Theory, experiment, and simulations in laboratory astrochemistry

Ever since the days of William Herschel (1738–1822) and Joseph von Fraunhofer (1787–1826), astronomy and atomic and molecular spectroscopy have been inseparably intertwined. Based on a combination of astronomical observations and laboratory spectroscopic studies, atoms, and then molecules, were identified in environments outside of the Earth’s atmosphere, with the analysis of optical spectra of the Sun being one of the earliest and foremost examples. The ongoing discovery of ever more complex molecules, ions and radicals using their rotational transitions summarizes the current stage of the molecular exploration of our astrophysical environment.

In the 1960s, very few molecules had been identified in astrophysical environments, and many astronomers thought that the harsh conditions of the interstellar and circumstellar medium would prevent the existence of stable molecular species, with very few exceptions such as H₂ or CO. This situation changed profoundly with the advent of telescopes in the cm–mm range. Evidence of rich rotational microwave spectra in many interstellar gases signalled the onset of molecular astrophysics, and of what is known today as astrochemistry. Today, in the various conditions of the interstellar medium (ISM), more than 200 molecular species have been identified, not including the many isotopologues that have also been identified. Organic molecules with three or more heavy atoms (so-called Complex Organic Molecules, COMs, in astrophysical parlance) are commonplace, even if they occur in the ISM as traces. Their relative abundance with respect to the main molecular gases, particularly H₂, is less than 10⁻⁶ at most, and usually less than 10⁻⁹. Many of these molecules are common organic building blocks such as methanol, methyl- and ethyl-cyanide, and methyl formate. Another special class of molecules are the so-called Polycyclic Aromatic Hydrocarbons (PAHs), for which the exact formulas are not known. However, the fullerenes C₆₀ and C₇₀ are exceptions, and are the largest individual molecular species identified in space to date.

The application of the physical chemist's toolbox to questions of astrophysical relevance reaches far beyond spectroscopy alone. Gas-phase and surface reaction dynamics, (surface) scattering studies and cluster, grain and ice formation are employed to address astrophysical and astrochemical issues, and yield parameters that are of direct use to astronomical models. This collection of papers focuses on the application of physical chemistry to understand the chemistry of interstellar media (ISM). Laboratory astrochemistry, experimental as well as theoretical, has developed and matured considerably over the past two decades, on account of, among other reasons, the vigorous impetus of astronomical data modellers and the recognition of many space and research funding agencies that are of key importance for laboratory astrophysics studies.

Chemistry in interstellar environments is in many ways very different from the chemistry under the conditions of terrestrial chemical laboratories that we are more accustomed to. While the basic concepts of physical chemistry evidently remain unchanged, the physical conditions are such that some of the usual hypotheses are no longer valid. Whether referring to interstellar gaseous matter, gases surrounding young or old stellar objects or smaller bodies such as comets and asteroids, the main constraints on the chemical processes are: (i) very dilute conditions with number densities ranging from 100 cm⁻³ to 10¹⁰ cm⁻³, (ii) a wide range of temperatures with molecules observed at gas temperatures as low as 7–15 K, and (iii) an overwhelming dominance of atomic and molecular hydrogen, which constitute more than 90% (in number) of the total number density, with all other elements except for ⁴He being less than 1% in number. The main isotopes of the chemical elements of relevance are ¹⁶O, ¹²C, ¹⁴N, ²⁰Ne, ²⁴Mg, ²⁸Si, ³²S, ³⁶Ar, and ⁵⁶Fe. Nearly no trace is found of elements with nuclei heavier than that of ⁵⁶Fe (the most stable nucleus).

As a consequence of the particular physical and chemical conditions of the ISM, a laboratory astrochemist must adapt their experimental strategies to mimic the astrophysical conditions as closely as possible. The most relevant issues to consider for the studies described in the present series of papers are summarized as follows:

(1) Because of the low density and temperature in the ISM, chemistry usually does not take place under the conditions of thermodynamic equilibrium. Chemical processes are dominated by the kinetics and branching ratios of the reactions at hand, and are at steady state at best even if the astrophysical timescales are relatively long, with characteristic times being of the order of 10⁴–10⁶ years. Only in planetary (or stellar) atmospheres is thermal equilibrium reached. In space, three-body collisions are absent or extremely unlikely. Reactions such as A + B → AB may only occur significantly with radiative stabilisation, or else on the surface of a grain. These restrictions incur fundamental differences in many of the common assumptions of synthetic/organic chemistry.

(2) The low temperatures that often prevail in interstellar space prevent many reactions from taking place, even those with modest activation barriers. However, some reactions proceed through tunnelling, especially if they involve atomic H (or D). However, tunnelling of heavier atoms should not to be excluded.

(3) Ion chemistry plays an important role in space. On one hand, ions are long lived in low-density interstellar environments and on the other hand, ion–molecule reactions present an important alternative due to their long-range interaction and low or absent reaction barriers. Cations are most common, although anions are also present. The main ions are atomic species (in diffuse clouds) and protonated species (in molecular clouds), but some radical cations may be very stable, especially for resonantly stabilized species such as the family of PAHs. An all-important ion is H₃⁺, as it is a strong acid (proton donor) and the main vehicle driving deuteration via a slightly exothermic reaction (H₃⁺ + HD ⇒ H₂D⁺ + H₂) and analogous reactions for HD₂⁺ and D₃⁺.

(4) In dilute interstellar environments, the ionization of elements such as C and S is driven by photons with E < 13.6 eV, with all of the photons with higher energy being absorbed by atomic H. However, cosmic rays (CR) are ubiquitous, even in the denser parts of the ISM. They form the primary source of energy that initiates chemical reaction networks (via a reaction summarized as H₂ + CR ⇒ e⁻ + H₂⁺ followed by H₂ + H₂⁺ ⇒ H₃⁺ + H).

(5) In regions of the ISM with a low ionisation fraction, neutral–neutral reactions are of importance, especially those involving radicals. Some tend to have higher rates at low temperatures. However, ion/electron chemistry dominates the chemical evolution in many sectors of molecular complexity, like the successive hydrogenation of NH_n⁺ or CH_n⁺.

(6) Because of the very low temperatures, nuclear spin statistics play an important role, yielding various nuclear modifications of H₂ or, less commonly, ND₃.

Let us also underline that the chemistry of the ISM is relatively rich. Among the almost two hundred molecules that have now been detected, there are some quite unusual ones from the viewpoint of the terrestrial laboratory: long carbon chains, polycyclic aromatic hydrocarbons, long chain anions and cations, some cyclic molecules, and many radicals and protonated species, like the common N₂H⁺ or HCO⁺ cations. It must not be underestimated that some heavier species, including amino acids and monosaccharides, which are present in meteorites, might also be present as traces in the ISM. However, confirming their detection using rotational spectroscopy is exceedingly difficult because of the unfavourable partition functions over many rotational levels.

The quantitative description of the chemistry of the ISM necessitates a non-equilibrium model of the radiative transfer of the most abundant molecular species. In order to achieve this, it has been known for a long time that the excitation/de-excitation of various molecular levels is determined both by photon emission/absorption and by collisions with the main components of the molecular gas, or with electrons. In the ISM, the projectiles are mainly H₂, H, and electrons. For planetary atmospheres, dense regions of proto-planetary disks and atmospheres of Solar System objects, the collision partners may be heavier species, such as H₂O or N₂. In any case, the rates of energy exchange between the projectile and the target, the molecule being observed, determine the line intensities for most rotational transitions, and also for some ro-vibrational transitions, if the FIR emission rate is low enough. This is especially true for forbidden electric-dipolar transitions, like magnetic (fine-structure) transitions in open shell atoms or radicals. Also, elastic/inelastic scattering computations open routes to ab initio computing of pressure broadening and other collisional properties. On the surface of grains, many reactions take place with or without radiation; these reactions are the focus of numerous experimental and theoretical studies, and their relevance in astrochemistry is subject to intense scrutiny. The importance of knowledge on the microscale is fully recognized nowadays, as reflected in the many recent reviews devoted to this topic.

It has been a major endeavour to model the rates of reactions and incorporate them into chemical reaction networks. The widely used networks tend to be comprehensive and even include surface reactions and adsorption/desorption. Other networks are specialised for the chemical conditions of interest in particular environments, such as Photon Dominated Regions (PDRs) or very cold pre-stellar cores, or even as specific as Titan's atmosphere. Modern simulation tools include chemical models coupled to physical dynamics, in particular hydro- and, in the future, magneto-hydro-dynamics. Changes in thermal or transport properties occur because the chemistry evolves and in turn influences the hydrodynamic properties of the considered medium, for example through cooling rates. Most of those networks necessitate the knowledge of 10³–10⁶ reaction rates, at variable temperatures and pressures.

The characteristic physical chemistry of astrophysical environments summarized so far makes it certainly different from regular terrestrial laboratory conditions. However, it shares some peculiarities with environments that are not uncommon to an experimental physical chemist. Let us mention here a few representative conditions: (i) physico-chemical studies of gaseous radicals have a long history and have traditionally focused on reactions of relevance in atmospheric and combustion processes. In fact, many of the chemical rates used in astrochemical databases can be utilised even if the physical conditions are different. In particular, the ubiquitous presence of O₂ and halogens make these networks very different; (ii) dilute and low temperature gas chemistry is prevalent in molecular beam instruments, which are used in many physical chemistry labs around the world to study spectroscopy in all wavelength ranges as well as scattering dynamics; (iii) ion chemistry, ion–molecule chemistry and ion spectroscopy studies involving mass spectrometry instrumentation naturally operate under low-density conditions. Cryogenic ion traps add to the possibility of obtaining interstellar conditions in the lab; (iv) and finally, high-level computational chemistry yields particularly accurate results when modelling molecular species in isolation and at low internal energies.

It is hoped that this special issue will acknowledge these synergies, and form a collection of experimental as well as theoretical physical chemistry investigations with an understanding of astrochemical processes as the common background.

Acknowledgements

We warmly thank Susanna Widicus-Weaver for the impetus she has given to this special issue and for many lively discussions. LW thanks the COST action CM1401 ‘Our Astrochemical History’ and N. Balucani for providing a starting point to gather this collection of papers. LW is partly supported by the PALMS Labex, Université Paris-Saclay. JO is supported by NWO through the Dutch Astrochemistry Network and the EU MSCA-ITN network “EUROPAH”.

---

Footnote

† Also at Laboratoire Aimé-Cotton, Université Paris Saclay and CNRS, Orsay, France.

---