We performed photoelectron spectroscopy using femtosecond XUV pulses from a free-electron laser and femtosecond near-infrared pulses from a synchronized laser, and succeeded in measuring Ar 3p photoelectron sideband spectra due to the two-color above-threshold ionization. In our calculations of the first-order time-dependent perturbation theoretical model based on the strong field approximation, the photoelectron sideband spectra and their angular distributions are well reproduced by considering the timing jitter between the XUV and the NIR pulses, showing that the timing jitter in our experiments was distributed over the width of + -1.0 0.4 0.2 ps. The present approach can be used as a method to evaluate the timing jitter inevitable in FEL experiments.

The multiphoton double ionization of He was investigated using a soft x-ray free-electron laser (SX-FEL), in the perturbative intensity regime (<1013 W cm−2). The photon energy was tuned to the transition between the 1s and 2p levels of He+ ions. At this resonance energy of 40.8 eV, both sequential three-photon double ionization (S3PDI) and two-photon double ionization (2PDI) were measured by electron spectroscopy and by ion time-of-flight mass spectroscopy. We distinguished S3PDI from 2PDI by electron spectroscopy. Based on these experimental data, SX-FEL pulse-energy dependences of electron and ion yields for both S3PDI and 2PDI were discussed. The angular distribution of 2p photoelectrons emitted from He+(2p) ions in the S3PDI process was examined as an example of the simplest photoemission dynamics.

We performed time-resolved photoelectron spectroscopy of valence orbitals of aligned CO 2 molecules using the femtosecond soft x-ray free-electron laser and the synchronized near-infrared laser. By properly ordering the individual single-shot ion images, we successfully obtained the photoelectron angular distributions (PADs) of the CO 2 molecules aligned in the laboratory frame (LF). The simulations using the dipole matrix elements due to the time dependent density functional theory calculations well reproduce the experimental PADs by considering the axis distributions of the molecules. The simulations further suggest that, when the degrees of alignment can be increased up to cos 0.8 2 q á ñ > , the molecular geometries during photochemical reactions can be extracted from the measured LFPADs once the accurate matrix elements are given by the calculations.

We performed photoelectron spectroscopy using femtosecond extreme-ultraviolet (XUV) pulses from a free-electron laser and femtosecond near-infrared (NIR) pulses from a synchronized laser, and succeeded in measuring photoelectrons from highly excited Rydberg states of molecular ions + CS .2 Such excited states are prepared via a sequential process of photoionization and subsequent photoexcitation during a short pulse of XUV, and then a band of the Rydberg excited states is probed with a synchronized NIR pulse. The photoelectron spectrum has been well explained by our hydrogenic Rydberg orbital model. The present result demonstrates that the two-color experiments of XUV free-electron lasers and NIR laser makes it possible to observe highly excited states of molecular ions. This implies that the present approach provides ones a new spectroscopic tool for studying excited states of various molecular ions.