Temporary Negative Ion States Research Articles

Low-energy electron-impact excitation spectra of formaldehyde, acetaldehyde and acetone

Threshold electron-impact excitation spectra, transmission spectra and n→ n * excitation functions are presented for formaldehyde, acetaldehyde and acetone. Formation of temporary negative ion states and their decay-channels are discussed. The electronic transition near 4 eV is shown to be due both to triplet and singlet n→ n * excitation. The energy positions of the 3n→ n * and first triplet Rydberg transition are accurately located. In formaldehyde a new transition is reported at 8.50 eV. Assignments are given for the observed electronic excitation processes.

Characteristic energy losses by slow electron impact on thin-film alkanes at 77 °K

Characteristic energy losses by slow electron impact have been measured on thin films of several alkanes at 77 °K. The cross section for the lowest loss at [inverted lazy s] 0.6 eV increases tenfold in the series n -pentane to n -nonane with increasing carbon number. For a given carbon number it decreases markedly with branching. Losses at [inverted lazy s] 3, [inverted lazy s] 6, [inverted lazy s] 9, and [inverted lazy s] 22 eV are common to all alkanes with no strong dependence on molecular size or configuration. The lowest losses may be due to temporary negative ion states or to pre-existing trapping potentials. Losses at [inverted lazy s] 3 eV may be due to the highest temporary negative ions or to triplets. If the former is correct, then [inverted lazy s] 6 eV losses can be attributed to triplets. The [inverted lazy s] 9 eV losses are due to upper singlet states, and the [inverted lazy s] 22 eV losses may be due to collective excitations.

Threshold electron impact excitation of argon in the region of the M 1 edge

The threshold electron impact spectrum of argon has been obtained in the energy range 24–35 eV using the sulphur hexafluoride electron scavenging technique. Structure is observed which may be due to single excitation, double excitation and temporary negative ion states.

Electron-molecule interactions in photoelectron spectroscopy

Anomalous peaks have been observed in the photoelectron spectra of certain diatomic and triatomic molecules when the pressure in the ionization chamber is relatively high. These are attributed to collisions between emitted photoelectrons and neutral molecules leading to the formation of temporary negative ion states. Both vibrational excitation and resonant electron transmission phenomena are reported. The precision of the measurements compares favourably with results obtained by conventional electron impact techniques.

Electron Attachment and Excitation Processes in Selected Carbonyl Compounds

Short-lived negative ions were observed in urea (2.2 eV), acetone (1.6 eV), acetaldehyde (1.2 eV), acetophenone (0.95 eV), benzophenone (0.75 eV), and benzaldehyde (0.72 eV), using the SF6 scavenger technique. The energy position of the peak of each resonance is given in parentheses. These resonances were assigned to the capture of an electron into the lowest vacant (π*) orbital of the carbonyl group. Hexafluoroacetone captures slow electrons (∼0 eV) into a long-lived temporary negative ion state with an autodetachment lifetime of 65± 10 μsec. Cl− was observed through dissociative attachment of 0.5-eV electrons to chloroacetone. Dissociative attachment to urea yielded CN− and NCO− around 2 eV and NH2−, NH−, OH−, and N2H3CO− around 6 eV. Peaks in the threshold excitation spectra were observed in acetone at 4.15, 6.13, 7.5, and 8.1 eV; in acetaldehyde at 3.8, 6.35, and 6.62 eV; and in urea 6.1 and 8.4 eV.

Electron Transmission Spectroscopy: Core-Excited Resonances in Diatomic Molecules

A high-resolution and high-sensitivity electron transmission spectrometer is used to study sharp structures in the total scattering cross section of ${\mathrm{H}}_{2}$, ${\mathrm{D}}_{2}$, ${\mathrm{O}}_{2}$, NO, ${\mathrm{N}}_{2}$, and CO. Many new resonances are found, some of which form bands which consist of progressions of negative-ion vibronic states. The existence of bands makes the identification of temporary negative-ion states easier, since the spacing and the magnitudes of the structure can be compared with the appropriate parameters of various positive-ion cores. The most prominent features observed consist of two Rydberg electrons attached to a particular positive-ion core.

Electron transmission studies of decay channels of molecular resonances

Measurements are reported of transmission of energy monochromated electrons of energy 0.1-4eV through oxygen, nitrous oxide, nitric oxide, nitrogen dioxide, carbon dioxide, and benzene. Energy loss spectra are measured, and the zero angle scattering functions for excitation of different vibrational levels are reported. The functions show oscillatory structure due to temporary negative ion states. The function for O2 is analysed in terms of the compound state theory, yielding O2-2 pi g nuclear separation re=1.39AA in disagreement with the photodetachment value re=1.34+or-0.01AA (Celotta et al). The causes of this discrepancy are probably significant. The transmission functions for zero energy loss are treated by Fourier analysis, which yields the approximate vibrational energies of the temporary negative ion states of O2-, NO-, NO2-, C6H6-. It is possible that a second state of NO2- contributes the scattering at energies above 1.1eV.

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.

961. Threshold electron impact excitation of atoms and molecules: detection of triplet and temporary negative ion states: R N Compton et al,J Chem Phys, 48 (2), 15 th Jan 1968, 901–909

961. Threshold electron impact excitation of atoms and molecules: detection of triplet and temporary negative ion states: R N Compton et al,J Chem Phys, 48 (2), 15 th Jan 1968, 901–909

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.

Electron Capture by Rotational Excitation of Polar Molecules

The problem of electron capture by a polar molecule with simultaneous rotational excitation of the molecule is analyzed. Capture cross sections and lifetimes of temporary negative-ion states so formed are calculated on the assumption that the electron interacts primarily at large distances from the molecule by means of the dipole field and forms only loosely bound ionic states. The capture probability is proportional to the ratio ${(\frac{D}{I})}^{2}$, where $D$ is the dipole moment and $I$ the moment of inertia of the molecule about a perpendicular axis passing through the center of the dipole. Electron capture by rotational excitation is most probable for polar molecules that have both a relatively large dipole moment and a small moment of inertia. In order for this process to make effective contributions to momentum-transfer cross sections measured, e. g., in electron-swarm experiments, the spacing of the rotational levels of the molecule must be of the order of thermal energies, so that a relatively large number of electrons can be captured and released. These circumstances, viz., the relatively large value of $\frac{D}{I}$ and the right spacing of rotational levels, appear to offer an explanation of the particular behavior of ${\mathrm{H}}_{2}$O, ${\mathrm{D}}_{2}$O, and ${\mathrm{H}}_{2}$S among a number of polar molecules with which recent swarm experiments have been made. According to the theory, some other molecules (e. g., N${\mathrm{H}}_{3}$, HF, HCl, and ${\mathrm{H}}_{2}$${\mathrm{O}}_{2}$) should exhibit a similar behavior. Lifetimes of negative ions formed by this process are estimated to be of the order of ${10}^{\ensuremath{-}13}$ sec.

Study of the N2O Molecule Using Electron Beams

Inelastic processes and negative ion formation in N2O are measured using the trapped-electron method and conventional techniques, respectively. A large inelastic process is observed at 2.2 ev and is interpreted as the formation of vibrationally excited N2O via the formation of a temporary negative ion state. Negative ions O− are observed beginning at zero ev with peaks of the negative ion current occurring at 0.7 and 2.2 ev. The latter peak is attributed to the formation of the temporary negative ion with subsequent decay into a stable O− plus N2 in various states of vibrational excitation. Measurements of the kinetic energy of the negative ions confirm this hypothesis. A value of the dissociation energy of the N2–O bond in N2O is found from the appearance potential of O− extrapolated to zero kinetic energy. This value, D (N2–O) = 1.2±0.2 ev, agrees within experimental error with the value of 1.3±0.2 found independently by Curran and Fox, but disagrees with the thermochemical value of 1.66 ev.

Measurement of Excitation ofN2, CO, and He by Electron Impact

The inelastic excitation of ${\mathrm{N}}_{2}$ and CO by electron impact is studied using the trapped-electron method. In this method those electrons which have lost a portion of their initial energy in an inelastic collision are trapped in a potential well. Well depths up to 3 volts are used in the present experiment. The operation of the apparatus is checked for helium, where the shape of the excitation function is known accurately. The shape of the excitation function for metastable helium atoms obtained by the trapped-electron method is in good agreement with previous results. A large inelastic peak is observed at 2.3 ev in ${\mathrm{N}}_{2}$ and 1.7 ev in CO. This phenomenon is discussed in terms of the formation of a temporary negative ion state of ${\mathrm{N}}_{2}$ or CO and subsequent decay into various vibrational levels of the molecule. This model explains the sharp peak in both the elastic and inelastic cross section in ${\mathrm{N}}_{2}$ and CO. Neither ${\mathrm{O}}_{2}$ nor ${\mathrm{H}}_{2}$ show such a sharp peak at low energies.