15 research outputs found
Theory of analytical curves in atomic fluorescence flame spectrometry
An explicit expression for the intensity of atomic resonance fluorescence as a function of atomic concentration in a flame is derived under certain idealized conditions. The expression is generally valid for a pure Doppler absorption line profile as well as for a combined Doppler and collisional broadened profile (Voigt profile) of the absorption line in the entire range of concentration values.
The expression derived was used to calculate analytical curves for a number of different values of the ratio of collisional line width to Doppler width. The calculations were carried out for a single resonance line in the extreme cases of a very narrow exciting line source and a continuum radiation source, respectively.
Furthermore, a method is given to determine the so called damping parameter “a” and the fluorescence yield factor p from the asymptotic behaviour of the analytical curves if the actual atomic flame content at the intersection point of the initial and final asymptote is calibrated
Quenching of excited alkali atoms and related effects in flames: Part I. theoretical analysis
An a.c. photoelectric detection device has been used to determine the yield factor p of resonance fluorescence of the Na(5890/96) Å and K(7665/99) Å and (4044/47) Å resonance doublets in atmospheric flames as a function of the composition and temperature of the flame. From these measurements the value (and temperature dependence) of the specific effective quenching cross-sections of the considered lines for N2, CO2, CO, O2, H2, Ar and H2O could be derived. The experimental procedure and the results obtained will be reported and discussed in Part II of this paper. In Part I a theoretical analysis is given of the effect of radiative non- equilibrium on the occupation of the excited state, as a function of the p-value, the metal concentration in the flame, and the distance from the flame surface. In particular, an approximate expression for the population (or source) function in the high density case is derived by an iteration procedure for assumed rectangular, Doppler and Lorentzian spectral line profiles respectively. It is assumed that the source function is independent of frequency and that the excited level combines with the ground state only. In the analysis a homogeneous and isothermal slab of flame gases between two plane parallel boundaries is considered. Diffusion effects of the atomic metal vapour at the flame border are considered semi-quantitatively. Also the effect of radiative non- equilibrium on the intensity of the outgoing radiation, the line-reversal temperature, the shape of the curve of growth, and the intensity ratio of the doublet lines is analysed. Finally, the expected influence of quenching collisions on the broadening of spectral lines in flames is calculated
The relationship between the fluorescence yield and the underpopulation of doublet excited states
The influence of radiative non-equilibrium on the occupation of excited metal states in flames can be investigated by measuring the deviation ΔT of line reversal (or excitation) temperature TL from “true” (or translation) temperature Tt of the flame as a function of metal concentration. In this paper it is shown that, under certain conditions concerning the rates of the de-excitation processes, the measurements of ΔT for atoms with close lying upper levels (such as the alkali atoms) may be interpreted by assuming a single excitation level. In that case, the fluorescence yield YD, obtained when both lines are used in excitation and measured in fluorescence, occurs as a parameter in the expressions which describe the behavior of ΔT for low and high metal densities.
The theoretical relationships were checked by comparing the experimental values of YD and ΔT for the yellow sodium doublet in a typical hydrogen-oxygen-argon flame. Satisfactory agreement was found. In order to determine more reliably the low density asymptote of the excitation temperature, the variation of ΔT with atomic density was theoretically analysed to first order in the density.
Fluorescence yield factors were again measured for several alkali lines with an improved experimental set-up and agreed with our previous results, (1,2,3) except for the sodium doublet. The effective cross sections (see Table 1) derived from these yield factors for the quenching of excited sodium (3P1/2,3/2), potassium (4P1/2,3/2) and rubidium (5P1/2,3/2) atoms by various flame molecules, are in satisfactory agreement with the results of Jenkins' flame experiments.(4,5
Quenching of excited rubidium (52P) atoms in flames
An alternating current photoelectric device (compare the work of Boers,(1) Hooymayers et al(2-4) and Hooymayers(5)) has been used for determining the yield factor p of resonance fluorescence for the infrared rubidium doublet (7800/7947 Å). From the p-values measured in five different hydrogen flames at about 2000°K and 1 atm pressure, the specific effective quenching cross-sections(2-5) S of the excited Rb atoms in collision with some molecules could be derived.
The cross section values for these molecules were found to be SH2 = 3·6(±0·4) Å2, SN2 = 25(±1·5) Å2, SO2 = 83(±5) Å2 and SH2 O = 4·0(±0·8) Å2 respectively. In the derivation of the S-values it was assumed that SAr is negligibly small.
The S-values for the diatomic molecules may be considered as valid for a temperature of about 1900°K, whole SH2O is valid at about 2100°K. The possibility of a resonance effect between the excited Rb-level and the molecular vibrational levels is discussed
Quenching of excited hydroxyl (2Σ+, υ′ = 0) radicals in flames
An alternating current photoelectric device (compare the work of Boers et al.,(1)Alkemade,(2) Hooymayers et al.(3,4) and Hooymayers(5)) has been used for determining the yield factor p of resonance fluorescence for some specific rotational transitions of the (0,0) band of the hydroxyl 2Σ+ → 2Π-band system. Excitation of the selected rotational levels was achieved by irradiating the flame with a hollow cathode bismuth-lamp emitting the 3067·72 Å Bi-line. From the p-values measured in three different hydrogen flames at about 1500–1800°K and 1 atm pressure the specific effective cross-sections(4,5) S of the excited radical in collision with N2, O2 and H2O molecules as quenchers could be derived with an accuracy of about 18 per cent. Here the quenching cross section S is defined as π times the square of the distance between the centers of the colliding species. In contrast with the results of our quenching experiments on excited alkali-atoms, (4,5) it was found that the S-value for H2O as quencher (SH2O = 37 ± 6 Å2) exceeds the values for the other molecules mentioned (SO2 = 10 ± 2 Å2 and SN2 = 7 ± 1 Å2) by a factor of at least 4. Combining our results with other data obtained by Zeegers(6–8) at our laboratory in OH-chemiluminescence experiments, it appeared that at our flame conditions, (4,5) quenching of excited OH(2Σ, υ′ = 0) radicals by H2O molecules is partly (≈ per cent) achieved by dissociative quenching according to the reaction OH* + H2O → OH + OH + H
Emission noise spectrum in a premixed H2-O2-N2 flame
Experimental noise spectra in the frequency range of 15–105 Hz are reported for the thermal emission of the first resonance doublet of Na and K in a premixed H2-O2-N2 flame, and for the flame background emission. Under certain conditions, low-frequency peaks arise in the noise spectrum below 100 Hz, while a minimum is found at about 3 kHz. This minimum is of interest with respect to the optimal modulation frequency in atomic fluorescence measurements. The possible sources of fluctuations are discussed, but definite conclusions cannot yet be drawn