Context. Ice mantles that formed on top of dust grains are photoprocessed by the secondary ultraviolet (UV) field in cold and dense molecular clouds. UV photons induce photochemistry and desorption of ice molecules. Experimental simulations dedicated to ice analogs under astrophysically relevant conditions are needed to understand these processes. Aims. We present UV-irradiation experiments of a pure CO 2 ice analog. Calibration of the quadrupole mass spectrometer allowed us to quantify the photodesorption of molecules to the gas phase. This information was added to the data provided by the Fourier transform infrared spectrometer on the solid phase to obtain a complete quantitative study of the UV photoprocessing of an ice analog. Methods. Experimental simulations were performed in an ultra-high vacuum chamber. Ice samples were deposited onto an infrared transparent window at 8K and were subsequently irradiated with a microwave-discharged hydrogen flow lamp. After irradiation, ice samples were warmed up until complete sublimation was attained. Results. Photolysis of CO 2 molecules initiates a network of photon-induced chemical reactions leading to the formation of CO, CO 3 , O 2 , and O 3 . During irradiation, photon-induced desorption of CO and, to a lesser extent, O 2 and CO 2 took place through a process called indirect desorption induced by electronic transitions, with maximum photodesorption yields (Y pd ) of ∼1.2 × 10 −2 molecules incident photon −1 , ∼9.3 × 10 −4 molecules incident photon −1 , and ∼1.1 × 10 −4 molecules incident photon −1 , respectively. Conclusions. Calibration of mass spectrometers allows a direct quantification of photodesorption yields instead of the indirect values that were obtained from infrared spectra in most previous works. Supplementary information provided by infrared spectroscopy leads to a complete quantification, and therefore a better understanding, of the processes taking place in UV-irradiated ice mantles.

The kinetics of amorphization in crystalline Si0 2 (oi-quartz) under irradiation with swift heavy ions (0 +1 at 4 MeV, 0 +4 at 13 MeV, F +2 at 5 MeV, F +4 at 15 MeV, Cl +3 at 10 MeV, CI* 4 at 20 MeV, Br +5 at 15 and 25 MeV and Br +8 at 40 MeV) has been analyzed in this work with an Avrami-type law and also with a recently developed cumulative approach (track-overlap model). This latter model assumes a track morphology consisting of an amorphous core (area

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The damage induced on quartz (c-Si0 2 ) by heavy ions (F, O, Br) at MeV energies, where electronic stopping is dominant, has been investigated by RBS/C and optical methods. The two techniques indicate the formation of amorphous layers with an isotropic refractive index (n = 1.475) at fluences around 10 14 cm 2 that are associated to electronic mechanisms. The kinetics of the process can be described as the superposition of linear (possibly initial Poisson curve) and sigmoidal (Avrami-type) contributions. The coexistence of the two kinetic regimes may be associated to the differential roles of the amorphous track cores and preamorphous halos. By using ions and energies whose maximum stopping power lies inside the crystal (O at 13 MeV, F at 15 MeV and F at 30 MeV) buried amorphous layer are formed and optical waveguides at the sample surface have been generated.

The refractive index changes induced by swift ion-beam irradiation in silica have been measured either by spectroscopic ellipsometry or through the effective indices of the optical modes propagating through the irradiated structure. The optical response has been analyzed by considering an effective homogeneous medium to simulate the nanostructured irradiated system consisting of cylindrical tracks, associated to the ion impacts, embedded into a virgin material. The role of both, irradiation fluence and stopping power, has been investigated. Above a certain electronic stopping power threshold (~2.5 keV/ nm), every ion impact creates an axial region around the trajectory with a fixed refractive index (around n = 1.475) corresponding to a certain structural phase that is independent of stopping power. The results have been compared with previous data measured by means of infrared spectroscopy and small-angle X-ray scattering; possible mechanisms and theoretical models are discussed.

El acceso a la versión del editor puede requerir la suscripción del recurso Access to the published version may require subscription AbstractIonoluminescence (IL) of the two SiO 2 phases, amorphous silica and crystalline quartz, has been comparatively investigated in this work, in order to learn about the structural defects generated by means of ion irradiation and the role of crystalline order on the damage processes. Irradiations have been performed with Cl at 10 MeV and Br at 15 MeV, corresponding to the electronic stopping regime (i.e., where the electronic stopping power S e is dominant) and well above the amorphization threshold. The lightemission kinetics for the two main emission bands, located at 1.9 eV (652 nm) and 2.7 eV (459 nm), has been measured under the same ion irradiation conditions as a function of fluence for both, silica and quartz. The role of electronic stopping power has been also investigated and discussed within current views for electronic damage. Our experiments provide a rich phenomenological background that should help to elucidate the mechanisms responsible for light emission and defect creation.

We have determined the cross-section a for color center generation under single Br ion impacts on amorphous SiO2. The evolution of the cross-sections, a(E) and a(S e ), show an initial flat stage that we associate to atomic collision mechanisms. Above a certain threshold value (S e > 2 keV/nm), roughly coinciding with that reported for the onset of macroscopic disorder (compaction), a shows a marked increase due to electronic processes. In this regime, a energetic cost of around 7.5 keV is necessary to create a non bridging oxygen hole center-E' (NBOHC/E') pair, whatever the input energy. The data appear consistent with a non-radiative decay of self-trapped excitons. The effects of swift heavy ion (SHI) beams on dielectric materials (electronic excitation regime) are the cumulative result (overlapping) of disorder tracks caused by individual ion impacts. 15 In fact, the fluxes used in ion accelerators and other irradiation technologies imply that the time interval between successive ion impacts is much longer than the time needed to modify the atomic network of the material, around 10 ps. In other words, every impact only sees the "ashes" that remain after the previous impacts. Moreover, for SHI, the damaged region around the ion trajectory is only of a few nanometers, so that below ^10 12 cm -2 , the tracks are well isolated and can be individually registered. , 7 Therefore, the key problem is to understand the physical mechanisms operating at the nanoscale, during single ion impacts, which are responsible for structural modifications. This is very relevant in many technologies such as fission and fusion installations, the microelectronics and photonic industries, and the degradation of component and devices in space missions and hadron therapies using heavy ions. 811

Ionoluminescence (IL) has been used in this work as a sensitive tool to probe the microscopic electronic processes and structural changes produced on quartz by the irradiation with swift heavy ions. The IL yields have been measured as a function of irradiation fluence and electronic stopping power. The results are consistent with the assignment of the 2.7 eV (460 nm) band to the recombination of self-trapped excitons at the damaged regions in the irradiated material. Moreover, it was possible to determine the threshold for amorphization by a single ion impact, as ∼1.7 keV/nm, which agrees well with the results of previous studies.