Home

Cross Sections

Quantum Yields

Errata

Contact, Imprint, Acknowledgements

Keller-Rudek, H., Moortgat, G. K., Sander, R., and Sörensen, R.: The MPI-Mainz UV/VIS spectral atlas of gaseous molecules of atmospheric interest, Earth Syst. Sci. Data, 5, 365–373, (2013), DOI: 10.5194/essd-5-365-2013

Scientific background

The photolysis rates of gaseous trace species in the atmosphere are important parameters in the atmospheric sciences. This is especially true for modeling atmospheric chemistry, as most chemical reactions are directly or indirectly driven by the sun’s radiation. Photolysis rates depend on the intensity of the actinic flux, and also on the properties of the absorbing molecules. Photodissociation rate coefficients are governed by the absorption cross section σ and the quantum yield φ of the photolysis.

The absorption cross section σ is defined by the Beer-Lambert law describing the attenuation of light by a homogeneous absorbing system:

I = I₀ exp -(σ d n)

where I₀ and I are the incident and transmitted light intensities, d is the absorption path length (in cm), n is the concentration of the absorber (in molecule/cm³), and σ is the absorption cross section (in cm² molecule^-1).

The quantum yield φ is the probability that a particular photochemical process will occur following the absorption of a photon by the molecule.

Both, σ and φ, depend on wavelength, temperature, and pressure. Thus, knowing their values under atmospheric conditions is essential. In addition, the experimental determination of kinetic and photochemical parameters of many elementary reactions requires the precise knowledge of the absorption cross sections of many species.

The database

The MPI-Mainz UV/VIS Spectral Atlas is a comprehensive collection of cross sections for gaseous molecules and radicals, primarily relevant to atmospheric research. The individual data sets were collected from the original publications, either copied from tabulations, or read from figures in those cases where numerical data could no longer be obtained. In addition, several spectra were obtained from databases of several research centers and via personal communication. As of today (2024-11-07), the database contains:

• 8082 cross section data files for 1383 species
• 154 quantum yield data files for 30 reactions
• 2144 references
• 3839 graphical representations

Data files

The numerical data are available in ASCII format. The *.txt files contain 2 columns. The first column gives the wavelength λ (in nm) (generally in air unless vacuum wavelengths are indicated). The second column contains the absorption cross section σ (in cm² molecule^-1) or quantum yield (dimensionless). The "E notation" is chosen for the absorption cross sections in the data files, e.g., 1.5e-18 is to be read as 1.5×10^-18. Some files contain a third column indicating the error limits of the cross sections. Each data set refers to an individual measurement per species. The names of the data files consist of four parts, separated by the underscore character:

formula_author(year)_temperature_wavelengths.txt

In some cases, additional specifications are added in brackets after the wavelength, e.g.:

CF2ClBr(Halon-1211)_GillotaySimon(1989)_295K_170-302nm(2nm).txt

CHOCHO_Horowitz(2001)_295K_210-470nm(int-c).txt

Graphical representations

The graphical representations show the absorption cross sections σ vs. wavelength λ. In contrast to the individual numerical data sets, the plots combine the data from different measurements. Two kinds of plots are presented, with linear ("lin") and logarithmic ("log") scaling of the σ axis, respectively. The names of the plot files are characterized by the notation

formula_lin.jpg or formula_log.jpg

e.g.,

CH2Br2_lin.jpg or CH2Br2_log.jpg

These plots refer to measurements at room temperature and show the entire UV/VIS wavelength range covered by the various experiments. For plots showing a particular wavelength range, a temperature range, or a publication period, the name of the file contains additional specifications after the formula, e.g.:

CH2Br2_250-348K_lin.jpg

CH2Br2_VUV_lin.jpg

Br2_504-553nm_lin.jpg

O3_Chappuis_400-800nm(1986-2003)_lin.jpg

Links to the plots are shown on the pages of the individual species.

Browsing and searching

To browse the Spectral Atlas, clicking the links to "Cross Sections" and "Quantum Yields" in the left column will show molecular categories and subcategories. Inside these (sub)categories are the species. The pages for the species contain links to the graphical representations and the data sets.

Five search options are provided in the right column:

1. Species: Find the search string in the species' name or formula.
2. Identifier: Find a species through its CAS registry number or InChIKey.
3. Reference: Find the search string somewhere in the reference (author, title, journal, doi) and return all data sets from that article.
4. Full Text: Search the species, the reference, and also the comments.
5. Recent Additions: Show recent additions to the Spectral Atlas.

All searches are case-insensitive.

Search cross sections