Infrared photodissociation of size-selected methylamine clusters

Infrared photodissociation spectra of (CH₃NH₂)_n clusters were measured from n=2 to n=6 near the absorption of the C-N stretching mode of the monomer at 1044 cm^-1 using a line tunable cw CO₂ laser. The clusters are size selected by scattering them from a helium beam. The dimer spectrum shows a double-peak structure with a red- (1038 cm^-1) and a blue- (1048 cm^-1) shifted peak which is attributed to the nonequivalent position of the C-N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 and 1046.0 cm^-1 which is caused by the equivalent position of the C-N in the cyclic structures of the larger clusters. Calculations of the minimum-energy configurations confirm these results. The linewidth increases by more than a factor of 3 for cold and internally excited dimers.