Gallium arsenide clusters have been studied on a Fourier transform ion cyclotron resonance(FT-ICR) mass spectrometer with an external supersonic cluster source. Reaction of GaAs cluster cations with ammonia revealed that all cluster cations larger than seven atoms were most reactive near the 1/1 composition ratio of gallium/arsenic. The results suggest that even at this small size the clusters begin to adopt the alternating gallium/arsenic bonding arrangement characteristic of bulk GaAs crystal surfaces where gallium-arsenic bonding activates gallium atoms for ammonia chemisorption. With excess NH\sb3, Ga\sbxAs\sby(NH\sb3)\sbz\sp+ with the same GaAs composition (x and y) was observed with several values of z, which is explained in terms of the existence of multiple isomers of GaAs clusters. However, the anions were found to be inert toward NH\sb3. Nitric oxide was found to etch singly charged Ga\sbxAs\sby(x + y = 9 to 16) clusters. A pronounced even-odd alternation in cluster ion reactivity towards NO was observed with the odd numbered cluster ions which are believed to be closed shell singlets being much less reactive than the even numbered cluster ions which are of necessity open shell species. The local density calculations of small Ga\sbxAs\sby(x + y ≤ 10) have been carried out. Multiple low energy structures were found and the electronic properties are in agreement with experimental results. A model which directly controls the profile width of a stored waveform inverse Fourier transform(SWIFT), which is used for ion excitation and ejection, was developed. The power of the SWIFT was found to be approximately proportional to the selected profile width. Two apodization functions have been designed to give narrow frequency cut-offs. The ions are excited successively instead of concurrently if quadratic phase shift is adopted.