Absolute cross sections for dissociative electron attachment to acetylene and diacetylene

Abstract

Absolute cross sections for the production of the two astronomy-relevant negative ions H u C w C − and H u C w C u C w C − by dissociative electron attachment to acetylene C 2 H 2 and diacetylene C 4 H 2 were measured ͑with a Ϯ25% error bar͒. Acetylene yielded the C 2 H − ion at an electron energy of 2.95 eV with a cross section of 3.6Ϯ 0.9 pm 2 and also the C 2 − ion at 8.1 eV with a cross section of 4.1Ϯ 1 pm 2 .Diacetylene yielded the C 4 H − ion at 2.5 eV with a cross section of 3.0Ϯ 0.8 pm 2 and at 5.25 eVwith a cross section of 73Ϯ 17 pm 2 . Weaker C 4 − , C 2 H − , and C 2 − signals were also observed from diacetylene. The identity of the negative ion resonances mediating the dissociation and the consequences for the production of these ions in discharges are discussed. An alternate path for C 4 H − formation, from the O − -C 4 H 2 ion-molecule reaction, was also observed.PACS number͑s͒: 34.80. Ht, 52.20.Fs, 52.20.Hv The identifications of the negative ions C 6 H − ͓1͔, C 4 H − ͓2͔ and C 8 H − ͓3,4͔ in outer space are among the most exciting recent discoveries in astronomy. A necessary prerequisite for the assignment of the observed astronomical bands were laboratory microwave spectra ͓1,5͔-recorded with negative ions prepared in discharges containing acetylene H u C w C u H and diacetylene H u C w C u C w C u H. The knowledge of electron-induced chemistry of the C 2n H 2 class of compounds is thus of great interest if we wish to address the question of how the C 2n H − ions are formed, primarily in laboratory discharges.The most important primary electron-induced process leading to negative ion fragments is dissociative electron attachment ͑DEA͒, and the present work reports experimental absolute cross sections for this process in acetylene and diacetylene. These two compounds were found also in the upper layers of planetary atmospheres ͓6,7͔ and in flames ͓8͔. Both environments contain free electrons and the present electron-induced processes could consequently also play a role there.The desired quantitative cross sections were obtained by combining the results from two mutually complementary instruments.͑a͒ A dissociative electron attachment spectrometer described previously ͓9͔. It employs a trochoidal electron monochromator to prepare a magnetically collimated beam of quasi-monoenergetic electrons, which is directed into a target chamber filled with a quasistatic sample gas. Fragment anions are extracted at 90°and directed into a quadrupole mass spectrometer.͑b͒ A newly constructed total ion collection tube having the same basic principle of operation as that of Rapp and Briglia ͓10͔. Fragment ions were collected at electrodes surrounding the electron beam in a collision chamber. A smoothly varying background of scattered electrons also reached the ion collecting electrodes after multiple collisions with the gas, and was subtracted. The cross section was calculated from the ion current, the incident electron beam intensity, and the sample gas pressure measured with a capacitance manometer. The ele...

Absolute angle-differential elastic cross sections for electron collisions with diacetylene

“…Sharp Feshbach resonances were observed at 6.7, 6.8, and 7.0 eV in the yield of slow electrons [31]. A more diffuse core excited resonance was postulated to be at the origin of the 5.25 eV dissociative electron attachment band [11,32]. A broad (width ≈ 2 eV) shape resonance was postulated experimentally at 4.3 eV, based on enhancement of the C-H stretch vibration, and assigned as 2 u (or 2 g ) [11].…”

Induction of strand breaks in DNA films by low energy electrons and soft X-ray under nitrous oxide atmosphere

“…44 However, Chantry (1969) found that electrons with near zero eV energy could attach to N 2 O and the yield of oxygen negative ions production has a broad peak extending from ~0.0 eV to ~1.2 eV. Another more intense peak for this reaction occurs at 2.2 eV (Chaney and Christophorou, 1969).Absolute cross sections for the production O − by DEA to O 2 and N 2 O were recently measured by May et al (2008). O 2 yielded the O − ion at electron energy 6.5 eV with a cross section of 126 × 10 −20 cm 2 while, the O − production from N 2 O occurs at 2.2 eV with a cross section of 830 × 10 −20 cm 2 .…”

Dissociative electron attachment to methylacetylene and dimethylacetylene: Symmetry versus proximity

“…The C 3 H − 3 cross section in methylacetylene is approximately half of that of the corresponding cross section for the C 2 H − + H channel in acetylene (3.45 pm 2 when measured with the present method [1] or 3.8 pm 2 when measured with the total ion collection method [2]). The π * shape resonance in acetylene lies at 2.6 eV (as seen in vibrational excitation spectra) and the threshold for this DEA channel is 2.74 eV.…”

“…This dissociation into ground-state fragments is symmetry forbidden in linear geometry and the H atom follows an out-of-line trajectory as the bond stretches [5]. Such behavior, where the resonant state cannot dissociate in original geometry and distorts during the fragmentation, has been seen in a handful of molecules, for instance C 2 H 2 [1,5], C 4 H 2 [2,3], HCCCN [4], and chlorobenzene [11]. A related case is HCN [7,12], where the outgoing H atom tunnels through the barrier with symmetry lowering playing a limited role.…”

“…Acetylene was shown to be a prototype molecule in which the dissociative electron attachment (DEA) proceeds via electron capture in a π * orbital and the dissociation is mediated via vibronic coupling. Such molecules are the subject of current interest both experimentally [1][2][3][4] and theoretically [5][6][7][8], since the dynamical situation is more complicated than in the molecules where the DEA proceeds via electron capture to a σ * orbital and direct cleavage of the involved bond. For some molecules-a notable example is formic acid [9,10]-there is an ongoing discussion as to which of the two processes is operative.…”