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I am currently reading this article, which discusses the functional impact of the $Q_x$ and $Q_y$ absorption bands of chlorophyll in photosynthesis. However, the article didn't say where on the spectrum these bands exactly are. It did, however, say that the $Q_x$ band is also known as the $S_2$ band, and the $Q_y$ band as the $S_1$ band. I tried to resolve my question with some Googling, and found the following two pictures on the Internet. The problem is that these pictures seem to imply opposite things: one is claiming that the $Q_x$ band is the blue/violet part of the absorption spectrum while the $Q_y$ band is the red part, while the other claims that $Q_x$ and $Q_y$ are both red and practically superimposed on each other. So which one of these graphics can I trust? And can you point me to any articles that explain the $Q_x$ and $Q_y$ bands definitively?

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    $\begingroup$ Did you see this para in the article: "Historically, two different assignments of the spectra of Chl-a have been proposed: the "traditional" 1960’s assignment identifies the higher-energy component as the Q x origin 10,13–16 , while the "modern" 1980’s assignment selects the lower-energy one, see Fig. 2" $\endgroup$
    – bob1
    Commented Feb 15, 2023 at 0:17
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    $\begingroup$ I don't pretend to understand the physics or chemistry of it, but I think you'll find that the Qx and Qy are vibrational states, sometimes referred to as S1 and S2, not absorption bands. Are you asking about the absorption spectra of chlorophyll or the transitional states? $\endgroup$
    – bob1
    Commented Feb 15, 2023 at 0:25
  • $\begingroup$ @bob1: yes, but I thought that the article gave the bands in terms of $\Delta v$, a parameter in Magnetic Circular Dichroism measurements, which doesn't directly correspond to a given wavelength (I think). Am I wrong about that? $\endgroup$ Commented Feb 15, 2023 at 14:08
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    $\begingroup$ Mu might be wavenumber rather than a spectral measurement. $\endgroup$
    – bob1
    Commented Feb 15, 2023 at 19:53

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