Spectral energetic properties of the X-ray-boosted photoionization by an intense few-cycle laser

We report a discovery that an intense few-cycle laser pulse passing through gas leaves a fingerprint of its field envelope on the photoelectron energy spectrum, which involves continuous X-ray radiations. The spectrum resulting from the photoionization processes includes significant quantum enhancement and interference and exhibits interesting energetic properties. The spectral cut-off energies reflect the strength, time, and interference of the laser field modulation on the photoelectron energy. These energetic properties suggest a new method for precise intense-laser-pulse measurement in situ. The method has the advantages of accuracy, simplicity, speed, and large dynamic ranges (up to many orders of intensity).