Ritz-Paschen series

Equation (2-6) led to the identification of other series of the lines for hydrogen, including the Paschen series (n = 3), the Brackett series (nj = 4), and the Pfund series (n = 5). The Balmer series is in the visible region of the spectrum, the Lyman series is in the ultraviolet, and the Paschen, Brackett, and Pfund series appear in the infrared. Their distribution is shown in Figure 2-2. Equation (2-6), which accounts for all presently known lines of hydrogen, led Ritz (1908) to propose his combination principle, that the wavenumbers of all lines in a series are the result of the difference in energy between a fixed and a running term. 

Representative emission spectra are shown schematically in Fig. 2.2 for hydrogen, potassium, and mercury on a common wavelength scale from the near infrared to the ultraviolet. Under the coarse wavelength resolution of this figure, the emitted light intensities are concentrated at single, well-defined emission lines. In H, the displayed emission consists of four convergent series of lines, the so-called Ritz-Paschen and Pfund series in the near infrared, the Lyman series in the vacuum ultraviolet, and the Balmer series in the visible. Johann Balmer, a schoolteacher in Basel in the late nineteenth century. 

Figure 2.4 Hydrogen atom energy levels and transitions. The Lyman, Balmer, Ritz-Paschen, and Brackett series occur in the vacuum ultraviolet, visible, near-infrared, and infrared regions of the electromagnetic spectrum, respectively.

Theoretical considerations of emission spectra were slow to develop, although they started in the later 1800 s and extended into the twentieth century. Balmer s equation for the Balmer series of lines of hydrogen started the search for an explanation for the origin of atomic spectra. Later Ritz (1908) noted that lines of hydrogen observed in the ultraviolet by Lyman (1904) fit the Balmer equation if the constant was changed. This work was extended by Rydberg, Kayser, Runge, and Paschen. It was the work of Bohr, with his concept of the astronomical atom and certain postulates... 

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