We demonstrate that electrons at energies below the threshold for electronic excitation (<3 eV) effectively decompose gas phase uracil generating a mobile hydrogen radical and the corresponding closed shell uracil fragment anion (U-H)(-). The reaction is energetically driven by the large electron affinity of the (U-H) radical. This observation has significant consequences for the molecular picture of radiation damage, i.e., genotoxic effects or damage of living cells due to the secondary component of high energy radiation.

We present experimental results for 80 keV proton impact ionization of nucleobases (adenine, cytosine, thymine and uracil) based on an event by event analysis of the different ions produced combined with an absolute target density determination. We are able to disentangle in detail the various proton ionization channels from mass analyzed product ion signals in coincidence with the charge-analyzed projectile. Thus, for the first time, cross sections and fragmentation patterns are compared for direct ionization (with no charge transfer between the target molecule and the projectile) and for single electron capture (with projectile neutralization) in proton-nucleobase collisions. In addition we are able to determine a complete set of cross sections for the ionization of uracil by 20-150 keV proton

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Dissociative electron attachment (DEA) to water in the gaseous phase has been studied using two different crossed electron–molecule beam apparatus. Ion yields for the formation of the three fragments H−, O− and OH− were measured as a function of the incident electron energy. The kinetic energies of the fragment ions were measured and compared with the values derived from ab initio calculations to provide information on the energy partitioning in the fragmentation process. Isotope and temperature effects on the attachment process are discussed and the production of OH− via DEA is confirmed.

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Gobet et al. ReplyThis Reply aims to clarify a key argument in two recent publications [1,2] which has been criticized in the preceding Comment by Chabot and Wohrer [3]. The Letters in question feature a novel method to derive a caloric curve from event-by-event ion decay data measured using multicoincidence techniques after high-energy collisions (60 keV=amu) between mass selected hydrogen cluster ions [H 3 H 2 n , n 6-14] and a helium target. Reactions corresponding to a given deposited energy E d are selected and grouped into statistical subensembles. A caloric curve can then be constructed by deriving corresponding temperatures for these microcanonical cluster ensembles. It is this method which is criticized by Chabot and Wohrer.The essence of their criticism is expressed in the following extract from the Comment: ''. . .The authors assume that the internal energy E is the energy deposited in the cluster by the collision, E d , . . .The deposited energy E d is the sum of the energy due to electronic excitation and, as a dominant contribution in their systems, to ionization. But the ionization energy should not be included in E .'' Apparently, Chabot and Wohrer have misinterpreted the work, arguing along a single event consideration and not taking into account the statistical ensemble-type situation which applies.As explained in detail in [1,2], the temperature derived from the data is that of a statistical ensemble comprising a large number of decaying H 3 H 2 n ions. Each subensemble is characterized by the deposited energy and includes all of the processes induced by the high-energy collision (ionization, excitation, etc.). This energy is deposited during a very short time (0.1 fs) and the system (n protons and n ÿ 1 electrons) is left isolated immediately after the collision. Therefore, the subsequent statistical analysis is carried out in the microcanonical frame [4,5]. The results obtained are averages for each statistical ensemble comprising a large number of cluster ions with the same total amount of energy deposited, albeit distributed in different channels. The total energy (in the frame of reference of a single cluster ion) is equal to the sum of the internal energy before the collision and the energy deposited by the collision. It can be assumed that the internal energy before the collision is low in comparison with the deposited energy.The temperature is derived from one observable: the size distribution of the largest fragment among residual cluster ions in the statistical ensemble. The residual cluster sizes are measured not only after the ejection of electrons (ionization) but also following the ejection of ions, atoms, and molecules. The correlation between the temperature and the size distribution of the largest fragment has been previously demonstrated both in cluster physics [6] and in nuclear physics [7,8]. Moreover, regarding the recent work of Thirring et al. [9] (triggered by [1,2]), it is worth noting that our measurements and

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.