{"id":"ebd30442-3320-4f89-9d72-e7adc61e01ec","arxiv_id":"2501.02864","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Acetylene's VUV absorption cross sections grow with temperature, and using the 773 K measurements in a hot Jupiter model lowers the predicted C2H2 abundance by up to 40% near 5 x 10^-5 bar.","lead":"This paper reports the first measurements of acetylene's vacuum ultraviolet absorption at temperatures from 296 to 773 K, covering 115 to 230 nm. The data give exoplanet photochemistry models a temperature-appropriate input and change predicted acetylene, methane, and ethylene abundances in a simulated hot Jupiter.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 773 K long-wavelength C2H2 data, which drive the headline FT~20 at 220 nm and the 40% modeled abundance decrease, may be contaminated by pyrolysis products; the authors themselves flag the 222.5 nm bump as possibly thermal degradation, and no independent check exists above 205 nm.","rationale":"The reader's weakest assumption identifies exactly the load-bearing concern: possible pyrolysis contamination of the 773 K long-wavelength data. This is not a manufactured objection because the authors themselves raise it and because the affected spectral region is where the most dramatic quantitative result (FT ~20 at 220 nm) appears. The modeled 40% abundance decrease is also driven by the increased opacity from 150 to 230 nm, making it sensitive to the same possible contamination. Independent support for the paper's overall value exists: the lower-temperature data (373-673 K) are less likely to be contaminated, the 156.7 nm hot-band identification is plausible, and the SOLEIL cross-check up to 205 nm at 573 K and 773 K agrees with LISA up to 195 nm. However, the headline quantitative claims at 773 K above 210 nm remain unvalidated, and the paper itself calls for additional work. Since the reader already issued a CONDITIONAL verdict based on this same concern, our stress-test does not change the verdict. We would emphasize that the condition should explicitly require either a time-dependence experiment to rule out pyrolysis or a re-analysis with the contaminated region excluded, before the FT~20 value and the 40% modeled decrease are cited as robust results.","tokens_in":19055,"tokens_out":4658,"duration_ms":84342,"concrete_test":"Hold the absorption cell at 773 K with a fixed C2H2 pressure and record the transmitted intensity in the 210-230 nm region, especially at 222.5 nm, as a function of residence time (e.g., every minute for 30 minutes). If the apparent absorbance at 222.5 nm grows with time, pyrolysis products are accumulating and the current 773 K long-wavelength spectrum is contaminated. A complementary check is to measure the same spectral region using a fast-flow or short-residence-time configuration (e.g., a shock tube or a flow cell) at 773 K; if the 222.5 nm bump and the steep FT rise above 215 nm disappear or markedly diminish, the static-cell data should be corrected or excluded from model use above 210 nm.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's most striking quantitative results are the factor-of-change FT reaching about 20 around 220 nm at 773 K and the 40% decrease in modeled C2H2 abundance near 5e-5 bar. Both are driven by the increased C2H2 opacity from 150 to 230 nm, especially the steep rise above 210 nm. However, the 773 K LISA spectra are the only data covering 205-230 nm at high temperature: the SOLEIL spectra stop near 205 nm, so there is no independent validation of this critical region. In Sect. 3.2, the authors explicitly state that the bump at 222.5 nm 'could be attributed to the thermal degradation of C2H2' and that 'additional work will be needed to confirm this hypothesis.' The static-cell design with residence times of minutes is exactly the configuration previously shown to be vulnerable to pyrolysis: Zabeti et al. (2017) concluded that Vattulainen et al. (1997) static-cell values were affected by C2H2 thermal decomposition. If pyrolysis products absorb in the 215-230 nm range, then the FT factor of 20 at 220 nm and the modeled 40% abundance decrease, both of which rely on this spectral region, are overestimated. The central claim that the absorption cross section 'rises sharply from 185 to 230 nm' at 773 K could then be an artifact of contamination. A secondary but related concern is the uniform-temperature assumption (Sect. 2.2.2), which the authors estimate could bias high-temperature cross sections by several tens of percent, affecting all wavelengths; however, the pyrolysis issue is the more direct threat to the headline numbers because it specifically targets the region where the sharp rise is reported.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports new VUV absorption cross-section measurements of acetylene (C2H2) at seven temperatures between 296 and 773 K over 115-230 nm, made with a new LISA static-cell spectrometer and, at selected temperatures, with synchrotron radiation at SOLEIL. The data are reduced via the Beer-Lambert law and compared with existing literature. The authors define a factor of change FT relative to the 296 K spectrum, find that the cross section increases with temperature, report a particularly sharp increase beyond ~185 nm with FT reaching about 20 near 220 nm at 773 K, and identify a hot band near 156.7 nm. They then use the 296 K and 773 K cross sections in the FRECKLL 1D thermo-photochemical model of a hot-Jupiter-like atmosphere, finding that the modeled C2H2 abundance decreases by up to 40% near 5e-5 bar when the 773 K data replace the 296 K data, with smaller changes for CH4 and C2H4.","tokens_in":19420,"tokens_out":5594,"duration_ms":59420,"significance":"If the high-temperature long-wavelength cross sections are reliable, the paper fills a genuine gap: previously, C2H2 VUV data above ~400 K were essentially absent, even though C2H2 is a key hydrocarbon in warm-exoplanet photochemistry. The work combines two independent facilities, and the 573 K comparison between LISA and SOLEIL gives confidence in the qualitative picture below 205 nm. The data are publicly deposited, the model application uses a published chemical scheme, and the cross sections are obtained from direct transmission measurements rather than from any fit to the atmospheric result, so there is no circularity in the laboratory part. However, the headline quantitative claims—the FT factor of about 20 near 220 nm at 773 K and the 40% modeled abundance decrease—rest on spectral regions and assumptions that are called into question by the authors' own remarks about possible thermal decomposition and by the acknowledged temperature-gradient uncertainty. The manuscript therefore needs additional validation or a quantitative bounding of these effects before its central quantitative results can be accepted.","major_comments":[{"comment":"The factor-of-change FT reaching about 20 near 220 nm at 773 K is supported only by the LISA dataset in the 205-230 nm range; the SOLEIL measurements stop near 205 nm, so there is no independent validation of this critical spectral window. The authors themselves state that the bump at 222.5 nm 'could be attributed to the thermal degradation of C2H2' and that 'additional work will be needed to confirm this hypothesis,' and they cite Zabeti et al. (2017) showing that static-cell C2H2 measurements with long residence times were affected by pyrolysis. Because the same 150-230 nm opacity increase drives the modeled 40% abundance decrease (Sect. 3.3.5 and Figs. 9-10), both headline results are not fully supported unless the authors provide time-resolved evidence of gas stability in the cell or an upper-limit estimate of absorption by possible pyrolysis products.","section":"3.2 (Figs. 4, 6, 8)"},{"comment":"FT is normalized by the 296 K cross section, yet the LISA and SOLEIL ambient spectra differ by roughly 25% in the continuum level in the 195-230 nm region, and the text states that the origin of these differences is difficult to establish. Since the 773 K long-wavelength spectra come from the LISA setup with its own 296 K normalization, the reported factor of about 20 could be substantially biased by this baseline discrepancy. The paper should report FT with the ambient-continuum uncertainty propagated, or show explicitly how FT changes when the 296 K continuum is shifted by the observed LISA-SOLEIL scatter.","section":"3.1.1 and Eq. (2)"},{"comment":"The uniform-temperature assumption is acknowledged to potentially bias high-temperature cross sections by 'several tens of percent' because the column density in the colder end sections may be about twice that in the hot central section. The modeled C2H2 abundance change is 40%, i.e., of the same order as this systematic uncertainty. The authors should propagate this systematic range through the model calculation, or at least state quantitatively how the 40% figure changes when the high-temperature cross sections are shifted within the plausible systematic range.","section":"2.2.2 (last paragraph)"}],"minor_comments":[{"comment":"The Abstract states the temperature range as 296 to 773 K, while Section 4 states '296 to 793 K' and Section 2.1 says the maximum studied temperature was 773 K; 793 K appears to be a typo in the Conclusions.","section":"Abstract vs. Section 4"},{"comment":"The caption reads 'absorption cross of C2H2 section' and should read 'absorption cross section of C2H2'.","section":"Figure 3 caption"},{"comment":"The model sets the C2H2 photodissociation quantum yield to zero for wavelengths longer than 217 nm while the cross-section grid is used up to 228 nm; the text should clarify that the 217-228 nm opacity change does not directly photolyze C2H2 in the model and can affect modeled abundances only through shielding of other absorbers or through the general opacity calculation.","section":"3.3.5 and 3.3.4"},{"comment":"The model is run with cross sections and stellar fluxes binned to 1 nm, which strongly smooths the sharp long-wavelength rise reported near 220 nm; a sentence stating that the 40% abundance result is stable to the binning choice would help the reader judge the atmospheric conclusion.","section":"3.3.4"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is a good fit for A&A, and the laboratory methodology is sound in spirit, with the important strength of two-facility comparison and public data deposition. However, the two most striking quantitative results—FT near 20 at 220 nm and the modeled 40% abundance decrease—are both sensitive to the 773 K long-wavelength LISA data, which the authors themselves flag as possibly contaminated by pyrolysis. A simple caveat will not be sufficient for the quantitative claims; the revision should provide either new diagnostic measurements (e.g., gas-composition monitoring or residence-time variation) or a quantitative upper bound on the pyrolysis-product absorption in the 205-230 nm range. If such evidence cannot be provided, the authors should scale back the central quantitative claims to the wavelength range that is independently validated."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Worth knowing: this is the first C2H2 VUV absorption dataset covering 115–230 nm at temperatures up to 773 K, measured at two independent facilities and deposited in ExoMol. That alone makes it a useful contribution. The 573 K LISA/SOLEIL cross-validation is clean, and the room-temperature comparison with existing literature is handled carefully.\n\nWhat is genuinely new: the systematic temperature series, the hot band at 156.7 nm, and the modeling application showing a modest (up to 40%) reduction in C2H2 abundance and changes in CH4/C2H4 when 773 K cross sections are used. The cross sections come from direct Beer-Lambert measurements with independently measured pressure, temperature, and path length; no parameters are fitted to the target result. The citation pattern is appropriate, including the prior Vattulainen and Zabeti work.\n\nSoft spots, in order of size. First, the 773 K LISA data above about 205 nm are the only high-temperature data in that region—SOLEIL stops near 204 nm—and the authors explicitly flag the 222.5 nm bump as possibly thermal degradation of C2H2. That is exactly the region where FT reaches about 20 at 220 nm and drives the modeled 40% abundance decrease. So the headline numbers should be treated as conditional until pyrolysis is ruled out. This is not a hidden flaw; the authors state it openly. But it means the strongest quantitative claims are not yet solid.\n\nSecond, the uniform-temperature assumption in the cell could bias high-temperature cross sections by tens of percent, as the authors concede. That is a known, quantified caveat and does not threaten the overall qualitative trend. Third, the 25% continuum discrepancy between LISA and SOLEIL at 296 K in the 195–230 nm region is underplayed; since FT normalizes to sigma_296K, that discrepancy propagates directly into the quoted factors.\n\nBottom line: the data are new, the methodology is sound apart from the acknowledged pyrolysis risk, and the paper is honest about its limitations. This deserves peer review, but the referee should require the authors to either provide time-resolved composition measurements in the cell at 773 K or explicitly restrict the usable data range to wavelengths where contamination is not suspected, and to publish uncertainty envelopes with the released dataset.","headline":"New high-temperature C2H2 VUV cross sections are a real step forward, but the headline factor-of-20 rise at 773 K near 220 nm rests on data the authors themselves suspect of pyrolysis contamination.","tokens_in":20056,"tokens_out":2112,"would_cite":true,"duration_ms":20529,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Heated acetylene absorbs increasingly more vacuum-ultraviolet light across 115–230 nm, and applying 773 K data to a hot-Jupiter model lowers predicted C2H2 by up to 40%.","keywords":["acetylene","C2H2","VUV absorption cross sections","temperature dependence","exoplanet atmospheres","photochemistry","hot Jupiters","thermo-photochemical modeling"],"falsifier":"Measure the composition of the gas inside the static cell as a function of time at 773 K (e.g., mass spectrometry or time-resolved spectroscopy), or repeat the 773 K long-wavelength measurement in a fast-flow or shock-tube cell with microsecond residence times. If the 222.5 nm bump and the steep rise above 210 nm disappear or shrink when pyrolysis products are absent, the reported factor-of-20 increase is inflated and the 40% modeled abundance decrease would be too large.","tokens_in":2133,"feed_emoji":"🌡️","tokens_out":2263,"duration_ms":101477,"temperature":0.7,"pith_summary":"This paper reports the first laboratory measurements of acetylene (C2H2) vacuum-ultraviolet (VUV) absorption cross sections at the high temperatures typical of warm exoplanet atmospheres, from 296 K to 773 K over 115–230 nm. The authors show that heating acetylene increases its absolute VUV absorption: the increase is roughly uniform from 115 to 185 nm, then climbs steeply from 185 to 230 nm, reaching about a factor of 20 near 220 nm at 773 K. Feeding these data into a one-dimensional thermo-photochemical model of a hot Jupiter raises the high-altitude photodissociation rate of C2H2 by about a factor of three and lowers the predicted C2H2 abundance by up to 40% near $5\\times10^{-5}$ bar, with smaller changes to CH4 and C2H4. The work matters because photochemical models of exoplanets currently rely on room-temperature cross sections, and temperature-dependent data change both the chemistry and the depth at which starlight penetrates.","feed_headline":"Hot acetylene soaks up 20x more VUV light near 220 nm","feed_subtitle":"New 773 K lab data cut modeled C2H2 abundances by 40% and shift where starlight drives photochemistry.","key_machinery":"The carrying mechanism is the temperature-dependent population of vibrationally excited ground-state levels, which produces hot bands in the $A{}^1A_u \\leftarrow X{}^1\\Sigma_g^+$ band system longward of 185 nm and a new band near 156.7 nm; these changes are quantified with the factor $F_T = (\\sigma_T - \\sigma_{296\\,\\mathrm{K}})/\\sigma_{296\\,\\mathrm{K}}$. Experimentally, the cross sections are derived from the Beer–Lambert law using a 165 cm quartz cell heated up to 773 K with MgF$_2$ windows, a deuterium lamp plus VUV monochromator, and synchrotron spectra for wavelength calibration and cross-checks. The atmospheric application uses the paper's one-dimensional thermo-photochemical model, which computes photodissociation rates and steady-state abundances from the new cross sections.","core_discovery":"The central discovery, on the paper's own terms, is that acetylene's absolute VUV absorption cross section is strongly temperature dependent across the full photodissociation window. As temperature rises from 296 to 773 K, the continuum absorption increases while the discrete band intensities decrease, so the net cross section grows; from 115 to 185 nm the relative increase is nearly wavelength-independent (average $F_T$ of 0.5 at 773 K, with band-to-band excursions up to 3.5), and from 185 to 230 nm it rises sharply to a maximum $F_T$ of about 20 near 220 nm, attributed to hot bands of the A–X system. A new hot band appears near 156.7 nm at 473 K and above, assigned to the $\\nu_2$ vibrational mode. In the model atmosphere, using the 773 K cross sections instead of the 296 K ones makes C2H2 photodissociate three times faster at pressures below $10^{-2}$ bar, pushes the photodissociation cutoff upward, reduces the C2H2 abundance by 40% near $5\\times10^{-5}$ bar, and alters CH4 and C2H4 abundances down to $10^{-2}$ bar. The authors flag that the 222.5 nm feature at 773 K could partly reflect thermal decomposition products rather than C2H2 itself, and that additional time-resolved composition measurements are needed to confirm this.","pith_inferences":["Editorial inference: the 40% abundance change is a single-model realization (C/O = 2× solar, F-star, $K_{zz}=10^8$ cm$^2$ s$^{-1}$); for planets with different C/O, eddy mixing, or cooler upper atmospheres, the effect could be larger or smaller, and the paper does not map that sensitivity.","Editorial inference: if the 222.5 nm bump is confirmed as pyrolysis-product absorption rather than C2H2 hot bands, the $F_T \\approx 20$ near 220 nm and the 40% modeled decrease would need downward revision; a time-resolved measurement of the cell gas composition at 773 K, which the authors call for, would settle this.","Editorial inference: the steep rise longward of 185 nm arises from hot-band populations, so at even hotter temperatures (1000–1500 K) the simple scaled cross-section approach may break down; a vibrationally state-resolved treatment or measurements at those temperatures would extend the result.","Editorial inference: the atmospheric model assumes a single C2H2 photodissociation channel with unit quantum yield; if the quantum yield for C2H + H also varies with temperature, the abundance impact could differ from the 40% estimate."],"forward_implications":["Warm-exoplanet photochemical models should replace 296 K C2H2 cross sections with temperature-appropriate ones; in the paper's hot-Jupiter test, the 773 K data triple the high-altitude C2H2 photodissociation rate and reduce the predicted C2H2 abundance by 40% near $5\\times10^{-5}$ bar.","Using hot cross sections shifts the 150–230 nm actinic-flux cutoff upward by roughly an order of magnitude in pressure, so photochemistry at deeper levels is suppressed and species that share that opacity window are shielded.","The new 156.7 nm hot band, growing with temperature, gives a spectral signature that can identify warm acetylene and test the assumed temperature in future laboratory or atmospheric spectra.","Because similar temperature sensitivity is already documented for CO2, the implication for modelers is that other major VUV absorbers (CH4, C2H4, NH3, H2S) need high-temperature measurements if photochemical predictions are to be trusted."],"supporting_citations":[{"why":"Supplies the high-temperature cell methodology and temperature-gradient treatment that the C2H2 measurements inherit, and demonstrates the same kind of thermal cross-section increase for CO2.","marker":"Venot et al. (2018)"},{"why":"Introduces the heated VUV cell and the maximum-temperature assumption used to convert pressure to gas density; also the first high-temperature CO2 cross-section study.","marker":"Venot et al. (2013)"},{"why":"Provides the earlier 370 K C2H2 cross-section increase at 120–140 nm and defines the $F_T$ factor-of-change formula used to quantify temperature dependence.","marker":"Wu et al. (2001)"},{"why":"Gives high-temperature C2H2 cross sections at 200–300 nm from shock-tube measurements and is the key comparison showing that static-cell pyrolysis can inflate cross sections.","marker":"Zabeti et al. (2017)"},{"why":"Earlier static-cell high-temperature C2H2 data above 200 nm; the paper cites it as possibly affected by pyrolysis, anchoring the contamination caveat.","marker":"Vattulainen et al. (1997)"},{"why":"Hot-band assignments in the A–X system that the paper uses to interpret the sharp 185–230 nm increase.","marker":"Watson et al. (1982)"},{"why":"Identifies the hot bands in the A–X system near 215–230 nm used to explain the long-wavelength temperature dependence.","marker":"Van Craen et al. (1985)"},{"why":"Provides comparison C2H2 cross sections across 185–235 nm and the finding that the photodissociation rate changes by <1% with spectral resolution, justifying 1 nm model binning.","marker":"Bénilan et al. (2000)"},{"why":"The model code used for the atmospheric simulations.","marker":"Al-Refaie et al. (2024)"},{"why":"Supplies the chemical scheme used in the one-dimensional thermo-photochemical model.","marker":"Veillet et al. (2024)"}],"fun_headline_variants":["Acetylene VUV absorption up to 20x higher at 773 K near 220 nm","Hot acetylene VUV cross section rises 20x at 220 nm, alters exoplanet chemistry","First high-temp acetylene VUV data show 20x increase near 220 nm","773 K acetylene VUV measurements reshape hot Jupiter photochemistry"],"cache_read_input_tokens":22016,"weakest_assumption_plain":"The load-bearing premise is that the strong absorption seen at 773 K above about 210 nm is truly acetylene, not absorption by molecules formed as acetylene thermally decomposes inside the static cell.","fun_headline_variants_meta":{"raw":{"variants":["Acetylene VUV absorption up to 20x higher at 773 K near 220 nm","Hot acetylene VUV cross section rises 20x at 220 nm, alters exoplanet chemistry","First high-temp acetylene VUV data show 20x increase near 220 nm","773 K acetylene VUV measurements reshape hot Jupiter photochemistry"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000316,"raw_usage":{"total_tokens":1930,"prompt_tokens":1229,"completion_tokens":701,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":845,"completion_tokens_details":{"reasoning_tokens":607}},"tokens_in":845,"tokens_out":701,"duration_ms":6995,"temperature":1.0,"reasoning_tokens":607,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T22:01:03.611873+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the composition of the gas inside the static cell as a function of time at 773 K (e.g., mass spectrometry or time-resolved spectroscopy), or repeat the 773 K long-wavelength measurement in a fast-flow or shock-tube cell with microsecond residence times. If the 222.5 nm bump and the steep rise above 210 nm disappear or shrink when pyrolysis products are absent, the reported factor-of-20 increase is inflated and the 40% modeled abundance decrease would be too large.","supporting_citations":[{"cited_title":"2018, A&A, 609, A34","cited_arxiv_id":null,"evidence_quote":"Supplies the high-temperature cell methodology and temperature-gradient treatment that the C2H2 measurements inherit, and demonstrates the same kind of thermal cross-section increase for CO2."},{"cited_title":"2013, A&A, 551, A131","cited_arxiv_id":null,"evidence_quote":"Introduces the heated VUV cell and the maximum-temperature assumption used to convert pressure to gas density; also the first high-temperature CO2 cross-section study."},{"cited_title":"2017, Proceedings of the Combustion Institute, 36, 4469 Zádor, J., Fellows, M","cited_arxiv_id":null,"evidence_quote":"Gives high-temperature C2H2 cross sections at 200–300 nm from shock-tube measurements and is the key comparison showing that static-cell pyrolysis can inflate cross sections."},{"cited_title":"1997, Ap- plied Spectroscopy, 51, 1311","cited_arxiv_id":null,"evidence_quote":"Earlier static-cell high-temperature C2H2 data above 200 nm; the paper cites it as possibly affected by pyrolysis, anchoring the contamination caveat."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Hot-band assignments in the A–X system that the paper uses to interpret the sharp 185–230 nm increase."},{"cited_title":"F., Venot, O., Changeat, Q., & Edwards, B","cited_arxiv_id":null,"evidence_quote":"The model code used for the atmospheric simulations."}],"review_version":1}