A simple method for the in situ analysis of femtosecond UV pulses in the pump-probe spectroscopy of solutions

It is shown that the two-photon absorption (TPA) process in water and other solvents can be applied in common pump-probe experiments for the in situ determination of the peak intensity and duration of femtosecond pump pulses. The TPA coefficient of liquid water for femtosecond irradiation at 282 nm is measured to be β(282nm + 282nm) = (1.9 ± 0.5) × 10^-12 m/W. The coefficient for simultaneous absorption of photons at 282 nm and 299 nm is determined to be β(282nm + 299nm) = (1.0 ± 0.4) × 10^-12 m/W. Various TPA coefficients of thin diamond samples are also reported for pump pulses at 282 nm and probing pulses in the range 282-750 nm that are useful for the estimation of pulse intensities. The described technique is especially advantageous for the transient spectroscopy of solutes.