AstroChem Lab @ NSRRC
Extended Red Emission & Blue Luminescence : Luminescence from Graphenes
Extended Red Emission (ERE) is a phenomenon commonly observed in a wide range of circumstellar and interstellar environments in the Milky Way galaxy and other galaxies. Despite numerous efforts, definitive identifications of the carriers responsible for ERE and the associated processes remain elusive. None of the proposed candidates have fully satisfied the observational constraints established through extensive astronomical observations or garnered direct support from laboratory experiments.
A proposed model suggests that far-ultraviolet (far-UV) irradiated graphene and very large polycyclic aromatic hydrocarbons (PAHs) are promising candidates for ERE carriers. This model aligns with all the key observational constraints, including a carbonaceous nature, a requirement for far-UV radiation, high photon-conversion efficiency, and a redshift in the high vacuum ultraviolet (VUV) region. This alignment has been demonstrated through laboratory experiments and is consistent with the top-down chemistry observed in space.
Low Ortho to Para Ratio (OPR) of Water in Space: Production of Non-equilbrium Nuclear Spin Isomers in Solid Hydrogen
Water is a crucial molecule found widely throughout various astro-environments. Due to its two equivalent protons (with nuclear spin I = 1/2), water exhibits two isomers with distinct total nuclear spin I values: I = 0 for para-H2O (p-H2O) and I = 1 for ortho-H2O (o-H2O). The Pauli exclusion principle dictates that the total wavefunction of such species must be antisymmetric with respect to the permutation of the two identical protons. Consequently, p-H2O is associated with states having an even rotational quantum number (Ka + Kc), while o-H2O corresponds to states with an odd number of Ka + Kc. In accordance with spin statistics, under high-temperature conditions (T > 50 K), normal water (n-H2O) consists of a 1/4 para form and a 3/4 ortho form.
In isolated molecules, nuclear spin conversion (NSC) between nuclear spin isomers occurs at an extremely slow rate. However, non-statistical Ortho-Para Ratio (OPR) values for water have been observed through the analysis of pure rotational lines of gaseous water in space. Astrochemists have suggested that the spin temperature of water may hold a thermal history of the solar system. Recent experimental findings indicate that the OPR of water is close to 3 upon sublimation from water ice at 10 K. It has been concluded that the OPR of H2O desorbed from interstellar ices is consistently 3, regardless of the formation temperature (Hama et al., 2016). The origin of the low OPR of interstellar H2O remains an unresolved challenge in modern astrochemistry.
We propose a plausible mechanism for the interstellar production of non-equilibrium spin isomers of H2O through the photolysis of O2 within solid H2 adsorbed on grain surfaces. The desorption of H2O from the grain surface may preserve its OPR. This new concept will be validated by investigating the OPR of a very small amount of H2O sublimated from a matrix using high-resolution VUV spectroscopy.
Diffuse Interstellar Bands: Electronic Spectra of Hydrocarbon Cations
Irradiation Chemistry of Interstellar, Cometary, and Planetary Ices with Photons and Electrons
In recent years, various frozen molecules on dust grains in the interstellar medium (ISM) have been detected and identified through ground- and space-based infrared observations. These dust grains are thought to play important roles in the interstellar chemistry of cold and dense molecular clouds; in particular, low-temperature chemical reactions can possibly take place in the presence of cosmic ray irradiation. Interstellar grains or icy mantles of cosmic objects are continuously exposed to energetic particles as well as photons that will lead to excitation, dissociation and ionization of molecules. As a consequence, a variety of new species can be produced by reactions among the fragments.
Nitriles and nitrile chemistry have attracted more attention compared to other compounds in astrochemistry because they are easily detected both in the gas and solid phases in the ISM, comets, planets, and satellites. In addition, nitrile derivatives are thought to be precursors in the synthesis of amino acids in astrophysical environments, although amino acids have not yet been conclusively detected beyond the Earth. One would expect a method to synthesis molecules containing –CN groups in astrophysical environments to start from the simplest molecules containing N and C atoms, i.e. N2 and CH4, especially as these molecules are common both in the ISM and in our solar system. when exposed to VUV irradiation. These nitriles might play an important role in the synthesis of prebiotic molecules. More importantly, we have measured the column densities of HNC and HCN formation and have investigated their ratios as a function of irradiation time for the first time. The presence of HNC and the variations of HNC/HCN ratios obtained in the present work may improve our understanding of the evolution of icy chemistry on icy bodies in cosmic environments.
