C. D. Cooper

Collisional ionization of Na, K, and Cs by CO2, COS, and CS2: Molecular electron affinities

The negative ion products resulting from collisions between orthogonal beams of alkali metal atoms (Na, K, Cs) and the linear triatomic molecules CO2, COS, and CS2 have been studied from threshold to ∼400 eV (lab). Ions with masses corresponding to the parent molecules CO2, COS, and CS2 are detected for all collision permutations except for Na colliding with CO2. The following electron affinities are deduced from measurements of the threshold for the ion pair production reactions: CO2(−0.60±0.2 eV), COS(+0.46±0.2 eV), and CS2(1.0±0.2 eV). The CO−2 ion was found to be metastable with respect to autodetachment. This result is compatible with the negative electron affinity for CO2 and in agreement with our earlier observations of CO−2* and with recent theoretical calculations. The lifetime of CO−2* (9±2×10−5 sec) was measured to be independent of collision energy over the region of energy studied (threshold to ∼20 eV c.m.). The fragment ions O−/CO2, O−/COS, S−/COS, and S−/CS2 were detected at a threshold ene...

Resonantly enhanced multiphoton ionization of pyrrole, N‐methyl pyrrole, and furan

The resonantly enhanced multiphoton ionization (REMPI) spectra of pyrrole (C4H5N), N‐methyl pyrrole (C5H7N), and furan (C4H4O) have been measured in the wavelength region from 365 to 680 nm. New and previously observed Rydberg states are reported for pyrrole and furan. Vibrational constants are presented for most of the Rydberg series. Accurate ionization potentials are derived for pyrrole (8.207±0.003 eV) and N‐methyl pyrrole (7.94±0.02 eV). A strong two photon allowed transition is observed in N‐methyl pyrrole at 41 193 cm−1 (0,0) and is attributed to an 1A2 state. The corresponding state is not seen in pyrrole; however, it may be obscured by overlapping Rydberg series. Mass spectra following REMPI for benzene, pyrrole, and furan are reported. The degree of ionic fragmentation depends upon laser power density and wavelength.

Negative ion properties of p‐benzoquinone: Electron affinity and compound states

Bound and excited negative ion states of para‐benzoquinone (PBQ) are studied from experiments involving collisions of electrons and cesium beams with PBQ. The electron affinity of PBQ was measured to be 1.89+0.2−0.3 eV using the cesium collisional ionization technique. PBQ−(C6H4O−2) was the dominant negative ion observed following collisions of cesium beams with PBQ. Direct electron attachment to PBQ produced the fragment ions C5H4O−, C5H3O−, C4O2H−2, C2HO−, and C2H− as well as the parent C6H4O−2 ion. Our observations showed that the cross section for producing the metastable parent negative ion, C6H4O−2, peaks at 1.40±0.1 eV which is in disagreement with a previous value of 2.1 eV. Attachment of thermal electrons to PBQ at low pressure (<10−4 mm Hg) was not observed. Compound negative ion states were observed to peak at 0.70, 1.35, and 1.90 eV using the SF6 scavenger technique. The energies of these compound states agree qualitatively with the energies if the manifold of unfilled π* orbitals. The long‐li...

Transient Negative‐Ion States in Alicyclic and Aromatic Fluorocarbon Molecules

The formation of short‐lived (lifetime ∼10−15 − 10− 13  sec) and long‐lived (lifetime ∼10 − 6−10−3 sec) temporary negative‐ion states in 12 cyclic fluorocarbon molecules is studied in the gas phase with monoenergetic electron beams. The long‐lived molecular ions exhibit a systematic increase of autoionization lifetime from 7 μsec for C4F6− possessing 24 vibrational degrees of freedom to 800 μsec for C7F14− having 57 vibrational degrees of freedom. This 100‐fold increase of lifetime with increasing number of degrees of freedom qualitatively conforms to the previous theoretical model describing nondissociative electron attachment in polyatomic molecules. All of the long‐lived ions (C4F6−, C6F6−, C4F8−, C5F8−, C7F8−, C6F10−, and C6F12−) have a maximum attachment cross section at some energy less than 0.05 eV, and the widths of the attachment resonances observed in the beam experiments were instrumental with the exception of C7F14−, whose width at 12 maximum was approximately 0.2 eV. The SF6 scavenger techniq...

Collisional ionization between alkali atoms and some methane derivatives: Electron affinities for CH3NO2, CF3I, and CF3Br

The negative ion products resulting from collisions between orthogonal beams of alkali atoms (Na, K, Cs) and the methane derivatives CH3CN, CH3NO2, CF3Br, and CF3I have been studied in the energy range from reaction thresholds to ∼40 eV (LAB). Stable negative ions with masses corresponding to the last three molecules were detected and the following electron affinities are derived from measurements of the energy threshold for the ion pair production reactions: E.A.(CH3NO2) =0.44+0.1−0.2 eV; E.A.(CF3Br) =0.91±0.2 eV, and E.A.(CF3I) =1.57±0.2 eV. From measurements of the difference between the energy threshold for the appearance of various fragment ions and the parent ion, the following bond dissociation energies are deduced: D(CH3–NO−2) =0.56±0.2 eV; D(CF3–Br−) =0.54±0.2 eV and D(CF3–I−) =0.32±0.2 eV. An argument is presented which adds further strength to the suggestions of Williams et al. and Jordan and Wendoloski that electron binding to CH3CN is dominated by the dipole field.

Electron attachment to cyclic anhydrides and related compounds

Negative ion production cross section are sections are tabulated as a function of the bombarding electron energy for dissociative and nondissociative electron attachment to various cyclic anhydrides and related compounds. The unsaturated compounds, maleic anhydride (250 μsec), phthalic anhydride (313 μsec), pyromellitic anydride (8000 μsec), and maleimide (43 μsec) attach thermal electrons to form parent negative ions which are metastable with autodetachment lifetimes that decrease with an increase in the energy of the bombarding electron. The number in parentheses following each compound is the lifetime at peak cross section. Compound negative ion states are reported for the saturated compounds succinic anhydride (1.3 eV), cyclobutane‐dicarboxylic anhydride (1.3 eV), glutaric anhydride (0.6 eV), and succinimide (1.1 eV) for electron energies given in the parentheses. In most of the anhydrides the most abundant anions produced by dissociatve electron attachment are metastable RCO2‐* ions which result from...

Negative ion properties of tetracyanoquinodimethan: Electron affinity and compound states

Bound and excited negative ion states of gaseous 7,7,8,8‐tetracyanoquinodimethan (TCNQ) are studied from experiments involving collisions of electrons and fast cesium beams with TCNQ. The electron affinity of TCNQ is measured to be 2.8+0.05−0.3 eV by the collisional ionization technique. TCNQ attaches electrons with energies of ∼0, 0.7, and 1.3 eV (values represent peaks in cross section) to form long‐lived compound negative ions which are metastable with respect to autodetachment. The lifetime for the decay of TCNQ−* decreases from ∼2×10−3 sec at ∼0 eV to ∼10−4 sec at ∼4 eV. Evidence for a third compound negative ion state is seen as a peak in the TCNQ− signal at ∼3.2 eV. This state is metastable with respect to autodetachment and dissociation into the products C11N3H−3+HCN0.

Attachment of Electrons to Substituted Benzenes

Attachment of slow (0–5 eV) electrons to a number of aromatic species has been studied in a time‐of‐flight (TOF) mass spectrometer using an automatic retarding potential difference (RPD) electron beam with resolution of ∼ 0.1 eV. Autodetachment lifetimes were measured for parent ions formed by two‐body attachment of thermal (∼ 0.03 eV) electrons to the following molecules (lifetimes in microseconds are given in parentheses): C6F5Br (21), C6F5Cl (17.6), C6F5CN (17), C6F5CHO (36), C6H5CN (≈ 5), C6H5NO2 (17.5), C6D5NO2 (22), m‐C6H4ClNO2 (47), and perfluoronaphthalene (123). The C10F8− ion and ions of the form C6F5X− had lifetimes exhibiting a systematic increase with the number of vibrational degrees of freedom consistent with the trend observed in a previous study of cyclic fluorocarbons. Cl−, Br− and I− were formed from C6F5X (X==Cl, Br, I) along with C6F5− at incident electron energies below 0.1 eV. Current ratios for X− to C6F5− were found to be approxmaltely 100/1, 3/1, and 1/30, respectively. No nondis...

Molecular electron affinities from collisional ionization of cesium. II. SF6 and TeF6

The relative cross sections for the production of mass selected negative ions resulting from collisions of Cs with SF6 and TeF6 have been studied as a function of the incident cesium atom energy from 0 to 40 eV. Electron affinities (E.A.) for SF6 and TeF6 are derived from accurate measurements of the threshold for ion‐pair production with the results E.A. (SF6)=0.54−0.17+0.1 eV and E.A. (TeF6)=3.34−0.17+0.1 eV. Dissociative ionization studies resulted in E.A.(SF5) ≥ 2.8 ± 0.2 eV and D(SF6−−F)=1.05± 0.1 eV.

Negative ion properties of fluoranil, chloranil, and bromanil: Electron affinities

Bound and excited negative ion states of p‐fluoranil (FA), p‐chloranil (CA), and p‐bromanil (BA) are studied from experiments involving collisions of electrons, Na, K, and Cs beams with FA, CA, AND BA. Reaction thresholds for producing FA−, CA−, and BA− are determined as a function of the temperature of the target molecules. The electron affinities corresponding to 0 °K were measured to be 2.92±0.2 eV for FA, 2.76±0.2 eV for CA, and 2.44±0.2 eV for BA. FA, CA, and BA attach ∼ 0 eV electrons and FA attaches 0.5 eV electrons to form metastable parent negative ions with lifetimes in the millisecond range. These lifetimes are a function of the electron energy. Dissociative and nondissociative resonances observed via electron attachment to p‐benzoquinone (PBQ) and the ’’anils’’ are correlated with the optically determined 2B3g′, 2B1u, and 2Au states of PBQ− and the anion radicals of several substituted quinones.