Negative Ion Resonances Research Articles

Compound-negative-ion resonance states and threshold-electron excitation spectra of N-heterocyclic molecules: Pyridine, pyridazine, pyrimidine, pyrazine, and sym-triazine

The threshold-electron excitation spectra of the N-heterocyclic molecules pyridine, pyridazine, pyrimidine, pyrazine, and sym-triazine have been studied with a newly constructed electron spectrometer. These are presented, discussed, and compared with photoabsorption and, whenever possible, with electron impact spectral data. Optically forbidden and n→ π* transitions have been observed. Two compound negative ion resonance (CNIR) states (``shape'' resonances) have been detected below the first excited electronic states of all N-heterocyclic molecules studied, except sym-triazine for which only one CNIR state has been observed. These CNIR states are discussed in terms of the effect of a perturbation introduced by the more electronegative nitrogen atom(s) in the benzene ring on the two lowest (degenerate) antibonding orbitals of benzene.

Long-lived parent negative ions formed via nuclear-excited Feshbach resonances. Part 2.—Aromatic molecules other than benzene derivatives and non-aromatic organic structures

Long-lived parent negative ions formed via nuclear-excited Feshbach resonances at thermal energies have been observed for 1-naphthaldehyde, 2-naphthaldehyde, benzoin, tetracyanoethylene, pentane-2,3-dione, acrolein, and acetic, adipic and malonic acids. The autodetachment lifetimes of these ions at thermal electron energies as determined by the slope method using a time-of-flight mass spectrometer are 15 ± 0.5, 7.6 ± 0.3, 10 ± 0.5, ∼60, 7 ± 0.3, 38 ± 3, 326 ± 30, 30 ± 3, and > 1 µs, respectively. The organic molecules benzil and quinoline formed long-lived parent negative ions whose autodetachment lifetimes at the peak of the negative ion resonances are 26 and 632 ± 74 µs, respectively, but for these two molecules the negative ion resonances peak at 0.7 and 0.6 eV, respectively, rather than at ∼0.0 eV. The autodetachment lifetimes of the benzil and tetracyanoethylene parent negative ions decrease with increasing electron energy. The measured lifetimes, as well as those reported by others on long-lived parent negative ions of organic molecules other than benzene derivatives, are summarized, discussed, and related to the respective molecular structures. From these data and those presented in Part 1, it can be concluded that NO2-containing benzene derivatives, CN-substituted organic molecules, higher aromatic hydrocarbons, strained structures, and organic molecules containing the functional groups —COCO—, —COCH(OH)—, —COOH, and CHCHO, capture thermal electrons and form long-lived parent negative ions via nuclear-excited Feshbach resonances.

Compound negative ion resonances and threshold-electron excitation spectra of quinoline and isoquinoline

Two compound negative ion resonances below 2 eV have been observed for quinoline, isoquinoline and naphthalene. Electronic transitions in quinoline and isoquinoline are reported.

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 nondissociative attachment was observed for C6F5I. Monochlorination of nitrobenzene essentially doubled the autodetachment lifetime and added a new mode of dissociation. Cl− and NO2− ion currents from m-chloronitrobenzene showed almost identical energy dependences with peaks at ∼ 1 eV and ∼ 3.5 eV, corresponding closely to the energy dependence of NO2− dissociation from nitrobenzene. This was taken as evidence that the Cl− and NO2− ion currents result from unimolecular dissociation of the same intermediate negative ion state with the incident electron in a π-electronic orbital. Iodine substitution in nitrobenzene had the effect of elimination of long-lived parent ion formation with dissociative attachment producing I− dominating at thermal electron energy. An SF6 scavenger spectrum of benzaldehyde indicated the presence of a temporary negative ion resonance at ∼ 0.65 eV. Negative ion breakdown curves were also determined for C6H5OH, C6F5OCH3, C6F5NH2, C6H5CN, o-C6H4ClF, m-C6H4ClF, m-C6F4ClBr, and m-C6H4ICl.

Energy dependence of the electron attachment cross section and the transient negative ion lifetime for p-benzoquinone and 1,4-naphthoquinone

Cross sections for nondissociative electron attachment to p-benzoquinone and 1,4-naphthoquinone are presented and discussed. Results are also reported showing the variation of the transient negative ion life time within the negative ion resonance for both nuclear- and electron-excited Feshbach resonances.

Electron impact excitation and ionization of helium near 60 eV

Double excitation levels and negative ion resonance have been observed in the electron impact excitation spectrum and the ionization cross section of He.

Energy lost by slow electrons in collissions with molecules

The mean energy lost by slow electrons in collisions with molecules has been calculated from published electron transport data. The general features of the large fractional energy losses are explained in terms of either direct molecular excitation or indirect via the decay of low-energy compound negative ion states. Evidence is also provided which suggests that the N 2O compound negative ion resonance at 2.3 eV predominantly decays elastically.

Temporary negative ion resonance of pyridine

Two temporary negative ion states of pyridine are observed with the electron scavenger technique. The two states are believed to arise from a splitting of the two lowest unfiled π-electron orbitals of the negative ion.

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 technique is employed to detect short-lived transient negative-ion states in the fluorinated benzenes, and the peak of the resonances occurred at 1.35 eV for C6H5F, 0.6 eV for 1,3-C6H4F2, 0.3 eV for 1,3,5-C6H3F3, and at ∼0 eV for 1,2,3,4-C6H2F4 and C6HF5. The peak energies of the temporary negative-ion resonances show an approximately linear decrease as a function of the number of fluorines added to the ring, and evidence is given that addition of each fluorine to the benzene ring increases the electron affinity by ∼0.4 eV. The completely fluorinated benzene ring attaches thermal energy electrons into a negative-ion state which has a lifetime of 12 μsec.

Threshold Electron Impact Excitation of Atoms and Molecules: Detection of Triplet and Temporary Negative Ion States

A technique which utilizes SF6 as a scavenger of low-energy electrons is employed to study the “threshold” excitation spectra of He, N2, HCl, H2O, D2O and a number of aromatic molecules. For electrons within ∼0.03 eV of threshold, it is found that the probability of exciting the 23S state of helium is approximately 1.5 times larger than that for the 21S state. For HCl all of the optically allowed transitions are observed including a Rydberg series leading to the ionization potential. Temporary negative ion resonances are observed below the first electronic state for all of the aromatic molecules studied. For benzene, in addition to the optically allowed transitions, the first (3.9 eV) and second (4.7 eV) triplet states are detected, while for naphthalene a new intense level at 5.4 eV as well as the lower triplet states are observed.

Temporary negative ion resonances in benzene and benzene derivatives

New, intense, energy loss resonances below the first excitation potential were discovered in the electron impact excitation spectra of benzene and six of its derivatives. The observed resonances for o-C 6H 4CH 3Cl, C 6H 5Cl, C 6H 5Br and o- 6H 4Cl 2 peak at exactly the same energy as the dissociative electron capture resonances previously reported for these compounds, suggesting that both resonances proceed via a common temporary negative ion state of the parent molecules.