  Radiation damage in biological systems by ionizing radiation is predominantly
caused by secondary processes such as charge and energy transfer leading to the
breaking of bonds in DNA. Here, we study the fragmentation of cytosine (Cyt)
and thymine (Thy) molecules, clusters and microhydrated derivatives induced by
direct and indirect ionization initiated by extreme-ultraviolet (XUV)
irradiation. Photofragmentation mass spectra and photoelectron spectra of free
Cyt and Thy molecules are compared with mass and electron spectra of Cyt/Thy
clusters and microhydrated Cyt/Thy molecules formed by aggregation in
superfluid helium (He) nanodroplets. Penning ionization after resonant
excitation of the He droplets is generally found to cause less fragmentation
compared to direct photoionization and charge-transfer ionization after
photoionization of the He droplets. When Cyt/Thy molecules and oligomers are
complexed with water molecules, their fragmentation is efficiently suppressed.
However, a similar suppression of fragmentation is observed when homogeneous
Cyt/Thy clusters are formed in He nanodroplets, indicating a general trend.
Penning ionization electron spectra (PIES) of Cyt/Thy are broad and nearly
featureless but PIES of their microhydrated derivatives point at a sequential
ionization process ending in unfragmented microsolvated Cyt/Thy cations.
