Ammonia physisorption on polycrystalline copper surface at 85 K has been studied by infrared photoacoustic spectroscopy. Much attention has been paid on the vibrational spectra of ammonia symmetric deformation mode and its band variation as a function of coverage. The different coverage dependent behaviors are exhibited for ammonia adsorption respectively on clean and on oxygen pre-adsorbed copper surface, and can be explained by assuming that ammonia follows different adsorption mechanisms. Studies of the minute discrepancy between them further illustrate the applicability of this technique in surface adsorption analysis.