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Characterization of ultrashort vacuum and deep ultraviolet pulses is important in view of applications of those pulses for spectroscopic and dynamical imaging of atoms, molecules, and materials. We present an extension of the autocorrelation technique, applied for measurement of the pulse duration via a single Gaussian function. Analytic solutions for two-photon ionization of atoms by Gaussian pulses are used along with an expansion of the pulse to be characterized using multiple Gaussians at multi-color central frequencies. This approach allows one to use two-photon autocorrelation signals to characterize isolated ultrashort pulses and pulse trains, i.e., the time-dependent amplitude and phase variation of the electric field. The potential of the method is demonstrated using vacuum and deep ultraviolet pulses and pulse trains obtained from numerical simulations of macroscopic high harmonic spectra.

 

Abstract Progress in ultrafast science allows for probing quantum superposition states with ultrashort laser pulses in the new regime where several linear and nonlinear ionization pathways compete. Interferences of pathways can be observed in the photoelectron angular distribution and in the past they have been analyzed for atoms and molecules in a single quantum state via anisotropy and asymmetry parameters. Those conventional parameters, however, do not provide comprehensive tools for probing superposition states in the emerging research area of bright and ultrashort light sources, such as free-electron lasers and high-order harmonic generation. We propose a new set of generalized asymmetry parameters which are sensitive to interference effects in the photoionization and the interplay of competing pathways as the laser pulse duration is shortened and the laser intensity is increased. The relevance of the parameters is demonstrated using results of state-of-the-art numerical solutions of the time-dependent Schrödinger equation for ionization of helium atom and neon atom. 

High harmonic spectroscopy utilizes the extremely nonlinear optical process of high-order harmonic generation (HHG) to measure complex attosecond-scale dynamics within the emitting atom or molecule subject to a strong laser field. However, it can be difficult to compare theory and experiment, since the dynamics under investigation are often very sensitive to the laser intensity, which inevitably varies over the Gaussian profile of a typical laser beam. This discrepancy would usually be resolved by so-called macroscopic HHG simulations, but such methods almost always use a simplified model of the internal dynamics of the molecule, which is not necessarily applicable for high harmonic spectroscopy. In this Letter, we extend the existing framework of macroscopic HHG so that high-accuracyab initiocalculations can be used as the microscopic input. This new (to the best of our knowledge) approach is applied to a recent theoretical prediction involving the HHG spectra of open-shell molecules undergoing nonadiabatic dynamics. We demonstrate that the predicted features in the HHG spectrum unambiguously survive macroscopic response calculations, and furthermore they exhibit a nontrivial angular pattern in the far field.