Determination of dipole moment change on the electronic excitation of isolated Coumarin 153 by pendular-state spectroscopy

The change (Δμ) in the permanent dipole moment on the S1←S0 electronic excitation is determined for Coumarin 153 (C153) in a molecular beam from the spectral change induced by applying a strong dc electric field up to 200 kV/cm. The comparison of the observed fluorescence excitation spectra under various external fields with the corresponding simulations for a pendular-state molecule yields Δμ=7.1±0.4 D for both the syn and anti conformers of C153. Previous experimental results on Δμ in the condensed phase, as well as the theoretical predictions reported in the literature, are discussed on the basis of the present value under an isolated condition.