XUV-Initiated Dissociation Dynamics of Molecular Oxygen (O₂)

Abstract

We performed a time-resolved spectroscopy experiment on the dissociation of oxygen molecules after the interaction with intense extreme-ultraviolet (XUV) light from the free-electron laser in Hamburg at Deutsches Elektronen-Synchrotron. Using an XUV-pump/XUV-probe transient-absorption geometry with a split-and-delay unit, we observe the onset of electronic transitions in the O 2+ cation near 50 eV photon energy, marking the end of the progression from a molecule to two isolated atoms. We observe two different time scales of 290 ± 53 and 180 ± 76 fs for the emergence of different ionic transitions, indicating different dissociation pathways taken by the departing oxygen atoms. With regard to the emerging opportunities of tuning the central frequencies of pump and probe pulses and of increasing the probe–pulse bandwidth, future pump–probe transient-absorption experiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociation.

Time-resolving state-specific molecular dissociation with XUV broadband absorption spectroscopy

“…The v = 1 tunneling dissociation is expected to take place on a femtosecond timescale (24,25), which is not resolved in the present experiment. In addition, these processes can be attributed to O þ 2 ðB 2 Σ À g Þ dissociation into the first dissociation channel (41), and two pump-photon processes and respective O + +O + dissociation (14), which are all known to dissociate faster than our 5-ps resolution. For all resonance lines, we find this fitted timescale to be τ fast ≲ 1 ps.…”

“…the “Fitting procedure of resonant lineouts Δ OD ( t, E r )” section], which gives an estimate of the combined effects of (i) both individual FEL and HHG pulse durations and (ii) the temporal jitter and drifts between the pulses. Potential electronic configuration changes during fragmentation, which would lead to shifts in the measured resonance positions, are expected to be on the same order of magnitude or faster ( 14 ) and thus cannot be resolved in this experiment. In addition, the position t 0 of the complementary error function allows for an in situ determination of the temporal overlap of FEL and HHG pulses.…”

“…The understanding of processes in chemically reactive environments, for example, atmospheric or biological processes (4)(5)(6)(7)(8)(9)(10)(11), is facilitated by their sensitivity to electronic structure and atomic sites (5)(6)(7) and the capabilities of time-resolved experiments (12). The oxygen molecule is a key component in such environments and its photodissociation is thus an active area of research (13)(14)(15)(16). Tunable extreme ultraviolet (XUV) free-electron lasers (FELs) (17) provide a unique tool to trigger electronic or molecular dynamics (18) by preparing specific initial states, but mostly lack the spectral bandwidth to detect all relevant resonances and fragments at the same time (14,19).…”

“…The oxygen molecule is a key component in such environments and its photodissociation is thus an active area of research (13)(14)(15)(16). Tunable extreme ultraviolet (XUV) free-electron lasers (FELs) (17) provide a unique tool to trigger electronic or molecular dynamics (18) by preparing specific initial states, but mostly lack the spectral bandwidth to detect all relevant resonances and fragments at the same time (14,19). In contrast, high-order harmonic generation (HHG) pulses (20) can be used as XUV broadband probes in transient absorption spectroscopy (21) to simultaneously detect several neutral and ionic fragments and chemical shifts within a molecule, but up to now have only been used in combination with pump pulses at lower (optical) frequencies (22), and thereby lose the benefit of a state-and/or site-specific excitation.…”

FLASH and the FLASH2020+ project—current status and upgrades for the free-electron laser in Hamburg at DESY

“…Understanding of the mechanisms in intense short wavelength light-matter interaction has been obtained in split and delay experiments. [25][26][27][28][29][30].…”