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A new synchrotron study for CH$_2$F$_2$ from has been combined with earlierdata. The onset of absorption, band I and also band IV, is resolved into broadvibrational peaks, which contrast with the continuous absorption previouslyclaimed. A new theoretical analysis, using a combination of time dependentdensity functional theory (TDDFT) calculations and complete active spaceself-consistent field, leads to a major new interpretation. Adiabaticexcitation energies (AEEs) and vertical excitation energies, evaluated by thesemethods, are used to interpret the spectra in unprecedented detail usingtheoretical vibronic analysis. This includes both Franck-Condon (FC) andHerzberg-Teller (HT) effects on cold and hot bands. These results lead to there-assignment of several known excited states and the identification of newones. The lowest calculated AEE sequence for singlet states is 1$^1$B$_1$$\sim$ 1$^1$A$_2$ < 2$^1$B$_1$ < 1$^1$A$_1$ < 2$^1$A$_1$ < 1$^1$B$_2$ <3$^1$A$_1$ < 3$^1$B$_1$. These, together with calculated higher energy states,give a satisfactory account of the principal maxima observed in the VUVspectrum. Basis sets up to quadruple zeta valence with extensive polarizationare used. The diffuse functions within this type of basis generate both valenceand low-lying Rydberg excited states. The optimum position for the site offurther diffuse functions in the calculations of Rydberg states is shown to lieon the H-atoms. The routine choice on the F-atoms is shown to be inadequate forboth CHF$_3$ and CH$_2$F$_2$. The lowest excitation energy region has mixedvalence and Rydberg character. TDDFT calculations show that the unusualstructure of the onset arises from the near degeneracy of 1$^1$B$_1$ and1$^1$A$_2$ valence states, which mix in symmetric and antisymmetriccombinations.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

The valence and Rydberg states of difluoromethane: A combined  experimental vacuum ultraviolet spectrum absorption and theoretical study by  ab initio configuration interaction and density functional computations