100 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The absorption of energy by molecules is more complex than atomic absorption, especially if the molecules are in the condensed liquid or solid states, or in solution. The total energy content of a molecule is made up of electronic bonding energy and kinetic energy. The latter has both vibrational and rotational components: E = Eboncling d- (E•'ibrational d- Ero,ati .... 1) (4) E TOTAL = EBONDiNG + IEvIBRATIONAL + EROTATIONAL I ENERGY KGAL/GM-MOLE RADIATION BONDING 200 FAR U-V I QQ NEAR U-V 5Q VISIBLE VIBRATIONAL 5 NEAR I-R ROTATIONAL 0..5 FAR I-R Quantitatively, the electronic bonding energies are the largest, and the vibrational energies are greater than the rotational energies. Thus bonding energies are associated with absorption bonds in the ultraviolet and visible regions (200 kcal./mole in the far ultraviolet, 100 kcal./mole in the near ultraviolet, 50 kcal./mole in the visible range. Vibrational changes are of the order of 5 kcal./mole, in the near infrared, and rotational . changes are of the order of 0.5 kcal./mole, in the far infrared. LINE a- BAND ABSORPTION SPEGTRA OF BENZENE 3OO 2OO I00 SOLUTION IN PEN TANE IVAPORI WAVELENGTH

DESIGN OF COSMETIC SUN SCREENS 101 It is important to note that absorption can take place only if the photon corresponds to the difference in energy between two possible energy levels of the molecule. The change from one such permissible level to the other is termed an electronic transition. Each transition gives rise to an absorption line of very narrow, though finite, width and for each such electronic transition, there is a progression of closely spaced absorption lines corresponding to different possible combinations of vibrational and rotational levels belonging to the same two electronic levels. In complex molecules, the number of possible combina- tions of vibrational and rotational levels are so great that the linear fine structure is blurred. In solutions, and in the condensed states, vibrations and rotations are blurred still further by solute-solvent interactions and by molecular association of the close-packed molecules. This gives rise to a diffuse absorption band which is observed in place of the system of discrete absorption lines. FIG.3, INTENSITY OF MDSUIVlV1ER NO(DN•Y SUN- 5 SKYI_IGHT AT • LEVEL ½' ---WASHINGTON D.C. ,,z•'.' -•-TUCSON ARIZ. •,- - ..--CLEVELAND O. IOOC 300 320 340 36•) 3@0 400 WAVELENGTH - MILLI MICRON S WAVE LI-NGTH-M[J 280 290 297,5 •'• I I I I _ •' • FIG.4. ERYTHEMAL I •-- EFFECTIVENESS OF - - ULTRAVIOLET RADIATION _ RELATIVE TO 2970 A.U, I -- , 330 320 340 350 380 400 WAVELENGTH - MILLIMICRONS ENERFoY- K,F.,AL/6M- MOLE 103,2 99,4 96.9 315 91.4 320 90.4