Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water

Ueta, Hirokazu; Watanabe, Naoki; Hama, Tetsuya; Kouchi, Akira

doi:10.1103/physrevlett.116.253201

Cited by 27 publications

(40 citation statements)

References 36 publications

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“…While the above reactions are considered to be the main paths for the H 2 nuclear conversion, in the last years the o-p conversion on dust has attracted a lot of attention (Watanabe et al 2010;Ueta et al 2016 …”

Section: Nuclear Spin Conversion Ratementioning

confidence: 99%

H₂ Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

Bovino

Schleicher

Caselli

2017

ApJL

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Deuterium fractionation, i.e., the enhancement of deuterated species with respect to non-deuterated ones, is considered to be a reliable chemical clock of star-forming regions. This process is strongly affected by the ortho-topara H 2 ratio. In this Letter we explore the effect of the ortho-para (o-p) H 2 conversion on grains on the deuteration timescale in fully-depleted dense cores, including the most relevant uncertainties that affect this complex process. We show that (i) the o-p H 2 conversion on grains is not strongly influenced by the uncertainties on the conversion time and the sticking coefficient, and (ii) that the process is controlled by the temperature and the residence time of ortho-H 2 on the surface, i.e., by the binding energy. We find that for binding energies between 330 and 550 K, depending on the temperature, the o-p H 2 conversion on grains can shorten the deuterium fractionation timescale by orders of magnitude, opening a new route for explaining the large observed deuteration fraction D frac in dense molecular cloud cores. Our results suggest that the star formation timescale, when estimated through the timescale to reach the observed deuteration fractions, might be shorter than previously proposed. However, more accurate measurements of the binding energy are needed in order to better assess the overall role of this process.

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Section: Nuclear Spin Conversion Ratementioning

confidence: 99%

H₂ Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

Bovino

Schleicher

Caselli

2017

ApJL

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“…The conversion mechanism on dust grains has been postulated to be the interaction with inhomogeneities of the surface (Fukutani & Sugimoto 2013), but it is not completely understood. Regardless of the conversion mechanism, it has been shown that in amorphous solid water (ASW), conversion can take place on laboratory timescales, 6.4 × 10 2 to 4.1 × 10 3 s (Watanabe et al 2010;Ueta et al 2016).…”

Section: Introductionmentioning

confidence: 99%

Binding energies and sticking coefficients of H₂ on crystalline and amorphous CO ice

Molpeceres

Zaverkin

Watanabe

et al. 2021

A&A

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Context. Molecular hydrogen (H2) is the most abundant interstellar molecule and plays an important role in the chemistry and physics of the interstellar medium. The interaction of H2 with interstellar ices is relevant for several processes (e.g., nuclear spin conversion and chemical reactions on the surface of the ice). To model surface processes, quantities such as binding energies and sticking coefficients are required. Aims. We provide sticking coefficients and binding energies for the H2/CO system. These data are absent in the literature so far and could help modelers and experimentalists to draw conclusions on the H2/CO interaction in cold molecular clouds. Methods. Ab initio molecular dynamics simulations, in combination with neural network potentials, were employed in our simulations. Atomistic neural networks were trained against density functional theory calculations on model systems. We sampled a wide range of H2 internal energies and three surface temperatures. Results. Our results show that the binding energy for the H2/CO system is low on average, − 157 K for amorphous CO and −266 K for crystalline CO. This carries several implications for the rest of the work. H2 binding to crystalline CO is stronger by 109 K than to amorphous CO, while amorphous CO shows a wider H2 binding energy distribution. Sticking coefficients are never unity and vary strongly with surface temperature, but less so with ice phase, with values between 0.95 and 0.17. With the values of this study, between 17 and 25% of a beam of H2 molecules at room temperature would stick to the surface, depending on the temperature of the surface and the ice phase. Residence times vary by several orders of magnitude between crystalline and amorphous CO, with the latter showing residence times on the order of seconds at 5 K. H2 may diffuse before desorption in amorphous ices, which might help to accommodate it in deeper binding sites. Conclusions. Based on our results, a significant fraction of H2 molecules will stick on CO ice under experimental conditions, even more so under the harsh conditions of prestellar cores. However, with the low H2–CO binding energies, residence times of H2 on CO ice before desorption are too short to consider a significant population of H2 molecules on pure CO ices. Diffusion is possible in a time window before desorption, which might help accommodate H2 on deeper binding sites, which would increase residence times on the surface.

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“…[12], displayed in Figure 1, which summarizes results from FIR and MIR spectroscopies, as well as INS (Ref [24,25,[38][39][40]43]). line) [26,[48][49][50]. At T<10K, the contribution from a reversible one-phonon process [i.e., the so-called "direct process" o-H 2 O↔p-H 2 O+hω Ar , whose temperature behavior scales as coth(hω Ar /2RT ) withhω Ar ∼ 23 cm -1 ] appears to dominate NSI interconversion rates ( Figure 3A, blue dash-dot-dot line).…”

Section: Resultsmentioning

confidence: 99%

Confinement Effects on the Nuclear Spin Isomer Conversion of H₂O

et al. 2017

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The mechanism for interconversion between the nuclear spin isomers (NSI) of HO remains shrouded in uncertainties. The temperature dependence displayed by NSI interconversion rates for HO isolated in an argon matrix provides evidence that confinement effects are responsible for the dramatic increase in their kinetics with respect to the gas phase, providing new pathways for o-HO↔p-HO conversion in endohedral compounds. This reveals intramolecular aspects of the interconversion mechanism which may improve methodologies for the separation and storage of NSI en route to applications ranging from magnetic resonance spectroscopy and imaging to interpretations of spin temperatures in the interstellar medium.

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Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water

Cited by 27 publications

References 36 publications

H₂ Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

H₂ Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

Binding energies and sticking coefficients of H₂ on crystalline and amorphous CO ice

Confinement Effects on the Nuclear Spin Isomer Conversion of H₂O

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Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water

Cited by 27 publications

References 36 publications

H2 Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

H2 Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

Binding energies and sticking coefficients of H2 on crystalline and amorphous CO ice

Confinement Effects on the Nuclear Spin Isomer Conversion of H2O

Contact Info

Product

Resources

About

H₂ Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

H₂ Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

Binding energies and sticking coefficients of H₂ on crystalline and amorphous CO ice

Confinement Effects on the Nuclear Spin Isomer Conversion of H₂O