Gas-phase absorption cross sections σ(λ) of about 1000 molecules, from the vacuum ultraviolet to the near infrared, gathered from five open datasets: the MPI-Mainz UV/VIS Spectral Atlas (2013 snapshot), NCAR's TUV-x photolysis data, the Leiden photodissociation and photoionization database, IUP Bremen's molecular spectroscopy data, and the Harvard CfA molecular database. Every dataset is credited to the paper that measured it. Overlay datasets on one plot, read σ at your laser line (point value or averaged over the line width), interpolate between measurement temperatures, convert between cm², Mb, molar absorptivity and base-e or base-10 coefficients, and turn a cross section into an absorbance, an optical depth, or the fraction of molecules a pulse photolyses.
Key terms
Cross section σ
The effective area a molecule presents to a photon: the fraction of light absorbed by a thin layer is σ times the number of molecules per unit area. Units cm² molecule⁻¹.
Molar absorptivity ε
The same quantity per mole and in base 10, as used in solution spectroscopy: ε = σNA/(1000 ln 10) in L mol⁻¹ cm⁻¹.
Optical depth τ
τ = σnL for number density n and path L; the transmitted fraction is e−τ. Absorbance (base 10) is τ/ln 10.
Quantum yield φ
The fraction of absorbed photons that lead to a given product. Absorption alone does not say what happens next.
Recommended
A dataset an evaluation panel (NASA JPL or IUPAC) chose as the best available at the time of the evaluation.
Beta, in review. How it is tested: the data archive behind this page is checksummed file by file, the Bremen and CfA references were checked against Crossref (the 2013 Mainz snapshot carries no DOIs), and spot checks compare the archived datasets at well-known wavelengths (ozone at the 253.65 nm mercury line, among others) with established values; the unit conversions use CODATA 2022 constants. There is no self-test tab yet. Values are shown as each source gives them; check the original paper before you rely on a number. Found a problem? Email foley.photon@gmail.com.
Molecule and datasets
Spectrum
nm
nm
At a laser line
nm
nm
Absorption and photolysis
cm²
K
cm
mJ/cm²
Convert units
How it works
Values between points are linear interpolations of the tabulated data. A value at a laser line with a width averages σ over a Gaussian of that full width at half maximum. If the data are tabulated more coarsely than the line, the tool says so: the average is then only an interpolation, and narrow structure (the bands of O₂, NO or SO₂) can make it wrong by large factors.
Units. ε (base 10) = σNA/(1000 ln 10); ε (base e) = σNA/1000; k (cm⁻¹ atm⁻¹ at 273.15 K) = σn₀, with Loschmidt's constant n₀ = 2.686 780 111 × 10¹⁹ cm⁻³ (CODATA 2022). Physicists' units: 1 Mb = 10⁻¹⁸ cm², 1 barn = 10⁻²⁴ cm², 1 Ų = 10⁻¹⁶ cm², a₀² = 2.800 285 × 10⁻¹⁷ cm², and the oscillator-strength density df/dE = σ/(2π²reħc), so σ in Mb ≈ 109.76 × df/dE in eV⁻¹ (Berkowitz's convention for VUV photoabsorption).
Photolysis per pulse. Photon fluence Φ = F/(hc/λ); fraction excited 1 − e−σΦ, fraction photolysed 1 − e−σΦφ, for an optically thin sample and one-photon absorption. The saturation fluence hν/σ is where a fraction 1 − 1/e of the molecules has absorbed.
Temperature. Where one series was measured at several temperatures, σ at an intermediate temperature is interpolated linearly in T, never extrapolated.
Assumptions and limits
Data are shown as their sources give them, including the wavelength scale (air or vacuum, where stated) and the spectral resolution of the measurement. Above 200 nm, air and vacuum wavelengths differ by about 0.06 nm at 250 nm and 0.11 nm at 400 nm.
Cross sections of banded spectra depend on resolution, temperature and pressure; compare like with like.
Absorption is not photolysis: what happens after the photon is absorbed is set by the quantum yields.
Data sources and terms
Related tools
The Thermochemistry Lab (beta) gives the photolysis thresholds, excited-product channels and photofragment energetics that go with these cross sections, from Argonne’s Active Thermochemical Tables.