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The photoabsorption cross sections of C3H6 isomers (cyclopropane and propylene) and C 4 Hg isomers (I-butene, iso-butene, cis-2-butene, and trans-2-butene) have been measured in the wavelength region from 300 A to the ionization potential of each molecule (about 1200--1400 A)using an ionization chamber with four ion-collection plates. The obtained photoabsorption cross sections show a maximum value at about 700--800 A for each molecule. In the wavelength region which is shorter than about 500 A the cross sections are almost the same among the isomers and equal to the sum of the cross sections for the constituent atoms. In the longer wavelength region, however, the cross sections have different peaks and shoulders depending on an isomer, i.e., on its molecular structure. The obtained cross section values, together with previously reported ones in the wavelength region outside of the present experiment, satisfy the Thomas-Kuhn-Reiche sum rule within experimental errors.

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