Inactivation cross sections of infectious <PX-174-DNA in the extreme vacuum-ultraviolet were determined by irradiation of thin layers with monoehromatic light down to 584 A by means of a powerful grating-monoehromator, the elements of whieh are described. Comparison of inactivation and Iight-induced electron emission shows that light of quantum energies below 7 eV inactivates by excitation only, whereas above 10.2 eV ionization is the predominant inactivation meehanism. Because of the

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... ry agreement of the curves for inactivation and electron emission, it is conducted that the remarkable increase of the inactivation cross section in the region of the short wavelength vacuum-uv is due to increasing ionization probability.