The Ã1B 2- X∼1A 1 two-photon fluorescence excitation spectrum of 1,3-difluorobenzene

The ùB ₂- X∼¹A ₁ system of 1,3-difluorobenzene has been observed using the technique of two-photon fluorescence excitation obtained with a pulsed dye laser. Calibration was achieved by a combination of the neon optogalvanic spectrum and etalon fringes. In circular, compared to linear, polarization the bands divide into two groups, those which are B₂- A₁ and which retain their intensity with circular polarization, and those which are A₁- A₁ and lose about 60% of their intensity under the same conditions. These two kinds of bands also show characteristic rotational contours. All of the A₁- A₁ bands whose assignments are established obtain their intensity through vibronic interaction in which the vibration ν' ₂₅ ( ν' ₁₄ in the Wilson numbering) mixes the à with, presumably, the X∼ state. There is an important Fermi resonance between the 9 ¹ and 10 ¹11 ¹ levels. Parts of the one-photon absorption spectrum have been photographed to identify sequences associated with the 0 ₀⁰ band for comparison with those observed in the two-photon spectrum, and to search for bands involving odd quanta of b₂ vibrations, including ν' ₂₅ ( ν' ₁₄); none was found.