Ultrafast interferometric pump–probe correlation measurements in systems with broadened bands or continua

Ultrafast (femtosecond) interferometric pump–probe techniques can be used to measure rates of population and quantum phase decay in complicated media such as liquids and solids. However, the levels probed in such systems are often inhomogeneously broadened or are part of a continuum of states. The use of broadband ultrafast lasers thus results in multiple levels being excited and detected. The inherent averaging that is due to this effect can alter the measured coherent response, thus affecting the information that can be retrieved on the phase decay. The importance of these effects is considered for the representative case of two-photon photoemission from metals. The effects of (i) continuum excitation; (ii) excitation from the Fermi level, i.e., a spectral step function; (iii) excitation from broadened levels with a finite width; and (iv) photoelectron energy analyzer resolution are determined.