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Is there any mechanism by which the vapor pressure of a liquid substance (lets say in a vacuum) is measured to be greater than the expected saturated vapor pressure at that temperature?

Specifically, in my work I'm measuring a number density of alkali atoms in an evacuated container, the measured number density exceeds what would be the expected number density due to the saturated vapor pressure of the alkali metal (Cs and K, both of which are liquid at the temperatures I'm measuring)

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  • $\begingroup$ How sure are you that you are at equilibrium? Any chance that you have local hot spots? $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented May 26, 2011 at 4:05
  • $\begingroup$ @dmckee Not very sure at all. In fact I'm almost positive the temperature is inhomogeneous. However I was under the impression that the rule of thumb was that the lowest temperature dictates the vapor pressure, I'm fairly positive that I have "cold spots" rather than hotspots. I.e. the measured temperature is the hottest point in the container. Some have mixed species (Cs and K vapor in a single container - for instance) if that matters. $\endgroup$
    – crasic
    Commented May 26, 2011 at 4:09
  • $\begingroup$ Alakali atoms can take a long time to migrate around a cell, so you may have to wait some time before the system reaches equilibrium and the pressure is dictated by the cold spot. Of course, I'm assuming that the hot spots are gravitationally elevated relative to the cold spots; if they're not, the liquid will run down and evaporate. One slightly more exotic possible cause: photodesorption. $\endgroup$
    – Anonymous Coward
    Commented May 26, 2011 at 17:42
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    $\begingroup$ Also, if you want strangers over the internet to diagnose why your experiment is working funny, it may be helpful to include more details (cell material, wall coating, temperatures, measurement method, etc.) $\endgroup$
    – Anonymous Coward
    Commented May 26, 2011 at 17:43

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That phenomenon does happen, and it is called supersaturation. It shows that the system is out of its thermodynamic equilibrium, just like the better-known phenomenon of supercooling.

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