The mass spectrum of positive ions formed by electron impact ionization of guanine (C5H5N5O) has been measured in a crossed electron–molecule beam experiment. The ionization energy of the molecule and the appearance potential (AP) of some fragment ions were obtained. The value of the ionization energy for the parent ion with m/z = 151 was determined experimentally to be 8.1 ± 0.2 eV, and it is in good agreement with vertical ionization energies obtained in previous photoelectron spectroscopy experiments. The temperature effect for the APs of guanine has been studied for the first time, and a remarkable difference of the APs for various fragment ions has been observed.

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