Richard A. Holroyd

Energy of Excess Electrons in Nonpolar Liquids by Photoelectric Work Function Measurements

The energy of the excess electron state has been determined for six nonpolar liquids by a photoelectric injection method. The functional dependence of the photocurrents on wavelength is the same in the liquid as in the vacuum and work functions can be evaluated from Fowler plots. For several liquids the work functions are less than the vacuum value and these lowerings are characteristic for each liquid. For tetramethylsilane, neopentane, cyclopentane, and 2,2,4‐trimethylpentane the work functions are lowered by 0.62, 0.43, 0.28, and 0.18 eV, respectively. For n‐pentane and n‐hexane the work functions are close to the vacuum value. The magnitude of the lowering is correlated with the energy of the first electronic absorption band. A variation of electron mobility with the work function changes is noted and discussed relative to the trap theory. The photocurrents measured in the liquids increase with voltage, and at the voltage employed the currents in neopentane and tetramethylsilane are comparable to the ...

Measurement of the absorption length and absolute quantum efficiency of TMAE and TEA from threshold to 120 nm

Several existing and planned high energy physics experiments incorporate detectors which use either TMAE (tetrakis-dimethylaminoethylene) or TEA (triethylamine) as their photosensitive agent. Understanding the operation of these devices requires knowledge of the absolute photoionization quantum efficiencies and absorption lengths of TMAE and TEA. In an experiment performed at the National Synchrotron Light source at Brookhaven National Laboratory, we have measured these parameters from 120 nm to 280 nm. The quantum efficiencies were normalized to the known photoionization yields of benzene and cis-2-butene. The results of these measurements and details of the experiment are presented in this paper.

Single‐photon induced conductivity of solutes in nonpolar solvents

Synchrotron radiation was used for determining single‐photon photoconductivity thresholds for various solutes in nonpolar solvents. For anthracene as a solute the thresholds (Eth) are 6.14, 6.07, and 5.87 eV in 2,2,4‐trimethylpentane, 2,2,4,4‐tetramethylpentane, and tetramethylsilane, respectively. The threshold decreases as the conduction band energy (V0) decreases. The polarization energy (P+) of the anthracene cation, as derived from the data for 2,2,4‐trimethylpentane, is one‐third larger in magnitude than that reported for the polarization energy of the anthracene anion in this solvent, which suggests that the cation is smaller than the anion. Thresholds were also measured for 1,2‐benzanthracene, azulene, perylene, triphenylamine, and diazabicyclo‐octane. The thresholds in solution and solvation terms are related quantitatively by the expression Eth=I.P.+V0+P+.

Chemical reaction rates of quasi-free electrons in non-polar liquids. The equilibrium CO2 + e− CO2−☆

The forward and reverse rates of the reaction CO2 + e− CO2− were determined in tetramethylsilane, neopentane and 2,2,4-trimethylpentane as a function of temperature. The forward reaction is exothermic in solution but the equilibrium constant shifts over four orders of magnitude with changes in solvent. The change in entropy for the forward reaction is approximately the same (−44 cal/mol degree) in the three solvents.

Correspondence of conduction band minima and electron mobility maxima in dielectric liquids

Conduction band energies are reported for neopentane and tetramethylsilane up to the critical point.(AIP)

Zero‐field mobility of excess electrons in dense methane gas

Measurements of the low‐field mobility of electrons are reported for dense methane gas above the critical temperature. Use of a thick‐walled stainless steel cell permitted measurements to 200 atm or a density of 0.3 g/cc at ‐77°C. At low densities the mobility follows a (density)−1 dependence and increases with increasing temperature. The mobility goes through a maximum of about 1000 cm2/V sec at a density of 1×1022 molecules/cc. The variation in mobility at high densities depends primarily on fluid density and less on either fluid structure or temperature. The position of the mobility maximum is expected to correlate with a minimum in the energy of the lowest electronic eigenstate.

Spectroscopic and kinetic studies of formic acid adsorption on Cu(110)

The adsorption of formic acid on Cu(110) has been studied using Fourier-transform reflection/absorption IR spectroscopy (FTRAIRS) and molecular beam measurements. The adsorption is inefficient at 300 K, having an initial sticking coefficient (S0) of ca. 0.1. Experiments with sub-ambient crystal temperatures show much more efficient adsorption with S0 being 0.9 at 180 K. The absorption appears to be of the precursor type. FTRAIRS indicates that the species adsorbed at 200 K is very different from that at 300 K. The latter is the usually reported bidentate formate with a simple IR spectrum and symmetrically equivalent oxygens. At 200 K we propose that formate is produced in the monodentate form, that is strongly anchored to the surface through one oxygen, with the other in the form of a carbonyl group. However, the carbonyl group may also be bonding to the surface weakly. The monodentate species converts to bidentate upon heating. If the bidentate formate is cooled to 200 K, formic acid can be adsorbed from the gas phase and this converts all the formate on the surface back to the monodentate form.

Laser photodetachment of electrons from O2− in nonpolar liquidsa)

The wavelength dependence of the photodetachment cross section of electrons from O2− in liquid tetramethylsilane, 2,2‐dimethlypropane, 2,2,4‐trimethylpentane and argon was measured from the conductivity change induced by a short laser pulse. The O2− was generated by exposing a solution of O2 to a pulse of 2 MeV x rays just prior to the laser pulse. Comparison with gas phase results from the literature shows that the threshold law is the same in the two phases, and that the size of the cross section is similar at comparable excess energies. However, the thresholds are shifted from the infrared in the gas phase to the visible region in solution. The observed values are: Eth=2.08, 2.02, 2.55 eV in tetramethylsilane, 2,2‐dimethylpropane, and 2,2,4‐trimethylpentane at 296 K, respectively, and 2.32 eV in tetramethylsilane at 200 K. The major energy term causing this shift is the solvation energy of O2−. If this is described by Born’s equation the effective radius of O2− is 1.54±0.02 A in the hydrocarbons and te...

The physics and chemistry of room-temperature liquid-filled ionization chambers

Abstract The properties of excess electrons in non-polar liquids, such as tetramethylsilane and 2,2,4,4-tetramethylpentane, which are suitable for room-temperature liquid-filled ionization chambers are reviewed. Such properties as mobility, ionization yield, conduction band energy, trapping, and the influence of the electric field are considered.

The Hall mobility of excess electrons in 2,2‐dimethylbutane, 2,2,4‐trimethylpentane, and 2,2,4,4,‐tetramethylpentane

The Hall mobility (μH) of excess electrons was measured in liquid 2,2‐dimethylbutane (22DMB), 2,2,4‐trimethylpentane (224TMP), and 2,2,4,4‐tetramethylpentane (2244TMP) at temperatures from 20 to 160 °C. Drift mobilities μD were also measured over this temperature range. At 20 °C the observed values of μH for 22DMB, 224TMP, and 2244TMP are 12, 22, and 32 cm2/V s, respectively. In all three liquids μH increases with temperature. The comparison of μH and μD indicates that electron traps are absent in 22DMB and 2244TMP. In contrast, in 224TMP μH is about 3.5 times μD at 18 °C. Thus, in this liquid, traps are present and the concentration of traps is about one per thousand solvent molecules. The Hall mobility results are in reasonable accord with the expectations of the deformation potential model and support the concept of scattering of electrons in the conduction band due to density fluctuations.