Ground-state OH Research Articles

Kinetic Studies of Ammonia Oxidation in Shock Waves. III. The Radiation of Excited OH Radicals

Abstract Mixtures of ammonia and oxygen in argon were heated by reflected shock waves to temperatures between 1600 and 2100°K, and the ultraviolet radiation from OH(2Σ+) was observed by means of monochromator and a photomultiplier. The radiation profiles showed two types of radiation: a strong spike which appeared after an induction period, and a weak and constant-level emission, which followed the spike and persisted for up to 500 μsec. The former emission was observed also in the oxidation of hydrogen, methane, and acetylene, but the latter emission was observed only in the oxidation of methane and acetylene. The temperature coefficients of the intensities of the latter emission divided by the sum of spectral emissivities were found, for the oxidation of ammonia, not large enough to be explained by the thermal excitation of the ground-state OH(2π) to the 2Σ+ state. Possible reaction processes for this chemiluminescence were proposed to explain the experimental results.

Einstein A Coefficient for the Λ Doublet Transitions of the Ground State of OH

FOR some time now there has been some uncertainty in the literature of the correct value for the Einstein A coefficients for the ground state Λ doublet transitions of OH. Barrett1 calculated A = 2.66 × 10−11 sec−1 for the 1,667 Mc/s line using a matrix element |μij|2 taken from Dousmanis, Sanders and Townes2. This element is incorrect both because the form of its dependence on the rotational quantum number J was incorrectly given as [(J + 1)(2J + 1)]−1, and also because the element was incorrectly defined as (1). The appropriate definition is given below. The correct J-dependence, which is [J(J + 1)]−1, was given by Meyer3, in connexion with the Stark-effect determination of the OH dipole moment, μ because Meyer used an electric field directed so that only transitions of type ΔMJ = 0 occurred between the magnetic sub-levels characterized by quantum number MJ, the problem of combining these elements with those for which ΔMJ = ±1 to form an overall matrix element μij was not encountered. To derive the overall matrix element between the Λ doublet levels of the ground state of OH, Goss and Weaver4 combined the correct J-factor of Meyer with standard relative intensity formulae for hyperfine transitions as given by Townes and Schawlow5 to derive A = 0.9640 × 10−11 sec−1 for the 1,667 Mc/s line. They also used the most recent values for the OH dipole moment and the fine structure interaction constant, the calculation making use of the theory for intermediate coupling between Hunds's cases (a) and (b), as given by Dousmanis, Sanders and Townes2.

Values of the Einstein A Coefficients for the Λ Doublet Transitions of the Ground State of OH.

view Abstract Citations (1) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Values of the Einstein A Coefficients for the Λ Doublet Transitions of the Ground State of OH. Goss, W. Miller ; Weaver, Harold Abstract In order to calculate abundances of OH from the four lines at 18 Cm, the Einstein A's are necessary. A recalculation of these quantities for the A doublet transitions arising from the ground state (2113/2) of OH has been made. The expression for the dipole matrix element as reported by Dousmanis, Sanders, and Townes (Phys. Rev. 100, 1735, 1955) is incorrect. The correct formulation has been given by Meyer (unpublished thesis, University of California, Berkeley, 1961). The Dousmanis, Sanders, and Townes expression must be multiplied by (2J+ 1) /J. This factor is 2.67 for the ground state of OH (J=3/2). Using recent experimental data on the dipole moment and the fine structure interaction constant of OH, the dipole matrix element is calculated for the case of intermediate coupling between Hund's cases a and b. The resultant A's are: 0.1614x 10-11 sec-', 0.8895 x 10-" sec-1, 0.9640x 10-" sec-1, and 0.1177x 10-" sec-1 for the 1612, 1665, 1667, and 1720 Mc/sec lines, respectively. The A value computed here for the 1667 Mc/sec line differs significantly from the value (2.66 X 10-" sec-1) given by Barrett (IEEE Trans. on Military Electron. MIL-8, 156, 1964). OH abundances computed with this earlier A value must be increased in the ratio 2.66x 10-"/0.964x 10-"=2.76. Publication: The Astronomical Journal Pub Date: 1966 DOI: 10.1086/110162 Bibcode: 1966AJ.....71T.162G full text sources ADS |

HFS Constants of OH Ground State

Fermi contact term at the proton in the OH ground state is pointed out to be negative either from a simple qualitative consideration or from an LCAO MO CI calculation. The calculated value agrees well with the experimental value revised recently by Radford. Other hfs constants, 〈1/r3〉H and 〈2P2(cosχ)/r3〉H, are also calculated by a single configuration approximation with the use of an analytical Hartree—Fock orbital. The results agree quite well with the experiment, although the agreement is somewhat worse for 〈sin2χ/r3〉H.

Relative population of OH X 2Π levels

Intensities of nightglow bands in the visible and the near infra-red region of the OH rotation vibration system are presented. Comparison with other similar measurements shows a fair consistency, and the absolute intensities given do not, except in a couple of cases, deviate seriously from those predicted previously. The relative population of the vibrational levels υ = 3 to 9 of the OH ground state is computed from the intensities by means of relative transition probabilities given by some previous investigators. The values obtained are, in descending order, from υ = 9:1·0, 1·25, 1·9, 2·6, 5·1, 7·0 and (8·1). The transition probabilities given by one investigator give somewhat larger values. Relative excitation numbers of the different levels have been evaluated from the relative population. It is concluded that excitation of the vibrational levels of the OH ground state takes place at all levels and that the relative number of molecules excited at different levels is about the same. The bearing of these results on the theories of Bates, Nicolet and Herzberg and of Krasovskii is discussed.

Abnormal Excitation of OH in H2/O2/N2 Flames

The emission from the first four vibrational levels of OH2Σ+ and the concentrations of ground state OH2II have been measured in a series of rich H2/O2/N2 flames held on flat porous burners as a function of distance from the burner. The intensity of emission of each of the bands is proportional to the cube of the concentration of ground state OH, but the constant of proportionality depends on the band and the flame conditions. This dependence, as well as absolute intensity measurements, establish the fact that the radiation is nonthermal. The dependence of the emission intensity on an integral power of OH strongly indicates that the excited OH is formed as a result of one or more of a set of possible radical recombination reactions. It is shown that in general it is not possible to obtain a unique determination of the excitation mechanism for any band. The only exception to this is that definite evidence for the preassociation reaction, O+H→OH2Σ, v=2, has been obtained. The vibrational distributions of excited OH are shown to be consistent with ``temperatures'' from 3000°K to 5000°K (compared with maximum flame gas temperatures of 1600°K), which is probably to be expected from a reaction with the exothermicity of H+OH+OH→H2+OH*. It is concluded from this and from the general behavior of the data, that this last reaction is most likely responsible for the nonthermal emission of all the bands, with the exception of that specifically known to be due to the preassociation reaction.