B.A. Thrush

Vacuum UV emission from reactions of metastable inert gas atoms: Chemiluminescence of ArO and ArCl

Abstract Vacuum UV chemiluminescence from bound upper states of ArO, KrO and ArCl is observed from the quenching of excited inert gas atoms by N 2 O, O 3 , Cl 2 and CCl 4 . In these systems, the metastable 3 P 2 states of Ar and Kr strongly resemble ground state alkali atoms in their chemical properties.

The recombination of nitrogen atoms and the nitrogen afterglow

The recombination of nitrogen atoms has been studied photometrically in a fast flow system. The concentration of nitrogen atoms was determined by nitric oxide titration. The measured bimolecular rate constant has the form kB = A + B[M] for M = N2, Ar and He with a constant value of A. The surface process was proved to be second order by inducing a small first order catalytic recombination involving CN and showing by computer analysis that a first order surface process would have been measured readily in our system. Third order rate constants (expressed as d[N2]/dt) had values: kN2 = kAr = (1.38 ± 0.11) × 1015, kHe = (1.92 ± 0.18) × 1015 in cm6 mole–2 s–1 units at 298 °K. The surface process in the 26 mm i. d. flow tube had a value of (4.4 ± 0.1) × 108 cm3 mole–1 s–1 at 298 °K. In the range 196 to 327 °K, activation energies were –(975 ± 140) cal/mole for the homogeneous process and +(620 ± 50) cal/mole for the surface reaction. The intensity of the nitrogen afterglow was shown to be proportional to [N]2 and independent of total pressure for nitrogen carrier in the range 2 to 10 mmHg. Partial replacement of the nitrogen carrier by helium or argon enhanced the nitrogen afterglow on a mole fraction basis. This effect was shown to be associated with an efficient quenching of the emitting B3Пg state by molecular nitrogen. This view is supported by Jeunehomme & Duncan’s work on the pressure dependence of the lifetime of this state. Measurement of the absolute intensity of the afterglow when combined with their data show that about 50% of the recombination passes through the B3Пg state. On this basis it is concluded that the A3Ʃ+u state and not the shallow 5Ʃ+g state is the precursor of the afterglow. Levels of the B3Пg state around v = 12, 6 and 2 are populated by collision induced transition from the A state.

The Association of Oxygen Atoms and their Combination with Nitrogen Atoms

The rate constants of the reactions N + O + M = NO + M (2) O + O + M = O2 + M (4) have been determined in active nitrogen systems, nitric oxide being added to result in the partial production of oxygen atoms. The concentrations of these atoms were monitored by measurements of the intensity of the N2 First Positive emission and NO β emission. The following rate constants (in cm6 mole–2 s–1) were obtained at room temperature (298 °K) N2 Ar He 10–15k2 3.88 ± 0.30 2.98 ± 0.35 1.36 ± 0.17 10-14k4 11.3 ± 1.1 6.0 ± 0.6 4.6 + 0.4 In the range 196 to 327 °K, the temperature coefficient of reaction (2) corresponds to a T-½ dependence or an activation energy of –270 ± 120 cal/mole. This is unusually small for a three body recombination and contrasts with more ‘normal’ activation energy of –1420 ±350 cal/mole found for reaction (4). The NO β emission associated with reaction (2) has a similar temperature coefficient to the overall reaction, but is slightly enhanced by replacing the nitrogen carrier by argon. Our kinetic studies of this emission generally confirm the mechanism of Young & Sharpless (1962).

The Detection of Free Radicals in the High Intensity Photolysis of Hydrogen Azide

The absorption spectra of the radicals NH and NH2 have been observed in the flash photolysis of hydrogen azide in the presence of an excess of inert gas. A new complex, but slightly diffuse, absorption has been detected around 2700 Å. This spectrum shows no shift when DN3 is substituted for HN3, and it is attributed to the N3 radical. The mechanism of the hydrogen azide photolysis is discussed briefly with reference to these observations.

The radiative lifetime of the metastable iodine atom I(52P1/2)

Abstract When traces of iodine are added to discharged oxygen from which atomic oxygen has been removed, the equilibrium is rapidly established. Comparison of the intensities of emission by O 2 ( 1 Δ g ) and I( 2 P 1/2 ) yields a radiative lifetime of 0.17 ± 0.04 sec for the latter.

Kinetics of the reactions of active nitrogen with oxygen and with nitric oxide

The rate of decay of nitrogen atoms in a fast-flow system in the presence of oxygen has been studied between 412 and 755°K. Nitrogen atom concentrations were estimated by titration with nitric oxide. The slow primary step can be represented by N + O2 = NO + O, (1) while the much more rapid secondary reaction (2) removes the nitric oxide formed in reaction (1) N + NO = N2 + O. (2) Reaction (1) was found to be first order in both nitrogen atom and oxygen molecule concentrations, and k1 could be represented by the expression k1 = 8.3 x 1012 exp (— 7100/RT) cm3 mole-1 s-1 between 412 and 755 °K. Under conditions of large oxygen flow rates and at high temperatures the air afterglow continuum was observed with low but easily measurable intensity in the gaseous products of reaction of oxygen with active nitrogen. Both nitric oxide and oxygen atoms are therefore present, and not all the nitric oxide formed in reaction (1) is consumed in reaction (2). These nitric oxide concentrations were determined by measuring the intensity of the air afterglow with a photomultiplier cell, which was calibrated by observation of the increase in the air afterglow intensity when known quantities of nitric oxide were added between the first mixing point and the photomultiplier. In this way a value of k2= 3.0 x 1013 exp( — 200/RT) cm3 mole-1 s-1 was determined. The mean value of k2 between 476 and 755 °K was 2.5 x 1013 cm3 mole-1 s-1, and was practically independent of temperature over this range, corresponding to a reaction occurring at about one sixth of the bimolecular collision frequency. It can be shown that both reactions (1) and (2) are expected to proceed through transition complexes having very similar molecular constants and vibration frequencies to those of nitrogen dioxide. However, the ratio of the frequency factors calculated on this basis, A1/A2 = 1.4, was much larger than the experimentally determined value of 0.3, and this discrepancy is outside the limits of experimental error.

Laser induced fluorescence of CFCl and CCl2 in the gas phase

Abstract The fluorescence of CFCl and CCl 2 excited by a tunable dye laser has been observed in the gas phase. The radiative lifetimes are 644 ± 18 and 3810 ± 300 ns respectively consistent with matrix results. The quenching rate of CCl 2 is greater than for CFCl and varies with the wavelength of excitation.

The study of substituted benzyl radicals by laser-induced fluorescence

Abstract The visible absorption spectra of all the monomethylbenzyl and monofluorobenzyl radicals in the gas phase have been studied by laser-induced fluorescence. The fluorescence lifetimes of the stronger vibronic transitions have been measured. It is concluded that, unlike benzyl itself, a single excited electronic state, probably 2 A 2 , is involved except for p -methylbenzyl and perhaps o -fluorobenzyl.

Local mode rotational structure in the (3000) Ge-H stretching overtone (“3ν3”) of germane

Abstract The (3000) Ge-H stretching overtone band of germane, GeH 4 (conventionally described as 3ν 3 ) has been observed near 6130 cm −1 by Fourier transform infrared spectroscopy with a resolution of 0.01 cm −1 . The rotational structure shows no sign of the tetrahedral splitting patterns normally characteristic of a spherical top. Instead the band exhibits a striking pseudo-symmetric-top structure. This had been predicted by Halonen and Robiette as the limiting case of a spherical top showing local-mode behaviour. The present results represent the first observation of this phenomenon.

The kinetics of elementary reactions involving the oxides of sulphur II. Chemical reactions in the sulphur dioxide afterglow

The main recombination reactions in the sulphur dioxide afterglow are shown to be O + SO2 + M = SO3 + M (1) and O + SO + M = SO2 + M, (2) with rate constants of (4·7 ± 0·8) x 1015 and (3·2 ± 0·4) x 1017 cm6 mole-2 s-1 respectively at 300°K for M = Ar. Reaction (2) is the dominant process removing sulphur monoxide (SO) which is otherwise remarkably unreactive. The absolute intensity of the sulphur dioxide afterglow is found to be I = 1·5 x 108[O] [SO] cm3 mole-1 s-1 for argon carriers at pressures between 0·25 an d 3·0 mmHg. The afterglow emission comes from three excited states of SO2. Spectroscopic and kinetic studies show that these are populated subsequent to or by the third order combination reaction (2). Excited SO2 is removed mainly by electronic quenching.