The CDMS database contains a catalog of transition frequencies and state energies for atoms and molecules of astrophysical and atmospheric interest. 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 regions.
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 is feasible.
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 selected molecule.
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 H2C18O.
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
REMARKS
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 spin-rotational degeneracy.
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 molecular weight.
The quantum numbers are given in the following order: J (or N); Ka and Kc (or ±; K); v; F1 . . . F 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.
Six quantum 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 selection rules ΔF = . . . = ΔJ = ΔN; for low values of N, other transitions may be comparatively strong.
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) rotor !
The a-type transitions are described by ΔKa ≡ 0 mod 2 and 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 and 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 described by ΔKa ≡ 1 mod 2 and 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 limit, respectively.
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.
The 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 increasing ambiguity.
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 instances.
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 at http://www.vamdc.eu/documents/standards/ 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 look like. Most importantly, a protocol for data-access (VAMDC-TAP), a common query language (VSS2), and a common output format (VAMDC-XSAMS) have been defined.
VAMDC-TAP
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 HTTPS) URIs.
Example:
VSS2
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.
VAMDC-XSAMS
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 – 890.
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 interaction.
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: