We revisit the excitation mechanism of NH 2 molecules in cometary comae. A fluorescence efficiency is one of the key parameters for estimating the amount of molecules from the observed flux of NH 2 emission in the optical wavelength region. Including rovibronic transitions, which so far have never been considered, we revised the fluorescence efficiencies of cometary NH 2 based on the fluorescence equilibrium model. Using the revised fluorescence efficiencies, we derived the abundance of the

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... nia from the estimated NH 2 abundance because the NH 2 is a photodissociation product of cometary ammonia. This revision not only helped us solve the socalled flux ratio anomaly in the NH 2 emission bands that were reported so far, but it also helped us to clear up the inconsistency between the ammonia abundances derived in situ by a spacecraft and those estimated from the ground-based observation of the NH 2 . The NH 2 abundances obtained by spectroscopic observations in the past are reevaluated based on our revised parameters and are revealed to be almost constant, with heliocentric distances from 0.5 through 3 AU. The average abundance in comets is found to be about 0.5% relative to the water, except for comet 21P/Giacobini-Zinner.