Picosecond Relaxation in Solvents

In fluid solvents at room temperature, spectral relaxation is usually comjdete prior to emission and occurs within abont 10 ps. This process is too rapid to be resolved with the usual instrumentation for TD or FD fluorescence. However, advances in laser technology and methods for ultrafast spectroscopy have resulted in an increasing interest in picosecond and femtosecond solvent dynaaiics. Becmise of the rapid timescale, the data on solvent dynamics are usually obtained using flurvescence upconversion. Hiis method is described in Section 4.7.C. lypical data are 

There is no comptehensive theoty which can be used to explain all time-dependent spectral shifts. This is because the spectral shifts can have their molecular origin in general solvent effects, qiecific solvent effects, or other excited-state processes. However, it is possible to predict the time dependence of spectral shifts due to general scdvent effects. The basic idea is that the leaction field around the excited molecule relaxes in a manner that is predictable firm the didectric idaxation times (To). Tb be more precise, the spectral relaxation tittle (Ts) is expected to be equal to the longitudinal relaxation time (It). The value of Ti is related to the dideclric telaxation time (to) by 

Values of xd and Xl for typical solvents are given in Table 7.3. For polar solvents, diespeSSal relaxation times are expected to be S- to 10-fold smaller than the value of To. Hence, the rate of spectral rdaxation is expected to be faster than the rate of dielectric relaxation. 

Spectral relaxation data arc frequently presented as the correlation fimetion 

This expression has the rrivantage of nonnaliziag the extent of relaxation to unity, allowing different experiments to be compared. However, when ushig Eq. (7.16), one loses the infotiuatioo on the emissian qiectta at r s 0 and r =. If some portion of the relaxation occtns more rapidly titan the time resolution of the meaautement, this portion of the eneigy will be missed and possiUy not noticed. 

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