Ultrafast Hydrogen Migration Research Articles

Ionization and ultrafast hydrogen migration of methylamine in few-cycle intense near-infrared laser fields

Double ionization and ultrafast hydrogen migration of methylamine induced by a few-cycle intense near-infrared laser pulse are investigated by the coincidence momentum imaging method. By reference to the results obtained by DFT calculations, the peak profiles in the kinetic energy release distributions have been assigned to the singlet and triplet decomposition channels of CH3NH22+. The anisotropy in the angular distribution of the ejection direction of the fragment ions is interpreted based on the symmetries of the (7a’) HOMO orbital. By the pump–probe measurements, it is revealed that the ultrafast hydrogen migration in CH3NH2+ proceeds within ∼ 30 fs.

Sequential deprotonation of the allene trication produced by 30-keV/u He2+ impact

The three-body fragmentation channel ${\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_{3}{\mathrm{H}}_{2}{}^{+}$ of the allene trication produced by 30-keV/u ${\mathrm{He}}^{2+}$ ion impact is investigated using cold-target recoil-ion momentum spectroscopy. The two protons with different kinetic energies are found emitted sequentially from the parent trication. The presence of such a sequential deprotonation process and the absence of corresponding concerted deprotonation may result from a major molecular deformation process, e.g., ultrafast hydrogen migration, which occurs in the parent ion prior to fragmentation, and finally leads to the formation of two different protons.

Imaging intramolecular hydrogen migration with time- and momentum-resolved photoelectron diffraction.

Imaging ultrafast hydrogen migration with few- or sub-femtosecond time resolution is a challenge for ultrafast spectroscopy due to the lightness and small scattering cross-section of the moving hydrogen atom. Here we propose time- and momentum-resolved photoelectron diffraction (TMR-PED) as a way to overcome limitations of existing methodologies and illustrate its performance in the ethanol molecule. By combining different theoretical methods, namely molecular dynamics and electron scattering methods, we show that TMR-PED, along with a judicious choice of the reference frame for multi-coincidence detection, allows for direct imaging of single and double hydrogen migration in doubly-charged ethanol with both few-fs and Å resolutions, all the way from its birth to the very end. It also provides hints of proton extraction following H2 roaming. The signature of hydrogen dynamics shows up in polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) as moving features that allow for a straightforward visualization in space.

Wave packet bifurcation in ultrafast hydrogen migration in CH3OH+ by pump-probe coincidence momentum imaging with few-cycle laser pulses

Ultrafast nuclear dynamics in CH3OH+ has been studied based on the released kinetic energy distributions of the fragment ions in the non-migration (CH3OH2+→CH3++OH+) and migration (CH3OH2+→CH2++OH2+) pathways obtained by pump-probe coincidence momentum imaging with few-cycle laser pulses (6.0(5)fs, 2.1(2)×1014W/cm2). A characteristic oscillatory structure with a bifurcation at ∼150fs in the kinetic energy distribution of the migration pathway is interpreted as the motion of a vibrational wave packet on a bound well around the migrated geometry, oscillating first along the CO bond and bifurcating into bound and dissociating components.

Dynamics of Glycine Dications in the Gas Phase: Ultrafast Intramolecular Hydrogen Migration versus Coulomb Repulsion

We present a combined experimental and theoretical study of the complex dynamics of excited doubly ionized glycine molecules in the gas phase. Multicoincidence mass spectroscopic techniques together with ab initio molecular dynamics simulations and density functional theory calculations allow us to show that an ultrafast intramolecular hydrogen migration (∼30 fs) appears in competiton with the expected Coulomb repulsion.

Breakdown of Born‐Oppenheimer Approximation as a Physical Mechanism for Ultrafast Hydrogen Migrations in Strong Laser Driven Molecules

AbstractWe propose a physical mechanism based on breakdown of the Born‐Oppenheimer approximation to rationalize the ultrafast hydrogen migration in strong laser driven isomerization reactions. A three nuclei (proton, donor, and acceptor) model is employed to develop a three step solution scheme. The proton‐donor Coulomb repulsion is shown to be responsible for the high proton mobility. We identify a proton tunneling process and use the Keldysh‐Faisal‐Reiss theory to calculate the tunneling probability. The effect of laser parameters (intensity, frequency, polarization, and pulse duration) has been studied and found to be consistent with recent experiments.

Ultrafast hydrogen migration in acetylene cation driven by non-adiabatic effects

Non-adiabatic dynamics of the acetylene cation is investigated using mixed quantum-classical dynamics based on trajectory surface hopping. To describe the non-adiabatic effects, two surface hopping methods are used, namely, Tully's fewest switches and Landau-Zener surface hopping. Similarities and differences between the results based on those two methods are discussed. We find that the photoionization of acetylene into the first excited state A(2)Σg(+) drives the molecule from the linear structure to a trans-bent structure. Through a conical intersection the acetylene cation can relax back to either the ground state of acetylene or vinylidene. We conclude that hydrogen migration always takes place after non-radiative electronic relaxation to the ground state of the monocation. Based on the analysis of correlation functions we identify coherent oscillations between acetylene and vinylidene with a period of about 70 fs after the electronic relaxation.

Effect of Laser Parameters on Ultrafast Hydrogen Migration in Methanol Studied by Coincidence Momentum Imaging

The effect of intensity, duration, and polarization of ultrashort laser pulses (795 nm, 40-100 fs, and 0.15-1.5 × 10(15) W/cm(2)) on the hydrogen migration in methanol is systematically investigated using Coulomb explosion coincidence momentum imaging. The ratio of the ion yield obtained for the migration pathway CH(3)OH(2+) → CH(2)(+) + OH(2)(+) with respect to the sum of the yields obtained for the migration pathway and for the nonmigration pathway CH(3)OH(2+) → CH(3)(+) + OH(+) exhibits a small (10-20%) but clear dependence on laser pulse properties, that is, the ratio decreases as the laser peak intensity increases but increases when the pulse duration increases as well as when the laser polarization is changed from linear to circular.

Communication: Two stages of ultrafast hydrogen migration in methanol driven by intense laser fields

Hydrogen migration in methanol induced by an intense laser field (0.2 PW/cm(2)) is investigated in real time by a pump-probe coincidence momentum imaging method. The observed temporal evolution of the kinetic energy spectra reveals that there are two distinctively different stages in the hydrogen migration processes in the singly charged methanol: ultrafast hydrogen migration occurring within the intense laser field ( approximately 38 fs) and slower postlaser pulse hydrogen migration ( approximately 150 fs).

Tracing ultrafast hydrogen migration in allene in intense laser fields by triple-ion coincidence momentum imaging

Ultrafast hydrogen migration in allene (CH(2)=C=CH(2)) in intense laser fields was investigated by triple-ion coincidence momentum imaging. The migrating proton covering the entire range of an allene molecule was visualized by the momentum correlation maps and by the geometrical structure of triply charged allene reconstructed from the observed momentum vectors of fragment ions. The extent of hydrogen migration was found to play a decisive role in breaking selectively one of the two initially equivalent C-C chemical bonds that become inequivalent in the course of the hydrogen migration.

Ultrafast hydrogen migration in allene in intense laser fields: Evidence of two-body Coulomb explosion

Two-body Coulomb explosion of allene induced by an ultrashort (∼40 fs) intense laser field is investigated by the coincidence momentum imaging (CMI) method. From the CMI maps, two types of two-body Coulomb explosion pathways, C 3 H 4 2 + → CH m + + C 2 H 4 - m + ( m = 1–3) and C 3 H 4 2 + → C 3 H 4 - n + + H n + ( n = 1–3), are securely identified. The formation of CH 3 + , C 2 H 3 + , and H 3 + shows that the chemical bond rearrangement associated with the ultrafast hydrogen migration occurs prior to the ionization into C 3 H 4 2 + and that the extent of the hydrogen migration determines either one of the two initially identical C C chemical bonds is broken.

Efficient ejection of H3+ from hydrocarbon molecules induced by ultrashort intense laser fields

The ejection processes of hydrogen molecular ion H(3)(+) from 12 kinds of hydrocarbon molecular species, methanol, ethanol, 1-propanol, 2-propanol, acetone, acetaldehyde, methane, ethane, ethylene, allene, 1,3-butadiene, and cyclohexane, induced by intense laser fields (approximately 10(14) W/cm(2)) have been investigated by time-of-flight mass spectroscopy. The observation of the H(3)(+) production with the kinetic energy range of 3.5-5.0 eV from doubly ionized ethylene, allene, 1,3-butadiene, and cyclohexane, which have no methyl groups, showed the existence of the ultrafast hydrogen migration processes that enables three hydrogen atoms to come together to form H(3)(+) within a hydrocarbon molecule.

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.

Visualizing recurrently migrating hydrogen by few-cycle intense laser pulses

Ultrafast hydrogen migration in deuterated acetylene dication (C2D22+) is studied by the pump-probe Coulomb explosion imaging with few-cycle intense laser pulses (9 fs, 0.13 PW/cm2, 800 nm). The temporal evolution of the momenta of the fragment ions produced by the three-body explosion, C2D23+ → D++ C+ + CD+, shows 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 (∼90 fs), and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation.

Coincidence momentum imaging of ultrafast hydrogen migration in methanol and its isotopomers in intense laser fields

Two-body Coulomb explosion processes of methanol (CH 3OH, CD 3OH, CH 3OD) in an intense laser field (0.2 PW/cm 2, 60 fs) are investigated by the coincidence momentum imaging method. The dissociation pathways with hydrogen/deuterium migration or hydrogen/deuterium exchange prior to the C–O bond breaking are securely identified. From the anisotropic angular distributions and the relative yields of the fragment ions, it is revealed that the hydrogen migration process is terminated within the period of an intense ultrashort laser pulse. A comparison of the results obtained for CH 3OH and those for the isotopomers shows that the hydrogen migration is decelerated by the isotope substitution.