In picosecond transient absorption spectroscopy using superbroadened continuum passed through an echelon to produce an optical interrogating probe beam, an accurate knowledge of the relative timing between the excitation and probe pulses is required. For instance, when changes in the transient absorption spectrum are studied as a function of time, particularly when the rate of transient formation is comparable to the excitation pulse duration, both the time spread of the continuum pulse and the time origin of the probe pulse must be accurately known. The time spread or "chirp" of the polychromatic continuum pulse is only occasionally estimated even though the variation due to group delay dispersion may be quite large especially for long pathlength continuum cells which are frequently used.
K. L. Sala, R. LeSage, and R. W. Yip, "Kerr Gating of the Continuum Pulse in Picosecond Spectroscopy: A Sensitive Null Method to Time the Probe and Excitation Pulses," Appl. Spectrosc. 38, 87-89 (1984)