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Photoemission, in which photons cause atoms or molecules to emit electrons, is not an instantaneous process. The emitted electron may take hundreds of attoseconds to traverse the local potential of its parent atom or molecule after photoabsorption. The exact time is sensitive to the potential landscape the electron encounters, which causes a delay in when the photoelectron’s wave packet emerges from different parts of the molecule. Here, we experimentally explore this delay in the hitherto elusive frame of reference of the molecule, which allows us to observe photoelectron emission relative to specific orientations of the molecule.As a prototype process, we choose the photoionization of the inner valence electron of the NO molecule. During the photoelectron’s excursion, it experiences an asymmetric molecular potential landscape and a dynamical centrifugal potential. We introduce an approach to clock the photoionization dynamics of the NO molecule in its molecular frame. A key finding is an asymmetric photoemission delay difference, around 150 attoseconds, at the opposite ends of the molecule. Our theoretical simulation illustrates that the asymmetric delay originates from the interference between multiple photoionization pathways.Despite being demonstrated in a heteronuclear diatomic molecule, our methods and findings are applicable to the photoemission dynamics in many molecules, surfaces, and interfaces with asymmetric potentials. Our approach opens a new avenue for exploring the attosecond photoelectron dynamics in complex systems and investigating the time-resolved quantum dynamics in solutions, complex materials, and biologic tissues.

We experimentally studied the three-body fragmentation dynamics of a noble gas cluster (ArKr2) upon its multiple ionization by an intense femtosecond laser pulse. The three-dimensional momentum vectors of correlated fragmental ions were measured in coincidence for each fragmentation event. A novel comet-like structure was observed in the Newton diagram of the quadruple-ionization-induced breakup channel of ArKr 2 4 + → Ar+ + Kr+ + Kr2+. The concentrated head part of the structure mainly originates from the direct Coulomb explosion process, while the broader tail part of the structure stems from a three-body fragmentation process involving electron transfer between the distant Kr+ and Kr2+ ion fragments. Due to the field-driven electron transfer, the Coulomb repulsive force of the Kr2+ and Kr+ ions with respect to the Ar+ ion undergoes exchange, leading to changes in the ion emission geometry in the Newton plot. An energy sharing among the separating Kr2+ and Kr+ entities was observed. Our study indicates a promising approach for investigating the strong-field-driven intersystem electron transfer dynamics by using the Coulomb explosion imaging of an isosceles triangle van der Waals cluster system. [ABSTRACT FROM AUTHOR]

We experimentally investigate the dissociative frustrated double ionization of N2Ar dimers exposed to strong laser fields by measuring the ejected charged and excited neutral Rydberg fragments in coincidence. The results show that the tunneled electron is more likely to be recaptured by the argon atomic ion than the nitrogen molecular ion to form the neutral Rydberg fragment in the dissociative frustrated double ionization of N2Ar dimers. It is attributed to the T-shaped molecular structure and the electron-localization-assisted enhanced ionization of the N2Ar dimer. [ABSTRACT FROM AUTHOR]

We report the stereodynamic control of D 3 + formation from the laser-induced bimolecular reaction in a weakly bound D 2 -D 2 dimer via impulsive molecular alignment. Using a linearly polarized moderately intense femtosecond pump pulse, the D 2 molecules in the dimer were prealigned prior to the bimolecular reaction triggered by a delayed probe pulse. The rotationally excited D 2 in the dimer was observed to rotate freely as if it were a monomer. It was demonstrated that the yield of photoreaction product D 3 + is increased or decreased when the molecular axis of D 2 is parallel or perpendicular to the probe laser polarization, respectively. The underlying physics of this steric effect is the alignment-dependent bond cleavage of D 2 + in the dimer induced by a photon-coupled parallel transition.

We experimentally studied the three-body fragmentation dynamics of a noble gas cluster (ArKr 2 ) upon its multiple ionization by an intense femtosecond laser pulse. The three-dimensional momentum vectors of correlated fragmental ions were measured in coincidence for each fragmentation event. A novel comet-like structure was observed in the Newton diagram of the quadruple-ionization-induced breakup channel of ArKr 2 4+ → Ar + + Kr + + Kr 2+ . The concentrated head part of the structure mainly originates from the direct Coulomb explosion process, while the broader tail part of the structure stems from a three-body fragmentation process involving electron transfer between the distant Kr + and Kr 2+ ion fragments. Due to the field-driven electron transfer, the Coulomb repulsive force of the Kr 2+ and Kr + ions with respect to the Ar + ion undergoes exchange, leading to changes in the ion emission geometry in the Newton plot. An energy sharing among the separating Kr 2+ and Kr + entities was observed. Our study indicates a promising approach for investigating the strong-field-driven intersystem electron transfer dynamics by using the Coulomb explosion imaging of an isosceles triangle van der Waals cluster system.

Rather than being freed to the continuum, the strong-field tunneled electrons can make a trajectory driven by the remaining laser fields and have certain probability to be captured by the high lying Rydberg states of the parent atoms or molecules. To explore the effect of molecular orbital on Rydberg state excitation, the ellipticity dependence of Rydberg state yields of N 2 and O 2 molecules are experimentally investigated using cold target recoil ion momentum spectroscopy and are compared with their counterpart atoms Ar and Xe with comparable ionization potentials. We found the generation probability of the neutral Rydberg fragment O 2 * was orders of magnitude higher than that of Xe* due to the butterfly-shaped highest occupied molecular orbital of O 2 . Meanwhile, our experimental and simulation results reveal that it is the initial momentum distribution (determined by the detailed characteristics of orbitals) that finally leads to the tendency that the Rydberg state yield of O 2 (Ar) decreased slower than that obtained for Xe (N 2 ) when the ellipticity of the excitation laser field is increased.

We experimentally investigate the ultrafast photodissociation dynamics of the SO 2 molecule induced by intense ultrashort laser pulses in a pump-probe scheme. Different three-body fragmentation pathways are discriminated using the time-dependent kinetic energy release spectrum with femtosecond time resolution. A nontrivial three-body fragmentation pathway, denoted as the bonding pathway, is unraveled, in which an intermediate fast rotating O 2 molecule is formed before complete fragmentation. The ultrafast chemical bond rearrangement after electron release is tracked in real time. The bonding pathway generally exists in the three-body fragmentation processes induced by strong laser fields of different wavelengths, which is observed in infrared, ultraviolet, and mixed two-color cases. Our findings are significant for understanding the photon-induced ultrafast processes of the SO 2 molecule in atmospheric chemistry.

We experimentally visualize the dissociative frustrated double ionization of hydrogen molecules by using few-cycle laser pulses in a pump-probe scheme, in which process the tunneling ionized electron is recaptured by one of the outgoing nuclei of the breaking molecule. Three internuclear distances are recognized to enhance the dissociative frustrated double ionization of molecules at different instants after the first ionization step. The recapture of the electron can be further steered to one of the outgoing nuclei as desired by using phase-controlled two-color laser pulses. Both the experimental measurements and numerical simulations suggest that the Rydberg atom is favored to emit to the direction of the maximum of the asymmetric optical field. Our results on the one hand intuitively visualize the dissociative frustrated double ionization of molecules, and on the other hand open the possibility to selectively excite the heavy fragment ejected from a molecule.