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I'm reading an article about two-dimensional infrared spectroscopy and I don't understand the following sentence.

Bleach or stimulated emission contributions yield negative signals.

What are "bleach contributions"? I have never heard of it and cannot find a suitable explanation on the Internet. Can somebody explain that to me?

The article I'm referring to is J. Chem. Phys. 121, 5935 (2004)

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If the source is on the web a link so that we can get context would be useful. – Brandon Enright May 13 '13 at 23:08
In general, on this site and anywhere else, please always, always, always cite your source when giving a quotation. – Nathaniel May 14 '13 at 7:42
up vote 2 down vote accepted

I feel obliged to post another answer since the first one contains a mistake.

As you deplete the ground state, you also populate the excited state, and when the photon interacts with the excited molecule, then instead of being absorbed it generates another photon through stimulated emission. The paper you cite treats this contribution as a negative signal.

Eventually, at 50/50 population you reach an equilibrium, where the amount of absorbed light is equal to the amount of light produced by stimulated emission, and it looks like your medium is not absorbing light at all, which is why it is called bleaching (i.e. loosing its color).

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Thanks (from the other answerer). You seem to imply that (Wikipedia's) "der Grundzustand wurde gebleicht" is actually a meaningless statement. Would my answer be better if I'd just remove that bit? I wouldn't want to leave a misleading answer here. – Keep these mind May 14 '13 at 12:06
In fact, comparing answers, it now strikes me that mine may only make sense in ultrafast laser spectroscopy. If you can confirm this I can consider editing or deleting my answer. – Keep these mind May 14 '13 at 12:20
@Gugg even with an ultrafast laser you cannot empty the ground state. As soon as you reach 50/50 equilibrium, you start pumping the molecules up and down with the same rate. The Wikipedia statement about moving all molecules from the ground to the excited state is certainly false. – gigacyan May 15 '13 at 10:00
This is why, by the way, it was so difficult to create laser (which was theoretically predicted in the beginning of the 20th century) - to amplify light, you need to have more than half molecules in the excited state and you cannot do it by optical pumping of a 2-level system (Nd:YAG lasers use a four-level system). – gigacyan May 15 '13 at 10:05

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