The Cologne Database for Molecular Spectroscopy (CDMS) provides in its catalog section transition frequencies of atoms and molecules mostly for radio astronomic observations. Species are covered which have been or may be observed in the interstellar medium (ISM), in the circumstellar envelopes (CSEs) of very young as well as very old stars, or in planetary atmospheres.
As of January 2015, almost 750 species are available which refer not only to the main isotopic species and the ground vibrational state, but also to minor isotopic species and excited vibrational states as far as these may be of interest for radio astronomers. The catalog is similar to that of the Jet Propulsion Laboratory. Duplication is avoided, nevertheless, some overlap exists. Frequently, entries in the CDMS catalog are more recent and then usually more reliable, some common entries exist, and in some cases, the JPL catalog entry is more recent and then usually more reliable.
Wavelengths reach from longer than microwaves to the millimeter, terahertz, and far-infrared region, in few cases also to the infrared.
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The physical and chemical conditions of the diverse regions in space determine which of the atoms or molecules can be observed in emission or absorption at certain wavelengths. In turn, observations of certain atoms or molecules provide information on densities, temperatures, degrees of ionization, or other properties in a given region in space. Another important aspect of astrochemistry is the complexity of molecules observable in space as molecules formed in a molecular cloud around a young star may contribute to the formation of life on a planet such as Earth.
Around 200 molecules have been detected in diverse media in space (ISM, CSEs, stellar, and planetray atmospheres (see. e.g. http://www.astro.uni-koeln.de/cdms/molecules or http://www.astrochymist.org/astrochymist_mole.html) since the late 1930s in the optical and since the late 1960s by means of radio astronomy.
As a consequence, multifrequency observations can be used to reconstruct the physical structure of the studied object under the assumption of local thermodynamical equilibrium (LTE). LTE is a reasonable assumption at elevated temperatures (> ~50 K) and at larger densities (> ~104/cm3). Collisional processes may have to be considered if one of the conditions is not met. Radiative processes may also be important.
With the advent of a new generation of heterodyne instruments possessing large instantaneous bandwidths, from the ground-based single-dish 30m IRAM [http://www.iram-institute.org/EN/30-meter-telescope.php] to the Atacama Large Millimeter/submillimeter Array [ALMA, http://www.almaobservatory.org/] interferometer, and to the Herschel Space Observatory [http://www.cosmos.esa.int/web/herschel/home], the simultaneous observations of several molecular transitions is routine nowadays. In fact, unbiased spectral surveys are becoming a privileged tool for studying the chemical and physical structure of astrophysical sources.
Finally, the ever-increasing sensitivity of receivers will also lead to an increasing number of molecular lines to interpret. The huge investment in these new observational facilities, makes the knowledge and availability of accurate transition frequencies and intensities essential. In this context, having easy access to reliable databases of transition frequencies of atoms and molecules has become more than an urgent need: it is a must.
Methodology for data-
Entries are usually generated from laboratory data, astronomical data may be employed if appropriate. Most of the entries are fit in-house to accepted Hamiltonian model to generate spectroscopic parameters describing the spectrum in the most reliable way possible from the existing data. Even for simple cases, it is important to scrutinize the reported uncertainties, provided any are given. It is found quite frequently that reported uncertainties are too optimistic or too pessimistic. The step also involves testing the correctness of the assignments. The Hamiltonian model needs to be scrutinized also thoroughly; sometimes small modifications in the model can result in greatly improved models.
Correct prediction of the intensities requires the dipole moment (components) to be known sufficiently well from experiment or from quantum chemical calculations. Rotational or vibrational corrections will be employed if they are known with sufficient certainty. In cases of vibration-rotation interaction or tunneling-rotation interaction, it may be important to evaluate the dependence of intensities on the signs of interaction parameters relative to those of the dipole moment components.
Manager: Christian Endres (endres[at]ph1.uni-koeln.de)