The effect of isotopic mercury on the quenching cross sections of the Hg 6[cubed] P1 state

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1967

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Abstract

A study has been made of the effects of isotopic mercury on the quenching of mercury 6 atoms with various gases. It had been observed that the rate at which mercury atoms were quenched upon collisions with substrate gas molecules was not the sars when discharge lamps containing different isotopes of mercury were used as the excitation source. In order to investigate this effect, it was necessary to modify the existing theoretical and experimental treatments so that they would be applicable to isotopic work. One of the basic problems in obtaining reliable quenching cross section values has been the proper treatment of radiation imprisonment. However, by extending the imprisonment theory developed by Holstein, it was possible to evaluate the contribution made by each separate isotope. Consideration was also given to the transport of resonance radiation through the isotopic discharge lamp, the resonance lamp and absorption cell used in this work. The complications produced by the overlap of the hyperfine lino components in the spectrum of mercury were also evaluated. These effects of radiation transport, hyperfine line overlap and radiation imprisonment were combined in order to develop a series of equations from which individual isotopic quenching cross section values could be determined. Modifications were made in the most accurate experimental methods available so that the quenching data obtained would have a high degree of reliability. Since the reliability of this data deperded strongly on the stability of the isotopic mercury electrcdeless discharge lamps used for excitation, a specialized technique was developed for making these lamps with a very stable plasma and a long life. A computer program was developed so that a rigorous least-squares treatment could be applied to the experimental data. The results provided the best fit of the quenching data. The program also gave an estimate of the experimental errors in the quenching cross section values that were computed. Tne possible effects of polarisation and magnetic fields on quenching values were also investigated. No effect was detected within the limits of experimental errors. However, the results supported certain assumptions made in bhe theoretical treatment concerning quenching rate constants. The isotopic quenching cross section values obtained did not indicate any predominance of one isotope over another. Therefore, a statistical study was made to determine if all the isotopic quenching cross section values were the same for a given gas within the limits of the experimental errors. The conclusion was that, with the possible exception of hydrogen, the ratio of any two isotopic quenching cross section values was the same for all the gases within a confidence level of 25% and that all the isotopic quenching cross section values for any given gas were the same with a confidence level of better than 5%. comparison was made of the quenching cross section values obtained using a natural mercury discharge lamp with those values which had been previously reported using other methods. The comparison was very good. It was also determined that the energy distribution of the isotopic radiation emitted by the discharge lamps and the imprisonment time for each hyperfine line could be determined by direct experimental measurement. This would greatly simplify the theoretical treatment by eliminating the need for the resonance lamp. It would also increase the precision of the quenching measurements. The results of this study indicate that with the exception of hydrogen, the rate, of collisional quenching for different mercury isotopes in the 6 state is the same for any given gas, within the limits of experimental accuracy when the effects of isotopic radiation imprisonment, hyperfine line overlap, and isotopic resonance radiation transport are considered. Furthermore, it was shown that these effects must be included in any quenching cross section determinations in order to obtain accurate absolute values.

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