Duration of an intense laser pulse can determine the breakage of multiple chemical bonds

Three-body deprotonation fragmentation dynamics of C6H63+ induced by electron-impact ionization

The three-body fragmentation dynamics of benzene trications C6H63+ induced by 200 eV electron-impact produced by a photoemission cathode is investigated. All three fragment ions are detected in coincidence, and their momentum vectors are determined by employing a COLTRIMS reaction microscope. The detailed kinematical information of three deprotonation fragmentation channels of H+ + C3H2+ + C3H3+, H+ + C2H3+ + C4H2+, and H+ + C2H2+ + C4H3+ are obtained. By analyzing the momentum and energy correlation spectra among all the three fragment ions, we find that all the three channels are primarily generated by sequential fragmentation processes. Each channel has two deprotonation pathways, corresponding to proton emission in the first or second step of sequential fragmentation, respectively. These results provide insight into the mechanisms and dynamics of deprotonation and ring-breaking reactions in the three-body fragmentation processes of aromatic ring molecules.

Carrier envelope phase sensitivity of photoelectron circular dichroism

We report on a combined experimental and numerical study of photoelectron circular dichroism (PECD) induced by intense few-cycle laser pulses, using methyloxirane as the molecular example. Our experiments reveal a remarkably pronounced sensitivity of the PECD strength of double-ionization on the carrier-envelope phase (CEP) of the laser pulses. By comparison to the simulations, which reproduce the measured CEP-dependence for specific orientations of the molecules in the lab frame, we attribute the origin of the observed CEP-dependence of PECD to the CEP-induced modulation of ionization from different areas of the wave functions of three dominant orbitals.

Isomerization Dynamics in the Symmetric and Asymmetric Fragmentation of Ethane Dications

Hydrogen- or proton-migration-induced isomerization has recently been of concern for its critical role in the dissociation of organic molecules of astrophysical or biological relevance. Herein we present a combined experimental and theoretical study of the two-body C-C bond breakdown dissociation of ethane dication. For the asymmetric fragmentation channel CH₂⁺ + CH₄⁺, the kinetic energy release measurements and ab initio quantum chemical calculations demonstrate that the reaction pathway involving hydrogen-migration-induced isomerization of [CH₃-CH₃]²⁺ to [CH₂-CH₄]²⁺ can be accessed via the lowest triplet state rather than the ground singlet state of ethane dication. Interestingly, it is found that a considerable proportion of the yield of symmetric fragmentation CH₃⁺ + CH₃⁺, which is usually considered from a direct Coulomb explosion and seemingly independent of isomerization, could come from the dissociation of ethane dication in the ground singlet state with the involvement of [CH₃-CH₃]²⁺ isomerization to intermediate [H₂C(H₂)CH₂]²⁺ of the diborane-like double-bridged structure.

Experimental Separation of Subcycle Ionization Bursts in Strong-Field Double Ionization of H_{2}

We report on the unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase, in combination with multiparticle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state-specific Coulomb distortions on emitted electrons in polyatomic molecules.

Subfemtosecond Tracing of Molecular Dynamics during Strong-Field Interaction

We introduce and experimentally demonstrate a method where the two intrinsic timescales of a molecule, the slow nuclear motion and the fast electronic motion, are simultaneously measured in a photoelectron photoion coincidence experiment. In our experiment, elliptically polarized, 750 nm, 4.5 fs laser pulses were focused to an intensity of 9×10^{14}  W/cm^{2} onto H_{2}. Using coincidence imaging, we directly observe the nuclear wave packet evolving on the 1sσ_{g} state of H_{2}^{+} during its first round-trip with attosecond temporal and picometer spatial resolution. The demonstrated method should enable insight into the first few femtoseconds of the vibronic dynamics of ionization-induced unimolecular reactions of larger molecules.

The importance of Rydberg orbitals in dissociative ionization of small hydrocarbon molecules in intense laser fields

Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which typically have electronic states that are relatively well separated in energy. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C₂D₂, C₂D₄ and C₂D₆ reveals that the photofragment angular distributions can only be understood when the field-dressed orbitals rather than the field-free orbitals are considered. Our measured angular distributions and intensity dependence show that these field-dressed orbitals can have strong Rydberg character for certain orientations of the molecule relative to the laser polarization and that they may contribute significantly to the hydrogen elimination dissociative ionization yield. These findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields.

Strong Field Molecular Ionization in the Impulsive Limit: Freezing Vibrations with Short Pulses

We study strong-field molecular ionization as a function of pulse duration. Experimental measurements of the photoelectron yield for a number of molecules reveal competition between different ionization continua (cationic states) which depends strongly on pulse duration. Surprisingly, in the limit of short pulse duration, we find that a single ionic continuum dominates the yield, whereas multiple continua are produced for longer pulses. Using calculations which take vibrational dynamics into account, we interpret our results in terms of nuclear motion and nonadiabatic dynamics during the ionization process.

Two-pulse control over double ionization pathways in CO2

We visualize and control molecular dynamics taking place on intermediately populated states during different sequential double ionization pathways of CO2 using a sequence of two delayed laser pulses which exhibit different peak intensities. Measured yields of CO2 (2+) and of fragment pairs CO(+)/O(+) as a function of delay between the two pulses are weakly modulated by various vibronic dynamics taking place in CO2 (+). By Fourier analysis of the modulations we identify the dynamics and show that they can be assigned to merely two double ionization pathways. We demonstrate that by reversing the sequence of the two pulses it becomes possible to control the pathway which is taken across CO2 (+) towards the final state in CO2 (2+). A comparison between the yields of CO2 (2+) and CO(+)/O(+) reveals that the modulating vibronic dynamics oscillate out-of-phase with each other, thus opening up opportunities for strong-field fragmentation control on extended time scales.