Orientation of pyrimidine in the gas phase using a strong electric field: Spectroscopy and relaxation dynamics

Orientation of pyrimidine in a strong electric field was measured using resonantly enhanced multiphoton ionization (REMPI) and laser induced fluorescence (LIF). The ion and fluorescence yields showed preference for perpendicular excitation relative to the orientation field, implying a perpendicular relationship between the permanent dipole and the transition dipole. Calculation results using a linear variation method reproduced the observed spectral features, overall transition intensity, and polarization preference of the excitation laser. The permanent dipole of the S-1 state of pyrimidine was thereby determined to be +0.6 Debye. Measurements of polarization preferences in photoexcitation, i.e., linear dichroism, provide a direct approach for determination of transition dipole moments. A general theory of deriving directions of transition dipoles relative to permanent dipoles based on this type of measurement/calculation was also developed. In addition, we report observations of complex relaxation dynamics of pyrimidine in an electric field. At 50 kV/cm, the overall fluorescence yield was quenched to a quarter of its value under field free conditions. The spectral intensity distribution exhibited dependence on the delay time of the ionization laser in the REMPI experiment. Qualitative assignments of the REMPI spectra revealed that the loss of signal strength with delay time was primarily from levels containing high rotational angular momenta. Elimination of contributions from levels with M(')greater than or equal to 3 in the calculation was sufficient to reproduce experimental spectra recorded with a delay time of 200 ns. These observations and interpretations agree with previous reports on photophysical properties of pyrimidine, including relaxation and quenching in a magnetic field. (C) 1999 American Institute of Physics. [S0021-9606(99)01224-6].