Reportedly for the first time, scientists have combined a broadband extreme-ultraviolet (EUV) probe pulse from high-order harmonic generation (HHG) with a free-electron laser (FEL) pump pulse to observe photodissociation pathways leading to fragments in different quantum states. The team, led by Christian Ott at the Max-Planck-Institut für Kernphysik, temporally resolved a quantum mechanical dissociation mechanism of a specific O2+ state into two competing channels by measuring the resonances of ionic and neutral fragments. The ability to initiate targeted electronic or molecular processes while performing an independent read-out of quantum-mechanical state information about a molecule or its individual fragments, using two different EUV light sources, could enable a better understanding and greater control of complex chemical reactions to light. Specifically, an increased understanding of the tunneling processes of charge carriers could be relevant to the production of attosecond pulses via HHG, the properties of transistors, semiconductors, and 2D materials, as well as to chemical reactions and charge transfer in proteins. Also, the team’s approach could be used to investigate convoluted dynamics in larger molecules, which are important to a diverse array of scientific fields. An EUV laser pulse (pink) excites an oxygen molecule (orange). The molecule dissociates into different atomic fragments, which can be 'photographed' by another EUV laser pulse (blue). Courtesy of the Max-Planck-Institut für Kernphysik. Unlike visible, UV, and IR light, EUV light is absorbed by the atmosphere before it can reach Earth’s surface. However, EUV light can be generated in a laboratory to selectively excite electrons and induce chemical reaction processes that do not occur naturally. The researchers generated laser pulses using HHG, a process that converts IR light to EUV by guiding the light through a gas cell. They also used an FEL to emit EUV light via accelerated electrons. Both lasing methods can generate EUV pulses with a duration of just femtoseconds, and both methods have their strengths and weaknesses. FELs can trigger electronic or molecular dynamics by preparing specific initial states, but they lack the spectral bandwidth to detect all relevant resonances and fragments at the same time. In contrast, HHG pulses can be used as EUV broadband probes in transient absorption spectroscopy to simultaneously detect several neutral and ionic fragments and chemical shifts within a molecule. “The HHG pulses have a very broad spectrum, which means they consist of light with many different frequencies. In the visible range, this could be understood as different colors,” researcher Alexander Magunia said. “The FEL pulses, on the other hand, are much more limited spectrally.” The researchers generated the FEL pulses at the FEL in Hamburg (FLASH@DESY) and used them to excite the electrons of the oxygen molecule into a state that is scientifically known to cause the molecule to dissociate via two different channels. The speed at which dissociation happens, however, has been unclear until now, because the atoms in the molecule must go through a quantum tunneling process. The team combined the strengths of the FEL pump and HHG probe pulses in all-EUV transient absorption spectroscopy to clock the fragmentation of the O2 state. By adding the second HHG pulse with an adjustable time delay to the first exciting FEL pulse, the researchers were able to record molecular dissociation experimentally. Like a fast photo series, the HHG pulses enabled the researchers to “photograph” all the resulting fragments at once through their spectral absorption fingerprints. The researchers observed that the bigger the time delay between the two pulses, the more molecules had already decayed. This increase in fragments made it possible for them to determine the duration of the process and the respective rates for the two decay channels. The results show how state-specific, ultrafast molecular dynamics can be extracted with spectrally and temporally resolved FEL pump-HHG probe transient absorption spectroscopy. They provide insight into state-specific molecular breakup, including experimentally distinguishing both competing dissociation channels and determining the dissociation time, which is strongly influenced by the interplay of the parallel tunneling and predissociation channel. In the future, the researchers’ approach could be applied to molecular systems, allowing precision tests of state-of-the-art quantum dynamics theory in small molecules and time-resolving state-specific molecular dynamics in more complex systems with a broad, dynamic range from nanoseconds to femtoseconds. It could be used to experimentally address questions about intermediate states or electronic changes faster than, or in interplay with, structural dynamics. Furthermore, the approach could aid the investigation of electronic charge transfer within intact neutral molecules, extending previous studies to a higher (i.e., EUV) photon energy or to more complex systems covering several atomic sites. The research was published in Science Advances (www.doi.org/10.1126/sciadv.adk1482).
