We studied dissociative electron attachment to a series of compounds with one or two hydroxyl groups. For the monoalcohols we found, apart from the known fragmentations in the 6-12 eV range proceeding via Feshbach resonances, also new weaker processes at lower energies, around 3 eV. They have a steep onset at the dissociation threshold and show a dramatic D/H isotope effect. We assigned them as proceeding via shape resonances with temporary occupation of s * O-H orbitals. These low energy fragmentations become much stronger in the larger molecules and the strongest DEA process in the compounds with two hydroxyl groups, which thus represent an intermediate case between the behavior of small alcohols and the sugar ribose which was discovered to have strong DEA fragmentations near zero electron energy [S. Ptasin´ska, S. Denifl, P. Scheier and T. D. Ma¨rk, J. Chem. Phys., 2004, 120, 8505]. Above 6 eV, in the Feshbach resonance regime, the dominant process is a fast loss of a hydrogen atom from the hydroxyl group. In some cases the resulting (M À 1) À anion (loss of hydrogen atom) is sufficiently energyrich to further dissociate by loss of stable, closed shell molecules like H 2 or ethene. The fast primary process is state-and site selective in several cases, the negative ion states with a hole in the n O orbital losing the OH hydrogen, those with a hole in the s C-H orbitals the alkyl hydrogen.

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...

Dissociative electron attachment (DEA) to diethyl ether yielded primarily the C 2 H 5 O À ion, with a strong Feshbach resonance band at 9.1 eV and a weaker shape resonance band at 3.89 eV. Very similar spectra were obtained for dibutyl ether, with C 4 H 9 O À bands at 8.0 and 3.6 eV. Some of these primary ions subsequently lost H 2 and yielded weaker signals of the C 2 H 3 O À and C 4 H 7 O À ions. In contrast, DEA to the cyclic ether tetrahydrofuran (THF) yielded mainly a fragment of mass 41, presumably deprotonated ketene, at 7.65 eV. The low-energy band was missing in THF. H À with two bands at 6.88 and 8.61 eV, and an ion of mass 43 (presumably deprotonated acetaldehyde) with two bands at 6.7 and 8.50 eV were also observed. We propose that in the primary DEA step the C-O bond is cleaved in both the open-chain and the cyclic ethers. In the open-chain ethers the excess energy is partitioned between the (internal and kinetic) energies of two fragments, resulting in an RO À ion cool enough to be observed. The CH 2 (CH 2 ) 3 O À ion resulting from cleavage of the C-O bond in THF contains the entire excess energy (more than 6 eV at an electron energy of 7.65 eV) and is too short-lived with respect to further dissociation and thermal autodetachment to be detected in a mass spectrometer. These findings imply that there could be a substantial difference between the fragmentation in the gas phase described here and fragmentation in the condensed phase where the initially formed fragments can be rapidly cooled by the environment.

A difference was observed in the reactivity of alcohols and ethers toward free electrons. Whereas the lowest core-excited state of the negative ion-a 2 (n,3s 2 ) Feshbach resonance-of the alcohols readily dissociates by losing a hydrogen atom, ethers show no observable signal from this resonance. This difference in reactivity has a parallel in the anomalous shapes and energies of the parent states of the Feshbach resonances, the 1 (n,3s) Rydberg states of the neutral alcohols. We explained this anomaly using potential surfaces of the alcohols and ethers calculated using the TD-DFT method as a function of the dissociation coordinate. The lowest excited state of alcohols was found to be repulsive, whereas a barrier to dissociation was found in the ethers. Rydbergvalence mixing and avoided crossings are decisive in determining the shapes of the potential surfaces. It is concluded that the reactivities of alcohols and ethers toward free electrons are rationalized by assuming that the potential surfaces of the daughter Feshbach resonances closely follow those of the parent Rydberg states, i.e., the lowest Feshbach resonance is repulsive, but a barrier occurs in ethers. The potential surfaces of both the Rydberg states and the Feshbach resonances thus differ dramatically from the non-dissociative surface of the grandparent 2 (n À1 ) positive ions, despite the nominally non-bonding character of the Rydberg electrons.

Absolute vibrational excitation cross sections were measured for diacetylene (1,3-butadiyne). The selectivity of vibrational excitation reveals detailed information about the shape resonances. Excitation of the C≡C stretch and of double quanta of the C−H bend vibrations reveals a 2 u resonance at 1 eV (autodetachment width ∼30 meV) and a 2 g resonance at 6.2 eV (autodetachment width 1-2 eV). There is a strong preference for excitation of even quanta of the bending vibration. Excitation of the C−H stretch vibration reveals σ * resonances at 4.3, 6.8, and 9.8 eV, with autodetachment widths of ∼2 eV. Detailed information about resonances permits conclusions about the mechanism of the dissociative electron attachment.