21 research outputs found
Femtosecond intramolecular rearrangement of the CH3NCS radical cation
Strong-field ionization, involving tunnel ionization and electron
rescattering, enables femtosecond time-resolved dynamics measurements of
chemical reactions involving radical cations. Here, we compare the formation of
CH3S+ following the strong-field ionization of the isomers CH3SCN and CH3NCS.
The former involves the release of neutral CN, while the latter involves an
intramolecular rearrangement. We find the intramolecular rearrangement takes
place on the single picosecond timescale and exhibits vibrational coherence.
Density functional theory and coupled-cluster calculations on the neutral and
singly ionized species help us determine the driving force responsible for
intramolecular rearrangement in CH3NCS. Our findings illustrate the complexity
that accompanies radical cation chemistry following electron ionization and
demonstrate a useful tool for understanding the cation dynamics after
ionization.Comment: Combined PDF file consisting of the main text (20 pages, 7 figures, 2
tables) and the supplementary material (7 pages, 1 figure, nuclear
coordinates of the calculated molecular structures). This article has been
accepted for publication in the Journal of Chemical Physics. After it is
published, it will be found at https://doi.org/10.1063/5.011787
Carrier-envelope phase control over pathway interference in strong-field dissociation of H
The dissociation of an H molecular-ion beam by linearly polarized,
carrier-envelope-phase-tagged 5 fs pulses at 4W/cm with a
central wavelength of 730 nm was studied using a coincidence 3D momentum
imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission
direction of H fragments relative to the laser polarization were observed.
These asymmetries are caused by interference of odd and even photon number
pathways, where net-zero photon and 1-photon interference predominantly
contributes at H+H kinetic energy releases of 0.2 -- 0.45 eV, and
net-2-photon and 1-photon interference contributes at 1.65 -- 1.9 eV. These
measurements of the benchmark H molecule offer the distinct advantage
that they can be quantitatively compared with \textit{ab initio} theory to
confirm our understanding of strong-field coherent control via the
carrier-envelope phase
The importance of Rydberg orbitals in dissociative ionization of small hydrocarbon molecules in intense laser fields
Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which typically have electronic states that are relatively well separated in energy. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C2D2, C2D4 and C2D6 reveals that the photofragment angular distributions can only be understood when the field-dressed orbitals rather than the field-free orbitals are considered. Our measured angular distributions and intensity dependence show that these field-dressed orbitals can have strong Rydberg character for certain orientations of the molecule relative to the laser polarization and that they may contribute significantly to the hydrogen elimination dissociative ionization yield. These findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields
Incorporating real time velocity map image reconstruction into closed-loop coherent control
We report techniques developed to utilize three-dimensional momentum information as feedback in adaptive femtosecond control of molecular dynamics. Velocity map imaging is used to obtain the three-dimensional momentum map of the dissociating ions following interaction with a shaped intense ultrafast laser pulse. In order to recover robust feedback information, however, the two-dimensional momentum projection from the detector must be inverted to reconstruct the full three-dimensional momentum of the photofragments. These methods are typically slow or require manual inputs and are therefore accomplished offline after the images have been obtained. Using an algorithm based upon an âonion-peelingâ (also known as âback projectionâ) method, we are able to invert 1040 Ă 1054 pixel images in under 1 s. This rapid inversion allows the full photofragment momentum to be used as feedback in a closed-loop adaptive control scheme, in which a genetic algorithm tailors an ultrafast laser pulse to optimize a specific outcome. Examples of three-dimensional velocity map image based control applied to strong-field dissociation of CO and O2 are presented
Mechanisms and dynamics of the NH + H and NH + H + H fragmentation channels upon single-photon double ionization of NH
We present state-selective measurements on the NH + H and
NH + H + H dissociation channels following single-photon double
ionization at 61.5 eV of neutral NH, where the two photoelectrons and two
cations are measured in coincidence using 3-D momentum imaging. Three dication
electronic states are identified to contribute to the NH + H
dissociation channel, where the excitation in one of the three states undergoes
intersystem crossing prior to dissociation, producing a cold NH fragment.
In contrast, the other two states directly dissociate, producing a
ro-vibrationally excited NH fragment with roughly 1 eV of internal
energy. The NH + H + H channel is fed by direct dissociation from
three intermediate dication states, one of which is shared with the NH
+ H channel. We find evidence of autoionization contributing to each of
the double ionization channels. The distributions of the relative emission
angle between the two photoelectrons, as well as the relative angle between the
recoil axis of the molecular breakup and the polarization vector of the
ionizing field, are also presented to provide insight on both the
photoionization and photodissociation mechanisms for the different dication
states.Comment: 18 pages, 21 figures, 3 table
Dissociation dynamics of molecular ions in ultrafast, intense laser fields: from diatomic to polyatomic molecules
Doctor of PhilosophyDepartment of PhysicsItzhak Ben-ItzhakOut of the many tools for probing molecular dynamics, intense, ultrafast laser pulses are particularly well suited for this purpose. First, these pulses have temporal durations shorter than the typical rotational and vibrational periods of molecules and therefore allow the observation of molecular dynamics on their native timescales. Further, the broad bandwidth and high peak intensities of these laser pulses can result in the excitation of many transition pathways that may interfere and enable control of dynamics.
The primary focus of this work is the ultrafast laser-induced dissociation of molecular ions. We generate these ions as âfastâ beam targets and study their fragmentation using a coincidence three-dimensional (3D) momentum imaging technique, which allows the measurement of all nuclear fragments, including neutrals. This approach is employed to study laser-induced processes in a variety of molecules. The goal of these efforts is not to study specific molecules but rather to use them as testing grounds to deepen our knowledge of laser-induced molecular dynamics in general.
For example, we find that permanent-dipole transitions, which are commonly overlooked in the interpretation of strong-field experiments, play a key role in laser-induced dissociation of metastable NOÂČâș ions. General consideration of these transitions in heteronuclear molecules is important in building our understanding towards more complex molecules. Speaking of more complex systems, we have also begun investigating the laser-induced dynamics of simple hydrocarbons. Our use of molecular ion beam targets gives us the unique ability to exercise control over the initial âconfiguration,â i.e., geometry of these molecules. Utilizing CâHâ^q ion beam targets (where q is the molecular ion charge state) prepared in various initial configurations, including acetylene (HCCH), vinylidene (HâCC), and cis/trans, we have determined that this property has an immense impact on the isomerization dynamics, a finding that we anticipate will lead to future work towards deeper understanding. More broadly, this approach of probing molecules in different initial configurations offers a unique perspective that could be complementary to mainstream methodsânot just in the case of CâHâ but other chemical systems as well.
We also describe some improvements to the 3D momentum imaging methods that facilitate the study of molecular dynamics. One of these developments is a method to distinguish and evaluate the momenta of neutral-neutral channels resulting from the fragmentation of negative ion beams. The second is a technique for imaging the breakup of long-lived metastable molecules decaying in flight to the detector and retrieving the lifetime(s) of the populated states.
Our collaborative efforts in adaptive closed-loop control are also discussed. Here, an evolutionary learning algorithm supplied with experimental feedback obtains optimally-shaped ultrashort laser pulses for driving targeted molecular dynamics. While the complexity of the shaped pulses can make interpretation challenging, the combination of these efforts with basic experiments like those we perform using ion beams can help.
In closing, the work presented in this thesis extends from diatomic to polyatomic molecules, following the natural progression of building from simpler to more complex systems. We believe that the results of these efforts aid in the advancement of understanding strong-field molecular dynamics and will stimulate future research endeavors along these directions
Note: Determining the detection efficiency of excited neutral atoms by a microchannel plate detector
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 C2H3 + + H+: ns and ns. For the corresponding channel in deuterated ethylene, lifetimes of ns and ns are obtained