Femtosecond polarization resolved spectroscopy: a tool for determination of the three-dimensional orientation of electronic transition dipole moments and identification of configurational isomers.

A method is presented that combines femtosecond polarization resolved UV/visible pump-IR probe spectroscopy and density functional theory calculations in determining the three-dimensional orientation of an electronic transition dipole moment (tdm) within the molecular structure. The method is demonstrated on the approximately planar molecule coumarin 314 (C314) dissolved in acetonitrile, which can exist in two ground state configurations: the E- and the Z-isomer. Based on an exhaustive search analysis on polarization resolved measurement data for four different vibrational modes, it is concluded that C314 in acetonitrile is the E-isomer. The electronic tdm vector for the electronic S(0)-->S(1) transition is determined and the analysis shows that performing the procedure for four vibrational modes instead of the minimally required three reduces the 1sigma probability area from 2.34% to 2.24% of the solution space. Moreover, the fastest rotational correlation time tau(c) for the C314 E-isomer is determined to be 26+/-2 ps.