A question is about positron chemistry. Does the pH of an aqueous solution affect the lifetime of positronium (Ps)? The following references reported that the interactions between Ps and MnO4- was almost the same at pH=2, 7 and 12.
- V. I. Goldanskii, et al., "Observation of positronium reactions in aqueous solutions", J. Experim. Theoret. Phys., vol. 12, pp. 1026 (1960). http://www.jetp.ac.ru/cgi-bin/e/index/e/12/5/p1026?a=list
- T. L. Williams, and H. J. Ache,
Article Positronium Chemistry in Aqueous KMnO4 Solutions
, J. Chem. Phys., vol. 50, pp. 4493 (1969).
Are there any other similar examples? I wonder if it is correct to assume that the lifetime of Ps is unaffected by pH. Or conversely, are there any reports that the lifetime of Ps changes with pH?
Dear Kengo,
what you observed is what I would expect.
If a positron "meets" an electron, an annihilation into 2 photons takes place. If the pH influences the density of electrons, the reaction probability would change. But this seems not to be the case.
With regard
R. Mitdank
Thanks for the multiple replies.
The probability of pick-off annihilation in positronium (Ps) is primarily due to electron density, as Rüdiger Mitdank has pointed out. It is described by the overlap of the positron and electron wave functions. However, in practical terms, it is not determined by that alone, but is also affected by various environmental factors. In this discussion, I would like to get into these factors.
For example, water and many other liquids expand as their temperature rises. Since expansion decreases the molecular density (electron density), it is assumed that the higher the temperature, the longer the Ps lifetime. However, this assumption is not true. In fact, the lifetime of Ps is shorter in warm water than in cold water [Ref. 1].
Article Measurement of positron lifetime to probe the mixed molecula...
Similarly, the relationship between Ps lifetime and pH is not easy to estimate. Therefore, we should base our decisions on experimental data.
Most of the basic Ps chemistry was done in the 1960s and 1970s. In those days, the resolution and stability of measuring instruments were not as good as they are today, and the use of computers was limited. Straight lines were drawn with a ruler on hand-drawn one-log graphs [Ref. 2]. Therefore, the environmental factors that were reported to be "independent" at that time may or may not be due to the lack of measurement accuracy. As the application of positron science to medicine has progressed, motivation to revisit some of the issues has arisen.
Article Positronium interactions in aqueous oxyacids and hydrogen compounds
I hope somebody will continue to let us know if you have any information on the relationship between pH and the lifetime of Ps. Thank you very much for your time.
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