Energetic Electron Irradiations of Amorphous and Crystalline Sulphur - Bearing Astrochemical Ices Duncan V. Mifsud12 Péter Herczku2 Rich árd Rácz2 K.K. Rahul2 Sándor T.S. Kovács2

2025-04-29 0 0 959.16KB 20 页 10玖币

侵权投诉

Energetic Electron Irradiations of Amorphous and Crystalline Sulphur-

Bearing Astrochemical Ices

Duncan V. Mifsud1,2,*, Péter Herczku2,*, Richárd Rácz2, K.K. Rahul2, Sándor T.S. Kovács2,

Zoltán Juhász2, Béla Sulik2, Sándor Biri2, Robert W. McCullough3, Zuzana Kaňuchová4,

Sergio Ioppolo5, Perry A. Hailey1, and Nigel J. Mason1,2,*

1 Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent,

Canterbury CT2 7NH, United Kingdom

2 Institute for Nuclear Research (Atomki), Debrecen H-4026, Hungary

3 Department of Physics and Astronomy, School of Mathematics and Physics, Queen’s University

Belfast, Belfast BT7 1NN, United Kingdom

4 Astronomical Institute, Slovak Academy of Sciences, Tatranská Lomnica SK-059 60, Slovakia

5 School of Electronic Engineering and Computer Science, Queen Mary University of London,

London E1 4NS, United Kingdom

* Correspondence: Duncan V. Mifsud dm618@kent.ac.uk

Péter Herczku herczku@atomki.hu

Nigel J. Mason n.j.mason@kent.ac.uk

ORCID Identification Numbers

Duncan V. Mifsud 0000-0002-0379-354X

Péter Herczku 0000-0002-1046-1375

Richard Rácz 0000-0003-2938-7483

K.K. Rahul 0000-0002-5914-7061

Sándor T.S. Kovács 0000-0001-5332-3901

Zoltán Juhász 0000-0003-3612-0437

Béla Sulik 0000-0001-8088-5766

Sándor Biri 0000-0002-2609-9729

Robert W. McCullough 0000-0002-4361-8201

Zuzana Kaňuchová 0000-0001-8845-6202

Sergio Ioppolo 0000-0002-2271-1781

Perry A. Hailey 0000-0002-8121-9674

Nigel J. Mason 0000-0002-4468-8324

Abstract

Laboratory experiments have confirmed that the radiolytic decay rate of astrochemical ice

analogues is dependent upon the solid phase of the target ice, with some crystalline molecular

ices being more radio-resistant than their amorphous counterparts. The degree of radio-

resistance exhibited by crystalline ice phases is dependent upon the nature, strength, and extent

of the intermolecular interactions that characterise their solid structure. For example, it has

been shown that crystalline CH3OH decays at a significantly slower rate when irradiated by 2

keV electrons at 20 K than does the amorphous phase due to the stabilising effect imparted by

the presence of an extensive array of strong hydrogen bonds. These results have important

consequences for the astrochemistry of interstellar ices and outer Solar System bodies, as they

imply that the chemical products arising from the irradiation of amorphous ices (which may

include prebiotic molecules relevant to biology) should be more abundant than those arising

from similar irradiations of crystalline phases. In this present study, we have extended our work

on this subject by performing comparative energetic electron irradiations of the amorphous and

crystalline phases of the sulphur-bearing molecules H2S and SO2 at 20 K. We have found

evidence for phase-dependent chemistry in both these species, with the radiation-induced

exponential decay of amorphous H2S being more rapid than that of the crystalline phase, similar

to the effect that has been previously observed for CH3OH. For SO2, two fluence regimes are

apparent: a low-fluence regime in which the crystalline ice exhibits a rapid exponential decay

while the amorphous ice possibly resists decay, and a high-fluence regime in which both phases

undergo slow exponential-like decays. We have discussed our results in the contexts of

interstellar and Solar System ice astrochemistry and the formation of sulphur allotropes and

residues in these settings.

Keywords: Astrochemistry; Planetary science; Electron irradiation; Radiation chemistry;

Amorphous ice; Crystalline ice; Sulphur

1 Introduction

It has been established for some time now that the laboratory irradiation of astrochemical ice

analogues using energetic charged particles (i.e., ions and electrons) or ultraviolet photons may

lead to the production of prebiotic molecules relevant to biology, such as amino acids or

nucleobases (e.g., Muñoz-Caro et al. 2002, Hudson et al. 2008, Nuevo et al. 2012). Motivated

by a desire to further understand the non-equilibrium chemistry leading to the formation of

these so-called ‘seeds of life’, many studies have sought to determine and quantify the influence

of various physical parameters on the outcome of such reactions. Perhaps the best studied of

these is ice temperature, with previous works having demonstrated the key influence of this

parameter on the abundance of product molecules formed after irradiation (e.g., Sivaraman et

al. 2007, Mifsud et al. 2022a).

Our recent work has also demonstrated that the solid phase of an irradiated ice plays a crucial

role in determining the outcome of astrochemical reactions mediated by ionising radiation.

Through a series of comparative electron irradiations, we have demonstrated that the radiolytic

decay rate of an astrochemical ice is dependent upon the nature, strength, and extent of the

intermolecular interactions that characterise its solid phase (Mifsud et al. 2022b, Mifsud et al.

2022c). For instance, the decay rate of α-crystalline CH3OH was found to be significantly less

rapid than that of the amorphous phase. This was attributed to the existence of an extensive

network of strong hydrogen bonds that exists in the α-crystalline phase. This network requires

an additional energy input from the projectile electrons to be overcome, thus leaving less

energy overall to drive radiolytic chemistry. Conversely, the amorphous CH3OH ice is

characterised only by localised clusters of hydrogen bonded molecules. Such a structure does

not benefit from the same stabilising effect supplied by the network of hydrogen bonds in the

α-crystalline phase, particularly as hydrogen bonding in CH3OH is known to be a cooperative

phenomenon in which the presence of one hydrogen bond in the network strengthens

successive hydrogen bonds through electrostatic effects (Kleeberg and Luck 1989, Sum and

Sandler 2000).

In the case of N2O ice, the decay rate of the amorphous phase was noted to be only moderately

more rapid than that of the crystalline phase (Mifsud et al. 2022b). The dominant

intermolecular forces of attraction in solid N2O are expected to be dipole-dipole interactions.

Although the orientation of these dipoles in the crystalline phase is anticipated to confer some

degree of resistance against radiolytic decay compared to the amorphous phase, this is

considerably less than that induced by the hydrogen bonding network in α-crystalline CH3OH.

This therefore explains the more similar radiolytic decay rates of amorphous and crystalline

N2O. Such results carry important implications for the radiation processing of astrochemical

ices, as they suggest that the irradiation of amorphous ices is more chemically productive than

that of crystalline ones; particularly in the case of those ices which are able to form strong and

extensive intermolecular bonds when crystalline. Extending this idea further, it is entirely

possible that those astrophysical environments in which space radiation-induced amorphisation

processes dominate over thermally-induced crystallisation may be characterised by a more

productive radiation chemistry. This idea is not unreasonable, particularly in light of the

discovery of several complex organic molecules in pre-stellar cores (e.g., McGuire et al. 2020,

Burkhardt et al. 2021).

In this present study, we have expounded upon our previous work by performing comparative

electron irradiations of the crystalline and amorphous phases of pure H2S and SO2

astrochemical ice analogues, thus simulating the processing such ices undergo during their

interaction with galactic cosmic rays, stellar winds, or magnetospheric plasmas as a result of

the production of large quantities of secondary electrons (Mason et al. 2014, Boyer et al. 2016).

Solid H2S is known to exhibit a number of stable crystalline phases under low temperature and

ambient pressure conditions (Fathe et al. 2006), but it is the crystalline phase III (hereafter

simply referred to as the crystalline H2S phase) which is of importance under conditions

relevant to astrochemistry. This phase is orthorhombic, having eight molecules per unit cell

and adopting the Pbcm space group. SO2 may also exist as an orthorhombic crystalline solid

under astrochemical conditions, but in this case the Aba2 space group is adopted and there are

only two molecules per unit cell (Schriver-Mazzuoli et al. 2003).

Although sulphur is one of the most abundant elements in the cosmos and is of importance in

both biochemical and geochemical contexts, much remains unknown regarding its chemistry

in interstellar and outer Solar System settings (Mifsud et al. 2021a). It is thought, for instance,

that H2S ice processing by galactic cosmic rays or ultraviolet photons accounts for the apparent

depletion of sulphur (relative to its total cosmic abundance) in dense interstellar clouds by

producing large quantities of atomic sulphur or molecular sulphur chains and rings which are

difficult to detect using current observation techniques (Jiménez-Escobar and Muñoz-Caro

2011, Jiménez-Escobar et al. 2014). H2S itself has not yet been definitively detected in

interstellar icy grain mantles (Boogert et al. 2015). Conversely, SO2 ice has been detected

within both the dense interstellar medium as well as on the surfaces of outer Solar System

bodies such as the Galilean moons of Jupiter (Boogert et al. 1997, Carlson et al. 1999).

However, the exact chemical mechanisms leading to its formation in these settings remain

widely debated (Mifsud et al. 2021a).

The purpose of this study is thus two-fold: (i) to determine whether the phase of irradiated

sulphur-bearing molecular ices influences the radiation-induced rate of decay as was previously

demonstrated for non-sulphur-bearing ices; and (ii) to contribute further to our (comparatively

poor) understanding of the extra-terrestrial chemistry of sulphur. To achieve these goals, the

amorphous and crystalline phases of pure H2S and SO2 ices were respectively irradiated with

2 and 1.5 keV electrons, and the resultant physico-chemical changes were followed in situ

using Fourier-transform mid-infrared (FT-IR) transmission absorption spectroscopy.

2 Experimental Methodology

The irradiation experiments were performed using the Ice Chamber for Astrophysics-

Astrochemistry (ICA); a custom-built experimental apparatus located at the Institute for

Nuclear Research (Atomki) in Debrecen, Hungary. This apparatus (Fig. 1) has been described

in detail in previous publications (Herczku et al. 2021, Mifsud et al. 2021b), and so only a brief

description of the most salient details will be provided here. The ICA is a UHV-compatible

chamber with a nominal base pressure of a few 10–9 mbar which is achieved by the combined

action of a dry rough vacuum pump and a turbomolecular pump. Within the centre of the

chamber is a gold-coated oxygen-free copper sample holder which supports up to four ZnSe

deposition substrates, onto which astrochemical ice analogues may be prepared. The

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

EnergeticElectronIrradiationsofAmorphousandCrystallineSulphur-BearingAstrochemicalIcesDuncanV.Mifsud1,2,*,PéterHerczku2,*,RichárdRácz2,K.K.Rahul2,SándorT.S.Kovács2,ZoltánJuhász2,BélaSulik2,SándorBiri2,RobertW.McCullough3,ZuzanaKaňuchová4,SergioIoppolo5,PerryA.Hailey1,andNigelJ.Mason1,2,*1CentreforAs...

展开>> 收起<<

Energetic Electron Irradiations of Amorphous and Crystalline Sulphur - Bearing Astrochemical Ices Duncan V. Mifsud12 Péter Herczku2 Rich árd Rácz2 K.K. Rahul2 Sándor T.S. Kovács2.pdf

共20页,预览4页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

Energetic Electron Irradiations of Amorphous and Crystalline Sulphur - Bearing Astrochemical Ices Duncan V. Mifsud12 Péter Herczku2 Rich árd Rácz2 K.K. Rahul2 Sándor T.S. Kovács2

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: