In the last three decades, the field of laboratory astrochemistry has been a significant contributor to our understanding of the origin, formation, and evolution of organic molecules in astrophysical environments, from the interstellar medium (ISM) to the protosolar nebula and cold bodies in the Solar System (comets, asteroids, icy satellites). More particularly, experiments in which ice mixtures representative of astrophysical media, such as H2O, CO, CO2, CH3OH, and NH3 are photoprocessed by vacuum ultraviolet (VUV) photons or high-energy particles at cryogenic temperature (10-100 K) showed that those ices chemically evolve toward the formation of more complex organic molecules (COMs), which are stable at room temperature.

    In astrophysical environments, ices are subjected to several types of energy processes, which can be divided into thermal or radiation-induced processes. This radiation is driven by photons and cosmic rays. In earlier studies, it was commonly believed that thermal processes dominate the evolution of astrophysical ices. This point of view has been changing progressively thanks to experimental studies whose results showed that the evolution of astrophysical ices is dominated by radiation processes. Indeed, photons provide a significant amount of energy to the ice, which leads to the excitation of the ice components into high-energy states, and in some cases to the rupture of chemical bond in those molecules, even at very low temperatures, to form radicals. Radicals will release energy if they are able to recombine with other radicals to form new molecules. As the photon energy is fed to the ice in a constant manner, ices provide an optimal environment for the formation of more complex molecules.

    In the case of cosmic ray radiation, the incident high-energy particles initiate a series of ionizations and excitations of the ice species, which can combine with other excited or ionized molecules, or radicals formed via photoprocessing, thus forming new molecules. Therefore, both radiation sources―photons and cosmic rays―can provide enough energy to induce chemical and physical changes to the ices and make them evolve towards the formation of new species. Although high-energy ions and photons radiations have different penetration depth in the ices, it has been shown experimentally that both types of radiation induce similar effects on icy materials, and yield similar products.

    A large number of photochemistry studies of interstellar ice analogs attempted to measure destruction/production yields (or rates) of species to be compared with astronomical observations or astrochemical models. Only a few of these studies used a monochromatic light source such as synchrotron radiation. From a theoretical point of view, studies using a monochromatic light source are usually more consistent with theoretical models, because experimental studies can be carried out with selected, narrow-band photon energies. Recent VUV absorption cross-section studies of molecules in the solid phase provided new insights for the possible mechanisms leading to the production of interstellar ices. These measurements showed that the absorption cross section of CO ice in the hydrogen molecular emission energy range is much higher than in the Ly-α emission range of atomic hydrogen. This result implies that the photon energy distribution of the VUV radiation field in different astrophysical environments may lead to significant discrepancy in the evolution of their respective ices.

    Recent works suggest that the stellar high-energy photon emission plays an important role in the formation and evolution of stars and planetary systems, as well as in the life origin and evolution. The higher penetrability of X-rays into the hydrogen rich interstellar gas should translate into a considerable chemical evolution both on the ice and gas phase, and thus needs to be taken into account in modeling and interpretation of observational data. In particular, young solar type stars emit X-rays at a level of 3−4 orders of magnitude higher than the present-day Sun. Such a copious hard emission must affect the processing of gas in the vicinity of the star. Although the energy emitted in the X-ray range is only a modest fraction of the entire stellar energy budget, because of their high energy, X-ray photons produce phenomena that cannot be caused by radiation at lower energies, regardless of their larger flux.

    Our research is mainly based on the design, construction, and scientific use of ultra-high-vacuum chamber equipped with a closed-cycle helium-flow cryostat to simulate astronomical environments, and study the photoprocessing of ice mixtures of astrophysical interest (e.g., H2O, CO, CO2, NH3) subjected to VUV, EUV and X-ray irradiation at low temperature (≥10 K). In 2018, we were working on the set-up of a novel Interstellar Energetic-Processes System (IEPS) which is equipped with electron gun in the range of 1-5000 eV, a microwave-discharge hydrogen flow lamp (MDHL) providing VUV photons with energies in the range of 5-11 eV, and a connecting port to be able to connect the IEPS with synchrotron radiation sources in the VUV, extreme ultraviolet (EUV) and X-ray ranges. This experimental device is thus devoted to processing of astrophysical ice analogs with different types of radiation.

    This access to various radiation sources and the synchrotron facility, providing photons and electrons that can be simultaneously used in the same and unique ultrahigh vacuum chamber allows me to better evaluate the relative importance of irradiation particles and their energy dependence. It has been shown in our previous works that the energy of charged particles and photons used to irradiate ice samples is the important parameter in these experiments, regardless of the nature of the source. In other words, icy parent molecules will be broken down into fragments that will recombine to form more complex molecules as soon as some type of energy is provided to the ice, no matter what particle(s) provide this energy. The study of the differences between VUV/EUV photon, X-ray and energetic electron irradiation under simulated astrophysical conditions will provide useful data to evaluate and simulate all the types of radiation that ice mantles experience during their lifetimes in the interstellar and circumstellar media.

    Ice photoprocessing includes two independent processes, namely, photolysis and photon-induced desorption. Photolysis leads to the destruction of the parent molecules to convert them into other products via radical-radical and radical-molecule reactions, whereas photodesorption does not involve any dissociation of the molecules, but provides them enough energy to be sputtered out of the ice surface and become free gas-phase particles. In addition to the photodesorption of the starting ice components, we also observed the photodesorption of some photoproducts. Photoprocessing studies have mainly been focused on the effects of (1) the Lyman-α emission line (121.6 nm) and the H2 molecular emission in the 110-180 nm range via the use of a MDHL, (2) a tunable VUV, EUV and X-ray synchrotron radiation provided by National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, and (3) energetic electrons in the range of 100−5000 eV. Since synchrotron radiation can provide photons with energies ranging from 4 to 1250 eV, we can study the photoprocesses of interstellar ice analogs at different monochromatic photon energies.