Laboratory Spectroscopy Techniques to Enable Observations of Interstellar Ion Chemistry
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REVIEWS Laboratory spectroscopy techniques to enable observations of interstellar ion chemistry Brett A. McGuire 1,2,3 ✉ , Oskar Asvany 4, Sandra Brünken 5 and Stephan Schlemmer 4 ✉ Abstract | Molecular ions have long been considered key intermediates in the evolution of molecular complexity in the interstellar medium. However, owing to their reactivity and transient nature, ions have historically proved challenging to study in terrestrial laboratory experiments. In turn, their detection and characterization in space is often contingent upon advances in the laboratory spectroscopic techniques used to measure their spectra. In this Review, we discuss the advances over the past 50 years in laboratory methodologies for producing molecular ions and probing their rotational, vibrational and electronic spectra. + We largely focus this discussion around the widespread H3 cation and the ionic products originating from its reaction with carbon atoms. Finally, we discuss the current frontiers in this research and the technical advances required to address the spectroscopic challenges that they represent. More than 200 molecular species are now known to The primary initiator in this network is the simple + + be present in the interstellar medium (ISM), but only polyatomic cation H3 . The importance of H3 in initia- slightly more than 15% of these are ionic: either posi- ting ion–molecule chemistry has long been recognized, 1 + tively or negatively charged . Yet these charged species and H3 has been invoked as a key intermediate in have a substantial role in driving chemical evolution in reaction networks for decades2,4,5. Although laboratory space, where low densities and temperatures present knowledge of its spectrum dates back to 1980 (REF.6), considerable obstacles to reactivity. In a relatively diffuse it was not detected in the ISM, where it is now widely (REF.7) + gas, the time between collisions for two species in space observed, until 1996 . The reaction of H3 with can often be measured in days or months, and even then, hetero atoms (X = O, C or halogens, among others) is only if one of them is hydrogen (H or H2). When these generally exothermic and barrierless, resulting in a species do find each other, they typically only react if protonated species XH+: 1Department of Chemistry, there is no energetic barrier to do so. Ions help to solve ++ Massachusetts Institute both of these problems. X+HX3 → H+H(2 1) of Technology, Cambridge, Because ions are charged, their long- range inter- MA, USA. actions induce dipole moments in nearby neutral species. A cascade of reactions can proceed from the for- 2National Radio Astronomy Thus, an attractive potential forms between the charged mation of XH+, building up larger and more complex Observatory, Charlottesville, VA, USA. and uncharged species, substantially increasing the fre- species, often through subsequent reaction with H2. quency of collisions. Reactions with ions are also often Alternative initial reaction steps can occur, depending 3Center for Astrophysics, Harvard and Smithsonian, barrierless and exothermic, requiring no input of energy on the astrophysical environment (Fig. 1). There is sub- Cambridge, MA, USA. to proceed. As a result, much of the initial build- up of stantial discussion in the literature as to the extent of 4I. Physikalisches Institut, chemistry in the ISM is initiated through ion–molecule complexity that can be built solely through ion–molecule Universität zu Köln, Köln, reactions, which are often faster by orders of magnitude reactions and the competition of these reactions with Germany. than those between neutral collision partners2. Figure 1 grain- surface and neutral–neutral reactions8–10. There 5Radboud University, Institute shows a portion of this early reaction chemistry, demon- is little debate, however, about the importance of for Molecules and Materials, strating the role of ion–molecule reactions in generat- molecular ions in kick- starting chemical complexity FELIX Laboratory, Nijmegen, Netherlands. ing small hydrocarbons, water and more complex species in the ISM. + ✉e- mail: [email protected]; such as protonated methanol (CH3OH2 ). The latter is In this Review, we examine many of the laboratory [email protected] koeln.de thought to be an important intermediate in the forma- techniques that have enabled the observation of mol- 3 https://doi.org/10.1038/ tion of still more complex species but has not yet been ecular ion spectra, the current status of the field and s42254-020-0198-0 detected in the ISM1. the impact of recent advances in the laboratory on the 402 | AUGUST 2020 | VOLUME 2 www.nature.com/natrevphys REVIEWS Key points (and thus the sensitivity) is directly proportional to the path length. A common technique for increasing the • Molecular ions have a key role in driving chemistry in the interstellar medium because path length, without increasing the physical size of their reactions are usually exothermic and the attractive potentials that they induce the cell beyond what can reasonably be housed in a lab- increase reaction rates. oratory, is to use a system of mirrors to ensure that the • The laboratory spectra needed to study ions in space are challenging to collect, light reflects through the absorbing material many times as ions are both difficult to produce in large quantities and highly reactive. before it is detected. Indeed, in Oka’s set-up, the effective • A boom in ion studies was aided by the combination of spectroscopy techniques that path length was 32 m from a 2- m- long cell. There are greatly increased the optical path length with others that modulated the signal at several variations of this optical set- up; the most com- known frequencies while leaving noise unmodulated. mon are White-type 12 and Herriott-type 13 arrangements, • Action spectroscopy in cryogenic ion traps indirectly measures absorption by observing as well as simpler double- pass set-ups that use a polariz- changes in counts of the mass- selected, stored ions. ing grid and a polarization- rotating rooftop reflector14. • Action spectroscopy is by far the most sensitive technique and has developed into a Oka also implemented a phase- locked detection scheme versatile tool in the full spectral range of molecular spectroscopy, covering rotational, vibrational and electronic motion. using frequency modulation, which is widely used in spectroscopy. Two more recent techniques, cavity ring-down spec- future observation of ionic species in the ISM. The dis- troscopy (CRDS) and noise- immune cavity- enhanced cussion centres on a few selected ions that we believe heterodyne velocity- modulation spectroscopy (NICE- exemplify not only the main spectroscopic techniques OHVMS), further exploit multi- pass geometries and and advances, but also the close connections between noise- rejection methods. CRDS uses a pair of high- laboratory spectroscopy and observational astronomy: reflectivity (R ≥ 99.99%) supermirrors to create an + + + + + 15 H3 , CH3 , CH , C3H and C60 . The selection of ions optical cavity with path lengths of many kilometres . also determines the techniques that we highlight herein. With each pass within the cavity, a fraction of the light As a consequence, many important experimental meth- leaks out of the back of one mirror and is detected as ods, such as cavity- enhanced Fourier- transform micro- a decay signal. A molecular absorber within the cavity wave (FTMW) spectroscopy and velocity- modulation increases this decay rate. Because the change in rate is infrared absorption spectroscopy, are outside our scope directly proportional to the molecular absorption, here and are only referenced or discussed briefly. an absorption spectrum can thus be determined. In the + case of H3 , CRDS techniques have been used to measure, + + H3 as the cornerstone of ion chemistry for example, the electron recombination rate of H3 + + The infrared (IR) spectrum of H3 was measured by by monitoring the decay rate of the H3 signal in the Oka6 in 1980 using a sophisticated direct- absorption cavity16,17. spectrometer outfitted with a multi- pass set- up and a In addition to cavity enhancement, NICE- OHVMS frequency- modulated detection system. The discharge (FIG. 2) incorporates two modes of modulation18. Its cell was liquid- nitrogen cooled in a manner similar to predecessor, NICE- OHMS (noise- immune cavity- earlier work on HCO+ (REF.11). In direct-absorption tech- enhanced optical heterodyne molecular spectroscopy), niques, the strength of the observed absorption signal uses a laser locked to the cavity resonant frequency19. A pair of frequency- modulated sidebands, 180° out of + + + phase with one another, is then added to the laser at a H5O2 CH3OHH CH5 + + spacing equal to an integer of the cavity free spectral N2H HCO XH+ H2O H2 range, allowing the sidebands and laser to be simul- + + taneously coupled into or out of the cavity. When no H3O CH3 H2 H2 absorber is present, the beat signals of the two side- + X + bands exactly cancel each other out, eliminating all H O H H CH2 2 2 2 noise from the system and creating a background- free OH+ CH+ scenario. An absorption signal is detected when one of the sidebands is absorbed by a molecular carrier. H O H + C 2 3 Compared with NICE-OHMS, NICE-OHVMS adds an H2 extra modulation component by varying the velocity of H2 molecular ions within the sample. This further increases O+ H + C+ 2 the sensitivity of the experi ment, as was pioneered by cr cr Saykally and co- workers, who applied velocity modula- O H2 C tion in standard absorption experiments to a large class of molecular ions20–22. NICE- OHVMS has been used to | Fig. 1 Astrochemical ion–molecule reactions. An astro- measure transitions of many molecular cations23–25, in chemical reaction tree depicting several pathways in which particular H + (REFS26–30), to sub- Doppler accuracy, ena- ion–molecule reactions build molecular complexity in the 3 109 + bling measurements made in the IR region to approach interstellar medium . The H3 cation has a fundamental + microwave- level accuracies on rotationally resolved role in initiating this network.