William H. Hamill

Molecular Ions in Radiation Chemistry. I. Formation of Aromatic‐Amine Cations in CCl4 by Resonance Charge Transfer at 77°K

Gamma‐irradiated polycrystalline CCl4 at 77°K provides evidence that positive charge migrates and is trapped by the matrix itself. The trapped positive hole absorbs at about λmax=400 mμ in pure CCl4 or dilute solid solutions of aromatic amines. The solvent band is depressed by aromatic amines or other additives which act as hole traps with appearance of the corresponding amine cation absorption. After γ irradiation the residual 400‐mμ band photobleaches at 77°K with increase in solute cation absorption. Brief heating to 143°K gives the same result. A kinetic treatment which assumes that the only labile species in the system is migrating positive charge can explain the experimental results in a satisfactory way. The kinetic analysis gives a rough estimate of total available positive charge created per 100 eV of absorbed energy as 1.9. Several aromatic amines have been used as test substances and their optical absorption spectra recorded.

Molecular Ions in Radiation Chemistry. III. Absorption Spectra of Aromatic‐Hydrocarbon Cations and Anions in Organic Glasses

Aromatic hydrocarbons (ArH) in γ‐irradiated, glassy butyl chloride at 77°K are efficient traps for migrating positive holes, forming the radical cations of 40 compounds in good yield. No ArH− ions appeared to form. Photobleaching is very inefficient, or nil. Resonance charge transfer and simple competition for holes by ArH, solvent, and anions are consistent with observation. In glassy methyltetrahydrofuran, ArH gave ArH− in good yields with very little ArH+. In 3‐methylpentane, both ArH+ and ArH− were formed. Addition of propyl chloride enhanced ArH+ and suppressed ArH−. Addition of ethanol had the reverse effect.

Molecular Ions in Radiation Chemistry. II. Aromatic‐Hydrocarbon Cations in CCl4 at 77°K

Solutions of aromatic hydrocarbons in a polycrystalline matrix of CCl4 at 77°K have been γ irradiated and well‐resolved optical absorption spectra measured in the interval 270–2000 mμ. All spectra characteristic of solute are attributed to molecular monopositive ions of benzene, toluene, bi‐ and triphenyl methanes, biphenyl, o‐, m‐, and p‐terphenyl, bi‐, tri‐, and tetraphenylethylene, biphenylacetylene, naphthalene, anthracene, phenanthrene, stilbene, diphenylstibene, and 1,3,5‐triphenylbenzene. Resonance charge transfer from CCl4+ to CCl4, terminated by transfer from CCl4+ to solute, is postulated.

Ionic Processes in γ‐Irradiated Organic Solids: Electrons in 3‐Methyl Pentane

γ‐irradiated 3‐methylpentane (3MP) and solutions of biphenyl (φ) have been examined at −196° at doses of 1 to 26×1018 eV ml−1. The half‐life for decay of the trapped electron (e−) in 3MP is independent of dose for t 1 h. This behavior, as well as very efficient formation of φ− at small % φ, requires electron track lengths of ∼10−4 cm. The 100 eV yield of (e−) shortly after γ irradiation is ∼0.9, in terms of the measured extinction coefficient e(e−)≅3.0×104 liter mole−1·cm−1 at 2.3×1018 eV ml−1, but only 60% as great at 10‐fold greater dose. At 0.04% φ, the rate of (e−) decay is much faster and approximates first order. The ratio of reaction cross sections of (e−) with positive holes and φ is ∼120. Addition of 2% 2‐methylpentene−1 as positive hole trap increases the yield of (e−) and decreases the rate of decay, which becomes second order throughout. Recombination luminescence intensity from TMPD at t>1 h decays at the same rate following uv and γ excita...

Ionization Potentials of Molecules and Free Radicals and Appearance Potentials by Electron Impact in the Mass Spectrometer

An improved method of measuring ionization efficiency (I.E.) curves by the retarding potential difference technique in the mass spectrometer (to ∼one ion/sec at onset) gives ionization potentials which agree within ± 0.02 eV of spectroscopic values and ± 0.03 eV of photoionization. Ionization potentials I have been measured for the following free radicals (eV): CH3, 9.87; C2H5, 8.34; n‐C3H7, 8.13; i‐C3H7, 7.57; n‐C4H9, 8.01; C2H5O, 9.11; n‐C3H7O, 9.20; i‐C3H7O, 9.20; n‐C4H9O, 9.22. Appearance potentials V have been measured for R+/RX, where R=CH3, C2H5, n‐C3H7, n‐C4H9 and X=H, Cl, Br, I, and OR; also for RO+/ROX where R=C2H5, i‐C3H7, n‐C4H9 and X=H, NO, and R; also for I+/n‐C4H9I, NO+/C2H5ONO, NO+/n‐C4H9ONO, and C3H6D+/CH3CD2CH3. Values of ΔHf(R+), ΔHf(RO+), D(R−X), and D(RO−X) are reported as well as electron affinities of NO, C2H5O, n‐C3H7O, i‐C3H7O, and n‐C4H9O. Whenever two or more independent measurements of V provide the same ΔHf or D, agreement is satisfactory. The linearity of onsets does not corr...

Ionization‐Efficiency Curves for Molecular and Fragment Ions from Methane and the Methyl Halides

The retarding potential difference method has been used to measure ionization efficiency curves of CH3X+ (X=H, F, Cl, Br, I), CH3+, and X+ from methane and the methyl halides in a time‐of‐flight mass spectrometer. Discontinuities between linear segments of the ionization efficiency curves are interpreted as appearance potentials (A.P.) of electronic states or processes. For all CH3+, A.P.s occur at 1.1, 1.7, 2.7, and about 3.4 eV above onset in the ion pair region. Empirical knowledge of this structure makes it possible to identify the A.P. for the ion—neutral pair, CH3+—X, in their ground states. Combining these data and the A.P.s for X+/CH3X gives the ionization potential for CH3 and the bond dissociation energies, D(CH3—X).

Ionic Processes in γ‐Irradiated Organic Solids: Positive‐Hole Migration in 3‐Methylpentane

The yields of positive and negative species trapped in γ‐irradiated organic glasses at −196° have been measured as a function of glass composition. Increased yields of solvent‐trapped electrons (e−) and biphenyl anions (φ2−), as well as decreased yields of various solute cations (A+), were noted when additives capable of undergoing charge‐transfer or proton‐transfer reactions were present in small amounts (<2 mole %) in 3‐methylpentane (3MP). All results can be understood qualitatively in terms of the mobile positive hole being transferred from 3MP to these additives and trapped. A simple scheme of competition for the total available number of positive holes accounts for the decrease in A+ when a second solute (B) is added and relative cross sections to form A+ and B+ can be determined. Additives affect the optical absorption spectrum of (e−) in 3MP. The spectra of the biphenyl positive and negative ions are similar but displaced, while the anion and cation from benzene are anomalous in 3MP glass. The rat...

Trapped Electrons in γ‐Irradiated 3‐Methylpentane at −196°

The yield of solvent‐trapped electrons in γ‐irradiated 3‐methylpentane at −196° was 0.75 per 100 eV when measured within a few seconds, and diminished to ∼0.6 after 3 min. The quantum yield for bleaching at 950 mμ decreased from 0.8 toward zero with progressive bleaching. Addition of 0.05% biphenyl maintained the quantum yield at >0.8 until more than 90% of the solvent trapped electrons had bleached. The initial quantum yield at λ>1.7μ was ∼0.01, both with and without added biphenyl. There is qualitative correlation between photobleaching and the rate of spontaneous decay.

Appearance Potentials of Positive and Negative Ions by Mass Spectrometry

Appearance potentials (AP) of ions were obtained by the retarding potential difference (RPD) technique using a time‐of‐flight mass spectrometer. Confirmatory measurements were made using a 180° magnetic instrument. Information about processes such as ion‐pair formation, electronic excitation of ions, rearrangement of ions, autoionization and metastable transitions is inferred from the structure of the ionization efficiency (IE) curves of Xe, H2, CH4, CD4, C2H4, C2H6, C3H8, and C4H10 over about a 4 eV range. The APs in Xe at 12.02, 12.35, 12.75, and 14.0 correspond in a qualitative way to autoionization levels. The structure observed in parent and fragment IE curves for hydrocarbons can, in some systems, be correlated with known processes. Structure due to vibrational excitation of ions was not resolved. The IE curves for polyatomic ions indicate that many ions formed by electron impact are formed in electronically excited states, and that appearance potentials of fragment ions often correlate with upward...

Paramagnetic Resonance of Alkyl Radicals from Dissociative Electron Attachment in γ‐Irradiated Organic Glass

The radicals CH3·, CH3CH2·, CH3CH2CH2·, (CH3)3C·, (CH3)3CCH2·, and C6H5ĊHCH3 have been identified by EPR in γ‐irradiated dilute solutions of the corresponding halides in 3‐methylpentane at 77°K. For 0.4 mole % CH3I, G(CH3·)=1.2. Methyl radicals disappear by some first‐order rate process for which k=4.6×10−4 sec−1. Other radicals did not decay measurably under similar conditions. The results are consistent with the mechanism of dissociative thermal electron attachment, RX+e−→R·+X−, which is an efficient, selective process whenever the electron affinity of X exceeds the bond dissociation energy of R—X.