J. A. Ghormley

Absorption Spectrum and Reaction Kinetics of the HO2 Radical in the Gas Phase

HO2 radicals were produced in the gas phase by flash photolysis of water vapor (3%) in an atmosphere of hydrogen, helium, or argon containing ∼ 2% oxygen. Water is dissociated in the first continuum to H and OH, and O2 converts the H atoms to HO2. Hydrogen nearly doubles the amount of HO2 produced by converting OH to H. The absorption spectrum of HO2 is a broad band with a peak at 2050 A. The molar extinction coefficient, emax, based on measurement of the H2O2 formed in the hydrogen system, is 1770 ± 150 M−1· cm−1. The rate constant for the bimolecular combination reaction, HO2+HO2 → H2O2 + O2, was evaluated as 5.7 ± 0.5 × 109 M−1· sec−1 at 298°K and for the reaction HO2+OH→ H2O+O2, k = 1.2 ± 0.2 × 1011M−1. sec−1. From auxiliary measurements of the rate of O3 formation it was also found that, in the flash photolysis of O2 (2%) in H2, hot O atoms react with H2 to form OH and H which are then converted to HO2.

Enthalpy Changes and Heat‐Capacity Changes in the Transformations from High‐Surface‐Area Amorphous Ice to Stable Hexagonal Ice

Amorphous ice prepared by slow condensation of water vapor onto a surface cooled with liquid nitrogen or liquid hydrogen undergoes several exothermal transformations on warming. Thermal analysis by means of warming curves indicates that the heat liberated on warming ice prepared at − 253°C is 8 cal/g from − 253° to − 196°C and 22 cal/g from − 196° to − 120°C.Ice prepared at − 196°C gives up 14 cal/g between − 185° and − 125°C. These enthalpy changes are attributed in part to decreases in the surface area of the amorphous ice. Beginning at − 120°C, 24 cal/g heat is evolved on crystallization to the cubic form (ice Ic). Between − 80° and − 50° an additional 3 cal/g is attributed to recrystallization of the cubic phase to larger crystals. In the cubic to hexagonal (ice Ih) transition between − 50° and − 5°, ΔH is less than ±0.3 cal/g, but a decrease occurs in the low‐temperature heat capacity. The heat capacity of amorphous ice or ice Ic in the range − 170° to − 120°C exceeds that of ice Ih by 27% and the he...

A Calorimetric Calibration of Gamma‐Ray Actinometers

The yield for gamma‐ray oxidation of ferrous sulfate in air saturated 0.4M sulfuric acid was found to be 15.6±0.3 ferrous ions oxidized per 100 electron volts absorbed. This value was based on calorimetric measurements of cobalt gamma‐ray energy absorption in water at intensities of 25 r/sec, 74 r/sec, and 225 r/sec. The yield for reduction of ceric sulfate in air saturated 0.4M sulfuric acid solution, determined by comparison with the ferrous sulfate yield was 2.52±0.05 ceric ions reduced per 100 ev absorbed. The effects of several variables on these yields were investigated.

Vibrationally Excited Ozone in the Pulse Radiolysis and Flash Photolysis of Oxygen

The kinetics of ozone formation have been studied in the pulse radiolysis and flash photolysis of gaseous oxygen by measuring the change in optical absorption following the pulse. The absorption spectrum of the ozone present immediately after the pulse is considerably broader and the peak is at longer wavelength (∼2860 A) than that of ground state ozone (∼2560 A). The initial absorption is attributed to ozone in the upper vibrational levels produced by O + O2⇋O3‡. It is clearly shown that the over‐all third‐order reaction O + O2+M→O3 + M occurs by a sequence of steps. The relaxation time for de‐excitation from the upper vibrational levels to the ground state is ≥6 × 10−6 sec in O2 at 740 torr and 24°C. De‐excitation requires at least 4.5 × 104 collisions, and assuming 20 vibrational levels, the average efficiency is about one quantum in 2200 collisions. The third‐order rate constant based on O atom disappearance k(−O) is several times larger than that based on formation of O3 in the ground vibrational sta...

Adsorption and Occlusion of Gases by the Low‐Temperature Forms of Ice

Water vapor condensed on a surface at −196°C in the presence of O2, N2, Ar, or CH4, forms a condensate which, on warming, evolves gas in three temperature ranges. The rate of gas evolution first goes through a maximum between −196° and −125°C probably from release of adsorbed gas due to decrease in surface area of the amorphous ice. A second peak in the gas evolution rate appears in the same temperature range as the formation of cubic ice between −120° and −110°C. The remainder of the occluded gas comes out between −70° and −40° (at 15° rise/min) where cubic ice transforms to the hexagonal form. Nitrogen adsorption isotherms indicate surface areas as high as 241 m2/g for amorphous ice prepared at −196°C, and annealing measurements suggest areas as high as 500 m2/g for lower‐temperature preparations. Water—oxygen mixtures subjected to a microwave discharge, then condensed at −196° and allowed to warm, evolve as much as 97% of the product oxygen between −70° and −40°C.

Pulse Radiolysis of NO : Production of NO2 and N2O3 and the Production and Relaxation of Vibrationally Excited NO

Radiolysis of NO with a 30‐nsec pulse of 2‐MV electrons produces absorption bands attributed to NO2, N2O3, and vibrationally excited NO. In pure NO at 1 atm, most of the NO2 forms during the pulse, but the formation of N2O3 is slower (τ = 0.3 μsec). The NO2 is produced by the reaction of O atoms with NO, and the pseudo‐first‐order rate constant for formation of NO2 in an 11% NO–Ar mixture is slightly lower than that calculated from published values. The N2O3 comes from the equilibrium NO2 + NO⇆N2O3, which is achieved in <1 μsec, orders of magnitude faster than the previously estimated lower limit. The sum of yields of the two products (10.6 ± 0.5 molecules/100 eV) was independent of the pulse dose, but the ratio G(NO2) / G(N2O3) increased with dose. This can be explained by a shift in the above equilibrium caused by the temperature increase produced by the pulse. Superimposed on the broad N2O3 band in the ultraviolet was a sharp band at 2362 A, indicating population of the first vibrational level of NO. T...

Yield of Ozone in the Pulse Radiolysis of Gaseous Oxygen at Very High Dose Rate. Use of This System as a Dosimeter

A yield of ozone of 13.8 ± 0.7 molecules/100 eV was determined in the pulse radiolysis of oxygen at 1 atm by high‐energy electrons at a dose rate of 1012 rads/sec. The absorbed dose was based on the energy absorbed in a thin aluminum calorimeter. While this yield is somewhat higher than a recently reported value of 10.2 ± 0.6 determined from the initial rate of formation in radiolysis by cobalt gamma rays at low dose rate, it is likely that there is little or no dose‐rate dependence over the range from 102–1012 rads/sec. This system provides a simple and accurate dosimeter, and the in situ measurement of ozone offers the added advantage for system in which the electron flux is nonuniform, that the average absorbed dose in the optical path is measured.

NUCLEATION OF BUBBLES IN SUPERHEATED AQUEOUS SOLUTIONS BY FAST PARTICLES

Abstract Energy loss along the track of a fission recoil or α-particle in superheated uranyl sulphate solution or the recoil from a fast-neutron collision in superheated ether can account for evaporation of sufficient liquid to form bubble nuclei under conditions where nucleation is observed. The contribution of charge repulsion cannot be determined from experimental results.