Resonance fluorescence, Raman spectra and relaxation of single vibronic levels in the condensed phase: Azulene in naphthalene

Abstract

Resonance fluorescence and Raman spectra for excitation of single vibronic levels of the first excited singlet state of azulene in a naphthalene mixed crystal at 2 K are described. The intensity distributions for the resonance emission spectra were used to verify and in some cases reassign previously proposed correlations of the ground and excited state a1 modes. Relaxed fluorescence from levels populated via vibrational relaxation from the resonant vibronic level was observed and measurements of the population ratios for the vibronic levels relative to the zero-point level are given. From these ratios the following two conclusions were made. (1) In addition to the zero-point level, only strongly absorbing nearby vibronic levels are significantly populated through vibrational relaxation from the resonantly pumped level for excitations up to ≈1560 cm−1 above the zero-point level. This suggests that there is little cascading within the S1 vibrational manifold and that vibrational relaxation occurs largely by V–V energy transfer to the naphthalene host modes. (2) The measurements of the population ratios indicate that the vibrational relaxation times of most of the pumped levels are longer than the mean nonradiative decay time for the levels into which their relaxation occurs. When the nonradiative decay time is taken as 2.6 ps, the following values for the total vibrational relaxation times of the lower a1 vibronic levels were found: 8.7 (384), 7.4 (665), 10.8 (856), 1.3 ps (912). Comparison of the relative intensities of the Raman scattering and relaxed fluorescence from the zero-phonon level upon excitation of the phonon sideband of the 0–0 transition led to a value of ≈0.2 ps for the phonon relaxation lifetime. Spectral distinctions between resonance Raman scattering and fluorescence were examined through the temperature dependence of the spectrum of the emission onto the 825 cm−1 ground state vibrational level over the range 4.2–35 K. The total dephasing decay constant T2 was calculated from the ratio of the contributions of the fluorescence and Raman scattering to the emission and from the fluorescence linewidth at various temperatures. The values ranged from ≈5.4 ps at 4.2 K to ≈1.5 ps at 35 K and were similar at a given temperature for both methods of calculation indicating that the same dephasing mechanisms are responsible for both effects. The temperature dependence of the fluorescence linewidth and frequency shift is consistent with mechanisms of dephasing involving phonons of either a single mode or all thermally accessible modes. Finally, vibrational and electronic dephasing are compared.