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Sub-Doppler, saturation dip, spectra of lines in the $v_1 + v_3$, $v_1 +2v_4$ and $v_3 + 2v_4$ bands of $^{14}$NH$_3$ have been measured by frequencycomb-referenced diode laser absorption spectroscopy. The observed spectral linewidths are dominated by transit time broadening, but show resolved orpartially-resolved hyperfine splittings that are primarily determined by the$^{14}$N quadrupole coupling. Modeling of the observed line shapes based on theknown hyperfine level structure of the ground state of the molecule shows that,in nearly all cases, the excited state level has hyperfine splittings similarto the same rotational level in the ground state. The data provide accuratefrequencies for the line positions and easily separate lines overlapped inDoppler-limited spectra. The observed hyperfine splittings can be used to makeand confirm rotational assignments and ground state combination differencesobtained from the measured frequencies are comparable in accuracy to thoseobtained from conventional microwave spectroscopy. One upper state level showsvery clear differences from the expected splittings. Examination of the knownvibration-rotation level structure shows there is a near degeneracy betweenthis level in $v_1+v_3$ and a rotational level in the $v_1 + 2v_4$ manifoldwhich is of the appropriate symmetry to be mixed by magnetic hyperfine termsthat couple ortho- and para- forms of the molecule.

Quadrupole Splittings in the near-infrared spectrum of $^{14}$NH$_3$