Using two step-tunable CO2 lasers, we have observed phase-matched generation of frequencies 70 <ν<110 cm−1 by nonlinear mixing in a birefringent ternary semiconductor, ZnGeP2. An observed power of ∼1.7 μW at 83.37 cm−1 gave a signal-to-noise ratio of ∼1000 with a Ge:Ga detector. In combination with tunable optical lasers, this technique should yield a tunable source of submillimeter wave radiation for high-resolution spectroscopy.

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Optical maser action on a number of rotational transitions of the S w + -Zg+ vibrational band of C0 2 has been recently reported. 1 The maser lines were identified as the rotational transitions from P(12) to P(38) of the 0 0°1 -1 0°0 band and from P(22) to P(34) of the 0 0°1 -0 2°0 band. We wish to give here a simple theoretical treatment which allows us to interpret the results and especially the fact that no R-branch transitions were seen in maser oscillation. The treatment satisfactorily explains the results and leads to an interesting conclusion that for the vibrational-rotational transitions, optical maser action can be obtained on the P-branch transitions even when no inversion exists between the total population densities in the two vibrational states. Figure 1 shows pertinent parts of the energy LASER 960.8 CM 10°0-2o + RECOMBINATION AND CASCADES RAD DECAY> 720.5 CM"' (M) N RAD DECAY 667.3 CM -1 (V.S.) -00°1 (POSSIBLE ELECTRON! IMPACT EXCITATION) 2349.3 CM" 1 (V. S.) C02 Sg + 00°0 (GROUND STATE) FIG. 1. Pertinent part of energy level diagram of CO z showing the maser transitions and other optical transitions with their respective strengths (reference 2). Patel , Murray Hill, New Jersey April 1964)level diagram of C0 2 (Herzberg 2 ). The rotational levels belonging to each of the vibrational states are not shown for the sake of simplicity. The upper maser level (for both the bands) S w + (0 0°1) is optically connected to the ground state S^+(0 0°0) of CO z through strongly allowed transitions at 2349. 3 cm" 1 . The lower laser levels 2g-+ (l 0°0 and 0 2°0) both decay to the 1^(0 1*0) levels through radiative transitions at 720. 5 and 618.1 cm" 1 , respectively, and these transitions are reported to be of medium strength (reference 2). The molecules in the 1^(0 l 1 *)) levels decay through strongly allowed transitions at 667. 3 cm" 1 to the ground state of C0 2 . Thus the maser scheme looks like a four-level system. The probable excitation and decay processes are shown in Fig. 1 with their appropriate strengths as obtained from reference 2. Alternate lines in the rotational spectrum of £ w + -2g-+ bands of C0 2 are missing because of symmetry considerations for the linear and symmetric molecule C0 2 . Also, the Q-branch-i.e., AJ=0-transitions are for bidden since both the upper and the lower levels have 1 = 0.Now consider a simplified model of a vibrational level in which the rotational level populations are described by a Boltzmann distribution at a temperature T. It can be shown 2 that for a linear and symmetric molecule like CO z ,where Nj is the population density of the Jth rotational level, N=ZJJNJ, h= Planck's constant, c= velocity of light, B = rotational constant for the particular vibrational level, fe = Boltzmann's constant, gj= statistical weight for the Jth rotational level, and F(J) = energy of Jth rotational 588

Spectroscopic measurements have been made of the nitric oxide and water vapor concentrations in the stratosphere at an altitude of 28 kilometers. The measurements, carried out in situ with the use of a spin flip Raman laser, represent the first accurate determination of nitric oxide as a function of time (as the sun rose) from about 6: 30 to 14: 00 C. D. T.

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