Reflection and Absorption Spectra of the Higher π → π* Transitions of Solid Benzene

Reflection and absorption spectra of C₆H₆ and C₆D₆ were obtained at temperatures near 4.2°K, in the wavelength region of $\text{1700 Å\hspace{0.17em}<\hspace{0.17em}λ\hspace{0.17em}<\hspace{0.17em}2700 Å}$ . The samples were prepared as films on a cold window. Wellresolved spectra were obtained with both methods. The reflectance data (obtained at near normal incidence) were converted, using a combination of Kramers–Kronig and curve‐fitting techniques, to yield the real and imaginary parts of the dielectric constant as well as the refractive index and the absorption coefficient. The results were in good correspondence with those obtained by direct absorption measurements. The absorption bands observed below 2200 Å belong to the electronic transitions ${}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_{\text{1u}}\text{(ν̃\hspace{0.17em}<\hspace{0.17em}50 000}{\mathrm{cm}}^{\text{−1}},\mathrm{weak\; system})\mathrm{and}{}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{E}_{\text{1u}}\text{(ν̃\hspace{0.17em}>\hspace{0.17em}50 000}{\mathrm{cm}}^{\text{−1}},\mathrm{strong\; system})$ , respectively. The possible assignments of the individual bands within each system are discussed on the basis of the present results, taking into account conclusions derived from spectra of benzene embedded in rare‐gas matrices. Semiempirical calculations are carried out on the shift of the different progressions within the ${}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_{\text{1u}}$ transition in the solid.