samson_1994 =========== J\. A\. R\. Samson, Z\. X\. He, L\. Yin and G\. N\. Haddad, "Precision measurements of the absolute photoionization cross sections of He", J\. Phys\. B\: At\. Mol\. Opt\. Phys\. 27, 887 \(1994\) `Source record `__ · :doc:`Catalogue ` · :doc:`Review register ` DOI: `10\.1088/0953\-4075/27/5/008 `__ **Source kind:** measurement **Recorded technique:** Total absorption cross sections from a double ion chamber, sigma = \(1/nL\) ln\(i1/i2\) \(Eq\. 1\), with a 6 kV spark\-discharge line source and a 2\.2 m grazing\-incidence monochromator \(discrete lines 10\-120 eV, free of higher orders and scattered light\); pressure from a calibrated capacitance manometer \(0\.5 %\), picoammeters interchanged to cancel their calibration; each point the average of 5\-12 measurements \(pp\. 887\-888\)\. Above 120 eV an extrapolation fitted to Watson's \(1972\) and Denne's \(1970\) data, and above 280 eV predicted values interpolated to Bearden's \(1966\) attenuation data corrected for N2 impurity and for Rayleigh and Compton scattering \(pp\. 892\-896\)\. **Materials in structured entries:** He **Dataset quantities:** photoionization\_cross\_section **Contents:** 3 datasets; 3 formulas. Curation notes -------------- Below the double\-ionization threshold these absorption cross sections are the photoionization cross sections \(the paper uses the terms interchangeably\); 'Below 1\.5 keV the cross sections can be considered to be equal to the photoionization cross sections' \(p\. 894\); above, scattering was subtracted\. Photon energies in eV \(the ionization threshold 24\.587 eV\)\. Accuracy \(pp\. 887\-889\)\: '\+\-1 % to \+\-1\.5 % from threshold to about 60 eV and \.\.\. \+\-2 % from 60 to 120 eV' \(p\. 887\); 'Each data point has an accuracy of \+\-1 %' between 24\.6 and 48 eV \(p\. 888\); '\+\-1 % and \+\-1\.5 %' in the 45\-75 eV resonance region \(p\. 889\); 'estimated error of \+\-2 % between 67 and 120 eV' \(p\. 889\)\. The values of Table 1 are the best fit to the data \(the resonance rows 60\.19\-65\.40 eV are measured points in the 2snp autoionizing region\: 0\.55 Mb at 60\.19 eV is the window of the first resonance\), extrapolated above 120 eV\. The predicted 280\-8000 eV values\: 'the absolute error of these cross sections are estimated to be less than \+\-10 % assuming equation \(3\) to be valid' \(p\. 895\)\. Comparisons \(pp\. 889\-896\)\: Chan et al\. \(1991, chan\_1991\) within 2\-5 % to 75 eV and \+\-2 % at 67\-110 eV, too high above; Marr and West \(1976\) up to 26 % low at 170 eV; Henke et al\. \(1993, henke\_1993\) 10\-30 % above the predicted values at 1\-8 keV; Bell and Kingston within 5 % at 240\-700 eV\. Table 3 \(not extracted\: oscillator strengths of energy ranges, not a vocabulary quantity\), p\. 897\: 24\.6\-120 eV 1\.4194 \+\- 0\.021; 120\-280 eV 0\.1053 \+\- 0\.003; 280 eV\-inf 0\.0249 \+\- 0\.0025; discrete lines \(Chan et al\. 1991\) 0\.4310 \+\- 0\.020; total 1\.981 \+\- 0\.047 against the Thomas\-Reiche\-Kuhn value 2\. A trapezoidal integration of Tables 1 and 2 \(df/dE = sigma/109\.76 Mb eV; computed here\) gives 1\.394 \(1\.410 without the 60\.19 eV window point\), 0\.113 and 0\.023\: the 120\-280 eV entry of Table 3 is not reproduced \(7 % lower than the tabulated cross sections imply\), the total continuum \(1\.53 vs 1\.55\) agrees to 1\.5 %\. **Recorded bibliographic identity status:** VALIDATED\_WITH\_WARNINGS. This concerns identification of the source; it does not establish the scientific correctness of its contents. Datasets -------- samson\_1994/measured ~~~~~~~~~~~~~~~~~~~~~ .. code-block:: text { "id": "measured", "material": "He", "file": "He/samson_1994__measured.csv", "table": "tbl_95893c11a7e1", "quantity": "photoionization_cross_section", "unit": "Mb", "abscissa_unit": "eV", "wavelength_medium": "vacuum", "use": "primary", "uncertainty": { "relative": 0.01, "level": "not stated", "relative_rows": { "49": 0.015, "50": 0.015, "51": 0.015, "52": 0.015, "53": 0.015, "54": 0.015, "55": 0.015, "56": 0.015, "57": 0.015, "58": 0.015, "59": 0.015, "60.19": 0.02, "62.87": 0.02, "64.31": 0.02, "64.74": 0.02, "65.20": 0.02, "65.40": 0.02, "66": 0.02, "67": 0.02, "68": 0.02, "69": 0.02, "70": 0.02, "71": 0.02, "72": 0.02, "73": 0.02, "74": 0.02, "75": 0.02, "76": 0.02, "77": 0.02, "78": 0.02, "79": 0.02, "80": 0.02, "81": 0.02, "82": 0.02, "83": 0.02, "84": 0.02, "85": 0.02, "86": 0.02, "87": 0.02, "88": 0.02, "89": 0.02, "90": 0.02, "91": 0.02, "92": 0.02, "93": 0.02, "94": 0.02, "95": 0.02, "96": 0.02, "97": 0.02, "98": 0.02, "99": 0.02, "100": 0.02, "101": 0.02, "102": 0.02, "103": 0.02, "104": 0.02, "105": 0.02, "106": 0.02, "107": 0.02, "108": 0.02, "109": 0.02, "110": 0.02, "111": 0.02, "112": 0.02, "113": 0.02, "114": 0.02, "115": 0.02, "116": 0.02, "117": 0.02, "118": 0.02, "119": 0.02, "120": 0.02 } } } Table 1, 24\.587\-120 eV \(97 rows\)\: the best fit to this work's measurements, including the resonance\-region points 60\.19\-65\.40 eV\. Relative uncertainty 1 % to 48 eV, 1\.5 % to 59 eV, 2 % from 60\.19 eV \(the paper's accuracy statements, level not stated\)\. samson\_1994/extrapolated ~~~~~~~~~~~~~~~~~~~~~~~~~ .. code-block:: text { "id": "extrapolated", "material": "He", "file": "He/samson_1994__extrapolated.csv", "table": "tbl_95893c11a7e1", "quantity": "photoionization_cross_section", "unit": "Mb", "abscissa_unit": "eV", "wavelength_medium": "vacuum", "use": "primary" } Table 1, 125\-280 eV \(32 rows\)\: 'We have extrapolated the solid line to higher energies to give the best fit to the data of Watson and Denne\. Both authors' quote error bars of \+\-3 %' \(p\. 890\); represented to \+\-1 % between 140 and 280 eV by Eq\. \(2\)\. The authors' recommended values; their own uncertainty is not stated \(the underlying data \+\-3 %\)\. samson\_1994/predicted ~~~~~~~~~~~~~~~~~~~~~~ .. code-block:: text { "id": "predicted", "material": "He", "file": "He/samson_1994__predicted.csv", "table": "tbl_168a48a2c629", "quantity": "photoionization_cross_section", "unit": "Mb", "scale": 1e-06, "abscissa_unit": "eV", "wavelength_medium": "vacuum", "use": "primary", "excluded_rows": { "280": "repeats Table 1's 280 eV value (0.0194 Mb = 19400 b), which is kept there" }, "uncertainty": { "relative": 0.1, "level": "bound" } } Table 2, 280\-8000 eV, printed in barns \(scale 1e\-6 to Mb\)\: the predicted photoionization cross sections, a smooth curve of increasing slope joining this work's E^\-3\.05 data to Bearden's attenuation data corrected for N2 and scattering \(Eq\. 3 above about 3 keV\); 'less than \+\-10 %' \(p\. 895\), stored as a bound\. Formulas -------- samson\_1994/eq2 ~~~~~~~~~~~~~~~~ .. code-block:: text { "id": "eq2", "material": "He", "equation": "Eq. (2), p. 892", "printed": "sigma (b) = 403 E (keV)^-3.05", "variable": "E: photon energy in keV; sigma: photoionization cross section in barns (1e-24 cm^2)", "state": "none (per atom)", "density": "none", "range": "140-280 eV ('with an accuracy of +-1 %')", "coefficients": { "a": "403", "p": "-3.05" }, "check": "Gives 0.1620, 0.05460 and 0.01956 Mb at 140, 200 and 280 eV against Table 1's 0.160, 0.0550 and 0.0194 (+1.3 %, -0.7 %, +0.8 %; checked 2026-09-24)." } A power law for the photoionization cross section; no form in the vocabulary \(cross\-section formulas are not dispersion forms\)\. samson\_1994/eq3 ~~~~~~~~~~~~~~~~ .. code-block:: text { "id": "eq3", "material": "He", "equation": "Eq. (3), p. 894", "printed": "sigma (b) = 410 E (keV)^-3.5", "variable": "E: photon energy in keV; sigma: photoionization cross section in barns", "state": "none (per atom)", "density": "none", "range": "the photoionization cross sections derived from Bearden's and McCrary et al.'s attenuation data, about 3-8 keV", "coefficients": { "a": "410", "p": "-3.5" }, "check": "Gives 8.767, 1.467 and 0.2831 b at 3, 5 and 8 keV against Table 2's 8.77, 1.47 and 0.28 (checked 2026-09-24). The paper: Bell and Kingston's velocity form gives 5.74 b at 3.5 keV against 5.11 b from Eq. (3), 12 % (p. 895)." } With Bearden's uncorrected data the constant would be 510 \(480 in Samson et al\. 1993, p\. 894\)\. samson\_1994/power\_700\_1800 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .. code-block:: text { "id": "power_700_1800", "material": "He", "equation": "text, p. 896", "printed": "the present data follows the curve 335 E (keV)^-3.25", "variable": "E: photon energy in keV; sigma in barns", "state": "none (per atom)", "density": "none", "range": "700-1800 eV", "coefficients": { "a": "335", "p": "-3.25" }, "check": "Gives 1068, 335 and 49.6 b at 0.7, 1.0 and 1.8 keV against Table 2's 1060, 339 and 49.4 (within 1.2 %; checked 2026-09-24)." } Bell and Kingston's \(1971\) calculation follows 364 E^\-3\.25 over the same range, 8\.6 % higher \(p\. 896\)\. Corrections ----------- No dataset errata are recorded in this source record. Formula corrections, where recorded, appear with their formulas above. Curation notes may also discuss corrections or unresolved interpretation. Consult the :doc:`review register ` for the checks and decisions.