experimental and theoretical DEA cross sections we derive rate coefficients for Rydberg electron transfer (RET) and the dependence of the rate coefficients for free electron attachment of a Maxwellian electron ensemble on the mean electron energy from 0.002 to 2 eV at the constant gas temperature TG = 300 K; in both cases good agreement is observed with direct RET and swarm measurements.

High resolution study of anion formation in low-energy electron attachment to SF6 molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF6 molecules in helium carrier gas, nozzle temperatures T0= 300–600 K, stagnation pressures p0= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF6 molecules at high energy resolution. Using the standard LPA method, the yield for SF6- as well as SF5- and F- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF6- and SF5- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF6- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF5- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T0= 300 K and shifting to lower energies with rising T0) and a monotonical decrease towards higher energies. SF5- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF6 molecules with rising pressure. The SF5-/SF6- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF5- spectra are used as thermometers for the internal temperature of the SF6 molecules in the seeded supersonic beam which (at p0= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F- formation is similar to that for SF6-, but the F- signals are three to four orders of magnitude lower than those for SF6-; in view of the rather high endothermicity of F- formation the origin of the F- signals is discussed in some detail.

High resolution study of cluster anion formation in low-energy electron collisions with OCS clusters

Using the laser photoelectron attachment (LPA) method involving a supersonic target beam with carbonyl sulfide molecules (OCS) seeded in helium, we have studied the formation of cluster anions in low-energy electron attachment (E=1–200meV) to molecular clusters of OCS at high resolution (energy width 1–2meV). Homogeneous cluster anions (OCS)q− (q=1–12) are predominantly formed. In addition, due to CS2 impurities in the OCS sample, mixed cluster anions ((OCS)q−1CS2−, q≥1) are observed at levels of a few percent. The energy dependences of cluster anion formation are characterized by a strong rise towards zero energy attributed to s-wave attachment as well as—below the onsets of the (010), (001), and (020) vibrational modes of the OCS molecule—by vibrational Feshbach resonances (VFR) whose importance decreases towards larger cluster sizes q. Formation of the (OCS)2− anion is especially enhanced at near-zero electron energies; moreover, the attachment spectrum exhibits a sharp VFR resonance just below the onset for excitation of the (020) mode in the OCS molecule. The electron attachment behaviour of OCS clusters is intermediate between that of CO2 clusters (which is dominated by VFRs) and that of CS2 (which exhibits a strong zero energy peak, but no VFRs). This finding is correlated with the properties of the respective molecular neutrals and anions which have been studied using high level ab initio methods. We also report the energetics of the neutral and the anion dimer species. The role of VFRs as doorway states into valence-type anion states is discussed.

Dissociative attachment and vibrational excitation in low-energy electron collisions withchlorine molecules

In a combined experimental and theoretical effort, we have investigated dissociative attachmentand vibrational excitation in low-energy electron collisions with chlorine molecules. Using the laserphotoelectron attachment method, we have measured the energy dependence of the cross sectionσDA(E) for dissociativeelectron attachment (Cl− formation) over the range 0–195 meV with an energy width of 1–3 meVand for Rydberg electron transfer at high principal quantum numbers(n>67). Nearzero energy, the cross section shows a behaviour compatible with the threshold law for p-wave attachmentvia the 2Σu+ resonance, reaches a maximum around 50 meV and declines towards higherenergies. These findings are in good agreement with the results of semi-empiricalR-matrix calculations. Measured rate coefficientsknl forCl− formation due toelectron transfer from K**(nl) Rydberg atoms were found to be nearly constant for high principal quantum numbers(n>67) incontrast to the behaviour expected within the quasi-free electron model for p-wave attachment. TheR-matrix calculations are extended to describe electron attachment through the2Πg and 2Πu resonances, and recommended absolute cross sections for dissociative attachment to chlorinemolecules at room temperature are provided over the energy range 0–9 eV. Furthermore, wepredict cross sections for vibrationally inelastic electron scattering through the2Σu+,2Πg and 2Πu resonances.

High-resolution study of dissociative electron attachment to dipolar molecules atlow energies: CH2Br2 and CCl3Br

Using the laser photoelectron attachment method with an energy width ≤ 1 meV at lowenergies (≤ 172meV) and an electron beam method (energy width about 70 meV) at higherenergies (up to 2 eV), we have determined absolute cross sections σe (E)for dissociative electron attachment to the dipolar molecules CH2Br2 andCCl3Br by normalization to known electron attachment rate coefficients.At thresholds for vibrational excitation of the CH2Br2 molecule,the DA cross section exhibits pronounced structure due to couplingof the attachment process with scattering channels; below the ν3 = 1onset for the symmetric CBr2 stretch vibration, a clear vibrationalFeshbach resonance is observed. At low energies the cross sections σe (E)show an energy dependence between E−1/2 andE−1,in essential agreement with predictions of an extended Vogt–Wannier (EVW)capture model which includes the long-range electron–dipole interaction inaddition to the polarization force. The determined absolute values are, however,substantially smaller than those predicted by the EVW model. SemiempiricalR-matrixcalculations have been carried out which show that at low electron energies(E < 0.15eV) Br− formation from CCl3Br proceeds by s-wave attachment to the anionground state while a broad peak, observed around 0.6 eV and evolvingpredominantly into the Cl− channel, is due to an excited anion state. Comparisonsare made with cross sections and rate coefficients obtained in previousphotoelectron attachment work (TPSA) and in electron beam as well asswarm experiments. Based on our joint experimental results for σe (E),we report the electron temperature dependence of the rate coefficientske (Te) forfree electron attachment involving a Maxwellian electron ensemble and a gas at room temperature(TG = 300K).

Clear experimental evidence for p-wave attachment-threshold behavior in electron attachment to chlorine molecules.

Using the laser photoelectron attachment method and chlorine molecules in a seeded supersonic beam, we have measured the energy dependence of the cross section for dissociative electron attachment (DA; Cl- formation) over the range 0-195 meV with an energy width of about 1 meV. Our data provide the first clear experimental evidence for p-wave behavior of a DA cross section near zero energy and are in good overall agreement with recent theoretical predictions.

High resolution study of cluster anion formation in low-energy electron collisions with molecular clusters of CO 2, CS 2, and O 2

Using a high resolution (Δ E=1–2 meV) laser photoelectron attachment (LPA) method, we have studied the formation of (CO 2) q − ( q=4–32), (CS 2) q − ( q=1, 2, 3, 5), and (O 2) q − ( q=5–14) cluster anions in collisions of low-energy electrons (0–200 meV) with molecular clusters of CO 2, CS 2, and O 2 molecules in a skimmed supersonic beam. For CO 2 clusters, the attachment spectrum is dominated by rather narrow vibrational Feshbach resonances (VFRs) of the type [(CO 2) N−1 CO 2( ν i )] − which involve a vibrationally excited molecular constituent ( ν i ≥1 denotes excited vibrational mode). The size-dependent values of the observed redshifts of the VFRs (about 12 meV per added CO 2 unit for small q) in conjunction with their narrow widths and with simple model calculations of the electron binding energies suggest that the size q of the observed (CO 2) q − anions is essentially the same ( q= N or N−1) as the size N of the neutral precursor (CO 2) N which captures the electron. For cluster sizes q above 24, a doubling of the VFR peak structure is observed which we tentatively attribute to the transition from icosahedral (low N) to bulk cubic (high N) cluster structure, previously predicted to occur for N around 30. The size distribution in the (CO 2) q − anion mass spectrum, due to attachment of electrons with well-defined energies, shows undulatory structure which reflects the presence or absence of VFRs at that particular energy. In contrast, the energy-integrated cluster anion mass spectrum shows a rather smooth dependence on cluster size q. The yield spectra for (CS 2) q − cluster anions exhibit an s-wave attachment peak near 0 eV and a smoothly falling cross section towards higher energies without significant structure. In the yield for (O 2) q − anions we observe a rise towards 0 eV, indicative of s-wave electron capture, and a slowly varying and essentially structureless attachment continuum between 10 and 200 meV. The attachment characteristics of the different clusters systems are discussed in the light of the basic properties of the monomers.