Stark-tuned Lamb-dip spectroscopy of the ν4 and 2ν2 bands of 14NH3

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Abstract
Results are presented of Stark‐tuned Lamb‐dip measurements in the 6 μm ν4 and 2ν2 bands of 14NH3 obtained using an isotopic CO laser with an intracavity Stark cell. Some 230 lines from the isotopes 12C 16O, 13C 16O, 12C 18O, and 13C 18O were investigated with fields up to 30 kV/cm. 71 coincidences with Stark‐tuned NH3 lines, involving several hundred Lamb dips, were assigned and analyzed. The positions of these lines relative to the CO laser lines are reported with a typical accuracy of 5–20 MHz. Model calculations of Lamb‐dip spectra are also given that include power and pressure broadening and the collisionally transferred resonances. These are directly compared with observed spectra, from which we deduce values for the pressure broadening coefficients and the collisional transfer efficiency. Nuclear hyperfine structure is resolved on several lines, and from fitting one of these we obtained the value eqQ=−4.83±0.3 MHz for the nuclear quadrupole coupling constant in the s2ν2 (J=10, K=9) state. We see numerous examples of weak ‘‘forbidden’’ transitions ΔMJ=0, ±2. These arise from the nuclear quadrupole coupling, which mixes MJ states.