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Carnes, K. D.

Publications and source records attributed to Carnes, K. D..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Native frames: An approach for separating sequential and concerted three-body fragmentation

Disentangling sequential and concerted three-body fragmentation has been a longstanding endeavor in studies of molecular dynamics. To accomplish this goal, we recently introduced a novel method to separate sequential from concerted breakup, where the sequential events are analyzed in their native frames [J. Rajput et al., Phys. Rev. Lett. 120, 103001 (2018)]. The essence of this method is the use of the Jacobi coordinates' conjugate momenta to reduce the dimensionality of the multibody fragmentation in combination with a clear signature for sequential breakup. To demonstrate this method, we employ coincidence momentum imaging to study the strong-field dissociative ionization of OCS into O + +C + + S + , which typically undergoes concerted fragmentation or sequential breakup involving either a metastable CO 2+ or CS 2+ molecule. We identify sequential breakup using a uniform distribution as a function of the angle between the conjugate momenta, associated with the first and second fragmentation steps, which is due to the rotation of the intermediate molecule in the fragmentation plane. Furthermore, by exploiting this uniform distribution, we separate the sequential and concerted distributions in any plot created from the measured momenta.

Atomic & molecular processes in external fields↗
DOE OSTI2022

Two-body dissociation of formic acid following double ionization by ultrafast laser pulses

We studied the fragmentation of planar formic acid (HCOOH) molecules following their double ionization by intense ultrashort laser pulses. Deuterium tagging (i.e., HCOOD) combined with coincidence momentum imaging measurements of all fragment ions enabled determination of the role of the hydroxyl and carboxyl hydrogen atoms in the breakup. Specifically, we observe a strong preference for the hydroxyl (OD) group to remain intact in a ${\mathrm{HCOOD}}^{2+}{\rightarrow}{\mathrm{OD}}^{+}+{\mathrm{HCO}}^{+}$ fragmentation, which is an order of magnitude more likely than ${\mathrm{OH}}^{+}+{\mathrm{DCO}}^{+}$. An even larger preference for breaking the H-C bond over the O-H bond is observed in the ${\mathrm{H}}^{+}+{\mathrm{DCO}}_{2}^{+}$ and ${\mathrm{D}}^{+}+{\mathrm{HCO}}_{2}^{+}$ deprotonation channels. Bond rearrangement, leading to ${\mathrm{H}}_{2}^{+}$ or ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ formation, exhibits no isotopic preference. The kinetic-energy-release distributions of the ${\mathrm{OH}}^{+}+{\mathrm{DCO}}^{+}$ and ${\mathrm{O}}^{+}+{\mathrm{H}}_{2}{\mathrm{CO}}^{+}$ breakup channels suggest that more than one process contributes to these final products, although further theoretical work is needed to identify the specific paths.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Direct evidence of the dominant role of multiphoton permanent-dipole transitions in strong-field dissociation of NO 2 +

We study laser-induced dissociation of a metastable NO 2+ ion-beam target into N + +O + , focusing on the prominent contribution by molecules breaking parallel to the polarization at high peak laser intensity (~10 15 W/cm 2 ). Our experimental results and time-dependent Schrödinger equation calculations show that, contrary to commonly held intuition that electronic transitions always prevail, the dominant process underlying this highly aligned dissociation is a multiphoton permanent-dipole transition involving only the electronic ground state and leading to its vibrational continuum. Strong-field permanent-dipole transitions should thus be considered generally, as they may play a significant role in other heteronuclear molecules. Moreover, their role should only grow in importance for longer wavelengths, a trending direction in ultrafast laser studies.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Importance of one- and two-photon transitions in the strong-field dissociation of NO 2+

Employing a coincidence three-dimensional momentum imaging technique, we investigate the ultrafast, intense laser-induced dissociation of a metastable NO 2+ ion beam into N + + O + . Based on the kinetic energy release and angular distributions, measured using both 774-nm and second-order-harmonic 387-nm pulses, we show that the main processes driving dissociation in pulses of about 10 14 W/cm 2 peak intensity are one- and two-photon transitions from the X 2 Σ + ground state to the A 2 Π first-excited state. First-order perturbation theory calculations also corroborate these findings.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Controlling $H_{3}^{+}$ Formation From Ethane Using Shaped Ultrafast Laser Pulses

An adaptive learning algorithm coupled with 3D momentum-based feedback is used to identify intense laser pulse shapes that control $H_{3}^{+}$ formation from ethane. Specifically, we controlled the ratio of D 2 H + to $D_{3}^{+}$ produced from the D 3 C-CH 3 isotopologue of ethane, which selects between trihydrogen cations formed from atoms on one or both sides of ethane. We are able to modify the D 2 H + :$D_{3}^{+}$ ratio by a factor of up to three. In addition, two-dimensional scans of linear chirp and third-order dispersion are conducted for a few fourth-order dispersion values while the D 2 H + and $D_{3}^{+}$ production rates are monitored. The optimized pulse is observed to influence the yield, kinetic energy release, and angular distribution of the D 2 H + ions while the $D_{3}^{+}$ ion dynamics remain relatively stable. We subsequently conducted COLTRIMS experiments on C 2 D 6 to complement the velocity map imaging data obtained during the control experiments and measured the branching ratio of two-body double ionization. Two-body $D_{3}^{+}$ + $C_{2}D_{3}^{+}$ is the dominant final channel containing $D_{3}^{+}$ ions, although the three-body D + $D_{3}^{+}$ + $C_{2}D_{2}^{+}$ final state is also observed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Adaptive strong-field control of vibrational population in NO 2+

An adaptive closed-loop system employing coincidence time-of-flight feedback is used to determine the optimal pulse shapes for manipulating the branching ratio of NO dications following double ionization by an intense laser pulse. Selection between the long-lived NO 2+ and the dissociative N + + O + final states requires control of the vibrational population distribution in the transient NO 2+ . The ability to both suppress and enhance NO 2+ relative to N + + O + is observed, with the effectiveness of shaped pulses surpassing near Fourier transform-limited pulses by about an order of magnitude in each direction, depending on the pulse energy. The control is subsequently investigated using velocity map imaging, identifying plausible dissociation pathways leading to N + + O + . Combining the information about the N + + O + dissociation with a well-defined control objective supports the conclusion that the primary control mechanism involves selectively populating long-lived NO 2+ vibrational states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

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 (D 2 O). Specifically, we study the formation of the molecular cation AC + from the edge atoms of a triatomic molecular dication ABC 2+ 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 CO 2 and OCS are linear molecules. The angular distribution of O 2 + fragments arising from CO 2 is essentially isotropic, while SO + from OCS and $D$ $^{+}_{2}$ from D 2 O are aligned with the laser polarization. In the CO 2 and D 2 O 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 + . As a result, 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.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Dependence on the Initial Configuration of Strong Field-Driven Isomerization of C 2 H 2 Cations and Anions

In this work, we have investigated the femtosecond laser-induced fragmentation of $C_{2}H_{2}^{q}$ ion beam targets in various initial configurations, including acetylene (linear HCCH), vinylidene (H 2 CC), and cis/trans. The initial configuration is shown to have a tremendous impact on the branching ratio of acetylene-like (CH q1 + CH q2 ) and vinylidene-like ($CH^{q'_{1}}$ + $CH^{q'_{2}}_{2}$) dissociation of a specific $C_{2}H_{2}^{q}$ molecular ion. In particular, whereas $C_{2}H_{2}^{+}$ monocation generated from C 2 H 2 , a linear HCCH target, exhibits comparable levels of acetylene-like and vinylidene-like fragmentation, vinylidene or cis/trans configuration ion beams preferably undergo vinylidene-like fragmentation, with an acetylene branching ratio ranging from 13.9% to zero.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Bond rearrangement during Coulomb explosion of water molecules

Bond rearrangement, namely the dissociation of water ions into H 2 + + (q – 1)+ (q = 1–4) following fast ion-impact ionization, unexpectedly occurs following multiple ionization of water in spite of the presumably fast “Coulomb explosion” of the transient molecular ion. Furthermore, the branching ratio of bond rearrangement is found to be nearly equal for each level of ionization, q. In addition, formation of H 2 + is more than twice as likely to occur from the lighter water isotopologue H 2 O + than D 2 + from D 2 O + . Furthermore, these findings are consistent with the ground state dissociation mechanism in which a fast projection of the ground state nuclear wave function onto the vibrational continuum of the cation potential energy surface is sometimes followed by H 2 + formation.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Nonunique and nonuniform mapping in few-body Coulomb-explosion imaging

Much of our knowledge of molecular geometry and interaction dynamics comes from indirect measurements of the molecular fragments following breakup. This technique—Coulomb-explosion imaging (CEI), i.e., determining the initial molecular configuration of a system from the momenta of the resulting fragments using knowledge of the particle interactions—is one of the fundamental tools of molecular physics. Moreover, CEI has been a staple of molecular studies for decades. Here we show that one often cannot assign a unique initial configuration to the few-body breakup of a polyatomic molecule given the measurement of the resulting fragments' momenta. Specifically, multiple initial configurations can result in identical momenta for a molecule breaking into three or more parts. Finally, the nonunique and nonuniform mapping from the initial configuration to the measured momenta also significantly complicates the determination of molecular alignment at the time of breakup.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

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 C 2 D 2 , C 2 D 4 and C 2 D 6 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. Thus, these findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Note: Determining the detection efficiency of excited neutral atoms by a microchannel plate detector

We present a method for determining the detection efficiency of neutral atoms relative to keV ions. Excited D* atoms are produced by D{sub 2} fragmentation in a strong laser field. The fragments are detected by a micro-channel plate detector either directly as neutrals or as keV ions following field ionization and acceleration by a static electric field. Moreover, we propose a new mechanism by which neutrals are detected. We show that the ratio of the yield of neutrals and ions can be related to the relative detection efficiency of these species.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗