The Development of the Understanding of the Origin of Infrared Band Spectrum

Abstract

I have dealt with the development of the understanding of the origin of the infrared band in the molecular spectrum during the period between 1880 and 1920. Deslandres' law (1886 and 1887) and P.Drude's extended study of dispersion theory,(1904) prepared the first step in understanding the origin of the infrared band spectrum. The second step was closely related to the quantum theories of specific heat of Einstein's (1907) and Nernst's (1911). Einstein's theory implicitly taught that the near infrared band was attributed to atomic vibration. On the other hand, Nernst suggested that molecular rotation energy had relation to specific heat of gases, and that its energy was quantized. In their theories, the infrared band was located as an indicator to prove the quantum theory of specific heat. In the first Solvay Conference(1911),the question about whether the quantization of rotational energy was possible or impossible, was discuussed. In 1911 and 1912, N. Bjerrum who was backed by those researches, attributed the infrared bands to atomic vibration, vibration-rotation and molecular rotation, and related the wavelengths of spectral lines of every band with the energy of every motion, by the aids of the quantum condition of Planck or Rayleigh's theory of 1892. He succeeded to shift the position of the infrared band to the position as the indicator of the molecular structure. Ehrenfest in 1913 revised Bjerrum's theory of rotational spectrum; and then by the aid of his adiabatic hypothesis, he could formulate rotational energy in the stationary state of the quantum number m. The formulation of the rotational energy contributed to the development of the later researches about rotational spectrum. The third step was prepared by the new theories of Bohr's and Sommerfeld's; the new quantum condition, the frequency condition, and the selection rule Schwarzschild in 1916 formed the theory of rotational spectrum, by the aids of th frequency condition and the quantum condition of Sommerfeld. He used Deslandres' law as the guide to prove the validity of his theory of rotational spectrum Younger theorists such as Heurlinger, Lenz and Kratzer formed theories to interprete the occurrence of vibration-rotation, or rotation bands, by the aids of two conditions and a rule (1919 and 1920). They contributed to forming the theoretical model of diatomic molecule The former could determine the quantum number of rotational spectnim, by the aid of the selection rule. Around the same time, Imes found for the first time the fine structure of the vibration-rotation band of hydrogen chloride by means of the infrared spectroscope equipping the diffraction grating (1919 and 1920) His finding proved that Bjerrum's theory of double and was incomplete. Kratzer attempted to form a theory which could interprete the fine structure, by the aids of those conditions and a rule. He contributed to the physical interpretation of "missing", the determination of the distance between nuclei of diatomic molecule, and the confirmation of the attribution of the far infrared band to molecular rotation.