By combining a newly developed two-color laser pulsed field ionization-photoion (PFI-PI) source and a double-quadrupole–double-octopole (DQDO) mass spectrometer, we investigated the integral cross sections ( σ s) of the vanadium cation (V + ) toward the activation of CO 2 in the center-of-mass kinetic energy ( E cm ) range from 0.1 to 10.0 eV. Here, V + was prepared in single spin–orbit levels of its lowest electronic states, a 5 D J ( J = 0–4), a 5 F J ( J = 1–5), and a 3 F J ( J = 2–4), with well-defined kinetic energies. For both product channels VO + + CO and VCO + + O identified, V + (a 3 F 2,3 ) is found to be greatly more reactive than V + (a 5 D 0,2 ) and V + (a 5 F 1,2 ), suggesting that the V + + CO 2 reaction system mainly proceeds via a “weak quintet-to-triplet spin-crossing” mechanism favoring the conservation of total electron spins. In addition, no J -state dependence was observed. The distinctive structures of the quantum electronic state selected integral cross sections observed as a function of E cm and the electronic state of the V + ion indicate that the difference in the chemical reactivity of the title reaction originated from the quantum-state instead of energy effects. Furthermore, this work suggests that the selection of the quantum electronic states a 3 F J ( J = 2–4) of the transition metal V + ion can greatly enhance the efficiency of CO 2 activation.
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Photoionization of nS and nD Rydberg atoms of Rb and Cs from the near-infrared to the ultraviolet spectral region
Abstract We present calculations of the photoionization (PI) cross sections of rubidium and cesium Rydberg atoms for light with wavelengths ranging from the infrared to the ultraviolet, using model potentials from Marinescu et al (1994 Phys. Rev. A 49 982). The origins of pronounced PI minima are identified by investigating the free-electron wavefunctions. These include broad PI minima in the nS to ϵP PI channels of both Rb and Cs, with free-electron energy ϵ , which are identified as Cooper minima. Much narrower PI minima in the nD to ϵF channels are due to shape resonances of the free-electron states. We describe possible experimental procedures for measuring the PI minima, and we discuss their implications in fundamental atomic physics as well as in practical applications. Measurements of PI cross sections of Rydberg atoms may serve as a sensitive probe for many-electron interactions of the Rydberg electron in the atomic core region.
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- Award ID(s):
- 1707377
- NSF-PAR ID:
- 10321640
- Date Published:
- Journal Name:
- New Journal of Physics
- Volume:
- 23
- Issue:
- 6
- ISSN:
- 1367-2630
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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A bstract We report the first measurement of the exclusive cross sections e + e − → $$ B\overline{B} $$ B B ¯ , e + e − → $$ B{\overline{B}}^{\ast } $$ B B ¯ ∗ , and e + e − → $$ {B}^{\ast }{\overline{B}}^{\ast } $$ B ∗ B ¯ ∗ in the energy range from 10 . 63 GeV to 11 . 02 GeV. The B mesons are fully reconstructed in a large number of hadronic final states and the three channels are identified using a beam-constrained-mass variable. The shapes of the exclusive cross sections show oscillatory behavior with several maxima and minima. The results are obtained using data collected by the Belle experiment at the KEKB asymmetric-energy e + e − collider.more » « less
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We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of E r ∼ 0.01–10 eV. We used photodetachment of C − to produce a pure beam of atomic C in the ground electronic 3 P term, with statistically populated fine-structure levels. The H 2 + and D 2 + were formed in an electron impact ionization source, with well known vibrational and rotational distributions. The experimental work is complemented by a theoretical study of the CH 2 + electronic system in the reactant and product channels, which helps to clarify the possible reaction mechanisms underlying the ICS measurements. Our measurements provide evidence that the reactions are barrierless and exoergic. They also indicate the apparent absence of an intermolecular isotope effect, to within the total experimental uncertainties. Capture models, taking into account either the charge-induced dipole interaction potential or the combined charge-quadrupole and charge-induced dipole interaction potentials, produce reaction cross sections that lie a factor of ∼4 above the experimental results. Based on our theoretical study, we hypothesize that the reaction is most likely to proceed adiabatically through the 1 4 A′ and 1 4 A′′ states of CH 2 + via the reaction C( 3 P) + H 2 + ( 2 Σ+g) → CH + ( 3 Π) + H( 2 S). We also hypothesize that at low collision energies only H 2 + ( v ≤ 2) and D 2 + ( v ≤ 3) contribute to the titular reactions, due to the onset of dissociative charge transfer for higher vibrational v levels. Incorporating these assumptions into the capture models brings them into better agreement with the experimental results. Still, for energies ≲0.1 eV where capture models are most relevant, the modified charge-induced dipole model yields reaction cross sections with an incorrect energy dependence and lying ∼10% below the experimental results. The capture cross section obtained from the combined charge-quadrupole and charge-induced dipole model better matches the measured energy dependence but lies ∼30–50% above the experimental results. These findings provide important guidance for future quasiclassical trajectory and quantum mechanical treatments of this reaction.more » « less
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Abstract Background Surface dose in megavoltage photon radiotherapy has a significant clinical impact on the skin‐sparing effect. In previously published works, it was established that the presence of medium atomic number (Z) absorbers, such as tin, decreases the surface dose. It was concluded that high‐Z absorbers, such as lead, increase the surface dose, relative to medium‐Z absorbers, due to the increased contributions from photoelectrons and electron‐positron pairs.
Purpose The purpose of this investigation is to revisit these conclusions in the context of photon beams from modern linacs.
Methods A metric estimating the relative intensity of charged particles emitted in the forward direction, , was proposed using cross‐sections for the photon interactions. The values were calculated for various absorbers using energy spectra of 6 and 10 MV photon beams from a Varian TrueBeam linac. Monte Carlo (MC) simulations were performed using TOPAS MC code to calculate the surface dose for various absorbers. Surface dose measurements were performed with 6 and 10 MV photon beams with tin and lead absorbers.
Results The values were found to decrease as a function of Z for both 6 and 10 MV photon beams indicating that the surface dose from electrons emitted in the forward direction consistently decreases with increasing Z. With the increasing Z of the absorbers, both experimental and MC‐calculated surface dose decreased without exhibiting a minimum at medium‐Z absorbers. The surface dose for lead and tin was determined to be within 1% of each other for both 6 and 10 MV photon beams using MC simulations and experimental measurements. Therefore, no statistically significant difference in surface dose was found between the tin and lead absorbers disproving the presence of any minima in the surface dose versus the Z curve as has been reported in the literature.
Conclusions Surface dose for modern photon beams can be reduced using both medium and high Z absorbers since a consistent decrease in surface dose was found with increasing absorber Z.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/1367-2630/ac00d5
