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Spectral hole-burning experiments have measured the electronic spectra of aggregates of different deuteriumsubstituted isotopomers of methane (14: 1) with perylene. At =2-cm-' resolution, detail in the spectrum reveals at least two isomers each at the 1:l level for the CH4, CH3D, and CHzD2 complexes. Splitting observed for the CH4 species (-2 cm-l) suggests the presence of distinct nuclear spin isomers. The 1,l aggregate contains two separate methane species, the motion of which is limited only by the perylene molecule. The detailed spectrum of this species differs radically from that of the other isomer (2,0), which contains a contacted methane dimer. Such complex spectra lead to the conclusion that rotational mo$on is weakly hindered in perylene/CH,. On the other hand, the spectra of all CD4 aggregates studied exhibited simple resonances. We conclude that, if different nuclear spin isomers are present for the CD4 aggregates, their spectra differ by <1 cm-*. The spectra of CH3D and CH2D2 aggregates also suggest the presence of several isomers at the 1:l level. In addition to nuclear spin isomers, these may include isomeric forms resulting from different adsorption symmetries, trapped by hindered rotation.

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