Is selective heating of a gas within a mixture possible? Using the absorption wavelength of the molecule Could you effectively concentrate a particular gas in a mixture by applying only the wavelength absorbed by the target gas?
Assuming that the target gas molecules heat up when absorbing the selected wavelength, would they rise up and concentrate at the top of the vessel or would the added energy disperse and warm up the whole gas mixture?
In this case, the target gas would be CO2 in air.
 A: no, it can't. the gas molecules in a mixture are continuously undergoing collisions with every gas molecule in the mixture, and thereby sharing their kinetic energy in every possible way. If you could heat only one gas in a mixture, it would share that energy with all the other molecules so quickly that the gases would have no time at all to segregate themselves in the way you propose.
A: "Investigation of gas separation technique based
on selective rotational excitation of different
species by a laser", Phys. Fluids 32, 087106 (2020)
Abstract
"In this work, a gas separation approach based on the selective rotational excitation of different species is investigated. The presented method is particularly suitable for separating gases of similar or equal masses, such as isotopes and isomers. The selective rotational excitation is
achieved by a targeted application of multiple non-resonant ultrashort laser pulses. Upon collision with a solid surface, a part of the excited rotational energy gets transferred into translational energy. By creating a discernible difference in average thermal velocities between the species of similar masses, an increased diffusivity of the excited species can be utilized for its successful separation. In order to test the
validity of the novel separation technique, a comprehensive computational framework was developed. The energy transfer in gas–surface collisions was analyzed in great detail using a state-of-the-art molecular dynamics code, and the obtained data offered invaluable insight into the nature of scattering dynamics. Furthermore, a novel data-driven approach to gas–surface interaction modeling based on the recently introduced distribution element tree method was proposed. Relevant numerical and experimental data on the selective rotational excitation were gathered, and they served as an input for the performed numerical simulations. Using the developed computational framework, the validity of the proposed separation scheme was tested on a mixture of two species with identical mass. The obtained data offer numerical evidence supporting the proposed separation concept."
