Absorption spectra in the far ultra-violet region of the spectrum have recently assumed an important role in fixing the electronic structures of polyatomic molecules. This has been especially true of organic molecules such as acetylene, ethylene, the alkyl halides, alcohols, ethers and ketones. While all “molecular electrons” (i. e. those not contained in inner shells) can be expected to give rise to absorption bands in the region 2000–1000 A, it most frequently happens that one special electron type dominates the absorption. For example, the excitation of non-bonding
pπ
electrons dominate the absorption of methyl and ethyl iodides (Price 1936
a
); so-called “lone pairs” located on the oxygen atoms are responsible for all the strong bands of water, formaldehyde, etc. (Mulliken 1935
a, b
; Price 1935
a
, 1936
b
). In order to obtain discrete absorption bands, which are desirable for the purposes of interpreting electronic structure, it is usually necessary to take the very simplest organic molecule containing the group we wish to study. Thus for molecules of the type
R
1
COO
R
2
(
R
being an alkyl group or a hydrogen atom) it has been found that only the simplest of these, namely formic acid, shows discrete absorption bands. The interpretation of the electronic structure of the carboxyl group will therefore depend to a considerable extent upon the analysis of these discrete bands. From the discussion which follows it will be easy to see why the continuous absorption from the larger molecules of the type
R
1
COO
R
2
follows roughly the envelope of the discrete absorption of HCOOH except in so far as it is enhanced in certain regions by additional absorption from C—C and C—H bonding electrons or suffers small shifts to longer wave-lengths as a result of the substitution of hydrogen atoms by alkyl groups. The experimental technique employed in obtaining absorption spectra in the vacuum ultra-violet has been described elsewhere (Collins and Price 1934). The Lyman continuum serves as the background against which the 19340. The Lyman continuum serves as the background against which the absorption is observed, and the gas under investigation is allowed to flow continuously through the body of the spectrograph.
The absorption spectra of conjugated dienes in the vacuum ultra-violet (1)