Absorption Spectra of the Chrome Alums at Low Temperatures

Abstract

Line absorption spectra of thirteen sulfate and selenate chrome alums have been measured at 15–20°K. Pronounced differences in the spectra are attributed to structural differences in the crystals and are related to the size of the anions and the univalent cations. Anomalous behavior on change of temperature is shown by potassium sulfate and ammonium sulfate and selenate alums. Transitions disrupting the lattice and significantly altering the spectrum occur in ammonium chromium sulfate at 81±1°K and in ammonium chromium selenate at 106±5°. Combination of electronic frequencies of Cr⁺⁺⁺ with internal oscillations of anions are weak. Combination of lattice vibrations with both upper and lower electronic states of Cr⁺⁺⁺ are found. Bands are observed in all the alums shifted about 330, 570 and 800 cm^—1 to higher frequencies than the electronic absorption lines. These are thought due to photo‐excitation of a nonhomopolar complex, probably Cr(H₂O)₆⁺⁺⁺. Repetition of the electronic multiplet and associated vibrational structure at some of these higher frequencies is found in several of the alum spectra.