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Old May 21st 2019, 04:19 AM   #1
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Usage of absorption and magnitude mode spectra

In nuclear magnetic resonance, when should one process spectra in absorption mode and when - in magnitude mode? What is benefit of using the first one and the second one? For example, I need to integrate spectrum. In this case, what should I use - absorption or magnitude mode?
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Old May 28th 2019, 08:24 AM   #2
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I think it depends on the application. In astrophysics, it is often the case where stars and other thermal objects emit in the black body spectrum, so the absorption and emission spectra are both very indicative of the presence of particular substances (e.g. in the stellar atmosphere) and their abundances can be calculated by considering deviations from the black-body radiation. The emission often reaches the telescope so both of them can be investigated.

However, in other cases, such as nebulae and gas clouds, the emission is not a black body but is some other emitting entity, like a supernova remnant or field stars. Gas clouds, dust and the interstellar medium then capture some of that radiation and emits it in all directions, so very little of the re-emission is heading towards the telescope. In those cases, the emission spectra is not observed at all (or only very slightly), but the absorption spectra can still be observed. The absorption spectra can be obtained by comparing the emission from an unobstructed part (or low density part of the cloud) with the more obstructed part in order to estimate the abundance.

In lab experiments, however, emission spectra are much more useful because the original emission can be very closely controlled using, for example, monochromatic lasers. In those cases, the laser is set to be some frequency which will be absorbed by the target and then the emission spectra is measured. That being said, it's also common to do white light experiments at cyclotrons or synchrotrons, in which case the absorption spectra are useful again.
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