One of the most important applications of molecular spectroscopy is its use in the remote sensing of environments containing molecular species. Three such applications have been of particular interest to our Microwave Laboratory; in order of "remoteness,"
( 343, 345, 384, 387, 390, 398, 405, 406, 436)
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Techniques based on mm/submm spectroscopy have been developed for the study of excitation and energy transfer in molecular lasers. The wide tunability, high resolution, sensitivity, and immunity to noise of this technique has allowed us to obtain information unique in its quality and direct relation to the important molecular processes. Both optically pumped (370, 398, 405, 406, 422, 428, 436, 445) and discharge systems (342, 343, 387, 389, 390) have been investigated.
The latter systems are of extreme physical and chemical complexity. For the most important FIR discharge laser, HCN, our work has shown many of the existing conjectures about its excitation and relaxation to be incorrect and that a simple, quantitative model with almost no adjustable parameters will explain not only our extensive data, but will also resolve apparent inconsistencies and oddities in earlier work.
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The figure shows the experimental system used for studies of rotational and fast vibrational energy transfer. It consists of two major elements. The spectrometer can probe the absorption profiles of the relevant rotational transitions necessary to map out energy transfer. Briefly, it consists of a reflex klystron operating at about 35 GHz, a harmonic generator which produces mm/submm radiation at multiples of the klystron fundamental, a diagnostic cell, and an InSb detector. The diagnostic cell is 2 inches in diameter, 1.5 m in length, and has dichroic windows to separate the infrared pump from the mm/submm probe. The detector, which is cooled to 1.6 K, has a frequency response of about 1 MHz. |
The second element of this experiment is a Q-switched CO2 laser which is used to produce 500 ns pulses of 10 micron radiation yielding about 40 watts per pulse every 200 ms. The laser beam entered the absorption cell through a dichroic window. The other end of the absorption cell has a second dichroic window. The shape of the pulse is monitored by a fast pyroelectric detector, and the average power is measured by a power meter.
The method of this experiment is to monitor the change in the strength of a rotational absorption line following the establishment of an excess of population in the pumped ro-vibrational state of the molecule. To do this the microwave system is locked to the frequency of the rotational transition in question, gas transferred into the absorption cell, the laser pulsed, and the time response of the transition recorded. This technique benefits from a number of factors: (1) the individual rotational transitions studied are easily resolved, (2) the spectrometer is extremely sensitive, and (3) the absorption of a line is easily measured and the absorption coefficients for rotational transitions are well known so that absolute number densities in the states can be calculated.
The underlying physics of these laser mechanisms is discuess in a section on Collisional Energy Transfer.
Ever since the initial suggestion by Rowland and Molina that chlorofluorocarbons play an important role in the destruction of atmospheric ozone, the study of the physical and chemical processes involved has become an active and important field. Because of the complexity of the processes involved, an important element of this research has been the development of remote sensing methodologies capable of detecting and measuring the many molecular constituents which contribute to the ozone cycle.
Our interest has been focused on the infrared, far infrared, and millimeter spectral regions where the rotational energy level structure of the molecules plays an important role. In these regions our laboratory studies (reported in our bibliography) provided the basis for the design and interpretation of many of these field measurements. Additionally, many of these measurements have been incorporated the Jet Propulsion Laboratories' spectral line catalogue.
The conventional wisdom was that the combination of a lack of efficient production method and a plethora of robust destruction mechanisms would result in few, if any molecular species in the interstellar medium. However, the discovery of ammonia and formaldehyde by Townes and Snyder and the subsequent suggestion of efficient formation mechanisms by Herbst and Klemperer has given rise to an active field of research which has identified more than 100 molecular species.
Because most interstellar molecule forming regions are quite cold, there is a natural symbiosis between the study of molecules in the interstellar medium and the millimeter and submillimeter spectral region. For detection of species a temperature of 4 K leads to a maximum emission near 1 mm (300 GHz). Because collisions are very rare in the interstellar medium and the energy flux high, many of the more abundant species are ions or radicals. Consequently, one of our laboratory interests has been the production and study of these species, under controlled laboratory conditions. This has led us to the development of several new laboratory environments. These include special ion production environments and the development of very low temperature environments.
The results of our work can be found in our laboratory bibliography. Additionally, many of these measurements have been incorporated in the Jet Propulsion Laboratories' spectral line catalogue.