We observe fragmentation of ${\mathrm{H}}_2$ molecules exposed to strong laser fields into excited neutral atoms. The measured excited neutral fragment spectrum resembles the ionic fragmentation spectrum including peaks due to bond softening and Coulomb explosion. To explain the occurrence of excited neutral fragments and their high kinetic energy, we argue that the recently investigated phenomenon of frustrated tunnel ionization is also at work in the neutralization of ${\mathrm{H}}^{+}$ ions into excited ${\mathrm{H}}^{*}$ atoms. In this process the tunneled electron does not gain enough drift energy from the laser field to escape the Coulomb potential and is recaptured. Calculation of classical trajectories as well as a correlated detection measurement of neutral excited ${\mathrm{H}}^{*}$ and ${\mathrm{H}}^{+}$ ions support the mechanism.

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (k_x, k_y, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å⁻¹ diameter in a binding-energy range of several eV, resolving about 2.5 × 10⁵ data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe₂ measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å⁻¹, and a system response function of 150 fs.

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220–250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

The photodissociation dynamics of CH₃I and CH₂ClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide m/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH₂ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.

We investigate double ionization of argon dimers in high-intensity ultrashort Ti:sapphire laser pulses. We are able to identify several strong-field excitation pathways of the dimer that terminate in atomic ion pairs from a Coulomb explosion. The explosion starts from two-site double-ionized dimers and from one-site double-ionized ones after radiative charge transfer at small internuclear separation. One-site double ionization is accomplished by laser-induced charge transfer in the high-intensity laser pulse following two-site double ionization. The highest energy ion pairs we observed can be attributed to ``frustrated triple ionization'' of the argon dimer.

Strong field single ionization of homo- and heteronuclear noble gas dimers with ultrashort infrared laser pulses is experimentally investigated. A pronounced photoelectron yield maximum is found for dimers in the momentum range $|\mathbf{p}|$\le${}0.1\text{ }\text{ }\mathrm{a}.\mathrm{u}.$ which is absent for the corresponding monomer. This yield enhancement can be attributed to a new two-step strong field ionization mechanism active only in the dimers. In the first step, frustrated tunnel ionization at one of the atomic centers populates Rydberg states, which then become ionized in a second step through charge oscillation within the dimer ion core.