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

Using the laser photoelectron attachment method with an energy width ≤ 1 meV at low energies (≤ 172 meV) and an electron beam method (energy width about 70 meV) at higher energies (up to 2 eV), we have determined absolute cross sections σe (E) for dissociative electron attachment to the dipolar molecules CH2Br2 and CCl3Br 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 coupling of the attachment process with scattering channels; below the ν3 = 1 onset for the symmetric CBr2 stretch vibration, a clear vibrational Feshbach resonance is observed. At low energies the cross sections σe (E) show an energy dependence between E−1/2 and E−1, in essential agreement with predictions of an extended Vogt–Wannier (EVW) capture model which includes the long-range electron–dipole interaction in addition to the polarization force. The determined absolute values are, however, substantially smaller than those predicted by the EVW model. Semiempirical R-matrix calculations have been carried out which show that at low electron energies (E < 0.15 eV) Br− formation from CCl3Br proceeds by s-wave attachment to the anion ground state while a broad peak, observed around 0.6 eV and evolving predominantly into the Cl− channel, is due to an excited anion state. Comparisons are made with cross sections and rate coefficients obtained in previous photoelectron attachment work (TPSA) and in electron beam as well as swarm experiments. Based on our joint experimental results for σe (E), we report the electron temperature dependence of the rate coefficients ke (Te) for free electron attachment involving a Maxwellian electron ensemble and a gas at room temperature (TG = 300 K).

[1]  I. Fabrikant,et al.  Dissociative electron attachment to dipolar molecules at low energies with meV resolution: CFCl3, 1,1,1-C2Cl3F3, and HI , 2001 .

[2]  F. Dunning,et al.  Low-energy dissociative electron attachment to BrCN and CBrCl3: Temperature dependences and reaction dynamics , 2001 .

[3]  T. Märk,et al.  The working principle of the trochoidal electron monochromator revisited 2 2 Dedicated to Professor Aleksandar Stamatovic on the occasion of his 60th birthday. , 2001 .

[4]  H. Hotop,et al.  Dissociative electron attachment to CCl4 molecules at low electron energies with meV resolution , 2001 .

[5]  I. Fabrikant,et al.  Low-energy behavior of exothermic dissociative electron attachment , 2001 .

[6]  K. Aflatooni,et al.  Total cross sections for dissociative electron attachment in dichloroalkanes and selected polychloroalkanes: The correlation with vertical attachment energies , 2000 .

[7]  I. Fabrikant,et al.  Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters , 2000 .

[8]  A. Rau,et al.  Collisions near threshold in atomic and molecular physics , 2000 .

[9]  Ö. Şimşek,et al.  A new method for the determination of incoherent differential scattering cross sections , 2000 .

[10]  G. Gallup,et al.  Comparative studies of dissociative electron attachment to methyl halides , 1999 .

[11]  I. Fabrikant,et al.  Vibrational resonance and threshold effects in inelastic electron collisions with methyl iodide molecules , 1999 .

[12]  H. Hotop,et al.  NUCLEAR-EXCITED FESHBACH RESONANCES IN ELECTRON ATTACHMENT TO MOLECULAR CLUSTERS , 1999 .

[13]  P. Nordlander,et al.  Low-energy dissociative electron attachment to CH2Br2, CHClBr2, and CCl3Br: Intermediate lifetimes and decay energetics , 1998 .

[14]  H. Hotop,et al.  LASER PHOTOELECTRON ATTACHMENT TO MOLECULES IN A SKIMMED SUPERSONIC BEAM :DIAGNOSTICS OF WEAK ELECTRIC FIELDS AND ATTACHMENT CROSS SECTIONS DOWN TO 2 0 MU E V , 1998 .

[15]  H. Shimamori,et al.  LOW-ENERGY ELECTRON ATTACHMENT TO BROMINATED METHANES , 1997 .

[16]  T. Märk,et al.  A coordinated flowing afterglow and crossed beam study of electron attachment to CCl3Br , 1997 .

[17]  S. Burns,et al.  RATE COEFFICIENTS FOR DISSOCIATIVE ELECTRON ATTACHMENT BY HALOMETHANE COMPOUNDS BETWEEN 300 AND 800 K , 1996 .

[18]  L. Christophorou.  The Dependence of the Thermal Electron Attachment Rate Constant in Gases and Liquids on the Energy Position of the Electron Attaching State , 1996 .

[19]  J. M. Wadehra,et al.  Electron Attachment to Molecules at Low Electron Energies , 1996 .

[20]  T. Märk,et al.  A crossed beam high resolution study of dissociative electron attachment to CCl4 , 1995 .

[21]  F. Dunning.  Electron-molecule collisions at very low electron energies , 1995 .

[22]  B. Mirbach,et al.  Comparison of rate coefficients for rydberg electron and free electron attachment , 1994 .

[23]  H. Hotop,et al.  A tunable laser-atomic beam photoelectron source with sub-milli-electron-volt resolution: design, operation and application to threshold electron attachment studies , 1994 .

[24]  Smith,et al.  Inelastic electron--dipole-molecule scattering at sub-milli-electron-volt energies: CH[sub 3]I and CH[sub 2]Br[sub 2] , 1993 .

[25]  H. Shimamori,et al.  Electron-energy dependence of electron attachment to c-C7F14, CH3I and CH2Br2 studied by the pulse-radiolysis microwave-cavity technique combined with microwave heating , 1992 .

[26]  H. Hotop,et al.  Attachment of electrons to molecules at meV resolution , 1992 .

[27]  H. Hotop,et al.  Attachment of electrons to molecules at submillielectronvolt resolution , 1992 .

[28]  W. Domcke.  Theory of resonance and threshold effects in electron-molecule collisions: The projection-operator approach , 1991 .

[29]  P. Burrow,et al.  Dissociative attachment of electrons in the chloromethanes , 1990 .

[30]  E. P. Grimsrud,et al.  Electron capture chemistry of CBrCl3 at atmospheric pressure by mass spectrometry and the photodetachment-modulated electron capture detector , 1989 .

[31]  N. Adams,et al.  Studies of dissociative electron attachment to the free radicals CCl3 and CCl2Br using the FALP apparatus , 1988 .

[32]  Stephen J. Paddison,et al.  Bond dissociation energies and radical heats of formation in CH3Cl, CH2Cl2, CH3Br, CH2Br2, CH2FCl, and CHFCl2 , 1987 .

[33]  P. Brimblecombe,et al.  Chemistry of Atmospheres. , 1986 .

[34]  Chutjian,et al.  s-wave threshold in electron attachment: Observations and cross sections in CCl4 and SF6 at ultralow electron energies. , 1985, Physical review. A, General physics.

[35]  N. Adams,et al.  Rate coefficients for the attachment reactions of electrons with c-C7F14, CH3Br, CF3Br, CH2Br2 and CH3I determined between 200 and 600K using the FALP technique , 1984 .

[36]  L. Christophorou,et al.  in Electron - Molecule Interactions and their Applications , 1984 .

[37]  I. Fabrikant.  Frame transformation effective-range theory: application to interaction of polar molecules with electrons , 1983 .

[38]  K. Jordan,et al.  Temporary negative ions in the chloromethanes CHCl2F and CCl2F2: Characterization of the σ* orbitals , 1982 .

[39]  A. P. Kudchadker,et al.  Ideal gas thermodynamic properties of CH4−(a+b+c+d)FaClbBrcId Halomethanes , 1978 .

[40]  C. Klots.  Rate constants for unimolecular decomposition at threshold , 1976 .

[41]  L. Christophorou,et al.  Electron Attachment to Halogenated Aliphatic Hydrocarbons , 1968 .

[42]  G. Schulz,et al.  Trochoidal Electron Monochromator , 1968 .

[43]  T. G. Lee,et al.  ELECTRON ATTACHMENT COEFFICIENTS OF SOME HYDROCARBON FLAME INHIBITORS , 1963 .

[44]  G. Wannier,et al.  Scattering of Ions by Polarization Forces , 1954 .

[45]  E. Wigner.  On the Behavior of Cross Sections Near Thresholds , 1948 .