Photoprocessing & Spectroscopy Laboratory employs an ultra-high vacuum chamber (P ~10-10 torr) equipped with a closed-cycle helium cryostat (10 – 300 K) to mimic the environment of outer space. The studied ice or ice mixture analogs can be prepared by a gas handling system having 4 stainless steel bottles and a Baratron gauge measuring pressures in the 0–100 torr range with a 2% accuracy. Then gas mixtures introduced into the vacuum chamber through a stainless steel tube and condensed on the cold substrate where the temperature is kept at a selected condition (15 – 150K), and the deposition rate is controlled by using a precision leak valve. The total thickness of deposited ice ﬁlms is measured by monitoring the variation of interference fringes of a solid-state laser (532 nm)/He-Ne laser (632.8 nm) light reﬂected by icy films and KBr substrate.
Photoprocess studies have mainly been focused on the effects of (1) the Lyman-α emission line (121.6 nm) and the H2 molecular emission in 110–180 nm range via the use of a microwave-discharge hydrogen flow lamp (MDHL), and (2) tunable UV/EUV/X-ray synchrotron radiation (Beamline 03A & 08B) provided by National Synchrotron Radiation Research Center (NSRRC) in Hsinchu. Since synchrotron radiation can provide photons with energies ranging from 4 to 45 eV (BL03A) and from 80 to 1250 eV (BL08B), we can study photoprocesses of interstellar ice analogs at different monochromatic photon energies.
A number of different photon sources have been used in laboratory astrophysical/astrochemical simulation experiments. The most widely used photon source is the microwave-discharge hydrogen flow lamp (MDHL) because it provides intense hydrogen Lyman α (H Ly-α, or HI line) emission at 121.6 nm and molecular H2 emission bands in the 140–170 nm range. However, a large majority of these studies does not provide any UV output spectrum of the light source used or any detailed information about their experimental conditions and assume that the UV photon source is dominated by Ly-α emission.
We have measured the photon flux distribution of MDHL as a function of photon energy and photon flux calibration in several operating configurations. The results show that MDHL provides hydrogen Lyman α (H Ly-α, 121.6 nm) and H2 molecular emission in the 110–180 nm range, and the spectral characteristics of the VUV light emitted in this range, in particular the relative proportion of Lyman α to molecular emission bands, strongly depend on the pressure of H2 inside the lamp, the lamp geometry (F type vs. T type), the gas used (pure H2 vs. H2 seeded in He), and the optical property of the window used (MgF2 vs. CaF2). These different experimental configurations of the MDHL are then used to study the VUV irradiation of CO ice at 14 K. In contrast to the large majority of previous studies dedicated to the VUV irradiation of astrophysical ice analogs, which did not take into consideration the emission spectrum of the MDHL, our experimental results show that the processes induced by photons in CO ice from a broad energy range are different and more complex than the sum of individual processes induced by monochromatic sources scanning the same energy range, due to the existence of multi-state electronic transitions and discrepancy of absorption cross-section between parent molecules and products in Ly-α and H2 molecular emission ranges.
UV/EUV/X-ray photon energy will be selected to identify the relation between photolysis products and the input photon energy. Products during photon irradiation can be in situ monitored by an FTIR spectrometer and a quadrupole mass spectrometer (QMS). The irradiated residue after photon irradiation can also be ex-situ analyzed by high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). According to our previous study, even in the simple ice mixture, namely H2O+CO2+NH3 ice mixture, several amino acids can be found in the residue after UV/EUV photon irradiation.
In 2018, we have set-up a novel Interstellar Energetic-Processes System (IEPS) which equipped with electron gun with energies in the 1–5-KeV range, an MDHL providing VUV photons with energies in the 5–11-eV range, and a connecting port to be able to connect the IEPS with synchrotron radiation sources in the VUV, EUV and X-ray ranges. Therefore, this new experimental device will be optimized to study the electron, heavy-ion, VUV and EUV photon, and X-ray irradiation of icy systems.
This access to various radiation sources and the synchrotron facility (NSRRC), providing photons and electrons that can be simultaneously used in a unique same ultrahigh vacuum chamber allows us to better evaluate the relative importance of irradiation particles and their energy dependences. 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, heavy ion and energetic electron irradiations under simulated astrophysical conditions will provide useful data to evaluate and simulate all the types of irradiation that ice mantles experience during their lifetimes in the interstellar and circumstellar media.