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We present experimental results for the cross phase modulation (xpm) induced transient absorption signal in a 1 mm thick fused silica plate using a white light continuum as a probe. The fused silica plate mimics the entrance window of a commercial flow cell commonly used in liquid-phase transient absorption measurements. The experimental results are compared with those obtained theoretically by numerically solving the set of nonlinear coupled wave equations describing the propagation of the pump and the probe. The simulations allow for the different group velocities of the pump and probe pulses, and include the influence of the first and second order dispersion on the continuum probe. From the calculations the physical origin of the complex oscillatory feature observed around the zero time delay of each wavelength of the (chirped) continuum has been accurately identified. The influence of propagation effects arising from the finite thickness of the sample is discussed in great detail, and the necessity to work with thin samples, preferably free-flowing jets, is emphasized. The good agreement between theory and experiment indicates that the xpm artifact may be useful for characterizing the continuum probe, in particular its chirp.

The transient absorption spectra of 2,2‘-bipyridine (22BPY) and 4,4‘-bipyridine (44BPY) following femtosecond excitation at 266 nm in a series of organic solvents have been measured in the 300−670-nm wavelength range with subpicosecond time resolution. The results have been correlated with those obtained in the microsecond time domain. The S1 states are characterized and attributed to nπ* levels in both molecules. The S1 state of 22BPY decays essentially via intersystem crossing to the T1 ππ* state in all solvents (τS 1 = 50−80 ps). The 44BPY S1 lifetime varies from 10 to 70 ps depending on the solvent. The shortest lifetimes are found in alcohols (10−20 ps) and alkanes (≈11 ps) where the fast formation of the N-hydro radical 44BPYH•, already evidenced in previous nanosecond experiments, is shown to occur with high yields from the S1 state in parallel to ISC. In alcohols this process is found to be activated by the protic character of the solvent and to take place within H-bonded solute/solvent complexes pre-existing in the ground state. A mechanism involving a surprisingly fast H-atom abstraction by the 44BPY S1 state via homolytic breaking of the alcohol OH bond is suggested.