Nuclear Dynamics In Molecules Research Articles

Tracing Photoinduced Hydrogen Migration in Alcohol Dications from Time-Resolved Molecular-Frame Photoelectron Angular Distributions.

The recent implementation of attosecond and few-femtosecond X-ray pump/X-ray probe schemes in large-scale free-electron laser facilities has opened the way to visualize fast nuclear dynamics in molecules with unprecedented temporal and spatial resolution. Here, we present the results of theoretical calculations showing how polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) can be used to visualize the dynamics of hydrogen migration in methanol, ethanol, propanol, and isopropyl alcohol dications generated by X-ray irradiation of the corresponding neutral species. We show that changes in the PA-MFPADs with the pump-probe delay as a result of intramolecular photoelectron diffraction carry information on the dynamics of hydrogen migration in real space. Although visualization of this dynamics is more straightforward in the smaller systems, methanol and ethanol, one can still recognize the signature of that motion in propanol and isopropyl alcohol and assign a tentative path to it. A possible pathway for a corresponding experiment requires an angularly resolved detection of photoelectrons in coincidence with molecular fragment ions used to define a molecular frame of reference. Such studies have become, in principle, possible since the first XFELs with sufficiently high repetition rates have emerged. To further support our findings, we provide experimental evidence of H migration in ethanol-OD from ion-ion coincidence measurements performed with synchrotron radiation.

Initial-site characterization of hydrogen migration following strong-field double-ionization of ethanol.

An essential problem in photochemistry is understanding the coupling of electronic and nuclear dynamics in molecules, which manifests in processes such as hydrogen migration. Measurements of hydrogen migration in molecules that have more than two equivalent hydrogen sites, however, produce data that is difficult to compare with calculations because the initial hydrogen site is unknown. We demonstrate that coincidence ion-imaging measurements of a few deuterium-tagged isotopologues of ethanol can determine the contribution of each initial-site composition to hydrogen-rich fragments following strong-field double ionization. These site-specific probabilities produce benchmarks for calculations and answer outstanding questions about photofragmentation of ethanol dications; e.g., establishing that the central two hydrogen atoms are 15 times more likely to abstract the hydroxyl proton than a methyl-group proton to form H[Formula: see text] and that hydrogen scrambling, involving the exchange of hydrogen between different sites, is important in H2O+ formation. The technique extends to dynamic variables and could, in principle, be applied to larger non-cyclic hydrocarbons.

Chemistry without the Born-Oppenheimer approximation.

Chemistry without the Born-Oppenheimer approximation.

A pump–probe scheme with a single chirped pulse to image electron and nuclear dynamics in molecules

A single chirped few-femtosecond pulse can be used to control and image coupled electron-nuclear dynamics. Using full ab initio simulations of the simplest molecule, as a prototype target, we show that for intermediate values of the chirp, interference between sequential and direct contributions enables significant control over ionization yields, even when taking into account the effective decoherence introduced by nuclear motion and the presence of an electronic continuum. For larger values of the chirp, the single chirped pulse reproduces a classical pump–probe setup, with the chirp parameter mapping an effective time delay between the pumping and probing frequencies of the pulse. After demonstrating this numerically, we present a full analytical solution for the two-photon ionization amplitudes that provides an intuitive analogy between the molecular dynamics induced by a single chirped pulse and a traditional pump–probe setup.

Control of molecular dissociation by spatially inhomogeneous near fields

Tailored intense laser fields can be used to steer electron and nuclear dynamics in molecules and control chemical reactions. Here we demonstrate that inhomogeneous nanoscopic near fields provide an additional mechanism for controlling light-induced chemical reactions. This is investigated by quantum-dynamical calculations on dissociation of H-2(+) in few-cycle near fields with a controlled carrier-envelope phase. Directional dissociation asymmetry analysis reveals that the spatial asymmetry of the field is critical in controlling and in modifying field-induced dissociation pathways such as zero-photon dissociation, bond softening, and above-threshold dissociation. The degree of impact strongly depends on the maximum length scale of coherent control in a dissociation channel. The results pave the way towards coherent control of molecular reactions in a wide range of molecules in inhomogeneous near fields.

Disentangling Strong-Field Multielectron Dynamics with Angular Streaking.

The study into the interaction between a strong laser field and atoms/molecules has led to significant advances in developing spectroscopic tools in the attosecond time-domain and methods for controlling chemical reactions. There has been great interest in understanding the complex electronic and nuclear dynamics of molecules in strong laser fields. However, it is still a formidable challenge to fully model such dynamics. Conventional experimental tools such as photoelectron spectroscopy encounter difficulties in revealing the involved states because the electron spectra are largely dictated by the property of the laser field. Here, with strong field angular streaking technique, we measure the angle-dependent ionization yields that directly reflect the symmetry of the ionizing orbitals of methyl iodide and thus reveal the ionization/dissociation dynamics. Moreover, kinematically complete measurements of momentum vectors of all fragments in dissociative double ionization processes allow access to electron-momentum correlations that reveal correlated multielectron dynamics.

Monitoring Nonadiabatic Electron-Nuclear Dynamics in Molecules by Attosecond Streaking of Photoelectrons.

Streaking of photoelectrons has long been used for the temporal characterization of attosecond extreme ultraviolet pulses. When the time-resolved photoelectrons originate from a coherent superposition of electronic states, they carry additional phase information, which can be retrieved by the streaking technique. In this contribution we extend the streaking formalism to include coupled electron and nuclear dynamics in molecules as well as initial coherences. We demonstrate how streaked photoelectrons offer a novel tool for monitoring nonadiabatic dynamics as it occurs in the vicinity of conical intersections and avoided crossings. Streaking can provide high time resolution direct signatures of electronic coherences, which affect many primary photochemical and biological events.

Multidimensional resonant nonlinear spectroscopy with coherent broadband x-ray pulses

New x-ray free electron laser (XFEL) and high harmonic generation (HHG) light sources are capable of generating short and intense pulses that make x-ray nonlinear spectroscopy possible. Multidimensional spectroscopic techniques, which have long been used in the nuclear magnetic resonance, infrared, and optical regimes to probe the electronic structure and nuclear dynamics of molecules by sequences of short pulses with variable delays, can thus be extended to the attosecond x-ray regime. This opens up the possibility of probing core-electronic structure and couplings, the real-time tracking of impulsively created valence-electronic wavepackets and electronic coherences, and monitoring ultrafast processes such as nonadiabatic electron-nuclear dynamics near conical-intersection crossings. We survey various possible types of multidimensional x-ray spectroscopy techniques and demonstrate the novel information they can provide about molecules.

Revealing correlated electronic and nuclear dynamics in molecules with energy-resolved population imaging

We explore a new fashion, named energy-resolved population image (EPI), to represent on an equal footing the temporary electronic transition and nuclear motion during laser-molecular interaction. By using the EPI we have intuitively demonstrated the population transfer in vibrational H$_2^+$ exposed to extreme ultraviolet pulses, revealing the energy sharing rule for the correlated electron and nuclei. We further show that the EPI can be extended to uncover the origins of the distinct energy sharing mechanisms in multi-photon and tunneling regimes. The present study has clarified a long-standing issue about the dissociative ionization of H$_2^+$ and paves the way to identify instantaneous molecular dynamics in strong fields.

Molecular interferometer to decode attosecond electron–nuclear dynamics

Understanding the coupled electronic and nuclear dynamics in molecules by using pump-probe schemes requires not only the use of short enough laser pulses but also wavelengths and intensities that do not modify the intrinsic behavior of the system. In this respect, extreme UV pulses of few-femtosecond and attosecond durations have been recognized as the ideal tool because their short wavelengths ensure a negligible distortion of the molecular potential. In this work, we propose the use of two twin extreme UV pulses to create a molecular interferometer from direct and sequential two-photon ionization processes that leave the molecule in the same final state. We theoretically demonstrate that such a scheme allows for a complete identification of both electronic and nuclear phases in the wave packet generated by the pump pulse. We also show that although total ionization yields reveal entangled electronic and nuclear dynamics in the bound states, doubly differential yields (differential in both electronic and nuclear energies) exhibit in addition the dynamics of autoionization, i.e., of electron correlation in the ionization continuum. Visualization of such dynamics is possible by varying the time delay between the pump and the probe pulses.

Monitoring long-term evolution of molecular vibrational wave packet using high-order harmonic generation

We present a method for probing ultrafast nuclear dynamics in molecules using transient enhancement of high-order harmonic generation (HHG). The method exploits strong dependence of the overall harmonic yield in a fixed spectral range on the nuclear separation in a molecule, which is shown to take place for both aligned and randomly oriented molecules. Our numerical simulations show that this method is capable of monitoring long-term evolution of the nuclear vibrational wave packets in molecules, even in light-weight ones, with very high time resolution. By the example of D2+ vibrational wave packet launched via tunnelling ionization of D2 and probed by a time-delayed 8 fs laser pulse with λ = 800 nm and the peak intensity of 1014 W cm-2, we show that the time-delay dependence of the high harmonic signal exhibits pronounced features, which are due to the collapses and revivals of the nuclear wave packet. The time-frequency analysis of the pump–probe signal reveals structures with a periodicity down to 6 fs, which correspond to the fractional revivals of orders 1/5 and 1/10. With the laser parameters used, the deuteron motion during the probe pulse is shown to have almost negligible effect on the resulting signal.

Electron-diffraction imaging of nuclear dynamics in molecules

We investigate the formation of the diffraction image of a molecule arising in the rescattering process during field-induced molecular ionization. The problem is solved numerically by integration of the non-stationary two-electron Schrödinger equation. The observed diffraction patterns are used to extract the information about molecular orientation and internuclear separation. The validity of a simple double slit diffraction rule is examined, and additional phase corrections are obtained to provide high accuracy. The role of the presence of the bound electron in the parent molecular ion in the scattering process is analysed.

Theoretical Investigations of the Electronic and Nuclear Dynamics of Molecules in Intense Laser Fields: Quantum Mechanical Wave Packet Approaches

Abstract We have developed a method for describing the reaction dynamics of a polyatomic molecule in intense laser fields. First, the dynamical behavior of H2+ and H2 in near-infrared, intense laser fields (I > 1013 W cm−2 and λ > 700 nm) was examined; accurate evaluation of the electronic and nuclear wave packet was achieved by the dual transformation method that we developed. Using “field-following” time-dependent adiabatic states defined as eigenfunctions of the “instantaneous” electronic Hamiltonian, we have clarified the dynamics of bound electrons, ionization processes, Coulomb explosion processes, and molecular vibrations of H2+ and H2. The analyses indicate that the multielectron dynamics and nuclear dynamics of polyatomic molecules in intense fields can be described by using the potential surfaces of time-dependent adiabatic states and the nonadiabatic coupling elements between those states. To obtain time-dependent adiabatic states of a molecule, one can diagonalize the electronic Hamiltonian including the interaction with the instantaneous laser electric field by ab initio molecular orbital (MO) methods. The time-dependent adiabatic potentials obtained are used to evaluate the multichannel nuclear dynamics until the next ionization process. We have applied the time-dependent adiabatic state approach to reveal the characteristic features of the dynamics of structural deformations of CO2 and its cations in a near-infrared intense laser field. The experimentally observed stretched and bent structure of CO23+ just before Coulomb explosions originates from the structural deformation of CO22+. We also revealed the mechanism of the experimentally observed bond dissociation of C2H5OH; we found that the relative probability of C–O bond cleavage to that of C–C bond cleavage becomes smaller with decreases in the pulse length. This example clearly shows that field-induced nonadiabatic transitions play a decisive role in the reaction dynamics of molecules in an intense laser field.

Quantum mechanical study of electronic and nuclear dynamics of molecules in intense laser fields

Abstract The dynamical behaviour of H 2 + in near-infrared, intense laser fields ( I >10 13 W/cm 2 and λ >700 nm) is examined with accurate evaluation of the electronic and nuclear wave packet by the dual transformation method we developed to deal with the dynamics in Coulombic systems. Using “field-following” time-dependent adiabatic states defined as eigenfunctions of the “instantaneous” electronic Hamiltonian, we have clarified the dynamics of the bound electron, ionization, Coulomb explosion, and molecular vibration of H 2 + . The present analysis of H 2 + indicates that the multi-electron dynamics and nuclear dynamics of polyatomic molecules in intense fields can be described by using the potential surfaces of time-dependent adiabatic states and the nonadaiabatic coupling elements between those states. To obtain time-dependent adiabatic states of a molecule, the electronic Hamiltonian including the interaction with the instantaneous laser electric field is diagonalized by ab initio molecular orbital (MO) methods. While the time-dependent adiabatic potentials calculated by MO methods are used to evaluate the multichannel nuclear dynamics until the next ionization process, the charge distributions on individual atomic sites can be used to judge whether ionization occurs or not at a given light intensity. We have applied the time-dependent adiabatic state approach to reveal characteristic features of the dynamics of structural deformations of CO 2 and its cations in a near-infrared intense laser field (∼10 15 W cm −2 ). In the CO 2 and CO 2 + stages, ionization occurs before the field intensity becomes high enough to deform the molecule. In the CO 2 2+ stage, simultaneous symmetric two-bond stretching occurs as well as one-bond stretching. Two-bond stretching is induced by an intense field in the lowest time-dependent adiabatic state |1〉 of CO 2 2+ , and this two-bond stretching is followed by the occurrence of a large-amplitude bending motion mainly in the second-lowest adiabatic state |2〉 nonadiabatically created from |1〉 at large internuclear distances by the field. It is concluded that the experimentally observed stretched and bent structure of CO 2 3+ just before Coulomb explosions originates from the structural deformation of CO 2 2+ . We also applied the present approach to investigate dissociative ionization of C 2 H 5 OH in near-infrared, intense laser fields. The calculated ratio of the probability of C–O bond cleavage to that of C–C bond cleavage becomes smaller with decreases in the pulse length, as was observed in the experiment by Itakura et al. [J. Chem. Phys. 119 (2003) 4179]. In the case where the C–C axis is parallel to the polarization direction of an applied laser field, upper C–C bond dissociative adiabatic states are created owing to field-induced nonadiabatic transitions even if the pulse length is as short as 30 fs. This example clearly shows that field-induced nonadiabatic transition plays a decisive role in the reaction dynamics of molecules in intense laser field.

Quantum mechanical study of electronic and nuclear dynamics of molecules in intense laser fields

Quantum mechanical study of electronic and nuclear dynamics of molecules in intense laser fields

Molecular photodissociation dynamics: The time-dependent formulation

The time-dependent formulation for nuclear dynamics in molecules induced by electronic excitation in a radiation field is reviewed. The present discussion is especially aiming at extracting physical observables for photodissociation and highlighting the connection to the nuclear dynamics of the process. The total dissociation probability, the probability associated with the formation of a given chemical product, and the probability that this product shows up in a specified quantum state is considered. The results are given as a function of the form of the light pulse, and special attention is given to situations where the duration of the light pulse is very short or very long.

A perturbational approach to the calculation of correlation functions describing short-time nuclear dynamics in molecules

A method on stationary perturbation theory is proposed to calculate the correlation functions (CFs) describing short-time nuclear dynamics in molecules