Molecule | Isotope | Isotope | Isotope | Isotope |
---|---|---|---|---|
CO | 13CO | C17O | C18O | 13C18O |
CN | 13CN | |||
CS | C33S | C34S | 13CS | |
SiO | 29SiO | 30SiO | ||
SiS | 29SiS | 30SiS | Si34S | |
C2H | ||||
HCN | H13CN | HC15N | DCN | D13CN |
HNC | HN13C | H15NC | DNC | |
HCO+ | H13CO+ | HC18O+ | DCO+ | |
N2H+ | N2D+ | |||
HCS+ | ||||
OCS | O13CS | OC34S | 18OCS | |
HC3N | H13CCCN | HC13CCN | HCC13CN | DC3N |
p-CH3CN | p-13CH3CN | p-CH313CN | ||
o-CH3CN | o-13CH3CN | o-CH313CN | ||
p-CH3CCH | p-CH3C13CH | |||
o-CH3CCH | o-CH3C13CH | |||
SO | 34SO | |||
SO2 | 34SO2 | |||
H2CO | H213CO | HDCO | ||
H2CS |
1. The frequencies listed on these pages are generally accurate to 5 MHz or better
(except for molecules with hyperfine structure; see note 3).
Use the JPL Molecular Spectroscopy webpage
to obtain more accurate values for line frequencies.
2. The relative optical depths for transitions are calculated for
a Boltzman population distribution at a temperature of 60 K.
The optical depth is normalized so that the highest optical depth
transition has log(tau) of unity. The true optical depth in a cloud
depends on the molecular abundance and H2 column density, as well as the
temperature and density. The sole purpose of the log(tau) column is to give
an indication of the relative strengths of different transitions under
interstellar conditions. More useful predictions of relative intensities
can be obtained by using the LVG program in miriad.
3. Hyperfine structure is not included in the graphs or calculations of line
frequencies. Molecules which include nitrogen will usually exhibit hyperfine
structure which is important in determining line frequencies and
transition probabilities.
4. Some molecules are not as simple as you might expect due to nuclear or
electronic spin. For example CO is simple but SO is not because its
ground electronic state is 3-sigma. Other molecules (e.g. SO2
and CH3OH have very rich
energy level diagrams and many transitions due to bent bonds and 3-dimensional
structure. O-H bonds in molecules are usually troublesome (e.g. CH3OH and
H2O)
Thermalized Level Populations | ||||
---|---|---|---|---|
CO | CN | CS | SiO | SiS |
C2H | HCN | DCN | HNC | DNC |
HCO+ | DCO+ | N2H+ | N2D+ | HCS+ |
OCS | HC3N |
Statistical Equilibrium Level Populations | ||||
---|---|---|---|---|
CO | CS | HCN | SiO | HCO+ |
N2H+ | HCS+ | SiS | OCS | HC3N |
The relative level populations can be characterized by an excitation temperature relative to the next lower state, regardless of the fact that the level populations are not thermal. The excitation temperature is defined between two levels such that the Boltzman factor, at the excitation temperature, times the ratio of the statistical weights yields the ratio of the populations. The excitation temperature defined in this way can be larger or smaller than the true gas kinetic temperature; it can even be negative. When the excitation temperature is greater than the kinetic temperature, the molecule is super-thermal in that transition. When the excitation temperature is negative, the transition is capable of masing. The following graphs show the behavior of the excitation temperature for a number of molecules.
Statistical Equilibrium Excitation Temperatures | ||||
---|---|---|---|---|
CO | CS | HCN | SiO | HCO+ |
N2H+ | HCS+ | SiS | OCS | HC3N |
OCS | HC3N |