Absolute Emission Intensity Research Articles

Measurement of the Photoionization Cross Section in the Resonance Continuum of C i Using a Wall-Stabilized Arc*

A wall-stabilized high-current (100 A) arc was operated in argon at a pressure of 1 atm. Different gases (CO2, O2) could be added to the plasma. Temperature, electron, ion, and atom densities have been determined by spectroscopic observations in the visible and in the vacuum-ultraviolet (vuv) regions of the spectrum, assuming local thermal equilibrium. These values, together with the absolute emission intensities in the vuv region between 980 and 1250 Å, allowed the calculation of the photoionization cross section of carbon. This experimental value compared well with previously published theoretical calculations: it was 19×10−18 cm2 at threshold.

Measurements on O2 1 ? g and O2 1 ? + g in discharge flow systems

The absolute intensity of dimole emission at 6340 A by two O21Δg is found to be 16±4 [O21Δg]2 in cm3 mol–1 s–1 units. The rate constants for their energy pooling to yield O21Σ+g, ν= 0 and 1 are (1.2±0.3)× 107 and 6 × 105 cm3 mol–1 s–1 respectively. The removal of O21Σ+g at the walls is partly diffusion controlled, rate constants for its deactivation by H2O and H2 are reported.

Absolute rate constant measurements for the radiative combination ofatomic oxygen with nitric oxide

The spectral radiant intensity and the absolute rate constant of the NO-O chemiluminescent reaction have been determined over the wavelength region 3875—20,000 A. A direct method of intensity calibration making use of an NBS Standard light source was employed. In order to eliminate any possible interferences due to wall effects, the reaction was investigated in a freejet. The spectral intensity distribution curve, which extended in the infrared up to 20,000 A, was independent of pressures from 0.1 to 1 torr and was found to be identical whether oxygen atoms were produced from discharged O2 or from the reaction N(4S) + NO( 27r) = N 2(1S) -f- O(3P). Additional measurements of the NO-O glow, performed in a hydrogen flame at 10 torr pressure indicated a substantial shift of the intensity maximum toward longer wave- lengths with increasing temperature. Four independent rate constant determinations were performed over the observed spectral range of the glow and gave an average value of 6.8 X 10~17 cm3 molecule1 sec1 at 367°K with a probable error of ±35%. HIS paper reports the redetermination of the rate con- stant of an important chemiluminescent reaction. It confirms earlier work that used different experimental tech- niques. The importance of the NO-0 chemiluminescent reaction stems from its use as a laboratory standard for chemiluminescence1 and from its recent applications in at- mospheric physics. For instance Golomb et al.,2 determined the atomic oxygen concentration in the upper atmosphere by analyzing the radiation intensity of chemiluminous nitric oxide trails deposited into the atmosphere from rocket ex- periments. The quantitative investigation of the NO-0 chemiluminescence is also of general interest in connection with the interpretation of observable phenomena occurring during missile re-entry in the gas phase flow around and be- hind the vehicle. The first accurate rate constant determination of this reaction was due to Fontijn and Schiff.3 Prior to their work virtually no rate data were available for chemiluminescent reactions with an accuracy greater than an order of magni- tude. In their first investigation Fontijn and Schiff3 deter- mined the absolute quantum yield of the reaction for the spectral range 3875-6200 A by a method of chemical ac- tonometry. They used as a standard a solution of 0.15 M-K3 Fe (0264)3 of which the quantum efficiency had been determined accurately by Hatchard and Parker.4 In their final study Fontijn, Meyer, and Schiff1 determined the emission intensity spectral distribution and found the emission to extend in the infrared up to at least 1.4 ju. On the basis of these data the actinometer measurements were extended over the observed emission range (3875-14,000 A), and the absolute rate constant over this range was calculated and found to be 6.4 X 10~ 17 cm3 molecule sec1. In the present work the reaction was investigated in a pre- mixed freejet. The jet defined a homogeneous zone of reac- tion which was observed with a spectrometer of large aper- ture. Absolute spectral emission intensities of the zone of reaction were determined by direct comparison with an NBS spectral light source. The quantum yield over the

Thermal and Recombination Emission of NO2

The visible emission from electronically excited NO2 has been studied both with thermal excitation in an electric oven and with recombination excitation from the nitric oxide–atomic oxygen reaction. In both cases the emission is continuous with weaker, diffuse bands superimposed. The correlation of these bands with NO2 absorption bands has been extended. The thermal emission continuum begins near 3300 Å and increases in intensity to the end of our range at 8600 Å. The recombination emission continuum begins near 3800 Å and has a broad maximum at 6200 Å, in general agreement with previous workers. The absolute intensity of the thermal emission has been obtained at 100-Å intervals between 4600 and 8600 Å for temperatures between 972 and 1335°K. Excitation energies, which are obtained from the variation of intensity with temperature, range from 66 kcal at 4600 Å to a minimum of 44 ± 1 kcal near 8300 Å. This latter value is E0, the minimum energy of the emitting state of NO2. Radiative lifetimes are obtained for thermal emission which range from 100 to 1 μsec depending on emission wavelength and which approximately agree with fluorescent lifetimes for comparable excitation energies. The results are compared with shock tube measurements and are discussed in relation to air emissivity and opacity problems.

Halogen Atom Reactions. IV. Recombination into Electronically Excited States

Absolute emission intensities of the luminescence resulting from the recombination of ground-state chlorine and bromine atoms have been measured in an effort to determine the proportion of the recombination occurring through electronically excited states. The emitting states [Cl2(3Π0+u), Br2(3Π1u), andBr2(3Π0+u)] are found to be efficiently quenched by the corresponding atomic species and the efficiency is found to be dependent on the total pressure. Direct quenching by ground-state molecules is found to be at least 103 times less efficient. Rate constants for recombination via electronically excited states of Cl2 and Br2 are found to be ≥ 1014 and ≥ 1015 cc 1mole−1·sec−1, respectively. Luminescence is also reported from the recombination of ground-state iodine atoms and assigned to the transition I2(3Π1u → 1Σg+).

Shock-Tube Study of Radiation from S2

Absolute emission intensity measurements of the radiation from sulfur molecules S2 have been obtained in the temperature range from 2300 to 4800°K at six wavelengths spaced between 3132 and 5461 Å. The radiation intensities were proportional to the square of the sulfur atom concentration and were independent of the total pressure of the buffer gas, argon. In addition, the band structure was recorded for the wavelength interval 3600–4100 Å with an image orthicon camera mounted in a spectrograph. The intensities between 3650 and 5461 Å have similar activation energies and are interpreted in terms of the radiative transitions from the B 3Σu− state of molecular sulfur, this state being populated by the mechanism of inverse predissociation. Theoretical calculations of the B 3Σu− bands predicted the band shapes observed with the image orthicon and, together with the measured intensities, provide the electronic f value for the bands. The square of the transition moment [Re(r) / ea0]2 is estimated to have a value of 5.0, which corresponds to an f value of 0.16 on the basis of published Franck–Condon factors. The intensities at 3132 Å have a temperature dependence close to that predicted for transitions from the b 1Δu electronic state.

Further Studies of the Absolute Intensity of Emission of Characteristic X-Radiation

The theory of Metchnik and Tomlin (1963) relating to the characteristic X-ray yield from thick targets bombarded with electrons is further discussed and a simple formula is developed from it. Some selected results from an extensive experimental study of X-ray yield are presented and these are compared with other published results and various theoretical discussions.

Shock-Tube Study of the Radiative Combination of Oxygen Atoms by Inverse Predissociation

Absolute emission-intensity measurements of the radiative combination of oxygen atoms O(3P) in the temperature range between 2500° and 3800°K are presented. The emission intensity was recorded simultaneously in six spectral intervals in the wavelength range between 2300 and 4511 Å. The absolute emission intensity was found to be proportional to the square of the oxygen atom concentration and to be characterized by an activation energy of 28.9 ± 2.2 kcal mole− 1. These results are interpreted in terms of an inverse predissociation mechanism in which the oxygen atoms combine along a repulsive potential energy surface with a transition to the B 3Σu− state of molecular oxygen and a subsequent radiative transition to the ground electronic state of oxygen. The rate constant for the over-all radiative combination of oxygen atoms was found to be 4.3 × 106exp(− 28 900 ± 2200 / RT) cm3mole− 1·sec− 1 for the wavelength range 2300–5000 Å.

Kinetic and Spectroscopic Constraints on the Origin of the N2 Afterglow

The large transition probability (10−1 to 10−2) for curve crossing such as that which occurs in N2 pre-dissociation, N2 (3B) ⇆N2(5Σ), guarantees that the rotationally hot members of the 12th vibrational level of the B state (3B(12))* will always be in equilibrium with free N atoms without the intervention of a third body. A kinetic scheme is suggested for these species that includes rotational and vibrational quenching in competition with electronic quenching. Fast rotational quenching gives rise to pseudoequilibrium population of normal B(12)3 species. The equilibrium constant KB for 2N⇆N2(2B(12)) is estimated at 10−1.2 liter/mole = 10−22.9 cc/particle at 300°K with a negative activation energy of − 2.2 kcal/mole, in good accord with recent data. Using 105.0±0.3 sec−1 as the measured radiative rate constant of the 12-8 transition yields an absolute emission intensity for this band of 102.9±0.3 liter/mole·sec = 10−17.9±0.3 cc/molecule·sec, again in good accord with observation. It appears from this that the 11th, 10th, and 9th vibrational levels are populated by successive collisional deactivation of the 12th level. This collisional deactivation is in competition with electronic quenching.

Shock-Tube Study of the Radiative Processes in Systems Containing Atomic Oxygen and Carbon Monoxide at High Temperature

Absolute intensity measurements of the radiation emitted in systems containing atomic oxygen and carbon monoxide were made by shock heating gas mixtures containing O2, O3, CO, CO2, and Ar to pressures between 0.5 and 1.5 atm and to temperatures of 2679°±16°K and 2943°±26°K. The emission intensity was recorded as a function of time simultaneously in selected spectral intervals in the wavelength range between 2500 and 8000 Å. The absolute spectral intensity distribution for the radiative combination of O and CO was derived. Contributions to the measured emission intensity at wavelengths below 4500 Å by the radiative combination of oxygen atoms and by the radiation resulting from the electronic excitation of O2 were evaluated; at wavelengths greater than 7000 Å an unexpected rise in the emission intensity was found. The absolute spectral emission intensity for the O–CO combination was proportional to the concentration product, (O)(CO), at all wavelengths employed and at 2943°K, a rate constant of 3.80×105cm3 mole−1·sec−1 for the over-all radiative combination, O+CO→CO2+hv was calculated for the wavelength interval 2500 to 7200 Å. The activation energy for the radiative combination of O and CO was found to be 2.5±0.6 kcal mole−1 by combining the present results with low-temperature, discharge-flow-tube data over the common spectral interval between 3200 and 5000 Å; also, an activation energy of 31±12 kcal mole−1 was determined for the energy-transfer process CO2*+O2→CO2+O2*, where the starred quantities represent electronically excited molecules.

Measurement of oscillator strengths of the MgO( B1Σ+ - X1Σ+) and MgH( A2π - X2Σ+) band systems

Absolute emission intensities of a portion of the flame at the exit plane of a rocket engine burning hydrogen, fluorine, and a slurry of powdered MgO in water were measured. From these measurements and thermochemical calculations we have found an ƒ-value for the MgO( B 1 Σ + - X 1 Σ +) band system of ƒ el≅ƒ Δv=0 = (1·3±0·5) x 10 -3 . For the MgH( A 2 π- X 2 Σ +) band system we find ƒ el ≅ ƒ Δv=0 ⪅ 2·0 x 10 -3 . A crude upper limit of ƒ el ⪅ 5·0 x 10 -3 for the MgO( B 1 Σ + - A 1 π) band system and results for bands of the F 2- molecule are also obtained.

THE RED EMISSION BANDS OF MOLECULAR OXYGEN

Abstract— The temperature dependence and the absolute emission intensity of the 6340 Å band of molecular oxygen have been measured. The results indicate that the emitting pair of molecules is not bound and possesses a radiative half life of about 25 msec. The implications of these results on some chemiluminescent reactions are discussed.

Double-crystal x-ray monochromator

A double-crystal monochromator is described and the values of the integrated reflection coefficient R for an LiF crystal given for Cu Kα, Ag Kα and Cr Kα emission. The values of R were used to calculate the absolute intensity of emission and in the case of Cu Kα emission results are compared with those obtained by using nickel and iron balanced filters.

On the Absolute Intensity of Emission of Characteristic X Radiation

The theoretical discussion of Worthington and Tomlin published in 1956 has been modified to allow for electron scattering within the target, and the results of the two theories are compared with those of an extensive series of measurements of the absolute intensities of emission of characteristic K α radiation from thick targets of copper, silver and chromium. Good agreement was found between various methods of intensity measurement and the experimental results are in satisfactory accord with the modified theory.

Absolute photometry of the aurora—II: Molecular nitrogen emission in the sunlit atmosphere

The absolute emission intensity of a sunlit aurora was determined for the 1.N.G. λ3914 band of N 2 + by subtracting the emission intensity originating in the dark atmosphere from the total measured intensity. The method is based on the constancy of the ratio of emission intensity of the oxygen green line O I λ5577 to the λ3914 emission in the dark atmosphere. The ion density of N 2+ is computed as a function of height from the emission rate per unit volume.

Absolute photometry of the aurora—I: The ionized molecular nitrogen emission and the oxygen green line in the dark atmosphere

A low latitude aurora was measured photometrically. Three prominent emission features, λ3914 band of 1.N.G. N 2+, λ5577 of O I and λ6300–6364 of O I, were isolated with narrow pass band filters and recorded continuously throughout the night in the meridian sweep. Absolute calibration was carried out, and corrections for the scattered light and atmospheric extinction were made. The absolute emission intensity for λ3914 and λ5577 is given at a selected zenith distance. The ratio of instantaneous intensities I(O)5577 to I(O)3914 was found to be essentially constant over an intensity range of three powers of ten and to be independent of the type of auroral form. The intensities and correlations obtained are consistent with the following excitation mechanisms. In the dark atmosphere the N 2+ is excited from ionizing collisions of incoming particles with atmospheric N 2. It is concluded that the O I λ5577 line results from secondary electrons, arising from the primary ionization process above, colliding inelastically with oxygen atoms.

Absolute Emission Intensity of Schumann-Runge Radiation from Shock Heated Oxygen

Absolute Emission Intensity of Schumann-Runge Radiation from Shock Heated Oxygen

CN Bands in the Chromosphere.

view Abstract Citations (6) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CN Bands in the Chromosphere. Pecker, J. -C. ; Athay, R. G. Abstract CN bands are observed in emission in the chromosphere between the photospheric limb and a height of 750 km. Spectrograms obtained by the High Altitude Observatory at the Khartoum eclipse in 1952 showed a scale height of 210 km for the intensity of the emission in the 2 2 (0, 0) band at X 3883. The emission spectrum at 100 km gives a rotation temperature of 4500 , and the photospheric absorption spectrum in the Utrecht Atlas gives a rotation temperature of 4000 . At eclipse heights other than 100 km, the data were used to determine the temperature gradient. The rotation temperature was found to increase with height. Photospheric and chromospheric models were used to predict the absolute intensity of the CN emission in this band. The predicted intensity was less than the observed intensity by a tactor of 20. The discrepancy was accounted for by uncertainties in the absorption coefficient, which was obtained from a study of the absorption profile in the Utrecht Atlas. Publication: The Astrophysical Journal Pub Date: March 1955 DOI: 10.1086/145998 Bibcode: 1955ApJ...121..391P full text sources ADS |