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I measured the UV-absorption-spectrum of benzene vapor as well as solved in methanol and cyclohexane. The absorption of the vapor was much stronger than in solution. I also noticed that the peaks get slightly shifted to longer wavelengths - with cyclohexane more than with methane [sic] methanol - and I have no explanation for this. Any help is appreciated :)

(vapor) (methanol) (cyclohexane)

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    $\begingroup$ Did you notice also that the vapour spectrum should have sharp lines and many more of them that the solution spectrum ? if you did not then you need to increase the resolution of the spectrometer, i.e. decrease slit widths. The spectra have nothing to do with the air. In solution there are many more collisions with the solvent vs the vapour, these collisions change the energy levels slightly and so smear out the spectrum. $\endgroup$
    – porphyrin
    Commented Jun 4, 2021 at 19:37
  • $\begingroup$ @porphyrin I added the spectra $\endgroup$
    – Christian
    Commented Jun 5, 2021 at 9:57
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    $\begingroup$ I suspect that the spectral resolution is limiting the shape of the lines in the vapour phase and possibly the others. Use as narrow slits as your instrument will allow and if the spectrum is noisy as a result average this out. If the instrument is at minimum slit width you should find a better instrument :) Benzene is non polar so is little influenced by solvents (1 nm in 250 nm is not much of a shift in energy ) other than that they cause line broadening. $\endgroup$
    – porphyrin
    Commented Jun 5, 2021 at 15:36

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The shift to longer wavelength is due to solvent polarity. Shift to longer wavelength is known as a "red shift", or bathochromic shift. Methanol is polar and will interact with the polar excited state lowering the energy required to reach the excited state from the ground state, thus we see longer wavelength peaks. I am not sure why cyclohexane has shifted the spectrum, however, because it is non polar.

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