Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The principal features of this transition are a loss of intensity of the positive HOOP mode at 965 cm21 (Dioumaev and Braiman, 1997a; Gerscher et al., 1997; Pande et al., 1989), an intensity increase for the positive band at 1556 cm21, with contributions from amide II and retinal CAC stretch modes…
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