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 res onantly enhanced multiphoton ionization (REMPI) and laser induced fluoresce nce (LIF). The ion and fluorescence yields showed preference for perpendicu lar excitation relative to the orientation field, implying a perpendicular relationship between the permanent dipole and the transition dipole. Calcul ation results using a linear variation method reproduced the observed spect ral features, overall transition intensity, and polarization preference of the excitation laser. The permanent dipole of the S-1 state of pyrimidine w as thereby determined to be +0.6 Debye. Measurements of polarization prefer ences in photoexcitation, i.e., linear dichroism, provide a direct approach for determination of transition dipole moments. A general theory of derivi ng directions of transition dipoles relative to permanent dipoles based on this type of measurement/calculation was also developed. In addition, we re port observations of complex relaxation dynamics of pyrimidine in an electr ic field. At 50 kV/cm, the overall fluorescence yield was quenched to a qua rter of its value under field free conditions. The spectral intensity distr ibution exhibited dependence on the delay time of the ionization laser in t he 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 fr om levels with M(')greater than or equal to 3 in the calculation was suffic ient to reproduce experimental spectra recorded with a delay time of 200 ns . These observations and interpretations agree with previous reports on pho tophysical properties of pyrimidine, including relaxation and quenching in a magnetic field. (C) 1999 American Institute of Physics. [S0021-9606(99)01 224-6].