Beginning in the 1930s, G.H. Dieke and his students carried out an extensive program of measuring the optical spectrum of molecular hydrogen and its isotopes. Parts of the work were published but the project was interrupted by Dieke’s death in 1965, with much of the latest and most accurate work unpublished. This paper gives the 27 488 lines of molecular deuterium, measured by Dieke, arranges the 8243 assigned lines into band systems, and derives rotational–vibrational energy levels for over 50 electronic states. It also derives energy levels from published vacuum ultraviolet spectra of D2.

The Balmer lines of H, when produced by electron impact dissociative ionization of low pressure H2, are much broader than the Doppler width of thermal H atoms. Indeed, they each have (a) a central peak and (b) broad wings, which result from two different groups of dissociative states. Excitation cross sections show structure as a function of electron energy. This helps to identify the states as follows; Predissociated and directly dissociated Rydberg states produce the central peaks and doubly excited states produce the wings.

We have measured relative excitation functions for the production of electronically excited fragments by electron impact dissociation of methane, ethylene and methanol from 0 to 2000 eV. At low impact energies, 10–50 eV, the measured appearance potentials are correlated to specific dissociation limits and, when possible, to specific excited states of the parent molecules. It is found that below ∼25 eV superexcited states play the dominant role in producing these fragments. Fano plots have been constructed to determine the types of molecular excitation involved. In general, most of these excited fragments are produced through optically forbidden transitions.

Nitrogen atoms in long-lived high-Rydberg states have been produced in the dissociative excitation of N2 by electron impact. Four principal features were found in the time-of-flight distributions of the dissociation fragments and in the corresponding translational (kinetic) energy distributions. Appearance potentials and excitation functions were measured for high-Rydberg atoms with known and well-defined translational energies; for the slowest high-Rydberg fragments the excitation function exhibits sharp, resonancelike structure near threshold. The core-ion model of high-Rydberg dissociation, which treats the Rydberg electron essentially as a spectator in the dissociation process, is described and used to interpret the data. Observed dissociation limits are assigned as one fragment being an N+(3P) core surrounded by a high-Rydberg electron and the other fragment being a nitrogen atom or ion. In addition, the high-Rydberg translational energy distributions are in reasonable qualitative agreement with kinetic energy distributions of N+ measured by dissociative ionization experiments. These observations provide substantial support for the core-ion model. The available data on the dissociative core-ion states of N2+ are discussed extensively. However, no unique assignments can be made for the molecular high-Rydberg states which participate in the dissociation processes.

B y J . A . S C H I A V O N E( M a n u s c r i p t r e c e i v e d J a n u a r y 1 5 , 1 9 8 1 ) Estimating obstruction fading on line-of-sight microwave radio paths requires statistics on positive refractivity gradients in the lowest 100 meters of the atmosphere. In this paper, we describe a semiempirical climatological model that predicts quantitatively the occurrence frequency of positive refractivity gradients and the geo graphical variation of this frequency within the contiguous United States. The occurrence frequency is parameterized by six factors, for which numerical values are obtained from available climatological and physiographical data. By modeling the two diurnal atmospheric boundary layer regimes with separate probability density functions, we produce cumulative probability distributions for refractivity gra dients. The model is normalized using measured refractivity gradient distributions available for 17 sites within the United States. The model can be used for line-of-sight microwave radio path engineer ing; a companion paper presents a method for determining antenna heights. I. I N T R O D U C T I O NLine-of-sight microwave radio transmission performance is suscep tible to atmospheric conditions, in particular to spatial variations in the microwave refractive index. Nearly always during the day and frequently at night, sufficient vertical mixing occurs of the low-level air layers, through which the microwave beam propagates, so that vertical refractive index gradients encountered are small. However, horizontal layers containing strong vertical gradients in air tempera ture and water vapor pressure, which detennine the refractive index, sometimes develop at night. The presence of these layers can cause the received signal to fade (decrease in strength).