Harold S. Johnston

Reduction of Stratospheric Ozone by Nitrogen Oxide Catalysts from Supersonic Transport Exhaust

Although a great deal of attention has been given to the role of water vapor from supersonic transport (SST) exhaust in the stratosphere, oxides of nitrogen from SST exhaust pose a much greater threat to the ozone shield than does an increase in water. The projected increase in stratospheric oxides of nitrogen could reduce the ozone shield by about a factor of 2, thus permitting the harsh radiation below 300 nanometers to permeate the lower atmosphere.

SPECTRA AND KINETICS OF THE HYDROPEROXYL FREE RADICAL IN THE GAS PHASE

The absorption spectrum of the hydroperoxyl radical (HO2) has been obtained by the molecular‐modulation technique. The radical was formed by the photolysis of hydrogen peroxide at 2537 A, by the photolysis of ozone in the presence of hydrogen peroxide at 2537 A, and by the photolysis of Cl2 in the presence of hydrogen peroxide at 3500 A. The vibrational frequencies of HO2 have been observed to be 1095, 1390, and 3410 cm−1. Details of the vibrational spectrum are consistent with the molecular geometry: H–O bond distance 0.96 A, O–O bond distance 1.3 A, and H–O–O angle approximately 108°. The absorption spectrum of HO2 in the ultraviolet has a maximum at 2100 A. Kinetic analysis of the modulated absorption signals shows that the HO2 radical decays by a process second order in HO2 concentration. The rate constant for the disproportionation reaction HO2+HO2→ H2O2+O2 was found to be 3.6± 0.5× 10−12 cm3/molecule· sec in agreement with a reported value of 3× 10−12 cm3/molecule· sec. The absorption cross section ...

KINETICS OF NITROGEN DIOXIDE FLUORESCENCE

The fluorescence lifetime and intensity of gas‐phase nitrogen dioxide (2B1) have been measured as a function of excitation wavelength, fluorescence wavelength, and pressure (0.5–50 mtorr). The phase‐shift method was used; this technique allows lifetime measurements to be obtained with signal intensities of 100 counts per sec and lower. The excitation source, tunable throughout the visible region, had a half‐width bandpass as low as 15 A. Fluorescence wavelength separation was accomplished with 15 interference filters between 400 and 8000 A. The radiative lifetimes range from 55 to 90 μsec for excitation from 3980 to 6000 A and tend to increase with excitation wavelength; however, the lifetimes exhibit considerable variation within a narrow excitation region. The fluorescence sample was contained in a 33‐cm‐diam spherical bulb; apparent fluorescence lifetimes in smaller cells were reduced because of migration of excited molecules (under collision‐free conditions) and wall quenching. In order that the measu...

Reactions and quenching of vibrationally excited hydroxyl radicals

Rate constants for the deactivation of vibrationally excited hydroxyl radicals by the inert gases Ar, O2, N2, H2, and D2 have been determined. Additionally the rate constants for the reaction HO(v)+O3→ products have been measured and found to decrease from 11×10−12 to 3.7×10−12 cm3 molecule−1⋅ sec−1 from v=9 to v=4. Hydroxyl radicals were generated in a large tank at low pressures from the reaction of hydrogen atoms and ozone. The radicals were formed in excited vibrational states (v?9), and the excited radicals, v=4 to 9, were observed by visible light emission from high overtone transitions. Vibrational populations and the rate constants for loss processes were deduced from observations over a wide range of wavelength and pressure fitted to a comprehensive model of the system.

Theoretical Pre‐Exponential Factors for Twelve Bimolecular Reactions

The Arrhenius pre‐exponential factor A has been calculated and compared with experiment for a group of twelve bimolecular gas phase reactions of simple inorganic molecules. The most detailed form of the activated complex theory, a cruder method of hydrocarbon analogy, and the simple collision theory were used. Necessary assumptions about the properties of the activated complex were made uniform for the whole set of reactions. Particular attention was paid to the treatment of internal rotations and vibrations. The agreement of the activated complex theory with experiment was found to be satisfactory.

Theoretical interpretation of reactions occurring in photochlorination

The Arrhenius A and E factors of over 60 elementary reactions in photochlorination systems have recently been evaluated experimentally in this and other laboratories. In this article a uniform treatment of most of these reactions is given by activated complex theories. Two parameters are fit to the data from two activation energies. The other activation energies follow the expected trend for reactions of chlorine atoms with hydrocarbons, but the reactions of chlorine atoms with chlorinated hydrocarbons do not follow the expected trends. For reactions with activation energy, the theory accounts very well for the magnitudes and trends of the Arrhenius A factors. For reactions with no activation energy, the activated complex theory as used by Gorin accounts for the order of magnitude of the rate constants of the group as a whole, but it fails to account for several pronounced trends in the data and is judged to be of no predictive value. For certain limited types of chemical reactions and over restricted ran...

Hydrogen—Deuterium Kinetic Isotope Effect, an Experimental and Theoretical Study over a Wide Range of Temperature

An experimental study of the kinetic isotope effect for the reaction CF3+CHD3–|→F3C–H–CD3→F3CH+···→F3C–D–CHD2→F3CD+··· has been carried out between 300° and 700°K, and also the Arrhenius parameters k=A exp (—E/RT) were obtained for the individual reactions. An especial effort was made to avoid systematic errors, and the precision obtained was good.These data were combined with those of Tschuikow‐Roux who studied the essentially same reaction between 1000° and 1800°K, and a detailed test was made of activated complex theory utilizing the method of London—Polanyi—Eyring—Sato with corrections for quantum mechanical tunneling. Satos parameter was evaluated from activation energy of an analogous reaction so that kH/kD and A provided an unambiguous, nonadjustable test of theory. Because reactants were identical for both reactions, all the kinetic isotope effect depended on differences in the two activated complexes. The vibrational frequencies of the two nine‐atom models of the complex were evaluated by a comp...

High-frequency-modulation spectroscopy with a lead-salt diode laser.

For the first reported time, high-frequency optical heterodyne spectroscopy has been performed with tunable lead-salt diode lasers. N2O transitions at 1259.2 cm−1 were observed by modulating the injection current at 50–200 MHz and detecting the heterodyne beat signal generated by the frequency- and amplitude-modulated light. By proper adjustment of the rf phase in the phase-sensitive detection circuitry, the background contributions due to laser amplitude modulation could be completely suppressed. The sample induced signal is measured against zero background and detected at radio frequencies at which diode lasers have little noise.

Kinetics of the Fast Gas Phase Reaction between Ozone and Nitric Oxide

The rate of reaction between ozone and nitric oxide was followed optically at —43 and —75°. The reaction appears to be elementary bimolecular NO+O3=NO2+O2 and the rate constant is k=0.8×1012e−2500/RT cc mole−1sec−1.The pre‐exponential factor for this reaction is compared with those of six other elementary gas phase reactions of the type X+Y=Z+W+··· involving the oxides of nitrogen for which the energies of activation were higher. For this series of similar reactions the molecular complexity of the reactants is about constant (five or six atoms), and there appears to be no systematic difference in pre‐exponential factor between the fast and the slow reactions.

Chemiluminescence of IF in the gas phase reaction of I2 with F2

Abstract Emission from both the B 3 Π 0 + state and the previously unreported A 3 Π 1 state of IF has been observed in the gas phase reaction of I 2 with F 2 at low pressures. For the B 3 Π 0 + state the transition moment and vibrational populations were extracted from the spectra by a least-squares method whereby theoretical band shapes were fit to the experimental data. The effect of flow rates of reactants and Ar on the relative emission from the two electronic states, the effect of pressure on the B 3 Π 0 + state, and extinction of emission near 470 nm all favor the population of excited electronic states through a four-center reaction complex, rather than association of F and I atoms. It is argued that there is an avoided curve crossing between the lowest two 3 Π 0 + states of IF, and that the ground state dissociation energy is 23 229 ± 100 cm −1 . The radiative lifetime of the B 3 Π 0 + state is estimated to be 10 −3 sec and to be much shorter than that of the A 3 Π 1 state.