Reflection and absorption spectra of C6H6 and C6D6 were obtained at temperatures near 4.2°K, in the wavelength region of 1700 Å < λ < 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 A1g1 → 1B1u(ν̃ < 50 000 cm−1, weak system) andA1g1 → 1E1u(ν̃ > 50 000 cm−1, 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 A1g1 → 1B1u transition in the solid.
In this paper we present the results of an experimental study of the absorption spectrum of benzene and deuterated benzenes in solid Ar, Kr, Xe, and N2 in the spectral region 2800–1700 Å, with special reference to the 2100- and to the 1850-Å transitions. Our main results are:
(a) On the basis of the observed vibrational structure the second excited singlet state of the benzene molecule is assigned to the A1g1 → 1B1u rather than to the A1g1 → 1E2g excitation.
(b) Theoretical calculations of the dynamic electronic–vibrational coupling between the B1u1 and the E1u1 states support the B1u1 assignment of the 2100-Å transition.
(c) The vibrational structure of the 1850-Å A1g1 → 1E1u transition was resolved.
(d) No experimental evidence for Jahn–Teller coupling in the π → π* 1E1u state was observed, in agreement with theoretical analysis.
(e) Information on site splittings for the higher π → π* excitation of benzene in rare-gas solids has accumulated.
(f) Analysis of matrix shifts for the A1g1 → 1B1u and A1g1 → 1E1u transitions indicates that the “solvent effect” is dominated by dispersion interactions.
(g) Information on deuteration effects on the B1u1 and E1u1 energy levels was obtained.
(h) Qualitative information on intramolecular radiationless decay in the two higher π → π* excited states of the benzene molecule has been inferred from the linewidths in the absorption spectrum.
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