The CDMS database contains a catalog of transition frequencies
and state energies for atoms and molecules of astrophysical and
The relevant laboratory data on these molecules is scattered over
many journals and very many issues, and the CDMS has the purpose
to provide the relevant data to the astronomical and
spectroscopical community in a standardized way. Generally,
emphasis has been put on creating new or updated entries taking
into account particularly the submillimeter and terahertz
The entries are not restricted to species already observed in the
ISM or CSM, but also include molecules that may be found there in
the future. Therefore, the catalog can be, and has been used to
identify new molecules in space. At present, the catalog covers
mostly rotational transitions from the radio-frequency to the
terahertz region and lower mid-infrared region, i.e. frequencies
lower than ~18 THz or ~600 cm-1, or wavelengths longer
than ~16.5 μm.
Vibration-rotation transitions in the far-infrared region have
been included for C3. They may be included in a
greater number in the near future. Vibration-rotation transitions
in the mid-infrared region are currently not considered to be
included in the database. If there is a genuine interest for such
information to be included in our database please sent us
comments and suggestions in order to suggest species. Information
on background literature is desirable !!
CDMS is part of the Virtual Atomic and Molecular Datacenter
(VAMDC) and thus supports all its standards. [more...]
Comments and suggestions are very welcome. Please visit the Contact section.
Please acknowledge use of the CDMS by citing the following
references. You are very welcome to state the web address
also. We recommend to cite the original sources of the data too,
which are given in the documentations, at least as far as this
The first reference describes the structure of the files which
have been used to generate the catalog, whereas the second, more
recent one, contains updates of the database.
The catalog section of this web portal gives access to all the data available in the catalog.
Data such as listings of transitions and states is
provided through a query form, which allows to specify
restrictions on the returned data, e.g. specify a frequency range.
In addition, there is a documentation for each species which contains
The catalog section starts with a listing of species, which
contains all the species available in CDMS. A link named doc guides
the user directly to the documentation of the species specified in
the table row.
To access the query form, the user has to select the species of
interest via mouse clicks (one or multiple) first and confirm the
selection with the corresponding submit-button. The selection can
also be done on the level of isotopologs or molecules. Pressing
one of the buttons above the listing will change the listing
correspondingly. Thus, if the level of molecules has been chosen,
the listing will show all the different molecules in the database
and queries will restrict the data output to all species of the
The query form offers the possibility to submit queries also to
all databases in the VAMDC network if chosen in the
DATABASE-Select box. Results from other databases is only obtained
if selections on the level of isotopologs or molecules have been
defined. Species define concrete entries (compilations) in CDMS
and thus only data from CDMS is returned by the network.
Further refinements on the query such as frequency range, can be
specified in the corresponding REFINEMENTS section. The page also
shows a short summary of data found in the databases for the given
query. Hit the SHOW-button to view the complete result. Different
output formats can be chosen.
Each line in a given catalog entry corresponds to one spectral
feature, some of which might be overlapped. The information given
for the spectral features is shown below for two lines of
Frequency of the line (usually in MHz, can
be in cm–1; see below); uncertainty of the line (usually in MHz, can be
in cm–1; see below); base 10 logarithm of the integrated intensity at
300 K (in nm2MHz); degree of freedom in the
rotational partition function (0 for atoms, 2 for linear
molecules, and 3 for non-linear molecules; lower state energy (in
cm–1); upper state degeneracy
gup; molecule tag
(see below) – a negative value indicates that both line
frequency and uncertainty are experimental values; coding of the quantum numbers; and
finally the quantum numbers.
1872169.0570 0.2000 -1.3865 3 887.6325165 -32503 30327 325 26 324
1872505.7621 0.2374 -2.3388 3 1107.9703 55 32503 30327 622 26 621
The line position and its uncertainty are either in units of
MHz, namely if the uncertainty of
the line is greater or equal to zero; or the units
are in cm–1, namely if the uncertainty of the line is less or
equal to zero !
with gI the spin-statistical weight and
gF = 2F + 1 the upper state
The six digit molecule tag consists
of the molecular weight in atomic mass units for the first three
digits (here: 2×1 + 12 + 18 = 32), a 5, and the last two
digits are used to differenciate between entries with the same
The quantum numbers are given in the
F1 . . .
for the upper state followed immediately by those for the lower state (see also below).
Since the program was written for asymmetric top molecules, some
of the quantum numbers may be redundant.
numbers are available at most to describe the upper and lower
state, respectively. In case more quantum numbers are needed, even
if some of them are redundant, the only spin designating quantum
numbers are n, F,
were n is an aggregate spin number.
The CDMS database contains predictions for atoms and molecules of
astrophysical and atmospheric interest as well as some
documentation on which data have been used for the predictions.
Its Catalog section contains mostly rotational transition
frequencies, uncertainties, intensities, and a wealth of other
information of atomic and molecular species that may occur in the
ISM or CSM or in planetary atmospheres.
The transition frequencies were predicted from fits of
experimental data to established Hamiltonian models.
Herb Pickett's programs SPFIT and
SPCAT have been used for the most part to
generate the predictions.
Vibration-rotation transitions in the far-infrared region have
been included for C3. They may be included in a greater
number in the near future. Vibration-rotation transitions in the
mid-infrared region are currently not considered to be included in
the database. If there is a genuine interest for such information
to be included in our database please sent comments and
suggestions in order to suggest species (email to one of the given
contacts). Information on background literature is desirable !!
Strictly speaking, only the final quantum number F, which
includes all effects of rotation etc. as well as electronic and
nuclear spins, is a good one; meaning that it has a well-defined
an unambiguous meaning. And this holds only in the absence of an
electric or magnetic field. Mixing effects mediated by,
e. g., vibration-, fine structure- (electronic spin), or
hyperfine structure-rotation interaction (nuclear spins)
frequently will prevent N, K, v, J, or
Fi from being good quantum numbers in
general. Of course, in many instances these quantum numbers are
reasonably meaningful over a large range of quantum number
combinations. Only the selection rule
ΔF = 0, ±1 holds
strictly ! Fine or hyperfine structure effects may cause
mixing of levels having different values of N, J,
etc., so that ΔN = 0, ±1 and
ΔJ = 0, ±1 only hold
approximately ! Note: certain hyperfine interactions can
cause mixing of ortho- and para-levels !
E. g. vibration-rotation interaction can cause mixing of
different values of K. The strong transitions obey the
ΔF = . . . = ΔJ = ΔN;
for low values of N, other transitions may be comparatively
The projections of the rotational angular momentum onto the
a- and c-axes are described by the quantum numbers
Ka and Kc. Usually
Kc, not Ka, is the more
meaningful quantum number for an oblate asymmetric rotor while
Ka is more appropriate for a prolate rotor. For
that reason, one may find Kp and
Ko instead of Ka and
Kc, respectively. For rotational levels near the
oblate (prolate) limit, i. e. low (high) values of
Ka and high (low) values of
Kc, it is more useful to use
Kc (Ka) as the designated
K even if the molecule is a prolate (oblate)
The a-type transitions are described by
ΔKa ≡ 0 mod 2
ΔKc ≡ 1 mod 2. Transitions
with ΔKa = 0 are the strongest
ones – by far so for a molecule close the the prolate limit,
e. g. H2CO. The c-type transitions
are described analogously:
ΔKc ≡ 0 mod 2
ΔKa ≡ 1 mod 2. Again,
transitions with ΔKc = 0 are the
strongest ones – by far so for a molecule close the the
oblate limit. Both a- and c-type transitions do not
conserve the parity. The b-type transitions are
ΔKa ≡ 1 mod 2
ΔKc ≡ 1 mod 2;
these do conserve the parity. And again, transitions with
ΔKa = 1 and
ΔKc = 1 are the strongest ones
– by far so for a molecule close the the prolate and oblate
The assignment of quantum numbers may seem to be a straightforward
issue. While this is true in many simple systems this is
not the case in general !! Mixing effects are
model-dependent, and in extreme cases the assignment of certain
levels can be altered based on very small changes in the parameter
– even more so if the parameter set is different.
assignment of, e. g., K quantum numbers is not always
unique. To avoid assignment of one quantum number to more than one
state, quantum numbers are assigned to levels in the order of
The presence of more than one
non-zero spin will cause assignment ambiguities. To minimize these
assignment ambiguities, certain rules apply how the various
spin-angular momenta are coupled to the rotational angular
momentum. Usually, they are coupled in order of decreasing
size. Exception: one set of equivalent nuclear spin-angular
momenta is coupled to the combined spin-rotational angular
momentum last. The same applies if two different spin-angular
momenta are coupled together before they are coupled to the
combined spin-rotational angular momentum. Thus, the electronic
spin-angular momentum is usually coupled to the rotation
first. Exception: if the effects of the nuclear
spin-electron spin coupling are larger than the effects caused by
the electronic spin alone. This may happen in particular in some
radicals with Σ electronic state, e. g. in
13CN, the order of the spins is 13C,
electron, 14N. Quantum number assignments always
refer to Hund's case (b). Alternative assignments may be available
as non-default in the future.
CDMS offers different ways how catalog data can be searched and
accessed. Probably, the easiest way is through this webportal or
alternatively through VAMDC's portal at (http://portal.vamdc.eu).
Alternatively, every software, e.g. web-browsers, which can send
http-requests, can be used and data can be retrieved just by
formulating the data request as an URL string. The data will be
returned as XML-file. Please have a look at the VAMDC standards
section for more information.
The predicted uncertainties of the transition frequencies are
model dependent. Therefore, an additional parameter employed in
the fit will cause these to increase in general. Basically for the
same reason, extrapolations should always be viewed with some
caution since these may be affected by spectroscopic parameters
that could not be determined thus far. In contrast, interpolations
should be reliable in most instances.
If a large number of transition frequencies has been measured and
is included in a fit the predicted uncertainties can be much
smaller than the experimental uncertainties for a large range of
quantum numbers. One should keep in mind that these smaller
uncertainties are only meaningful if the experimental
uncertainties are caused by purely statistical effects. For that
matter, the predictions in the CDMS catalog have experimental
transition frequencies and uncertainties nerged into the catalog
file to indicate a more conservative means.
Recent catalog entries often contain some estimates as to how far
the predictions should be reliable. Here "reliable"
means that the transition frequencies should be found within three
to ten times the predicted uncertainties.
The partition function Q is very important to calculate
intensities of molecular lines at a given temperature. In general,
only data for the ground vibrational state have been considered in
the calculation of Q for a certain species. If excited
vibrational states have been taken into account this will be
mentioned in the documentation. Usually, individual contributions
of the vibrational states are given in the documentation, too, or
a link is given to a separate file containing the information.
Spin-statistical weight-ratios have been considered in most
In very cold regions of the ISM it may be important to consider
ortho and para states separately. The energy of the
lowest rotational or rotation-hyperfine level is 0 by default. The
energy of the lowest level for the other spin-modification(s) is
usually given in the documentation. Strictly speaking, one should
consider Q values for the different spin-modifications
separately – at least at low temperatures. We intend to
provide this information in the near future.
CDMS supports the standards defined by VAMDC. Please have a look
for detailed information.
All databases in the VAMDC network can be queried simply via
http-request. Thus every simple web-browser is capable of sending
data request to the databases in retrieve the result just by
formulating the request as a URL and a XML-file will be
returned. Within the VAMDC standards it is defined how the URL
string has to be formulated and how the returned XML-file will
Most importantly, a protocol for data-access (VAMDC-TAP), a common
query language (VSS2), and a common output format (VAMDC-XSAMS)
have been defined.
is a protocol for data-access services that provide the common
table model matching VAMDC-XSAMS and which can return the results
of queries in VAMDC-XSAMS. It is based on IVOA's Table Access
Protocol (TAP), which already provides virtual data and allows to
plug in the query language VSS2 and the data model VAMDC-XSAMS.
VAMDC-TAP is defined as a web-service protocol. That means that
VAMDC-TAP services are driven by GET and POST requests to HTTP (or
VAMDC SQL Subset 2 (VSS2) is a query language designed for the
VAMDC-TAP web-services. It specifies how queries to VAMDC
databases have to be formulated.
The VAMDC-XSAMS standard defines the data exchange format which is
used by VAMDC databases. All databases within the VAMDC network
will return data as XML file according to this standard.
Started with the International Atomic
Energy Agency's XML Schema for Atomic, Molecular and Solid Data
(XSAMS) version 0.1.1 [XSAMS], VAMDC consortium has found that
modifications/additions were necessary in order to meet the needs
of implementation and queries. This effort has resulted in the
so-called VAMDC-XSAMS schema that is used within VAMDC.
Here are some equations the user of the CDMS catalog may find helpful. They are
taken from SUBMILLIMETER, MILLIMETER, AND MICROWAVE SPECTRAL
LINE CATALOG by H. M. Pickett, R. L. Poynter, E. A. Cohen,
M. L. Delitsky, J. C. Pearson, and H. S. P. Müller;
J. Quant. Spectrosc. Radiat. Transfer 60 (1998) 883
Note: While frequently it
is straightforward to calculate Sg from
Sg µ g2 by
deviding through the respective µ
g2, this is not always correct or
applicable, for example in cases of strong vibration rotation
Note: Numerical problems
may occur in eq. 1, 2, 3 and 5 if the frequency ν is
small with respect to the lower state energy E". It
is advisable to take the following expressions into accout: