Digitale Medien
College Park, Md.
:
American Institute of Physics (AIP)
The Journal of Chemical Physics
107 (1997), S. 9807-9817
ISSN:
1089-7690
Quelle:
AIP Digital Archive
Thema:
Physik
,
Chemie und Pharmazie
Notizen:
Time-resolved resonance Raman and transient absorption spectra of photoexcited S1 quaterphenyl in solution have been measured in a single series of experiments over a range of probe wavelengths at various time delays and solvent temperatures. Increases of 0.4% in the energy of the 0–0 electronic resonance transition to the higher (Sn) state and 0.1% in some vibrational frequencies are observed to take place on a 17 picosecond timescale following photoexcitation, and electronic and vibrational bandwidths both reduce by a few percent. Comparisons of measured Raman excitation profiles with profiles calculated from the transient absorbance spectra are used to interpret the time dependence of Stokes resonance Raman band intensities. The electronic resonance shift and width change and relaxation of Franck–Condon displacements all contribute. All parameters vary with bath temperature, but bandshifts are small on cooling and the 0–0 resonance shifts to the red. The change in S0–Sn resonance frequency is taken to imply a change in S0–S1 potential and to be a solvation effect which is also responsible for the displacement and Raman frequency shifts. The anti-Stokes Raman band at 766 cm−1 shows additional intensity changes due to population relaxation on two distinct timescales: 〈1 ps and ∼17 ps. The fast component is attributed to intramolecular redistribution of v〉1 excitation energy and the slow component to the decay of a hot v=1 population with an average excess energy of ∼60 cm−1 per molecule. This is much smaller than the initial excess photoexcitation energy of ∼5000 cm−1 but corresponds to a temperature much greater than indicated by the bandwidth changes, implying a non-equilibrium distribution of vibrational energy whose decay is not limited by thermal diffusion. The slow component of the population relaxation matches approximately with the change in potential in both energy and timescale but no causal connection is identified. This experiment links the dynamics of Raman frequency shifts observed in an excited state molecule directly to a change in electronic potential. It is suggested a similar mechanism may operate in other systems, such as stilbene. © 1997 American Institute of Physics.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1063/1.475279
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