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REVIEW 2 major objections 5 minor 13 references

Ionic photofragmentation cross sections of the HS+ H2S+ and HCl+ molecular ions near the 2p threshold

T0 review · 2 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read The paper reports absolute cross sections for X2+ and X3+ production in HS+, H2S+ and HCl+ near the 2p thresholds and shows that summed photofragmentation spectra closely match synthetic spectra from CI-S+SO calculations.

desk verdict First absolute X2+/X3+ yields for HS+, H2S+, HCl+ near the 2p edges, with honest caveats about the missing X+ channel; the theory comparison is shape-based, not an absolute scale test. read the letter →

arxiv 2507.16993 v1 pith:TS5AH4FF submitted 2025-07-22 physics.atom-ph

Jean-Paul Mosnier , Eugene T. Kennedy , Denis Cubaynes , Jean-Marc Bizau , Segolene Guilbaud , Stephane Carniato This is my paper · ORCID
classification physics.atom-ph PACS 33.80.Eh32.80.Fb
keywords ionicphotofragmentationcrosssectionsHS+molecularionHCl+H2S+2pthresholdphotoionizationspin-orbitcouplingFranck-Condonvibrationaldynamicsabinitioconfigurationinteraction
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper measures, for the first time, absolute cross sections for producing doubly and triply charged sulfur and chlorine ions by 2p photoabsorption of the molecular ions HS+ and HCl+, along with similar data for H2S+. The central claim is that the summed S2++S3+ and Cl2++Cl3+ photofragmentation cross sections closely reproduce synthetic photoabsorption spectra built from computed oscillator strengths, meaning both the measurements and the core-excited-state calculations are reliable. The agreement only holds when spin-orbit coupling and vibrational (Franck-Condon) dynamics are included. Because these hydride ions are known interstellar tracers, the results provide quantitative reference data for x-ray absorption and Auger-driven fragmentation in astrophysical environments.

What carries the argument

The central object is the summed photofragmentation cross section sigma(X2+) + sigma(X3+) measured with a merged-beam apparatus, compared against synthetic spectra generated from the calculated absorption oscillator strengths (f-values) by assigning normalized Gaussian profiles and Franck-Condon vibrational envelopes. The f-value calculation uses DFT-optimized geometries, an augmented correlation-consistent polarized core-valence quintuple-zeta basis set, and Breit-Pauli spin-orbit coupling at the post-Hartree-Fock CI-S level. The key diagnostic is the match between theoretical and measured resonance positions and intensities, which fixes the assignment of the core-excited states.

What would settle it

A measurement of the X+ photofragment yield, for example with a higher-resolution demerger or coincidence detection, that shows the X+ channel carries 15% or more of the total absorption would shift the summed cross section closer to the atomic continuum and reopen the comparison with the synthetic spectra.

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Extended reading notes

Core claim

The paper establishes that the absolute ionic photofragmentation cross sections of HS+ and HCl+ near the 2p ionization thresholds closely match the photoabsorption cross sections obtained from ab initio configuration-interaction calculations with single excitations and spin-orbit coupling (CI-S+SO), once spin-orbit splitting of the 2p core hole and Franck-Condon vibrational envelopes are incorporated. The dominant spectral features are an intense broad 2p to sigma-star resonance, which indicates a strongly dissociating upper state, and sharp atomic-like 2p to 3d sigma resonances whose positions and relative intensities agree with the calculated oscillator strengths. The agreement assigns each measured resonance to a specific core-hole electronic configuration and validates both the experimental absolute scale and the theoretical method.

Load-bearing premise

The measured S2+/S3+ and Cl2+/Cl3+ yields are assumed to represent essentially the whole photoabsorption cross section, so the missing singly charged X+ channel is negligible in the comparison with theory.

Editorial extensions

If this is right

  • The absolute cross sections for HS+ and HCl+ near the 2p edges can now serve as reference data for x-ray absorption modelling of these interstellar hydride ions.
  • The good theory-experiment agreement demonstrates that CI-S+SO calculations with Franck-Condon vibrational envelopes are sufficient to describe core excitations in open-shell diatomic hydride ions.
  • The isoelectronic comparisons show that the 2p to sigma-star resonance is a molecular fingerprint while the 2p to 3d sigma resonance keeps its atomic character upon protonation, informing spectral interpretation in more complex molecular environments.
  • For H2S+, the summed S2++S3+ cross section reaches only about a quarter of the atomic S+ values, indicating that a large fraction of the absorption goes to other fragmentation channels and that the complete photoabsorption for the triatomic species remains to be mapped.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The 15-18% shortfall of the summed molecular cross section relative to the atomic S+/Cl+ continuum, though within combined error bars, is exactly the size one would expect if the unmeasured singly charged X+ channel contributed; a dedicated measurement of the X+ yield would test whether the summed spectra truly equal the photoabsorption cross section.
  • The same merged-beam measurement plus simulation recipe could be applied to other interstellar hydride ions, such as HF+, HBr+, or hydrocarbon cations, to produce absolute 2p absorption data where none currently exist.
  • The kinetic-energy-release analysis suggests the fast-beam method only begins to truncate high-KER fragments above several tens of eV; repeating the experiments at higher parent beam velocity or with slower beams would extend the usable KER range and sharpen the absolute cross sections.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. The paper reports absolute photofragmentation cross sections for the production of S2+ and S3+ from HS+ and H2S+, and Cl2+ and Cl3+ from HCl+, in the photon-energy region of the X 2p thresholds. Because the X+ single-ionization channel could not be measured with the merged-beam apparatus, the authors present the summed X2+ + X3+ yields as an approximate total photoabsorption cross section. The HS+ and HCl+ data are interpreted with ab initio CI-S+SO calculations of absorption oscillator strengths, including spin-orbit coupling, Franck-Condon vibrational progressions, and dissociative broadening for the sigma* states. The synthetically broadened theoretical spectra are compared visually with the summed experimental yields, and the paper reports very good agreement for HS+ and HCl+. Comparisons with isoelectronic atomic ions (Cl+, Ar+ and S+) and with H2S+ are also discussed.

Significance. If the central claim is taken at face value, this would be the first absolute measurement of ionic photofragmentation cross sections for HS+, H2S+ and HCl+ in the 2p region, with a nontrivial theoretical interpretation that is not fitted to the experimental spectra. The experimental section is unusually detailed, including absolute calibration, background subtraction, form-factor determination, and an appendix analyzing kinetic-energy-release effects on fragment transmission. The oscillator strengths are obtained from independent ab initio calculations, and the Gaussian widths used to synthesize spectra are tied to the independently known experimental band pass. These are genuine strengths. The main limitation is that the measured quantities are partial ionic yields, not total photoabsorption, and the paper's own comparisons show that the unmeasured X+ channel can be large enough to affect the absolute conclusion.

major comments (2)
  1. [Section 2.2 and Conclusions] The central comparison in the Conclusions equates the summed X2+ + X3+ yields with the total photoabsorption cross section and claims 'very good agreements' with the calculated total-absorption f-value spectra. However, Section 2.2 explicitly states that the X+ + H+ + e channel (Eq. 2) could not be measured and that the measured sum is only a 'fair approximation' of the absorption cross section. The paper's own checks in Section 2.2 show that the HS+ sum is 18% below the atomic S+ continuum at 190 eV, the HCl+ sum is 15% below the Cl+ value at 260 eV, and for H2S+ the measured S2+ + S3+ sum is only about 24% of the atomic S+ value, leaving 76% to unmeasured channels. Because the synthetic spectra in Figs. 6 and 9 are normalized relative spectra, the apparent good agreement does not constrain the absolute scale and could arise from a compensation between an overestimated X2+/X3+ branching fraction in the theory and a substantial missing X+ contribution in the experiment. The claim that total approximate photoabsorption cross sections are determined therefore needs either a quantitative estimate of the X+ branching fraction, for example from Auger/decay calculations or a dedicated measurement, or a clear reframing of the headline result as applying to the measured partial ionic yields only. I regard this as the load-bearing issue for the paper's central claim.
  2. [Sections 4.1.1 and 4.2.1, Figs. 6 and 9] The agreement between theory and experiment is described as 'very satisfactory' and 'very good' on the basis of visual comparison of normalized spectra. The synthetic spectra are constructed by assigning Gaussian profiles with FWHM set to the experimental band pass, and the only free parameter is the dissociative broadening width for the 2p5 sigma* states (Sections 4.1.1 and 4.2.1). No quantitative measure, such as a scaling factor, residuals, or reduced chi-square, is reported for the comparison. Without such a measure, the strength of the claim 'very good agreements' is difficult to assess, particularly because the theoretical quantity is a total absorption cross section while the experimental quantity is a partial ionic yield. I ask the authors either to provide a quantitative comparison or to characterize the comparison as qualitative.
minor comments (5)
  1. [Abstract] The abstract contains the word 'sulfanyumil' for H2S+; elsewhere the text uses 'sulfaniumyl'. Please correct the spelling.
  2. [Table 1] The ECRIS RF power is listed as 63 µW and 71 µW, which is implausibly low for an ECR ion source; this is likely a typo for W (watts). Please check and correct the units.
  3. [Reference 42] The quantitative comparison of the HCl+ data with the atomic Cl+ continuum in Section 2.2 and Fig. 11 relies on Ref. 42, which is described as unpublished work. Please either provide the data in a supplement or cite a published source, or state clearly what level of confidence the comparison carries.
  4. [Data availability] The data availability statement contains the placeholder URL 'https://doras.dcu.ie/xxxxx/'; the final repository link should be provided.
  5. [Equation (1)] The expression 'BG = 31.845IA - 20.618 = 5.806 q' sqrt(mEK)' is dimensionally unclear; the role of q' (which appears as a multiplier rather than a divisor) should be clarified with parentheses.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the ab initio f-values are independent of the measured photofragmentation yields, and the acknowledged unmeasured X+ channel is an interpretation caveat, not a circular reduction.

full rationale

The claimed derivation chain is not circular. The HS+ and HCl+ synthetic spectra are generated from absorption oscillator strengths computed ab initio with CI-S+SO, spin-orbit coupling, and Franck-Condon vibrational intensities (Sec. 3.2), not from the measured photofragmentation cross sections. In comparing to experiment, the Gaussian widths are fixed by the independently known monochromator band pass (130 meV in Sec. 4.1.1; 150 meV versus the measured 165 meV in Sec. 4.2.1), and no energy shift is reported as a fitted parameter. The central agreement is therefore a genuine test of the calculated energy positions and relative f-values against the measured X2+/X3+ yields. The explicit limitation in Sec. 2.2 - that the X+ yield 'cannot be obtained' and the summed X2+ + X3+ cross sections only provide 'a fair approximation' of photoabsorption - is a physical caveat, not a circular step: it affects how strongly the agreement can validate total photoabsorption, especially given the 15-18% atomic-continuum deficit and the 76% missing fraction noted for H2S+, but the theory was not constructed from the yields. Self-citations to the apparatus (Ref. 39) and to the authors' atomic S+/Cl+ measurements (Refs. 40 and 42) are auxiliary comparators and calibration references, not load-bearing premises that define the f-values. No uniqueness theorem, ansatz, or renamed empirical pattern is imported from the authors' prior work. Hence no circular step is identified.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central claims rest on the merged-beam absolute calibration, the proxy assumption that summed X2+ and X3+ yields represent total absorption, and the accuracy of the CI-S+SO/Franck-Condon calculations. No new entities are introduced. The only possibly undetermined quantity is the dissociative broadening of the sigma* features, whose computation is not specified.

free parameters (1)
  • Dissociative broadening width for 2p5 sigma* states = not stated
    The paper states that dissociation-induced broadening was included in the synthetic spectra (blue curves in Figs 6 and 9), but does not specify whether a width parameter was computed from the potential gradient or adjusted to match the experiment.
assumptions (5)
  • domain assumption The merged-beam absolute calibration method (form factor, detector efficiencies) yields absolute cross sections with 15% relative uncertainty (Ref 39).
    Underpins all absolute values; relies on prior apparatus calibration, not independently verified in this paper.
  • domain assumption The sum of X2+ and X3+ fragment yields approximates the total photoabsorption cross section, with the X+ channel negligible or within error bars.
    X+ channel cannot be measured due to 1 amu mass resolution limit (Section 2.2); supported indirectly by atomic comparisons (Section 4.1.2), not directly.
  • domain assumption CI-S with Breit-Pauli spin-orbit coupling, using 2s-1 reference orbitals, accurately describes S/Cl 2p core-excited states and oscillator strengths.
    Restricted to single excitations; no benchmark against known core-excitation spectra of these cations is provided.
  • domain assumption Franck-Condon approximation and computed numerical gradients capture the vibrational broadening, including the dissociative sigma* features.
    Section 3.2 states gradients and Franck-Condon calculations; the dissociative broadening is not shown in detail.
  • domain assumption The strong atomic character of 2p photoabsorption in HX+ far above threshold permits comparison with atomic S+ and Cl+ cross sections.
    Used in Section 2.2 to argue missing channels are small; reasonable but not proven.

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Cite this review

Pith. "Pith review of Ionic photofragmentation cross sections of the HS+ H2S+ and HCl+ molecular ions near the 2p threshold." pith.science (2026). https://pith.science/paper/TS5AH4FF

@misc{pith2026250716993,
  author       = {Pith},
  title        = {Pith review of: Ionic photofragmentation cross sections of the HS+ H2S+ and HCl+ molecular ions near the 2p threshold},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/TS5AH4FF}},
  note         = {Machine review of arXiv:2507.16993}
}
read the original abstract

The absolute cross sections for the production of X2+ and X3+ ions following absorption of monochromatized SOLEIL synchrotron radiation by the HX+ hydride molecular ions (X = S,Cl) are presented as a function of photon energy in the region of the X 2p ionisation thresholds (180 eV and 220 eV for sulphur and chlorine, respectively). The experimental results are interpreted with the help of extensive ab initio density functional theory (DFT) and post-Hartree-Fock configuration interaction calculations including spin-orbit coupling to compute the absorption oscillator strengths of the X 2p core excitations to valence and Rydberg states. In order to account for all the experimental features,the calculations must include vibrational dynamics and spin-orbit coupling. Similar experimental data are also presented for the sulfanyumil H2S+ molecular ion.

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