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This works shows a new approach to study the electron recapture process during double and multiple ionization of atoms. This scheme is applicable to non-dissociative processes in molecules, which are not accessible with existing methods based on the measurement of kinetic energy released from fragmentation processes. The experiments show a transition of frustrated double ionization in argon from a non-sequential scenario to a sequential scenario.

We report on an experimental and theoretical study of the ionization-fragmentation dynamics of argon dimers in intense few-cycle laser pulses with a tagged carrier-envelope phase. We find that a field-driven electron transfer process from one argon atom across the system boundary to the other argon atom triggers sub-cycle electron-electron interaction dynamics in the neighboring atom. This attosecond electron-transfer process between distant entities and its implications manifest themselves as a distinct phase-shift between the measured asymmetry of electron emission curves of the $\text{Ar}^{+}+\text{Ar}^{2+}$ and $\text{Ar}^{2+}+\text{Ar}^{2+}$ fragmentation channels. Our work discloses a strong-field route to controlling the dynamics in molecular compounds through the excitation of electronic dynamics on a distant molecule by driving inter-molecular electron-transfer processes.

We report on the results of a joint experimental and numerical study on the sub-cycle laser field-driven electron dynamics that underlie the population of highly excited electronic states in multiply ionized argon dimers by electron recapture processes. Our experiments using few-cycle laser pulses with a known carrier-envelope phase (CEP) in combination with reaction microscopy reveal a distinct CEP-dependence of the electron emission and recapture process and, furthermore, a small but significant CEP-offset to the scenario in which no excited argon dimers are produced. With the help of classical ensemble trajectory simulations we trace down these different CEP-dependencies to subtle differences in the laser-driven sub-cycle electron trajectory dynamics that involve in both cases the transfer of an electron from one argon ion across the system boundary to the neighboring ion and its transient capture on this ion.

We experimentally investigate dissociation of ${\mathrm{H}}_{2}^{+}$ in the (near-)zero proton energy region. Based on our experimental results we conclude that the underlying mechanism for the production of protons with such low energy in strong two-color and broadband laser fields is a stimulated Raman-scattering-based zero-photon-dissociation process, ${\mathrm{ZPD}}_{\text{stR}}$, taking place on the electronic ground state. It is furthermore shown that in the (near-)zero energy region the asymmetry in proton ejection induced by asymmetric laser fields is due to the interplay of several processes, rather than only pathway interferences, with vibrational trapping (or bond hardening) taking a key role.

We report kinematically complete measurements of frustrated double ionization of argon atoms with a reaction microscope. Experimental results show much higher electron trapping probability during the strong-field double ionization than that during the single ionization.

We demonstrate kinematically complete measurements on frustrated double ionization of argon atoms in strong laser fields with a reaction microscope. We found that the electron trapping probability after strong field double ionization is much higher than that after strong field single ionization, especially in case of high laser intensity. The retrieved electron momentum distributions of frustrated double ionization show a clear transition from the nonsequential to the sequential regime, similar to those of strong field double ionization. The dependence of electron momentum width on the laser intensity further indicates that the second released electron has a dominant contribution to frustrated double ionization in the sequential regime.

Laser-ionization of molecules is one of the most important processes in the strong-field and attosecond sciences. One of the key mechanisms governing molecular ionization is enhanced ionization, where for diatomic molecules the ionization probability becomes strongly enhanced at a critical internuclear distance [1, 2]. Experiments on polytatomic molecules have revealed remarkably high charge states and indicated the existence of a different ionization-enhancement mechanism [3-5]. However, until now this mechanism has not been fully understood and has been a subject of intense debate.

On the Earth, oxygen molecules are generated mostly via the photosynthesis by green plants and algae from carbon dioxide and water: nCO 2 + nH 2 O light→ (CH 2 O)n + nO 2 [1]. Although, theoretical studies showed the possibility of O 2 production via CO 2 fragmentation [2] and experimental investigations achieved the detection of C+ (referring the creation of O 2 on the other side) [3], the direct observation of O2 formation did not yet happen to our knowledge. In this submission, we report the direct experimental observation of molecular oxygen formation from carbon dioxide molecule after being doubly ionized by a strong laser pulse.

We report experiments on the direct observation of molecular oxygen formation from ${\mathrm{CO}}_{2}$ in strong laser fields with a reaction microscope. Our accompanying simulations and pump-probe measurements suggest that ${\mathrm{CO}}_{2}$ molecules undergo bending motion during strong-field ionization which supports the molecular oxygen formation process. The observation of molecular oxygen formation from ${\mathrm{CO}}_{2}$ may trigger further experimental and theoretical studies on such processes with laser pulses, and provides hints in studies of the ${\mathrm{O}}_{2}$ and ${\mathrm{O}}_{2}{}^{+}$ abundance in ${\mathrm{CO}}_{2}$-dominated planetary atmospheres.

C

We experimentally investigated a delayed fragmentation of hydrocarbon molecules on nanosecond to microsecond timescale after double ionization by femtosecond laser pulses. Quantum chemical simulations suggest that it originates from meta-stable high-lying vibrational states of dications.

Single and double electron recapture processes into Rydberg states driven by few-cycle laser pulses are studied for argon dimers. The electron emission and recapture dynamics show a strong dependence on the pulses’ carrier-envelope phase.

C7H4O2+ and C7H5O2+ ions and the respective neutrals have been investigated by absorption spectroscopy in neon matrixes following mass selection of ions produced from salicylic acid. Three electronic transitions starting at 649.6, 431.0, and 372.0 nm are detected for C7H4O2+ and assigned on the basis of CASPT2 energies and Franck–Condon simulations as the excitations from the X 2A″ to the 1 2A″, 2 2A″, and 3 2A″ electronic states of 6-(oxomethylene)-2,4-cyclohexadien-1-one ion (A+). Absorptions commencing at 366.4 nm are observed for C7H5O2+ and assigned to the 1 2A′ ← X 2A′ electronic transition of (2-hydroxyphenyl)methanone ion (J+). Neutralization of J+ leads to the appearance of four absorption systems attributed to the 4 2A″, 3 2A″, 2 2A″, and 1 2A″ ← X 2A″ transitions of J with origin bands 291.3, 361.2, 393.8, and 461.2 nm.

Using a double-pulse scheme we visualize and control molecular dynamics taking place on intermediately populated states during different sequential double ionization pathways of CO2. Exchanging the pulse-sequence can almost completely switch the pathway.

We demonstrate the detection of high-lying Rydberg states produced in strong laser fields with coincidence spectroscopy. Electron emission after the interaction of strong laser pulses with atoms and molecules is measured together with the parent ions in coincidence measurements. These electrons originate from high-lying Rydberg states with quantum numbers from $n\ensuremath{\sim}20$ up to $n\ensuremath{\lesssim}120$ formed by frustrated field ionization. Ionization rates are retrieved from the measured ionization signal of these Rydberg states. Simulations show that both tunneling ionization by a weak dc field and photoionization by blackbody radiation contribute to delayed electron emission on the nano- to microsecond scale. Furthermore, the dependence of the Rydberg-state production on the ellipticity of the driving laser field indicates that such high-lying Rydberg states are populated through electron recapture. The present experiment provides detailed quantitative information on Rydberg production in strong-field interaction.

We experimentally and theoretically investigated the deprotonation process on nanosecond to microsecond timescale in ethylene and acetylene molecules, following their double ionization by a strong femtosecond laser field. In our experiments we utilized coincidence detection with the reaction microscope technique, and found that both the lifetime of the long-lived ethylene dication leading to the delayed deprotonation and the relative channel strength of the delayed deprotonation compared to the prompt one have no evident dependence on the laser pulse duration and the laser peak intensity. Quantum chemical simulations suggest that such delayed fragmentation originates from the tunneling of near-dissociation-threshold vibrational states through a dissociation barrier on a dication electronic state along C--H stretching. Such vibrational states can be populated through strong field double ionization induced vibrational excitation on an electronically excited state in the case of ethylene, and through intersystem crossing from electronically excited states to the electronic ground state in the case of acetylene.

The alignment-dependent ionization of acetylene and ethylene in short laser pulses is investigated in the framework of the time-dependent density-functional theory coupled with Ehrenfest dynamics. The molecular alignment is found to have a substantial effect on the total ionization. Bond stretching is shown to cause an increase of the ionization efficiency, i.e., enhanced ionization, in qualitative agreement with previous theoretical investigations. It is also demonstrated that the enhanced ionization mechanism greatly enhances the ionization from the inner valence orbitals, and the ionization of the inner orbitals is primarily due to their extended weakly bound density tails.

We experimentally and theoretically demonstrate channel-selective control over strong-field induced fragmentation of a polyatomic molecule, acetylene, using impulsive laser alignment as the control mechanism.

We report experiments on the direct observation of O2 formation from CO2 in strong laser fields. Our simulations and pump-probe measurements suggest that CO2 undergos bending motion during strong-field ionization which supports the O2 formation.

We investigate the possibility of using molecular alignment for controlling the relative probability of individual reaction pathways in polyatomic molecules initiated by electronic processes on the few-femtosecond time scale. Using acetylene as an example, it is shown that aligning the molecular axis with respect to the polarization direction of the ionizing laser pulse does not only allow us to enhance or suppress the overall fragmentation yield of a certain fragmentation channel but, more importantly, to determine the relative probability of individual reaction pathways starting from the same parent molecular ion. We show that the achieved control over dissociation or isomerization pathways along specific nuclear degrees of freedom is based on a controlled population of associated excited dissociative electronic states in the molecular ion due to relatively enhanced ionization contributions from inner valence orbitals.

We experimentally observed high-lying (n>238) Rydberg states which are formed through recapture of electrons after strong field interaction in atoms and small molecules using electron and ion coincidence detection. The high-lying Rydberg states are identified through weak DC field-induced ionization.

We report experimental observation of "slow" fragmentation from ethylene molecules on nanosecond or even microsecond timescale after their interaction with femtosecond laser pulses. Our results indicate that such fragmentation process originates from meta-stable high-lying vibrational states of ethylene ions.

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.

We show that the interplay of electron removal and nuclear dynamics on the few femtosecond range, controlled by the duration of few-cycle laser pulses, determines the ratio of C2H43+ fragmentation into two respectively three moieties.

Laser-induced electron excitation is a general process in strong laser field interactions with atoms and molecules. Previous studies revealed that up to 10% released electrons can be populated into high-lying excited states through electron trapping by the Coulomb potential of their parent ions [1,2]. In case of dissociative double ionization of molecules, one of the two liberated electrons may be trapped and localized to one of the ionic fragments. This process is referred as dissociative frustrated double ionization. So far, researches about dissociative frustrated double ionization mainly focused on homonuclear diatomic molecules, such as H 2 , D 2 and Ar 2 [3–5]. In this work, we investigate electron dynamics in dissociative frustrated double ionization of CO 2 . The non-symmetry two-body dissociation of CO 2 allows us to study not only the electron trapping process itself but also electron localization to different ionic fragments and the importance of such electron dynamics to laser-induced molecular dissociation.

We observed high-lying (n>238) Rydberg states which survived from strong field interaction of atoms and molecules with electron and ion coincidence detection. The high-lying Rydberg states are measured through field ionization with weak DC fields.