C. H. Townes

Determination of Electronic Structure of Molecules from Nuclear Quadrupole Effects

Nuclear quadrupole coupling constants in molecules depend on the nuclear quadrupole moments and the variation in electrostatic field at the nucleus. It is shown that this variation of electric field is usually simply related to the molecular electronic structure, being primarily dependent on the way in which valence electrons fill the lowest‐energy p‐type orbits. Structural information which can consequently be obtained from known quadrupole coupling constants is discussed. Hybridization of the normal covalent bonds of N, Cl, and As with at least 15 percent s character is clearly shown. The alkali halides appear to be almost purely ionic; the quadrupole coupling data allow no more than 3 percent covalent character. In addition to molecular structure, some nuclear quadrupole moments are approximately evaluated by use of the theory developed here.

The 158 micron forbidden C II line: A measure of global star formation activity in galaxies

Some 158 micron forbidden C II fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the C II line is bright amounting to approximately 0.1 to 1 percent of the FIR luminosity of the nuclear regions of galaxies; the C II line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the C II line tracks the FIR continuum in a manner consistent with the PDR models; the integrated C II to isotope (C-12)D (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the C II line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given. 156 refs.

The Ionic Character of Diatomic Molecules

A relation between the electronegativity difference of two bonded atoms and the ionic character of the bond is obtained for singly bonded diatomic molecules in the gaseous state. The relation is based primarily on the wide variety of nuclear quadrupole coupling constants recently measured by microwave techniques. However, dipole moments of diatomic molecules are also used and discussed. Both quadrupole coupling constants and dipole moments indicate strongly that bonds involving electronegativity differences greater than 2 are almost completely ionic. Certain effects of hybridization, overlap, and polarization on considerations of ionicity are discussed.

Fluctuations in Amplification of Quanta with Application to Maser Amplifiers

Fluctuations in the amplification and absorption of waves by quantum processes are considered. Assuming for each quantum the probability (per unit time) a of producing another quantum, probability b of being absorbed, and assuming a probability c that a new quantum is introduced, a set of differential equations is obtained. By solving these equations, a complete expression for the probability of distribution of quanta is obtained, as well as expressions for the average values and fractional fluctuation. The expressions developed are applied in particular to maser-type amplifiers, and certain fluctuations in the amplification of electromagnetic waves are pointed out which are important when their quantum character becomes significant. This condition can occur in maser-type amplifiers, where thermal and extraneous noises may be very small. For such an amplifier, a is proportional to the number of excited molecules, whereas b consists of a term proportional to the number of molecules in the ground state plus...

Detection of Water in Interstellar Regions by its Microwave Radiation

A report of the detection of microwave radiation from water molecules in space, by the group which recently detected interstellar ammonia emission.

[C II] 158 Micron Observations of IC 10: Evidence for Hidden Molecular Hydrogen in Irregular Galaxies

We have mapped the [C II] 158 μm line over 85 × 65 in the Magellanic irregular galaxy IC 10, thus presenting the first complete [C II] map of an entire low-metallicity galaxy. The total luminosity in the [C II] line in IC 10 is 1.5 × 106 L☉. We discuss the origin of the [C II] emission toward different regions in the galaxy. Overall, about 10% of the [C II] emission can originate in standard H I clouds (n ~ 80, T ~ 100 K), while up to about 10% of the emission can originate in ionized gas, either the low-density warm gas or the denser H II regions. For the two brightest regions, most of the [C II] emission is associated with dense photodissociation regions (PDRs). For several regions, however, the [C II] emission may not be explained by standard PDR models. For these regions, emission solely from the atomic medium can also be precluded because the cooling rate per hydrogen atom would be much greater than the heating rate provided by photoelectric UV heating. We speculate that in these regions the presence of an additional column density of H2, 5 times that observed in H I, is required to explain the [C II] emission. The ambient UV fields present in these regions, combined with the low metallicity, create a situation where small CO cores exist surrounded by a relatively large [C II]-emitting envelope where molecular hydrogen is self-shielded. This additional molecular mass is equivalent to at least 100 times the mass in the CO core that one would derive from the CO integrated intensity alone using the standard CO-to-H2 conversion factor. These [C II] observations may, therefore, make a more reliable inventory of the gas reservoir in dwarf irregular galaxies where use of CO alone may significantly underestimate the molecular mass.

Far-infrared spectroscopy of galaxies - The 158 micron C(+) line and the energy balance of molecular clouds

Observations of the 158 microns fine-structure line of C(+) toward the nuclei of six gas-rich galaxies are presented. The observations are compared with observations of the CO J = 1-0 and H I 21 cm lines, observations of far-IR continuum emission, and observations of forbidden C II emission with the Galaxy. The forbidden C II line comes from dense, warm gas in UV-illuminated photodissociation regions at the surfaces of molecular clouds. This line is probably optically thin in all but the brightest of galactic sources. The variation of forbidden C II brightness from source to source and its ratio to the integrated infrared continuum intensity agree well with the theoretical prediction that UV absorption by dust controls the C(+) column density. The forbidden C II line is a tracer of molecular clouds, especially those near intense sources of UV radiation.

Neutral gas in the central 2 parsecs of the Galaxy

We have mapped the 1.1 mm HCN J=3→2 line with IRAM 30 m telescope at 12″ resolution and the 63 μm [O I] 3 P 1 → 3 P 2 line with the Kuiper Airborne Observatory at 20″ resolution within a projected distance of 2 pc of IRS 16/SgrA * . The HCN J=3→2 data, together with the previous HCN J=1→0 data of Gusten et al., indicate that the circumnuclear molecular material is arranged in several kinematically distinct streamers

Nuclear Quadrupole Effects and Electronic Structure of Molecules in the Solid State

Measured nuclear quadrupole effects in solids are discussed, compared with nuclear quadrupole coupling in gases, and correlated to some extent with molecular structure in the solid state. The iodine crystal affords a good example of intermolecular interactions in the solid state. In this crystal, nuclear quadrupole effects combined with crystallographic information show two intermolecular covalent bonds per atom, each of about 9 percent importance.

The nucleus of our Galaxy

The subject of this review is the central 100 parsecs of our Galaxy, with a strong focus on the central few parsecs. Observations of the electromagnetic spectrum over 13 orders of magnitude in wavelength show a broad range of phenomena involving a number of physical processes. We discuss the stellar and interstellar components, the importance of magnetic and gravitational forces, the evidence for stellar formation and a central massive black hole, and the origin and nature of ionization, outflows and interstellar gas dynamics.