hot molybdenum foil with an emissivity of about 0.13. In 1977, Shirahata et al. [19] measured an extremely small gain coefficient for optically-pumped CO2 mixtures which was considerably lower than the theoretical prediction. Recently, measurements of gain in a CO2 medium, excited by a blackbody cavity, indicate that gains in the order of 5X10-3 cm-1 are possible, supporting the theory reported by Yesil & Christiansen [20]. They utilized the blackbody radiation emitted from the interior surface of a graphite oven of emissivity approximately 1.0 to energize the gas medium. In the following section, measurements of optical gain for a CO2 + He gas mixture, pumped by radiation from an electrically heated blackbody cavity, are presented to demonstrate the concept of solar-powered gas lasers. 3.2 Theory of Blackbody-pumped CO2 Laser Mixtures An intermediate blackbody pumping scheme is applicable to many gas laser systems. The general theory for each scheme is the same, only differing in the specifics of kinetics and spectroscopy. In this work, the CO2 laser system was chosen because it produces high gain with high beam power, and it is well understood. The energy level and mode diagram of CO2, showing the important processes is depicted in Fig. 4. This model may be used for theoretical calculations of small-signal gain coefficients. ts is the collisional relaxation or radiative lifetime from the modes of the CO2 molecule. Both t8 and Tj were obtained from the data published by Taylor & Bitterman [21], Because of Fermi resonance between the levels (100) and (020), both the v, and v2 modes maintain a single vibrational temperature Tvl. The CO2 molecules in the asymmetric stretching mode are represented by TvJ. Using the results of Ref. [22] and making appropriate assumptions, the approximate equations for the rate of change of energy per unit volume for the modes of CO2, interacting with a radiating field in the absence of stimulated emission, are: where (1/tp)[£3(7'p)—P3] is the ‘source term' due to radiative pumping, and Tp is the temperature of the pumping source. Ev (v=l, 2 or 3) is the mode energy of the CO2 molecule defined by the equations of statistical mechanics [23] and £12 is the sum of E{ and E2. Tp, a time constant for pumping, is a measure of the time required to establish the mode temperature Tv3. In equilibrium, (TJ^T (gas temperature) and Ev— Ev. Finally ff is the characteristic vibrational temperature of the CO2 modes. The CO2 molecules can be excited vibrationally in the presence of an optical pumping source. Pumping of the mode v3 of CO2 is achieved by 4.3 //m radiation. Because of strong absorption in this mode, the (001) level is strongly coupled to the radiation field, and, as a result, its vibrational temperature approaches the temperature of the source. The radiative lifetime of the (001) level for combined P- and P-branches, T43,is about 2.5 X 10-3 s [24], which is very short compared to the radiative lifetimes of the other neighboring lower vibrational levels of CO2, which are on the order of 0.2-1.1 s [24] and can be neglected [ 15]. As the (100) level does not have such radiative coupling, its vibrational temperature
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