Molecular clouds (MCs) in space are the birthplace of various molecular species. Chemical reactions occurring on the cryogenic surfaces of cosmic icy dust grains have been considered to play ...important roles in the formation of these species. Radical reactions are crucial because they often have low barriers and thus proceed even at low temperatures such as ∼10 K. Since the 2000s, laboratory experiments conducted under low-temperature, high-vacuum conditions that mimic MC environments have revealed the elementary physicochemical processes on icy dust grains. In this review, experiments conducted by our group in this context are explored, with a focus on radical reactions on the surface of icy dust analogues, leading to the formation of astronomically abundant molecules such as H
, H
O, H
CO, and CH
OH and deuterium fractionation processes. The development of highly sensitive, non-destructive methods for detecting adsorbates and their utilization for clarifying the behavior of free radicals on ice, which contribute to the formation of complex organic molecules, are also described.
Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding ...the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear spin conversion (NSC) processes of H2 molecules on diamond-like carbon and graphite surfaces are investigated experimentally by employing temperature-programmed desorption and resonance-enhanced multiphoton ionization methods. For the diamond-like carbon surface, the NSC time constants were determined at temperatures of 10–18 K and from 3900 ± 800 s at 10 K to 750 ± 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp3 or sp2 hybridization) have little effect on the NSC rates.
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•Crystallinity of CO2 under UV-ray irradiation was directly observed by TEM.•Amorphization of crystalline CO2 upon UV-ray irradiation was not observed.•Stronger UV-ray irradiation in ...IR experiments did not cause amorphization either.•UV-fluence expected in molecular clouds will not cause amorphization of CO2 ice.
The Crystallinity of ices, amorphous and crystalline, can be altered not only by heat but also by irradiation of UV-rays or charged particles. In this work, the effect of UV-irradiation on the crystallinity of CO2 ice was investigated by using a transmission electron microscope. A crystalline CO2 ice was produced by annealing amorphous CO2 ice. We found that UV-ray irradiation of CO2 crystals at approximately 10 K does not cause amorphization in contrast to the reported amorphization of crystalline water ice below 70 K. We discuss the difference based on the expected UV photochemistry of CO2 ice.
Mass spectra of n-pentane and n-hexane ionized through femtosecond-laser pulses were measured using a time-of-flight mass spectrometer. Fragment ions ejected with large kinetic energies were ...identified as side peaks in which a two-body dissociation pathway, C5H12 ++ → C2H5 + + C3H7 +, was identified for n-pentane, and two for n-hexane, C6H14 ++ → C2H5 + + C4H9 + and C3H7 + + C3H7 +, based on momentum matching of the fragments. The two-body dissociation pathways were observed when the polarization direction of the linearly polarized laser light was perpendicular to the molecular axis. However, when the polarization direction was parallel to the molecular axis or the laser light was circularly polarized, these signals were weak or difficult to identify. These results suggest that the two-body dissociation pathways are caused by nonsequential double ionization (NSDI), which begins with ionization from the π-type second highest occupied molecular orbital (HOMO−1) via the laser electric field perpendicular to the molecular axis rather than bonding the σ-type HOMO. Quantum chemical calculations show that the dication has a triplet metastable state with the same formula as the neutral state (i.e., 3CH3–(CH2) n –CH3++). Therefore, the relevant two-body dissociation channels open through transition states with the (HOMO)1(HOMO−1)1 electron configuration and the estimated kinetic energy release values correlate with those observed.
It has been implicitly assumed that ices on grains in molecular clouds and protoplanetary disks are formed by homogeneous layers regardless of their composition or crystallinity. To verify this ...assumption, we observed the H2O deposition onto refractory substrates and the crystallization of amorphous ices (H2O, CO2, and CO) using an ultra-high-vacuum transmission electron microscope. In the H2O-deposition experiments, we found that three-dimensional islands of crystalline ice (Ic) were formed at temperatures above 130 K. The crystallization experiments showed that uniform thin films of amorphous CO and H2O became three-dimensional islands of polyhedral crystals; amorphous CO2, on the other hand, became a thin film of nano-crystalline CO2 covering the amorphous H2O. Our observations show that crystal morphologies strongly depend not only on the ice composition but also on the substrate. Using experimental data concerning the crystallinity of deposited ices and the crystallization timescale of amorphous ices, we illustrated the criteria for ice crystallinity in space and outlined the macroscopic morphology of icy grains in molecular clouds as follows: amorphous H2O covered the refractory grain uniformly, CO2 nano-crystals were embedded in the amorphous H2O, and a polyhedral CO crystal was attached to the amorphous H2O. Furthermore, a change in the grain morphology in a protoplanetary disk is shown. These results have important implications for the chemical evolution of molecules, nonthermal desorption, collision of icy grains, and sintering.
The para-hydrogen (p-H2) matrix-isolation technique has been scarcely used to record electronic absorption and emission spectra. It is expected that its small matrix shifts due to diminished ...molecular interactions and the softness of the lattice might be advantageous to help identify the carriers of the diffuse interstellar bands. In this article, we present infrared, fluorescence excitation, and dispersed fluorescence spectra of sumanene (C21H12), a bowl-shaped polycyclic aromatic hydrocarbon and a fragment of C60, isolated in solid p-H2. The recorded vibrational wavenumbers from infrared and dispersed fluorescence agree with the scaled harmonic vibrational wavenumbers calculated with the B3PW91/6-311++G(2d,2p) and B3LYP/6-311++G(2d,2p) methods. The recorded fluorescence excitation spectra are consistent with the spectra of jet-cooled gas-phase C21H12 reported previously by Kunishige et al. We found a rather small matrix shift of 55 cm–1 for the S1–S0 electronic transition origin located at 27 888 cm–1. Vibrational wavenumbers associated with the S1 state of C21H12 inferred from the experimental spectrum can be assigned mostly to fundamental normal modes; they are in satisfactory agreement with scaled harmonic vibrational wavenumbers calculated at the TD-B3PW91/6-311++G(2d,2p) level of theory. Significantly more vibrational modes of the S1 state were identified as compared with those in the reported gas-phase work. The potential of p-H2 matrix-isolation spectroscopy to provide electronic excitation spectra suitable for comparison to astronomical observations is discussed by comparing the spectra of C21H12 isolated in solid p-H2 and in solid Ne, a matrix host commonly employed in astrochemistry.
We report the first measurements of the complex refractive index (RI) at 375, 405, 532, and 781 nm for secondary organic aerosol (SOA) generated from isoprene/NOx photooxidation. At all wavelengths ...studied, slightly greater real components of the RI were observed for the SOA generated in the absence of SO2 compared with those generated in its presence. Considering the chemical properties, the differences in the oxidation state and/or ratio of particle density to molecular weight of compounds in the SOA are considered to be the main factors determining the real components. The imaginary components at ≤532 nm were found to increase with increasing initial SO2 concentration. The highly conjugated oligomers are suggested to be plausible chromophore candidates. This study suggests that when large amounts of SOA are generated after mixing of isoprene with NOx and SOx, light absorption of these SOAs may compete with that of black carbon, especially at ultraviolet wavelengths.
Key Points
Complex refractive index (m = n − ki) of the isoprene SOA is determined
Larger n values are obtained in the absence of SO2 than in the presence of SO2
Significant k values at 375, 405, and 532 nm are observed in the presence of SO2
ABSTRACT The mid-infrared emission from galactic objects, including reflection nebulae, planetary nebulae, proto-planetary nebulae, molecular clouds, etc, as well as external galaxies, is dominated ...by the unidentified infrared (UIR) emission bands. Large protonated polycyclic aromatic hydrocarbons (H+PAHs) were proposed as possible carriers, but no spectrum of an H+PAH has been shown to exactly match the UIR bands. Here, we report the IR spectrum of protonated ovalene (7-C32H15+) measured in a para-hydrogen (p-H2) matrix at 3.2 K, generated by bombarding a mixture of ovalene and p-H2 with electrons during matrix deposition. Spectral assignments were made based on the expected chemistry and on the spectra simulated with the wavenumbers and infrared intensities predicted with the B3PW91/6-311++G(2d,2p) method. The close resemblance of the observed spectral pattern to that of the UIR bands suggests that protonated ovalene may contribute to the UIR emission, particularly from objects that emit Class A spectra, such as the IRIS reflection nebula, NGC 7023.
•We investigated multiple ionization of the acetic acid dimer in intense laser pulses.•Two-body Coulomb explosion pathway into two monomer cations was identified.•It is suggested that the dimer ...structure deformed in the laser fields.•Quantum chemical calculations are performed for acetic acid dimer dication.
Fragment ions produced after the ionization of the acetic acid dimer (CH3COOH)2 in a near-infrared intense femtosecond laser field were measured using time-of-flight mass spectrometry. The two-body Coulomb explosion (CE) process (CH3COOH)22+→2CH3COOH+ with a kinetic energy release of 3.4eV, similar to the case of (HCOOH)22+ (Hoshina et al., 2012), was identified. The two-body CE of (CH3COOH)22+ was induced when the laser polarization direction was perpendicular to the C⋯C intermolecular axis in contrast to the case of (HCOOH)22+. The preferential configurations may be those that deform dimer structures close to dimer dications during the double-ionization process.
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