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

Citation: Ablikim, U., Bomme, C., Xiong, H., Savelyev, E., Obaid, R., Kaderiya, B., . . . Rolles, D. (2016). Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging. Scientific Reports, 6, 8. doi:10.1038/srep38202An 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

State-selective dissociation dynamics of an oxygen molecular ion studied with single-harmonic pump and infrared-probe pulses

Citation: Malakar, Y., Wilhelm, F., Trabert, D., P., K. R., Li, X., Pearson, W. L., … Rudenko, A. (2018). State-selective dissociation dynamics of an oxygen molecular ion studied with single-harmonic pump and infrared-probe pulses. Physical Review A, 98(1), 013418. https://doi.org/10.1103/PhysRevA.98.013418Laser-induced dissociation of a photoionized oxygen molecule is studied employing an extreme-ultraviolet-pump–near-infrared-probe (EUV-NIR pump-probe) technique. A combination of a narrow-band 11th harmonic pump centered at 17.3 eV and a moderate-intensity NIR probe restricts the dissociation dynamics to the pair of low-lying cationic states, a4Πu and f4Πg. The measured kinetic energies of the O+ fragments reveal contributions from one-, two-, and three-photon dissociation pathways (1ω, 2ω, and 3ω) involving these two states. While the yields of the two- and three-photon channels initially rise and then decrease as a function of EUV-NIR delay, the yield of the single-photon pathway rises slower but keeps increasing over the whole delay range studied. This behavior reflects the evolving probability density of the ionic nuclear wave packet at the internuclear distances, where it can undergo resonant 3ω and 1ω transitions from the a4Πu to the f4Πg state of O2+

Three-dimensional momentum imaging of dissociation in flight of metastable molecules

Citation: Jochim, B., Erdwien, R., Malakar, Y., Severt, T., Berry, B., Feizollah, P., … Ben-Itzhak, I. (2017). Three-dimensional momentum imaging of dissociation in flight of metastable molecules. New Journal of Physics, 19(10), 103006. https://doi.org/10.1088/1367-2630/aa81a

Three-dimensional momentum imaging of dissociation in flight of metastable molecules

We investigate dissociation in flight of metastable molecular dications formed by ultrashort, intense laser pulses using the cold target recoil ion momentum spectroscopy technique. A method for retrieving the lifetime(s) of the transient metastable state(s) as well as the complete three-dimensional momenta of the dissociating fragments is presented. Specifically, we demonstrate and discuss this approach by focusing on dissociation in flight of the ethylene dication going to the deprotonation channel. Two lifetimes are found to be associated with this process, C2H${}_{4}^{2+}\,\to$ C2H3 + + H+: ${\tau }_{1}=202\pm 10$ ns and ${\tau }_{2}=916\pm 40$ ns. For the corresponding channel in deuterated ethylene, lifetimes of ${\tau }_{1}=269\pm 29$ ns and ${\tau }_{2}=956\pm 83$ ns are obtained

Native Frames: Disentangling Sequential from Concerted Three-Body Fragmentation

Citation: Rajput, J., Severt, T., Berry, B., Jochim, B., Feizollah, P., Kaderiya, B., … Ben-Itzhak, I. (2018). Native Frames: Disentangling Sequential from Concerted Three-Body Fragmentation. Physical Review Letters, 120(10), 103001. https://doi.org/10.1103/PhysRevLett.120.103001A key question concerning the three-body fragmentation of polyatomic molecules is the distinction of sequential and concerted mechanisms, i.e., the stepwise or simultaneous cleavage of bonds. Using laser-driven fragmentation of OCS into O++C++S+ and employing coincidence momentum imaging, we demonstrate a novel method that enables the clear separation of sequential and concerted breakup. The separation is accomplished by analyzing the three-body fragmentation in the native frame associated with each step and taking advantage of the rotation of the intermediate molecular fragment, CO2+ or CS2+, before its unimolecular dissociation. This native-frame method works for any projectile (electrons, ions, or photons), provides details on each step of the sequential breakup, and enables the retrieval of the relevant spectra for sequential and concerted breakup separately. Specifically, this allows the determination of the branching ratio of all these processes in OCS3+ breakup. Moreover, we find that the first step of sequential breakup is tightly aligned along the laser polarization and identify the likely electronic states of the intermediate dication that undergo unimolecular dissociation in the second step. Finally, the separated concerted breakup spectra show clearly that the central carbon atom is preferentially ejected perpendicular to the laser field

Strong-field-induced bond rearrangement in triatomic molecules

A comparative study of bond rearrangement is reported for the double ionization of three triatomic molecules: carbon dioxide, carbonyl sulfide, and water (D2O). Specifically, we study the formation of the molecular cation AC+ from the edge atoms of a triatomic molecular dication ABC2+ following double ionization by intense, short (23 fs, 790 nm) laser pulses. The comparison is made using the double ionization branching ratio of each molecule, thereby minimizing differences due to differing ionization rates. The rearrangement branching ratio is highest for water, which has a bent initial geometry, while CO2 and OCS are linear molecules. The angular distribution of O2+ fragments arising from CO2 is essentially isotropic, while SO+ from OCS and D+2 from D2O are aligned with the laser polarization. In the CO2 and D2O cases, the angular distributions of the bond rearrangement channels are different from the angular distributions of the dominant dissociative double ionization channels CO++O+ and OD++D+. Only the angular distribution of SO+ from OCS is both aligned with the laser polarization and similar to the angular distribution of the largest dissociative channel, CO++S+. The mixed behavior observed from the angular distributions of the different molecules stands in contrast to the relative consistency of the magnitude of the bond rearrangement branching ratio

Mechanisms and time-resolved dynamics for trihydrogen cation (H 3 + ) formation from organic molecules in strong laser fields

Citation: Ekanayake, N., Nairat, M., Kaderiya, B., Feizollah, P., Jochim, B., Severt, T., … Dantus, M. (2017). Mechanisms and time-resolved dynamics for trihydrogen cation (H 3 + ) formation from organic molecules in strong laser fields. Scientific Reports, 7(1), 4703. https://doi.org/10.1038/s41598-017-04666-wStrong-field laser-matter interactions often lead to exotic chemical reactions. Trihydrogen cation formation from organic molecules is one such case that requires multiple bonds to break and form. We present evidence for the existence of two different reaction pathways for H3 + formation from organic molecules irradiated by a strong-field laser. Assignment of the two pathways was accomplished through analysis of femtosecond time-resolved strong-field ionization and photoion-photoion coincidence measurements carried out on methanol isotopomers, ethylene glycol, and acetone. Ab initio molecular dynamics simulations suggest the formation occurs via two steps: the initial formation of a neutral hydrogen molecule, followed by the abstraction of a proton from the remaining CHOH2+ fragment by the roaming H2 molecule. This reaction has similarities to the H2 + H2 + mechanism leading to formation of H3 + in the universe. These exotic chemical reaction mechanisms, involving roaming H2 molecules, are found to occur in the ~100 fs timescale. Roaming molecule reactions may help to explain unlikely chemical processes, involving dissociation and formation of multiple chemical bonds, occurring under strong laser fields

Mechanisms and time-resolved dynamics for trihydrogen cation (H 3 + ) formation from organic molecules in strong laser fields

Strong-field laser-matter interactions often lead to exotic chemical reactions. Trihydrogen cation formation from organic molecules is one such case that requires multiple bonds to break and form. We present evidence for the existence of two different reaction pathways for H3 + formation from organic molecules irradiated by a strong-field laser. Assignment of the two pathways was accomplished through analysis of femtosecond time-resolved strong-field ionization and photoion-photoion coincidence measurements carried out on methanol isotopomers, ethylene glycol, and acetone. Ab initio molecular dynamics simulations suggest the formation occurs via two steps: the initial formation of a neutral hydrogen molecule, followed by the abstraction of a proton from the remaining CHOH2+ fragment by the roaming H2 molecule. This reaction has similarities to the H2 + H2 + mechanism leading to formation of H3 + in the universe. These exotic chemical reaction mechanisms, involving roaming H2 molecules, are found to occur in the ~100 fs timescale. Roaming molecule reactions may help to explain unlikely chemical processes, involving dissociation and formation of multiple chemical bonds, occurring under strong laser fields