This paper introduces alphabet spectra which are the 26 frequencies of occurrence of scientific papers in a given sample with at least one author of each initial, A, B,...Z. The sum of these frequencies exceeds unity because of multiple authorships. Formulae are given relating this sum to the mean number of authors per paper in the sample. The method is applied to show the increase in this number over the last 15 years in different fields of science and for different countries. The alphabet spectra vary greatly depending on the nationality of the scientists concerned and can be compared to frequency absorption spectra for chemical elements or molecules. The spectra can be used to determine the national composition of a country's scientific authors and how this has changed with time.