Abstract
Double resonance experiments on NH3 are described in which a CO laser pumps a 6 μm ν4 transition while a tunable diode laser probes a 10 μm ν2 transition having a common lower level. Four different combinations of pump–probe transitions are studied. The CO laser is Lamb-dip stabilized on the pump transitions, which are tuned into coincidence with it using a precision intracavity Stark cell. The pump and probe beams overlap colinearly in the Stark cell. The double resonance signals appear as narrow transmission peaks on the diode laser scans. The narrowest observed widths are ≂3 MHz (FWHM), a large portion of which is due to unresolved hyperfine structure. An analysis of the various broadening mechanisms indicates that the diode laser contributes less than 1 MHz to the widths. Resonances due to velocity-preserving but state-changing collisions are seen. Asymmetries between co- and counterpropagating linewidths are shown to arise from a combination of field inhomogeneity and coherent narrowing effects. The data are recorded using a computer-aided, rapid-scan, digital signal averaging technique.
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