The Pickering series (also known as the Pickering–Fowler series) consists of three
lines of singly
ionised helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
found, usually in absorption, in the spectra of hot stars like
Wolf–Rayet star
Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectroscopy, spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very ...
s. The name comes from
Edward Charles Pickering and
Alfred Fowler. The lines are produced by transitions from a higher energy level of an electron to a level with
principal quantum number ''n'' = 4. The lines have wavelengths:
*10124
Ã… (''n'' = 5 to ''n'' = 4) (infrared)
*6560 Ã… (''n'' = 6 to ''n'' = 4)
*5412 Ã… (''n'' = 7 to ''n'' = 4)
*4859 Ã… (''n'' = 8 to ''n'' = 4)
*4541 Ã… (''n'' = 9 to ''n'' = 4)
*4339 Ã… (''n'' = 10 to ''n'' = 4)
*3645.56 Å (''n'' = ∞ to ''n'' = 4, theoretical limit, ultraviolet)
The transitions from the even-n states overlap with
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
lines and are therefore masked in typical absorption stellar spectra. However, they are seen in emission in the spectra of Wolf-Rayet stars, as these stars have little or no hydrogen.
In 1896, Pickering published observations of previously unknown lines in the spectra of the star
Zeta Puppis. Pickering attributed the observation to a new form of hydrogen with
half-integer transition levels. Fowler managed to produce similar lines from a hydrogen–helium mixture in 1912, and supported Pickering's conclusion as to their origin.
Niels Bohr
Niels Henrik David Bohr (, ; ; 7 October 1885 – 18 November 1962) was a Danish theoretical physicist who made foundational contributions to understanding atomic structure and old quantum theory, quantum theory, for which he received the No ...
, however, included an analysis of the series in his 'trilogy'
on
atomic structure and concluded that Pickering and Fowler were wrong and that the spectral lines arise instead from singly ionised
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
, He
+. Fowler was initially skeptical but was ultimately convinced that Bohr was correct,
and by 1915 "spectroscopists had transferred
he Pickering seriesdefinitively
rom hydrogento helium."
Bohr's theoretical work on the Pickering series had demonstrated the need for "a re-examination of problems that seemed already to have been solved within classical theories" and provided important confirmation for his atomic theory.
Wavelength formula
The energy differences between levels in the Bohr model, and hence the wavelengths of emitted or absorbed photons, is given by the Rydberg formula:
where
For helium,
, the Pickering-Fowler series is for
and the reduced mass for
is
thus
, which is usually approximated as
(in fact, although this number changes for each
isotope of helium, it is approximately constant). A more accurate description may be used with the
Bohr–Sommerfeld model of the atom.
The theoretical limit for the wavelength in the Pickering-Fowler is given by:
, which is approximatedly 364.556 nm, which is the same limit as in the
Balmer series (
hydrogen spectral series for
). Notice how the transitions in the Pickering-Fowler series for n=6,8,10 (6560Ã… ,4859Ã… and 4339Ã… respectively), are nearly identical to the transitions in the Balmer series for n=3,4,5 (6563Ã… ,4861Ã… and 4340Ã… respectively). The fact that the Pickering-Fowler series has entries inbetween those values, led scientist to believe it was due to hydrogen with half transitions ("half-hydrogen"). However, Niels Bohr showed, using his model, it was due to the singly ionised helium
, a
hydrogen-like atom. This also shows the predictability of Bohr model.
References
External links
PROTO-HYDROGEN
Astronomical spectroscopy
Helium