ACCIDENTAL 3-LEVEL DOUBLE RESONANCES IN THE $6-\mu m$ LASER-STARK SPECTRUM OF $^{15} NH_{3}$

$^{1}$ W. H. Weber, Chem. Phys. Letters 122, 469 (1985). Address of Weber: Physics Department, Research Staff, Ford Motor Company, Dearborn, MI 48121.Author Institution:A series of accidental 3-level double resonances are observed in the non-linear Stark spectrum of $^{15}NH_{3}$ obtained with a CO laser and an intracavity cell. They are assigned as Doppler-free-2photon absorptions originating in the a(4, 3) ground state level and terminating in the $s2v2 +v_{4}^{+1} (6,4)$ level, with an exactly resonant s2v2 (5, 3) intermediate state. These resonances occur in pairs, symmetrically located in the Doppler wings on either side of the usual Lamb dips. They are comparable in strength and width to the Lamb dips, but they have the opposite phase. One of each pair is associated with co-propagating absorption. the other being its counter-propagating partner. The co-propagating features are particularly interesting, since they can be observed without a cavity and their resonant field strengths do not depend on the laser frequency. They also span a wide range in electric field, from 22 kV/cm to over 90 kV/cm, and thus constitute an ideal series for establishing calibration standards in field strength. The data are analyzed to determine term values and Stark coefficients for the excited $states.^{1}$ The resulting fit gives an rms error of $\pm 290$ KHz for 18 lines