Photoelectron-photoion momentum spectroscopy as a clock for chemical rearrangements: isomerization of the di-cation of acetylene to the vinylidene configuration

Tracking ultrafast non-adiabatic dissociation dynamics of the deuterated water dication molecule

We applied reaction microscopy to elucidate fast non-adiabatic dissociation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV with synchrotron radiation. For the very rare D+ + O+ + D breakup channel, the particle momenta, angular, and energy distributions of electrons and ions, measured in coincidence, reveal distinct electronic dication states and their dissociation pathways via spin-orbit coupling and charge transfer at crossings and seams on the potential energy surfaces. Notably, we could distinguish between direct and fast sequential dissociation scenarios. For the latter case, our measurements reveal the geometry and orientation of the deuterated water molecule with respect to the polarization vector that leads to this rare 3-body molecular breakup channel. Aided by multi-reference configuration-interaction calculations, the dissociation dynamics could be traced on the relevant potential energy surfaces and particularly their crossings and seams. This approach also unraveled the ultrafast time scales governing these processes.

Probing Conical Intersection in the Multipathway Isomerization of CH3Cl Using Coulomb Explosion

Ubiquitous ultrafast isomerization is paramount in photoexcited molecules, in which non-adiabatic coupling among multiple electronic states can occur. We use the pump-probe Coulomb explosion imaging method to study the isomerization of CH3Cl molecules. We find that the isomerization under our strong field pump-probe scheme proceeds along multiple pathways, which are encoded in several distinct branches of the time-resolved kinetic energy release spectra for the CH2++HCl+ Coulomb explosion channel. Apart from the isomerized dissociative pathway in neutral and cationic excited states, the pump laser can also induce coherent vibrational dynamics in two coupled intermediate states and set up the initial conditions for the two concurrently proceeding isomerization pathways. The isomerization of CH3Cl provides an intriguing example of a chemical reaction consisting of multiple pathways and non-adiabatic dynamics.

Sequential mechanism in H3+ formation dynamics on the ethanol dication

Two- and three-body Coulomb explosion dynamics of isolated ethanol dications are studied via single-photon double-ionization with ultrafast extreme-ultraviolet pulses. The measured 3-body momentum correlations obtained via 3D coincidence imaging of the ionic products provide evidence for several concerted and sequential mechanisms: (1) a concerted 3-body breakup mechanism, with dominating channels such as CH₃⁺ + COH⁺ + H₂; (2) sequential dissociation in which the ejection of a low-kinetic-energy neutral OH precedes the Coulomb explosion of C₂H₅²⁺ → CH₃⁺ + CH₂⁺; and (3) a sequential 3-body breakup mechanism that dominates H₃⁺ formation from the ethanol dication via a mechanism that is different from the well-studied H₃⁺ formation in the 2-body Coulomb explosion of the methanol dication. Furthermore, we report surprising branching ratios of the competing C-O bond dissociation channels, resulting in H₃O⁺, H₂O⁺ and OH⁺ formation.

Controlling Fragmentation of the Acetylene Cation in the Vacuum Ultraviolet via Transient Molecular Alignment

An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the consequence of changes in the electronic response with molecular orientation relative to the ionizing field. We show that stable C2H2+ cations are mainly produced when the molecules are parallel or nearly parallel to the FEL polarization, while the hydrogen fragmentation channel (C2H2+ → C2H+ + H) predominates when the molecule is perpendicular to that direction, thus allowing one to distinguish between the two photochemical processes. The experimental findings are supported by state-of-the art theoretical calculations.

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.

Proton migration in hydrocarbons induced by slow highly charged ion impact

Different from most of the previous studies using light or photons, we use highly charged ions as projectiles to activate proton migration in the smallest saturated and unsaturated hydrocarbon molecules, i.e., CH₄ and C₂H₂. The H₃ ⁺ formation channel (H₃ ⁺ + CH⁺) and isomerization channel (C⁺ + CH₂ ⁺), serving as indicators of proton migration, are observed in the fragmentation of CH₄ and C₂H₂ dications. Corresponding kinematical information, i.e., kinetic energy release, is for the first time obtained in the collisions with highly charged ions. In particular, for the C⁺ + CH₂ ⁺ channel, a new pathway is identified, which is tentatively attributed to the isomerization on high-lying states of acetylene dication. The kinetic energy release spectra for other two-body breakup channels are also determined and precursor dication states could thus be identified.

Dependence on the Initial Configuration of Strong Field-Driven Isomerization of C2H2 Cations and Anions

We have investigated the femtosecond laser-induced fragmentation of C₂H₂ ^q ion beam targets in various initial configurations, including acetylene (linear HCCH), vinylidene (H₂CC), and cis/ trans. The initial configuration is shown to have a tremendous impact on the branching ratio of acetylene-like (CH ^q₁ + CH ^q₂) and vinylidene-like (C ^q₁' + CH₂ ^q₂') dissociation of a specific C₂H₂ ^q molecular ion. In particular, whereas C₂H₂⁺ generated from C₂H₂, a linear HCCH target, exhibits comparable levels of acetylene-like and vinylidene-like fragmentation, vinylidene or cis/ trans configuration ion beams preferably undergo vinylidene-like fragmentation, with an acetylene branching ratio ranging from 13.9% to zero.

Site-selective-induced isomerization of formamide

We theoretically demonstrate the possibility to site-selectively induce and track isomerization in formamide by using a femtosecond X-ray-pump/X-ray-probe scheme.

Ultrafast isomerization in acetylene dication after carbon K-shell ionization

Ultrafast proton migration and isomerization are key processes for acetylene and its ions. However, the mechanism for ultrafast isomerization of acetylene [HCCH]²⁺ to vinylidene [H₂CC]²⁺ dication remains nebulous. Theoretical studies show a large potential barrier ( > 2 eV) for isomerization on low-lying dicationic states, implying picosecond or longer isomerization timescales. However, a recent experiment at a femtosecond X-ray free-electron laser suggests sub-100 fs isomerization. Here we address this contradiction with a complete theoretical study of the dynamics of acetylene dication produced by Auger decay after X-ray photoionization of the carbon atom K shell. We find no sub-100 fs isomerization, while reproducing the salient features of the time-resolved Coulomb imaging experiment. This work resolves the seeming contradiction between experiment and theory and also calls for careful interpretation of structural information from the widely applied Coulomb momentum imaging method.

The timescale of isomerization in molecules involving ultrafast migration of constituent atoms is difficult to measure. Here the authors report that sub-100 fs isomerization time on acetylene dication in lower electronic states is not possible and point to misinterpretation of recent experimental results.

A perspective for investigating photo-induced molecular dynamics from within with femtosecond free electron lasers

Photo-induced molecular dynamics can now be investigated using free electron lasers (FELs) whose attributes are unprecedented brightness, few femtosecond pulses duration and in the near future few hundreds of attosecond pulse duration.

Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging

An experimental route to identify and separate geometric isomers by means of coincident Coulomb explosion imaging is presented, allowing isomer-resolved photoionization studies on isomerically mixed samples. We demonstrate the technique on cis/trans 1,2-dibromoethene (C₂H₂Br₂). The momentum correlation between the bromine ions in a three-body fragmentation process induced by bromine 3d inner-shell photoionization is used to identify the cis and trans structures of the isomers. The experimentally determined momentum correlations and the isomer-resolved fragment-ion kinetic energies are matched closely by a classical Coulomb explosion model.

Steering Proton Migration in Hydrocarbons Using Intense Few-Cycle Laser Fields

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to manipulate the movement of nuclei with tailored light within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely, acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics is monitored using coincident 3D momentum imaging spectroscopy and described with a widely applicable quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wave packet by the intense off-resonant laser field.

Visualization of bond rearrangements in acetylene using near single-cycle laser pulses

The migration of hydrogen atoms resulting in the isomerization of hydrocarbons is an important process which can occur on ultrafast timescales. Here, we visualize the light-induced hydrogen migration of acetylene to vinylidene in an ionic state using two synchronized 4 fs intense laser pulses. The first pulse induces hydrogen migration, and the second is used for monitoring transient structural changes via Coulomb explosion imaging. Varying the time delay between the pulses reveals the migration dynamics with a time constant of 54 ± 4 fs as observed in the H⁺ + H⁺ + CC⁺ channel. Due to the high temporal resolution, vibrational wave-packet motions along the CC- and CH-bonds are observed. Even though a maximum in isomerization yield for kinetic energy releases above 16 eV is measured, we find no indication for a backwards isomerization — in contrast to previous measurements. Here, we propose an alternative explanation for the maximum in isomerization yield, namely the surpassing of the transition state to the vinylidene configuration within the excited dication state.

Channel-resolved above-threshold double ionization of acetylene

We experimentally investigate the channel-resolved above-threshold double ionization (ATDI) of acetylene in the multiphoton regime using an ultraviolet femtosecond laser pulse centered at 395 nm by measuring all the ejected electrons and ions in coincidence. As compared to the sequential process, diagonal lines in the electron-electron joint energy spectrum are observed for the nonsequential ATDI owing to the correlative sharing of the absorbed multiphoton energies. We demonstrate that the distinct channel-resolved sequential and nonsequential ATDI spectra can clearly reveal the photon-induced acetylene-vinylidene isomerization via proton migration on the cation or dication states.

Hydrogen migration in formation of NH(A³Π) radicals via superexcited states in photodissociation of isoxazole molecules

Formation of the excited NH(A(3)Π) free radicals in the photodissociation of isoxazole (C3H3NO) molecules has been studied over the 14-22 eV energy range using photon-induced fluorescence spectroscopy. The NH(A(3)Π) is produced through excitation of the isoxazole molecules into higher-lying superexcited states. Observation of the NH radical, which is not a structural unit of the isoxazole molecule, corroborates the hydrogen atom (or proton) migration within the molecule prior to dissociation. The vertical excitation energies of the superexcited states were determined and the dissociation mechanisms of isoxazole are discussed. The density functional and ab initio quantum chemical calculations have been performed to study the mechanism of the NH formation.

Strong-field dissociative double ionization of acetylene

We investigate dissociative double ionization of acetylene, one of the smallest organic molecules yet with a rich electronic structure, in strong laser fields by measuring two fragment ions and two electrons in coincidence. The two-body fragmentation channels are dominated by the removal of electrons from the lower-lying molecular orbitals rather than from the highest occupied one. The electron localization-assisted enhanced ionization mechanism plays a central role for the strong-field deprotonation ionization of acetylene by releasing the second electron from the up-field potential well of the hydrogen site at the internuclear distance near twice the equilibrium value of the C-H bond.

Fundamental role of the H-bond interaction in the dissociation of NH3 on Si(001)-(2×1)

Further insight into the dissociative adsorption of NH3 on Si(001) has been obtained using a combined computational and experimental approach. A novel route leading to the dissociation of the chemisorbed NH3 is proposed, based on H-bonding interactions between the gas phase and the chemisorbed NH3 molecules. Our model, complemented by synchrotron radiation photoelectron spectroscopy measurements, demonstrates that the low temperature dissociation of molecular chemisorbed NH3 is driven by the continuous flux of ammonia molecules from the gas phase.

A study of selected fragmentation paths of the ethyne dication: theory and experiment

The fragmentation of the C(2)H(2)(2+) dication, formed upon inner shell ionization and the subsequent Auger decay, has been studied by means of Auger electron-ion and Auger electron-ion-ion coincidence spectroscopy at four different kinetic energies of the Auger electron. The experimental investigation of three fragmentation paths leading to the C(2)H(+)/H(+), C(2)(+)/H(+) and C(+)/H(+) pairs has been complemented by theoretical calculations of the Potential Energy Surfaces (PES). It is found that when the amount of internal energy of the dication increases this is mainly transferred into the kinetic energy of the fragments of the second step of the dissociation.

Ultrafast extreme ultraviolet induced isomerization of acetylene cations

Ultrafast isomerization of acetylene cations ([HC=CH](+)) in the low-lying excited A(2)Σ(g)(+) state, populated by the absorption of extreme ultraviolet (XUV) photons (38 eV), has been observed at the Free Electron Laser in Hamburg, (FLASH). Recording coincident fragments C(+) + CH2(+) as a function of time between XUV-pump and -probe pulses, generated by a split-mirror device, we find an isomerization time of 52±15 fs in a kinetic energy release (KER) window of 5.8<KER<8 eV, providing clear evidence for the existence of a fast, nonradiative decay channel.

Visualizing recurrently migrating hydrogen in acetylene dication by intense ultrashort laser pulses

We demonstrate the visualization of ultrafast hydrogen migration in deuterated acetylene dication (C2D2{2+}) by employing the pump-probe Coulomb explosion imaging with sub-10-fs intense laser pulses (9 fs, 0.13 PW/cm{2}, 800 nm). It is shown, from the temporal evolution of the momenta of the fragment ions produced by the three-body explosion, C2D2{3+}-->D{+} + C{+} + CD{+}, that the migration proceeds in a recurrent manner: the deuterium atom first shifts from one carbon site to the other in a short time scale (approximately 90 fs) and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation.

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.

Femtosecond coincidence imaging of multichannel multiphoton dynamics

The novel technique of femtosecond time-resolved photoelectron-photoion coincidence imaging is applied to unravel dissociative ionization processes in a polyatomic molecule. Femtosecond coincidence imaging of CF3I photodynamics illustrates how competing multiphoton dissociation pathways can be distinguished, which would be impossible using photoelectron or ion imaging alone. Ion-electron energy correlations and photoelectron angular distributions reveal competing processes for the channel producing (e(-)+CF+3+I). The molecular-frame photoelectron angular distributions of the two major pathways are strikingly different.