My question is illustrated as the figure. As far as I know, due to the Franck-Condon principle, the absorption spectrum is related to the ground state to the excited state with the same geometry. Here I marked as $|X \nu_0\rangle \rightarrow |A \nu_m \rangle$, where $\nu_0$ is the ground vibrational state, $\nu_m$ is the vibrational state in the first electronic excited state with the same geometry as the ground state.

The emission spectrum is related to the excited state to the ground state with the same geometry. Here it is labelled as $|A \nu_0 \rangle \rightarrow |X \nu_n \rangle$, where $\nu_n$ is the vibrational state with the ground electronic state geometry.

The difference between absorption and emission spectrum is known as Stokes shift.

My question is, is there any type of spectroscopy has the resolution for many vibrational states around the electronic states? E.g., $|X \nu_0\rangle \leftarrow \rightarrow |A \nu_0\rangle$ , $|X \nu_0\rangle \leftarrow \rightarrow |A \nu_1\rangle$ , $|X \nu_1 \rangle \leftarrow \rightarrow |A \nu_0\rangle$ etc. If yes, what is the name of it?

  • $\begingroup$ I think it depends what state of matter are you investigating. In gas phase, the vibrational structure of electronic transitions is visible in UV-VIS spectroscopy for example gas phase benzene. researchgate.net/figure/… $\endgroup$
    – M. Farooq
    Feb 15 at 5:39
  • $\begingroup$ Thank you so much for your link. My background is simulation and not familiar with experimental techniques. Is my understanding correct, that at room temperature, the molecules are at a distribution of vibration states, hence, in a lab, we can see all these vibrational structures, namely, peaks? $\endgroup$
    – kote2021
    Feb 15 at 6:05
  • $\begingroup$ At room temperature one can assume a molecule is at the electronic ground state and its vibrational energies are distributed due to thermal energy. If you are in gas phase, you can see rotational structure too. I have never done any theoretical simulations of molecular spectrum. But I like experimental spectroscopy. People mostly study gas phase spectrum. $\endgroup$
    – M. Farooq
    Feb 15 at 6:56
  • $\begingroup$ As well as UV visible spectroscopy (using a spectrophotometer for example) , fluorescence also exhibits vibrational levels. For the best resolution vapour phase spectra are best; search for 'benzene vapour absorption' spectrum and look for 'images'. $\endgroup$
    – porphyrin
    Feb 15 at 8:06
  • $\begingroup$ You can also include Raman as for all begins with a quasi-resonant or not excitation. $\endgroup$
    – Alchimista
    Feb 15 at 10:08

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