Definition of transient absorbtion

  • Thread starter prehisto
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Hi, could someone help me to define what is transient or in other words short-lived absorbtion?

Maybe the general defintion could sound something like: In transient absorbtion the sample is excited with some kind of source(electrons, laser ,x-ray and so on) and then flashed with wide spectra light.
And then absorbtion is measured as a function of time.
 

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  • #2
Simon Bridge
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The absorbance of a substance may vary over time ... thus the value would be "transient".

Junior physics is almost entirely kept to the steady state situation - before a system reaches steady state it passes through transient states that are usually very short lived. The time-scales involved in transient spectroscopy are of order ##10^{-16}##s.
 
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There are also other slower types of transient absorption, for example as a result of what is called spectral holeburning. This process happens often in materials that have several long lived ground states, and as you excite one of the levels it may fall down to another of the ground states and if those are outside the frequency width of the laser it will stay there, thus no longer contributing to absorption. In this process the time scales are given by the ratio of the pumping power vs the relaxation rate of the upper level and between the ground states respectively, and this can be very slow, even minutes.

The wide spectra light you mentioned are often used in this situation, in order to force a "reset" of the system, since wide spectra implies that you would target all ground states, thus refilling the spectral hole again.
 

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