P. W. Reinhardt

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...

Nondissociative Electron Capture in Complex Molecules and Negative‐Ion Lifetimes

The formation and decay of parent negative ions have been studied with electron‐swarm and electron‐beam techniques. The rate of attachment at thermal energies under electron‐swarm conditions for SF6 and C6H5NO2 was determined to be 1.24×109 and 2.1×107 sec−1·torr−1, respectively. Autoionization lifetimes for SF6−, C6H5NO2−, and (CH3CO)2− are reported to be 25, 40, and 12 μsec, respectively. The absolute rate of electron attachment in SF6 was found to be independent of the temperature (298°≤T≤418°K), and is thus consistent with an electron capture cross section for SF6 which varies inversely with the speed of the electron. The attachment rates and lifetimes were combined through the principle of detailed balance to calculate the ratio of density of states of the negative ion to that of the neutral plus electron. A simple theoretical treatment of long‐lived negative ions is presented, and the possibility of estimating electron affinities from measurements of attachment cross sections and negative‐ion lifeti...

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.

Collisional ionization between fast alkali atoms and selected hexafluoride molecules

Negative ion products resulting from collisions between orthogonal, crossed beams of alkali metal atoms (Na, K, Cs), and the octahedral hexafluorides MF6 (M=S, Se, Te, Mo, W, Re, Ir, and Pt) have been examined in the energy range from ∼0 to 40 eV (lab). Studies of the dependence of the reaction thresholds upon the temperature of the target molecules SF6, SeF6, and TeF6 have provided electron affinities for these molecules; E.A.(SF6) =0.46±0.2, E.A.(SeF6) =2.9±0.2, and E.A.(TeF6) =3.3±0.2 eV. Energy loss measurements of the alkali, A, in the reaction A+MF6→A++MF−6 at small scattering angles are consistent with these values. Measurements for SF4 together with temperature dependent thresholds for the formation of SF−5 from SF6 and SF−3 from SF4 combined with known bond dissociation energies for D (SF5–F) and D (SF3–F) yield electron affinity values for the SFn series; E.A.(SF6) =0.46±0.2, E.A.(SF5) =2.71±0.2, E.A.(SF4) =0.78±0.2, and E.A.(SF3) =3.07±0.2 eV. Lower limits of ∼5 eV for the electron affinities o...

Dissociative Electron Capture by Benzene Derivatives

We have studied dissociative electron capture in a series of benzene derivatives using the combined swarm—beam method. The benzene derivatives C6H5Cl, o‐C6H4Cl2, o‐C6H4CH3Cl, C6H5Br, C6D5Br, and o‐C6H4CH3Br, capture low‐energy (including thermal) electrons dissociatively in a capture process which peaks at about 1 eV and yields only Cl− or Br− ions. The energies (in electron volts) at which the capture cross sections σc(e) for the ∼1‐eV processes peak and the values of σc(e) (in cm2×1017) at peak energy for the above compounds are 0.86, 1.4; 0.36, 43; 1.1, 2.2; 0.84, 9.6; 0.8, 10.4; 0.95, 6 for C6H5Cl through o‐C6H4CH3Br, respectively. The rates of capture at thermal electron energies are also given for each compound (but because of impurities are in question). For comparison with the benzene derivatives, C2H5Cl was also studied with the swarm—beam method. Beam data are also presented for dissociative electron capture by C6H5I, C6H5NO2, and o‐ and m‐C6H4CH3NO2.

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.

Determination of Electron‐Capture Cross Sections with Swarm‐Beam Techniques

A method is described which combines electron‐swarm and electron‐beam techniques to obtain absolute electron‐capture cross sections as a function of electron energy. As examples of the method, data on Cl− from ortho‐chlorotoluene (approximately 1 eV) and O− from oxygen (approximately 7 eV) are presented and discussed.

Collisions of Monoenergetic Electrons with NO2: Possible Lower Limits to Electron Affinities of O2 and NO

The yields of O−, NO−, and O2− ions produced by dissociative attachment of electrons to NO2 have been studied as a function of the electron energy from 0 to 6 eV. A modulated retarding‐potential‐difference technique permitted electron‐energy resolution of approximately 0.1 eV. The measured appearance potentials for NO− and O2− were found to be consistent with an electron affinity of ≥0.65 eV for NO and ≥1.1 eV for O2. These lower limit measurements to the electron affinities of O2 and NO are discussed in relation to previous studies of electron attachment to these molecules.

Reactions of fast cesium atoms with polymers of antimony pentafluoride and gold pentafluoride

The observation of SbF6− and AuF6− produced in collisional ionization reactions of fast Cs atomics with polymers of SbF5 and AuF5 is reported. (AIP)

Mass spectrometry utilizing collisional ionization of cesium: Maleic anhydride and succinic anhydride

The translational energy dependence of the relative cross section for production of negative ions by collisions of fast cesium atoms with maleic anhydride (C4H2O3) and succinic anhydride (C4H4O3) has been studied from threshold up to approximately 20 eV (c.m.). Accurate measurements of the threshold energy for the creation of Cs+ and C4H2O3− from collisions of cesium atoms with maleic anhydride yield the electron affinity of maleic anhydride to be 1.4±0.2 eV. A number of fragment negative ions are observed from both compounds and the results are compared with our recently reported electron impact studies. CO2−* ions are observed to be unstable with respect to autodetachment, and the autodetachment lifetimes are measured to be 62±10 and 71±10 μsec for CO2−* from maleic and succinic anhydride, respectively. In both cases the lifetime of CO2−* is independent of the collision energy from threshold up to 15 eV.