Energy Level Spectroscopy

Tunable, high resolution techniques and spectroscopy were extended into the submillimeter region more than 40 years ago by the Microwave Laboratory (74, 82) (the reference numbers are keyed to the bibliography). This technology was based upon semiconductor harmonic generators and point contact diode detectors. With these techniques most of the fundamental linear and symmetric top species were studied. In addition, many measurements on important species such as water ( 88), oxygen (46, 52), ozone (75), etc. were made. A comprehensive summary of these results was published by Gordy (247). The introduction of liquid helium temperature detectors into these systems (283) significantly improved their sensitivity and reliability. This combination of phase-locked harmonic generator source and sensitive helium temperature detector has been extremely successful and has been widely adopted. (See also Claude Woods, Lucy Ziurys, and JPL Molecular Spectroscopy Home Page) More recently, a number of additional improvements have been reported (368).


Among the earliest studies with systems similar to that shown in the figure were the isotopes of water and hydrogen sulfide (289, 291, 294, 295, 299). This work was the first for light molecules in which modern centrifugal distortion theory, statistical and computational methods, and extensive experimental data sets were combined in order to produce models capable of calculating unobserved lines to microwave accuracy. This system was also used to make the first pressure broadening studies in the submillimeter spectral region (339) and Basic Submillimeter Spectrometer Design has more recently been used to observe pressure broadening due to collisions near absolute zero (386, 423, 441), as well as over the range of temperatures of atmospheric interest (426, 435, 438, 443). These techniques have also been used to study important light asymmetric rotors in non-singlet ground electronic states. These include species such as NO2 (362), HO2 (364) and HCO (374). Two light asymmetric rotors, HOOH (350) and CH3OH ( 441, 446), are also prototype internal rotors. The Microwave Laboratory has also carried out a number of studies of unstable species and ions. These include studies of "hot cell" diatomics such as NaCl (222), PN (300), and SiO (335) as well as studies of species such as CCH (361), and NH2 (352).


The magnetically lengthened negative glow (see Ion Production Environment) approach has been used to study a number of ions. The gains in ion density produced by this technique, coupled with the gains in sensitivity achievable at shorter wavelengths change the required search times for new ions by orders of magnitude and have brought many new ions beyond their threshold for study. For example, even though the signal strength of ArD+ is much more than an order of magnitude weaker than that of the "standard" molecular ions (HCO+ , N2H+ , etc.), it has been possible to observe both 36ArD+ (abundance 1/298) and 38ArD+ (abundance 1/1590) in natural abundance with only modest signal averaging (~1 minute) and HC18O+ (abundance 1/490) in real time on an oscilloscope and HC17O+ (abundance 1/2674).

Results of our work can be found in the bibliography of papers from the Microwave Laboratory. For reference by molecule, use the index of the molecular species.

To select from our bibliography, enter the number(s) of the publication(s) in the form xxx, yyy, zzz, ...

These data have also been included in the widely used Jet Propulsion Laboratory spectral line catalogue.

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