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Very interesting. A further question … when energy disturbs a body of gas in thermal equilibrium, would both emission and absorption spectra be observed to be occurring simultaneously ? (I'd guess the answer must be yes .. to varying luminosities .. and depending on the different states of the different parts of the gas body and the nature of the imposed energy).
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The answer is no.
Quantum mechanics states that the time for an energy transistion to occur such as an electron moving back to it's ground state is related to the energy difference between the excited and ground state. The larger the difference the faster the time for the transistions to occur.
Since a gas can undergoe various transistions at different energy levels none of the transistions occur simultaneously.
A spectrum is simply a record or snapshot of these transistions over a particular exposure time.
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The presence and nature of such imposed energy must be precisely shown by the spectral lines, which in the case of the Tempel 1 impact event, clearly varies over time (30, 40 minutes to hours, days). If it doesn't vary, then the process causing the spectra would be continuous (or smoothly varying), over that same timeframe and the conditions of the gas must not vary, either.
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The intensity of the peaks in either an IR absorption spectrum or IR emission spectrum relates to the concentration of the functional group causing the peak. Unfortunately this is not an absolute value.
By taking two IR spectra at different time intervals, any variations in the peaks allows one to calculate the emission rate of material from a comet.
As seen in the Japanese paper this is not a straightforward process.
Regards
Steven