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
The quantum yield φ is the probability that a particular
photochemical process will occur following the absorption of a photon by
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 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 (2016-08-25), the database contains:
- 6141 cross section data files for 1026 species
- 154 quantum yield data files for 30 reactions
- 1737 references
- 2674 graphical representations
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:
In some cases, additional specifications are added in brackets after
the wavelength, e.g.:
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
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.:
Links to the plots are shown on the pages of the individual
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:
- Species: Find the search string in the species'
name or formula.
- Identifier: Find a species through
its CAS registry number
- Reference: Find the search string somewhere in the
reference (author, title, journal) and return all data sets from that
- Full Text: Search the species, the reference, and
also the comments.
- Recent Additions: Show recent additions to the
All searches are case-insensitive.