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Although the biological hazard of alpha-particle radiation is well-recognized, the molecular mechanisms of biodamage are still far from being understood. Irreparable lesions in biomolecules may not only have mechanical origin but also appear due to various electronic and nuclear relaxation processes of ionizeds tates produced by an alphaparticle impact. Tw os uch processes were identified in the present study by considering an acetylene dimer,abiologically relevant system possessing an intermolecular hydrogen bond. The first process is the already well-established intermolecular Coulombic decayo fi nner-valence-ionized states.T he other is an ovel relaxation mechanism of dicationic states involving intermolecular proton transfer.B oth processes are very fast and trigger Coulomb explosion of the dimer due to creation of charge-separated states.T hese processes are general and predicted to occur also in alpha-particle-irradiated nucleobase pairs in DNAmolecules.