Particle bombardment (FAB) mass spectra of methanol at subambient temperatures

The spectra produced by methanol when bombarded with a mixed beam of argon ions and atoms with a nominal energy of 8 keV have been obtained at temperatures between 179 and 100 K. The spectrum of liquid methanol at 179 K consists primarily of clusters of methanol around the proton, H(CH/sub 3/OH)/sub n//sup +/, where the maximum value of n is 8. The spectrum of solid methanol at 169 K is very similar to that of liquid methanol. Mixed methanol-water clusters produced as a result of water impurity are also observed. The spectrum of solid methanol at 100 K is quite different, for the H(CH/sub 3/OH)/sub n//sup +/ ions lose their prominence, and the high mass spectrum (above m/z 60) is without character, consisting of ions at more or less every mass. The change in the spectrum is reversible. At an intermediate temperature (137 K) the spectrum is a mixture of the features observed at 169 and 100 K. This change in the character of the spectrum can be represented by plotting ..sigma../sub n/H(CH/sub 3/OH)/sub n//sup +//TIC (TIC = total ion current) as a function of T, and such a plot shows a sharp decrease between approximately 150 and 130 K. Evidence is presented that the phenomenon is not caused by surface melting produced by the particle bombardment or by surface charging.