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.

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 H2O and NH3 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:

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. (cm2) 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