Conditions Im interested in: air, 1 atm, 300K, very low ionization percentage, electrostatic source of ionization, reduction in ionization percentage two fold. But general answer is ok too.

Data I found for somewhat similar conditions:

Time till collision is about 0.1 nanosecond in oxygen at room temperature and 1 atm, http://www.3rd1000.com/chem101/chem102i.htm

While recombination is 30ns as I see in a graph, but mentioned time is more than a hundred nanoseconds, synthetic air with 20% oxygen and 80% nitrogen, atmospheric pressure and temperature https://iopscience.iop.org/article/10.1088/1361-6463/aae134/pdf

So the main question is: why is recombination time so much longer than collision time? 1000 times

Naively i would think that just a few collisions would be sufficient, and that charged particles will 'seek' each other even better than random collisions due to electrostatic attraction.

Bonus question: am I right that ionization percentage strongly affects recombination time? Strongly ionized gas would re-ionize itself often due to temperature and radiation. Weakly ionized gas would need to spend a long time till rare ionized particles would find each other, not sure which effect wins

P.S. by recombination I mean reunification of an electron and nitrogen or oxygen ion with one electron missing, and emmitting a photon to get rid of energy, or staying in a metastable state with electron at higher energy state. Main condition is non-conductivity, non-interaction with electrostatic field. So that it behaves more like a gas and less like a plasma. And just half of such particles to be united, to avoid the 'last pair will never find each other' complication.


1 Answer 1


why is recombination time so much longer than collision time?

As seen here, gas kinetic theory predicts that collision frequency $$ Z \propto T^{0.5}$$, however in this research of ion mixture in troposphere, it is noticed that recombination coefficient : $$ \alpha_{_T} \propto T^{-3} $$ So this tells us that certainly NOT a collision rate is responsible for a recombination rate increase. And it's most probably a reverse situation,- as temperature increases - recombination rates drops down. I don't know exact reasons, but I suspect free electron gas temperature also increases, so it becomes more hard for ions to capture them. Besides along recombination, ionization is also going on, which- contrary to recombination,- this time may be assisted by collision rates, because hypothetically atom could be ionized by a good punch to it by a phonon quasi-particle.


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