Laser induced fragmentation: from dissociation of neutrals to three-body breakup

Master of ScienceDepartment of PhysicsItzhak Ben-ItzhakUltrafast lasers allow us to study molecular dynamics on their natural timescale. The electronic dynamics can be studied using attosecond pulses, while the vibrational and rotational dynamics can be probed using tens of femtosecond and picosecond laser pulses, respectively. This capability has led to a broad understanding of the electronic dynamics in atoms and molecules as well as vibrational and rotational dynamics of molecules, which is one of the important goals in basic science. Moreover, it is possible to control quantum mechanical processes using ultrafast intense lasers. In this thesis, we focus on a couple of experiments. The first involves quantum control of the formation of neutral molecular fragments while the second focuses on three-body fragmentation of molecules employing the native-frames analysis method, which was recently introduced by our group [J. Rajput et al., Phys. Rev. Lett. 120, 103001 (2018)]. Experimental studies focused on the formation of excited neutral D fragments from D2 molecules are presented. We show that by manipulating the chirp of the intense laser pulses, i.e. the “time order” of the frequency components within the pulse, the formation of these fragments is controlled. To achieve this control we implement a single-prism compressor to manipulate the chirp of the laser pulses. Three-body fragmentation of CO₂ resulting in C+ + O+ + O+ is also studied. We show that if the two bonds break in a two-step process, i.e. a sequential breakup, the pathways from which the two identical O+ fragments originate can be separated using the native-frames analysis method. In contrast, the two O+ fragments cannot be distinguished if the two C-O bonds break simultaneously

Electromagnetically induced modification of metal optical properties

Doctor of PhilosophyDepartment of PhysicsMatthew J. BergThe reflection of light from a metal film is among the most fundamental and well-understood effects in optics. If the film thickness is greater than the wavelength, reflection is explained in simple terms with the electromagnetic boundary conditions. For film thickness much less than the wavelength, reflection is incomplete and more exotic physical effects become possible. This is especially so if the light illuminating the film is pulsed at the femtosecond time-scale. In this work, a new phenomenon is proposed where few-femtosecond laser pulses temporarily modify a thin metal film's optical properties. By casting a pulsed standing-wave pattern across the metal surface, conduction electrons are redistributed to create temporary regions of partially enhanced or depleted density. This constitutes a temporary change to the conductivity of the metal, and thus, a change to the transmittance and reflectance of the film. In regions where the density is enhanced (depleted), the transmittance is decreased (increased). The process is possible because the period of action of the applied electric field is shorter than the relaxation time for the conduction electrons. An experiment is conducted that tests the concept by measuring the change in reflectance and transmittance for films with thickness ranging from 20-400 Angstroms. A pair of calibrated photodiodes are used to monitor the reflection and transmission modulation of the sample. The data is collected over many laser pulses and is averaged which cancels the random power fluctuation effects of the laser. Our findings show that the film's transmittance decreases only when the standing-wave pattern is present. In other words, the metal sample is found to be less transparent hence a ``better" conductor in the presence of the conditioning beams compared to when there is no standing wave on the sample. As the pulse length of the pattern is increased, or as the film thickness is increased, these changes disappear. To gain further insight, the Drude free-electron model is used to develop a theoretical description for the process, which qualitatively agrees with the observed changes in reflectance and transmittance

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

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

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