Cross sections
Photoabsorption, photodissociation, and photoionisation cross sections are collected from published experimental and theoretical work. The selection and combination of data is described in various overview papers. References to the data sourced for each cross section are also embedded in their data files and relate to this bibliography.
Data format
The cross sections are available in multiple formats, as machine readable text files and binary hdf5 files. The explanation of these data formats is given in Heays et al. 2017 and in this guide including examples of how to use the binary data.
- Text format (.pd and .pi) files are as provided previously with this database. These divide cross sections into continuum and line absorption parts, and are on a reduced wavelength scale for computational efficiency.
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- Binary (.h5) files containing photodissociation and ionisation cross sections in a single file with sometimes a very large size.
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- Text (.txt) files containing photodissociation and ionisation cross sections in a single file with sometimes a very large size.
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- Text (_0.1nm.txt) files containing photodissociation and ionisation cross sections in a single file but reduce to a uniform 0.1nm wavlength grid for computational efficiency.
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Thresholds
The given thresholds are for the longest wavelength channels which are accessible by photoabsorption. The Lyman-alpha cross sections are taken at 121.4 nm.Cross sections in the database are given to a short-wavelength limit of 91.2 nm or shorter (the Lyman limit where all shorter-wavelength radiation is expected to be absorbed by photoionising interstellar H-atoms). All cross sections in .pd and .pi format are restricted to this limit of interstellar relevance in order to minimise the computational load of using this data.
Photodestruction products
The listed photodissociation products are the dominant fragments at the threshold wavelength. Significant branching to other fragments is possible at shorter wavelengths and is sometimes dominant. The wavelength-dependent branching of H_{2}O and NH_{3} are given on their own pages. Some other branching information is available from the PHIDRATES database. In all cases threshold photoionisation is dominated by the nondissociative ejection of an electron, but successively more dissociative-ionisation channels may be come significant at shorter wavelengths.
Uncertainties
Uncertainties are estimated from the known or approximate uncertainties of the measured and experimental cross sections compiled for each atom or molecule. These are estimated over the wavelength ranges relevant to stellar radiation (91.2 to the long wavelength threshold), cosmic-ray induced radiation (80 to 150 nm) and the Lyman-alpha wavelength (121.4 nm). These are categorised as follows:- A^{+}. Accurate to within 20%
- A. Accurate to within 30%
- B. Accurate within a factor of 2
- C. Accurate within a factor of 10
Cross section summary data
This table summarise some properties of the photodissociation and ionisation of atoms and molecules in the database.
[Download this data as a csv file.]
Photodis. | Threshold (nm) | Ly-alpha cross sect. (cm^{2}) | Cross section uncertainty relevant to | |||||
---|---|---|---|---|---|---|---|---|
Species | products | Photodis. | Photoion. | Photodis. | Photoion. | ISRF | Ly-alpha | Cosmic-ray UV |
Al | -- | — | 207 | 0.00e+00 | 8.71e-18 | B | B | B |
AlH | Al + H | 392 | 156 | 5.00e-20 | 0.00e+00 | C | C | C |
C | -- | — | 110 | 0.00e+00 | 0.00e+00 | B | -- | B |
C2 | C + C | 193 | 102 | 4.98e-19 | 0.00e+00 | B | B | B |
C2H | C2 + H | 253 | 109 | 3.21e-18 | 0.00e+00 | B | C | C |
C2H2 | C2H + H | 217 | 109 | 5.78e-17 | 0.00e+00 | A+ | B | B |
C2H4 | C2H2 + H2 | 258 | 118 | 2.36e-17 | 0.00e+00 | C | C | C |
C2H5OH | C2H5O + H | 208 | 119 | 2.56e-17 | 0.00e+00 | A | A | B |
C2H6 | C2H5 + H | 287 | 108 | 2.26e-17 | 0.00e+00 | A+ | A+ | A |
C3 | C2 + C | 268 | 102 | 1.38e-17 | 0.00e+00 | C | C | C |
C3H7OH | C3H7O + H | 208 | 123 | 5.16e-17 | 2.80e-19 | A | A | B |
CH | C + H | 358 | 117 | 5.00e-20 | 0.00e+00 | C | C | C |
CH+ | C + H+ | 303 | — | 5.00e-20 | 0.00e+00 | A | C | B |
CH2 | C + H | 417 | 119 | 5.00e-20 | 0.00e+00 | C | C | C |
CH2+ | CH+ + H | 204 | — | 5.00e-20 | 0.00e+00 | B | C | B |
CH3 | CH + H2 | 271 | 126 | 5.00e-20 | 5.31e-18 | B | B | B |
CH3CHO | HCO + CH3 | 342 | 121 | 2.03e-17 | 1.23e-18 | A | A | B |
CH3CN | CH2CN + H | 308 | 102 | 1.09e-17 | 0.00e+00 | A | A | B |
CH3NH2 | CH3NH + H | 244 | 135 | 0.00e+00 | 2.44e-17 | A | A | B |
CH3OH | CH3O + H | 280 | 113 | 1.39e-17 | 0.00e+00 | A+ | A+ | B |
CH3SH | CH3S + H | 331 | 131 | 2.18e-17 | 2.37e-17 | A | B | A |
CH4 | CH3 + H | 277 | 98 | 1.80e-17 | 0.00e+00 | A+ | A+ | B |
CH4+ | CH3+ + H | 1031 | — | 5.00e-20 | 0.00e+00 | B | C | B |
CN | C + N | 160 | 87 | 4.00e-19 | 0.00e+00 | B | C | B |
CO | C + O | 110 | 88 | 0.00e+00 | 0.00e+00 | A+ | -- | B |
CO+ | C+ + O | 149 | — | 5.00e-20 | 0.00e+00 | B | C | B |
CO2 | CO + O | 227 | 90 | 6.54e-20 | 0.00e+00 | A+ | A+ | B |
CS | C + S | 169 | 110 | 5.36e-17 | 0.00e+00 | C | C | C |
CS2 | CS + S | 278 | 123 | 1.71e-17 | 1.04e-18 | A | A | A |
Ca | -- | — | 203 | 0.00e+00 | 9.13e-19 | B | B | B |
Ca+ | -- | — | 104 | 0.00e+00 | 0.00e+00 | B | -- | B |
Cl | -- | — | 96 | 0.00e+00 | 0.00e+00 | B | -- | B |
Co | -- | — | 158 | 0.00e+00 | 1.77e-19 | B | B | B |
Cr | -- | — | 183 | 0.00e+00 | 7.35e-18 | B | B | B |
Fe | -- | — | 158 | 0.00e+00 | 4.86e-18 | A | A | B |
H | -- | — | 91 | 0.00e+00 | 0.00e+00 | A | -- | B |
H- | -- | — | 1470 | 0.00e+00 | 4.75e-18 | B | B | B |
H2 | H + H | 274 | 82 | 0.00e+00 | 0.00e+00 | A+ | -- | B |
H2+ | H + H+ | 290 | — | 6.83e-18 | 0.00e+00 | A | A | B |
H2CO | H2 + CO | 361 | 114 | 9.43e-18 | 0.00e+00 | B | B | B |
H2O | H + OH | 242 | 98 | 1.53e-17 | 0.00e+00 | A+ | A+ | B |
H2O2 | OH + OH | 556 | 117 | 9.65e-18 | 0.00e+00 | A | A | B |
H2S | H + SH | 318 | 119 | 5.39e-17 | 0.00e+00 | A | A | A |
HC3H | l-C3H + H | 400 | 138 | 5.00e-20 | 0.00e+00 | B | C | C |
HC3N | H + C3N | 244 | 107 | 1.98e-17 | 0.00e+00 | A | B | B |
HCN | H + CN | 243 | 91 | 3.40e-17 | 0.00e+00 | A | A | B |
HCO | H + CO | 2037 | 152 | 5.00e-20 | 0.00e+00 | C | C | C |
HCO+ | H + CO+ | 145 | — | 0.00e+00 | 0.00e+00 | B | -- | B |
HCl | H + Cl | 279 | 97 | 9.19e-19 | 0.00e+00 | A | A | B |
HCl+ | H + Cl+ | 267 | — | 8.44e-19 | 0.00e+00 | C | C | C |
HF | H + F | 211.4 | 77.5 | 3.10e-18 | 0.00e+00 | A | A | A |
HO2 | H + O2 | 476 | 109 | 5.00e-20 | 0.00e+00 | C | C | C |
K | -- | — | 286 | 0.00e+00 | 2.22e-19 | B | B | B |
Li | -- | — | 230 | 0.00e+00 | 1.33e-18 | A | A | B |
LiH | Li + H | 579 | 152 | 5.00e-20 | 0.00e+00 | A | C | B |
Mg | -- | — | 162 | 0.00e+00 | 7.13e-20 | B | B | B |
MgH | Mg + H | 623 | — | 5.00e-20 | 0.00e+00 | C | C | C |
Mn | -- | — | 167 | 0.00e+00 | 4.42e-20 | B | B | B |
N | -- | — | 85 | 0.00e+00 | 0.00e+00 | A | -- | B |
N2 | N + N | 127 | 79 | 0.00e+00 | 0.00e+00 | A+ | -- | B |
N2O | N2 + O | 735 | 96 | 3.85e-18 | 0.00e+00 | B | C | B |
NH | N + H | 362 | 92 | 3.10e-19 | 0.00e+00 | B | B | B |
NH+ | N+ + H | 281 | — | 4.68e-20 | 0.00e+00 | C | C | C |
NH2 | NH + H | 314 | 111 | 3.03e-20 | 0.00e+00 | C | C | C |
NH2CHO | NH3 + CO | 208 | 121 | 1.82e-17 | 1.88e-19 | A | A | B |
NH3 | NH + H2 | 301 | 122 | 8.37e-18 | 3.11e-19 | A+ | A+ | B |
NO | N + O | 191 | 134 | 4.81e-19 | 2.08e-18 | A | B | B |
NO2 | NO + O | 398 | 127 | 5.00e-20 | 1.06e-19 | B | C | B |
Na | -- | — | 241 | 0.00e+00 | 1.07e-19 | B | B | B |
NaCl | Na + Cl | 293 | — | 5.00e-20 | 0.00e+00 | C | C | C |
NaH | Na + H | 606 | 179 | 5.00e-20 | 0.00e+00 | A | C | B |
Ni | -- | — | 162 | 0.00e+00 | 3.12e-19 | B | B | B |
O | -- | — | 91 | 0.00e+00 | 0.00e+00 | B | -- | B |
O2 | O + O | 242 | 103 | 2.14e-19 | 0.00e+00 | A | B | B |
O2+ | O + O+ | 186 | — | 2.52e-18 | 0.00e+00 | B | C | B |
O3 | O2 + O | 1180 | 99 | 2.97e-17 | 0.00e+00 | B | B | B |
OCS | CO + S | 280 | 111 | 7.14e-18 | 0.00e+00 | A | A | A |
OH | O + H | 279 | 95 | 4.07e-18 | 0.00e+00 | A | B | B |
OH+ | O+ + H | 247 | — | 2.58e-25 | 0.00e+00 | A | A | B |
P | -- | — | 118 | 0.00e+00 | 0.00e+00 | C | -- | C |
PH | P + H | 398 | 122 | 1.13e-19 | 0.00e+00 | C | C | C |
PH+ | P+ + H | 369 | — | 1.06e-18 | 0.00e+00 | C | C | C |
Rb | -- | — | 297 | 0.00e+00 | 1.49e-19 | B | B | B |
S | -- | — | 120 | 0.00e+00 | 0.00e+00 | A | -- | B |
S2 | S + S | 283 | 133 | 0.00e+00 | 1.80e-18 | B | C | B |
SH | S + H | 345 | 119 | 8.07e-18 | 0.00e+00 | B | C | C |
SH+ | S+ + H | 320 | — | 7.41e-20 | 0.00e+00 | B | C | C |
SO | S + O | 233 | 120 | 1.06e-16 | 0.00e+00 | B | C | C |
SO2 | O + SO | 219 | 100 | 3.86e-17 | 0.00e+00 | A | A | A |
Si | -- | — | 152 | 0.00e+00 | 3.61e-17 | B | B | B |
SiH | Si + H | 417 | 155 | 5.00e-20 | 0.00e+00 | B | C | B |
SiH+ | Si+ + H | 391 | — | 2.22e-17 | 0.00e+00 | A | B | B |
SiO | Si + O | 150 | 108 | 5.00e-20 | 0.00e+00 | B | C | B |
Ti | -- | — | 182 | 0.00e+00 | 5.72e-18 | C | C | C |
Zn | -- | — | 132 | 0.00e+00 | 5.91e-20 | B | B | B |
c-C3H | C3 + H | 289 | 129 | 5.00e-20 | 0.00e+00 | B | C | B |
c-C3H2 | c-C3H + H | 284 | 136 | 5.00e-20 | 0.00e+00 | B | C | B |
l-C3H | C3 + H | 379 | 144 | 5.00e-20 | 0.00e+00 | B | C | B |
l-C3H2 | l-C3H + H | 320 | 119 | 5.00e-20 | 0.00e+00 | B | C | B |
l-C4 | C3 + C | 263 | 116 | 5.00e-20 | 0.00e+00 | B | C | B |
l-C4H | C4 + H | 267 | 129 | 5.00e-20 | 0.00e+00 | B | C | B |
l-C5H | C5 + H | 348 | 168 | 5.00e-20 | 0.00e+00 | B | C | B |