NIST: Freq. Interstellar Molec. Microwave Trans.

1. Introduction

The present tabulation of recommended transition frequencies for interstellar molecular species is the third revision of the previously published tables [1,2,3]. Since the last revision in 1991, approximately 5600 new transitions and 31 new molecular species have been added to the table which now lists more than 10,100 entries. This report updates the previous summaries, provides a current source of radio-astronomical molecular line observations, and improves the accuracy for many previously tabulated transition frequencies, important for determining physical properties of the molecular clouds investigated.

2. Sources and Selection of the Transition Frequencies

The present tabulation covers the astrophysical literature through December 2002. The 114 molecular species listed in Table 1 have now been identified in interstellar and circumstellar astronomical sources by means of their microwave spectra. The 22 additional interstellar species, identified by their infrared or ultraviolet spectra, and comet molecular species are listed in Table 2. Since no microwave transitions of these species have been reported yet, there will be no entries in Table 4 for these species.

The sources of the transition frequencies selected are as follows: laboratory measurements and predictions from the literature data, previously published tabulations of spectral frequencies [4-27], spectral predictions of transition frequencies from reanalysis of the literature data carried out in the present work, and web-based catalogs [26,27]. The primary criterion for selection of the transition frequencies is the magnitude quoted for the estimated uncertainty in the measured frequency or the standard deviation of calculated frequencies. For well-behaved species, i.e., those whose spectra are well fit by established Hamiltonians, the calculated frequencies are often more accurate than individual measurements, thus many of the entries in Table 4 are calculated values and are identified with an asterisk (*) following the frequency entry. In entries where it was determined that the measured value had the lowest uncertainty value, a reference to the literature value is given.

For many of the interstellar species the previously published tabulations of critically evaluated laboratory data [4-25] were the source of both measured and calculated frequencies cited here. In many cases for species treated in the publications indicated above, new spectral data have been reported and have been combined with the earlier data sets and reanalyzed to provide predicted frequencies employed here. Some of the earlier reviews also have a frequency limit of 200 GHz or 300 GHz, while new interstellar observations range up to 725 GHz. Thus, for most of the smaller species (diatomic, triatomic, etc.) calculations were extended to higher frequencies with new laboratory data included where available. The earlier reviews on CH₃OH [6], CH₃CHO [12], and HCOOCH₃ [19] are out-dated and new reviews were used (ref. [Xu_97], [Kle91], and [Oes99]). In a few cases, e.g., for radical species such as C₂H, C₃N, C₄H, etc., the on-line catalogs developed at the Jet Propulsion Laboratory (JPL) [26] and at the University of Köln [27] were used as the source of calculated frequencies.

Introduction | Transition Frequencies | Description of Tables | References to Text