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Recent reports have shown how thermal lensing spectroscopy (TLS) can be applied to the study of weak transitions in Liquids, such as highly excited vibrational overtones or spin forbidden transitions. We report here the observation of the low-energy weak absorption in $\beta$-carotene in the range $1600-18600cm-1$ as seen by TLS. The weak absorption features observed on the red edge of the strongly allowed $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bu\leftarrow Ag1$ transition are assigned as vibronic levels of the symmetry forbidden $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag\leftarrow 1Ag$ transition, thus supporting the reordering of $\beta$-carotene excited electronic states suggested by other experimental $evidence.1$