skip to main content

This content will become publicly available on July 27, 2023

Title: Coincidence ion pair production (cipp) spectroscopy of diiodine
Coincidence ion pair production (I + + I − ) (cipp) spectra of I 2 were recorded in a double imaging coincidence experiment in the one-photon excitation region of 71 600–74 000 cm −1 . The I + + I − coincidence signal shows vibrational band head structure corresponding to iodine molecule Rydberg states crossing over to ion-pair (I + I − ) potential curves above the dissociation limit. The band origin ( ν 0 ), vibrational wavenumber ( ω e ) and anharmonicity constants ( ω e x e ) were determined for the identified Rydberg states. The analysis revealed a number of previously unidentified states and a reassignment of others following a discrepancy in previous assignments. Since the ion pair production threshold is well established, the electric field-dependent spectral intensities were used to derive the cutoff energy in the transitions to the rotational levels of the 7pσ(1/2) ( v ′ = 3) state.
; ; ; ;
Award ID(s):
Publication Date:
Journal Name:
Physical Chemistry Chemical Physics
Page Range or eLocation-ID:
17569 to 17576
Sponsoring Org:
National Science Foundation
More Like this
  1. Coincidence ion pair production (cipp) spectra of F 2 were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975–126 210 cm −1 . The F + + F − signal shows a rotational band head structure, corresponding to F 2 Rydberg states crossing over to the ion pair production surface. Spectral simulation and quantum defect analysis allowed the characterization of five new molecular Rydberg states (F 2 **): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed ( T 0 = 125 999 cm −1 ) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.6229 4 ± 0.0004 3 eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F–F dissociation energy of 1.6012 9 ± 0.0004 4 eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F 2 and the dissociative photoionization threshold to F + + F was determined as 19.0242 ± 0.0006 eV. Using the atomic fluorine ionization energy, this can be converted to an F 2 dissociation energy of 1.6013 2 ± 0.0006 2 eV, further confirming the cipp-derivedmore »value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.6013 0 ± 0.0003 6 eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.25 1 ± 0.01 7 kJ mol −1 . This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three.« less
  2. null (Ed.)
    Photoelectron–photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p -coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO 2 + C 6 H 5 + e − , with a very small amount of stable C 6 H 5 CO 2 showing that the radical ground state is stable and the excited states are dissociative. For p -coumarate ( p -CA − ) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO 2 + HOC 6 H 4 CHCH + e − , characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOC 6 H 5 + HCC − + CO 2 . The ion beam contained both the phenolate and carboxylate isomers of p -CA − , but DPDmore »only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state.« less
  3. Acetaldehyde cations (CH 3 CHO + ) were prepared using single-photon vacuum ultraviolet ionization of CH 3 CHO in a molecular beam and the fragmentation dynamics explored over the photolysis wavelength range 390–210 nm using velocity-map ion imaging and photofragment yield (PHOFY) spectroscopy. Four fragmentation channels are characterized: CH 3 CHO + → C 2 H 3 O + + H (I), CH 3 CHO + → HCO + + CH 3 (II), CH 3 CHO + → CH 3 + + HCO (III), CH 3 CHO + → CH 4 + + CO (IV). Channels (I), (II), and (IV) are observed across the full photolysis wavelength range while channel (III) is observed only at λ < 317 nm. Maximum fragment ion yields are obtained at ∼250 nm. Ion images were recorded over the range 316–228 nm, which corresponds to initial excitation to the B̃ 2 A′ and C̃ 2 A′ states of CH 3 CHO + . The speed and angular distributions are distinctly different for each detected ion and show evidence of both statistical and dynamical fragmentation pathways. At longer wavelengths, fragmentation via channel (I) leads to modest translational energies ( E T ), consistent with dissociation overmore »a small barrier and production of highly internally excited CH 3 CO + . Additional components with E INT greater than the CH 3 CO + secondary dissociation threshold appear at shorter wavelengths and are assigned to fragmentation products of vinyl alcohol cation or oxirane cation formed by isomerization of energized CH 3 CHO + . The E T distribution observed for channel (III) products peaks at zero but is notably colder than that predicted by phase space theory, particularly at longer photolysis wavelengths. The colder-than-statistical E T distributions are attributed to contributions from secondary fragmentation of energized CH 3 CO + formed via channel (I), which are attenuated by CH 3 CHO + isomerization at shorter wavelengths. Fragmentation via channels (II) and (IV) results in qualitatively similar outcomes, with evidence of isotropic statistical components at low- E T and anisotropic components due to excited state dynamics at higher E T .« less
  4. Abstract

    Ion irradiation is a versatile tool to introduce controlled defects into two-dimensional (2D) MoS2on account of its unique spatial resolution and plethora of ion types and energies available. In order to fully realise the potential of this technique, a holistic understanding of ion-induced defect production in 2D MoS2crystals of different thicknesses is mandatory. X-ray photoelectron spectroscopy, electron diffraction and Raman spectroscopy show that thinner MoS2crystals are more susceptible to radiation damage caused by 225 keV Xe+ions. However, the rate of defect production in quadrilayer and bulk crystals is not significantly different under our experimental conditions. The rate at which S atoms are sputtered as a function of radiation exposure is considerably higher for monolayer MoS2, compared to bulk crystals, leading to MoO3formation. P-doping of MoS2is observed and attributed to the acceptor states introduced by vacancies and charge transfer interactions with adsorbed species. Moreover, the out-of-plane vibrational properties of irradiated MoS2crystals are shown to be strongly thickness-dependent: in mono- and bilayer MoS2, the confinement of phonons by defects results in a blueshift of theA1gmode. Whereas, a redshift is observed in bulk crystals due to attenuation of the effective restoring forces acting on S atoms caused by vacanciesmore »in adjacent MoS2layers. Consequently, theA1gfrequency of tri- and quadrilayer crystals is statistically invariant on account oft competition between phonon confinement effects and interlayer interactions. TheA1glinewidth is observed to decrease in bi- and trilayer crystals after low dose irradiation and is attributed to layer decoupling. This work shows that there is a complex interplay between defect production, crystal thickness and interlayer interactions in MoS2. Our results demonstrate that ion irradiation is an effective tool to modulate the electronic, vibrational and structural properties of MoS2, which may prove beneficial for practical applications.

    « less
  5. Abstract

    We perform particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizationsσ≲ 10−4and pair-loading factorsZ±≲ 10 are studied, whereZ±is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσexceeds a critical valueσLthat decreases withZ±. AtσσLthe shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease withZ±, leading to lowerσL. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales asE¯eZ±+11. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low asσ≈ 5 × 10−6. The ions then become essentially thermal with mean energyE¯i, while electrons form a nonthermal tail, extending fromEZ±+11E¯itoE¯i. Whenσ= 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here,more »the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows.

    « less