It is a fair conclusion that no one who attended the astrochemistry lecture by Paolo Caselli at the American Astronomical Society 236th meeting in 2020, e.g.
Ever heard of the "tetrahertz gap" in the radio portion of the electromagnetic spectrum. Which suggests either that this gap is of no compelling importance, except to the few astrochemists -astrophysicists who work in this area, or that the academic discipline itself lacks sufficient resources and empirical 'objects of inquiry' to pursue at a very rigorous level. Never mind. We have Prof. Sussana Widicus Weaver, based at the Univ. of Wisconsin =Madison, to thank for setting the record straight. (At least from her frame of reference!)
This was in her recent Physics Today essay, Astrophysics in the Tetrahertz Gap.
In terms of relevance and importance, Prof. Weaver reminds us that "studies of the atomic and molecular universes rely heavily on various spectra recorded in the long-wavelength range of the electromagnetic spectrum. At those wavelengths, astronomers identify the fingerprints of organic molecules, determine the conditions inside stellar nurseries, and detect redshifted transitions of atoms in distant galaxies." The subjects include complexes of molecular clouds and star-forming regions in the Milky Way. See e.g. some introductory material here:
Much of the previous work, Weaver notes, "has been conducted in the microwave regime, which covers 300 MHz to 300 GHz in frequency, or 1 m to 1 mm in wavelength. But recent insights into the molecular universe have come from far-IR (FIR) observations in the range of 300 GHz to 20 THz in frequency, or 1 mm to 15 μm in wavelength."
The era of far infrared astronomy was brought about by the Herschel Space Observatory, in tandem with the Atacama Large Millimeter/Submillimeter Array (ALMA), and the Stratospheric Observatory for Infrared Astronomy (SOFIA). Together we're told these have "led to a recent heyday of molecular astronomy."
But still, there's this gap. Weaver elucidates (ibid.):
"In the past, advancements in this field have been held back by the “gap” in the terahertz regime arising from the relative lack of molecular spectroscopic information in this range as compared with other regions of the electromagnetic spectrum. The gap, illustrated in figure 1, had arisen because of historical limitations in the technology available for laboratory and observational studies.
Because of advances in telecommunications, security, and astronomical instrumentation design in the past 20 years, however, astronomers are now making rapid improvements. New tunable terahertz light sources, higher-power terahertz amplifiers, more sensitive detectors, and rapid and broadband data-acquisition capabilities have revolutionized the field. Researchers are starting to fill the terahertz gap."
This brings up the properties of molecular spectral lines. But what is a basic spectral line? Atoms as well as molecules absorb only at certain wavelengths that correspond to specific energy transitions. For example I show some specific transitions of spectra in the diagram below:
As shown in the diagram, emission occurs when an electron in the atom, say hydrogen, makes a transition from a higher to a lower energy level. This is always accompanied by the emission of a photon with a defined energy E = hf = h (c/ l). For both emission and absorption multiple aspects may need to be considered, including transition probabilities. See e.g. my earlier post:
For molecules, as Prof. Weaver observes, the line production can also arise from energy changes associated with rotation, vibration, or electronic energy. Molecular transitions are quantized, which simply means that the transitions occur only at particular amounts, or quanta, of energy. The transitions are sharp, meaning that they happen over a narrow range of frequencies.
What I hadn't been aware of previously is that a number of important features have been identified already within this "FIR" regime including the electron spin flip of the hydrogen atom at a wavelength of 21 cm; the ammonia structural inversion transitions at a frequency of 23 GHz; the pure rotational lines of carbon monoxide, which are the signposts of telescope receiver bands; and the rotational and rovibrational lines of organic molecules that have been seen in comets and the interstellar medium and may be the precursors to life throughout the universe.
That's a lot to pack in as well as appreciate.
Molecular spectral transitions appear as narrow spikes (hence the use of the word “line” to describe them). Each line in a spectrum corresponds to a specific transition of the molecule. Figure 2 below shows an example molecular spectrum as shown in Weaver's PT article:
The specific energies of rotational transitions are related to a molecule’s structure. As such, each rotational spectrum is molecule specific. If several lines for a given molecule are observed, the temperature and density in a sample of gas can be quantified by comparing the lines’ relative intensities As Weaver goes on to note:
"Molecules, therefore, are routinely used as probes of the physical properties of objects in space. For astrochemists, the molecular information gained through such measurements improves their understanding of how chemistry evolves as stars and planets form.
To study a particular molecule in space, one must first obtain a laboratory spectrum and assign the lines therein. Spectral lines are measured across a given frequency range and matched to a spectral prediction based on a quantum mechanical model of the molecule’s energy levels. Usually not all of a molecule’s spectral lines are measured."
This is in line with what I already discussed in the previous blog post link. Further we know transition probabilities, Frequency coverage and sample conditions all influence what lines can be observed in the lab and in space. But even with only partial spectral information from the laboratory, one can assign the spectrum and predict the rest of the lines. Also important to note:
"Simple molecules require only a few parameters for a full spectral assignment. Complex molecules, however, demand dozens of parameters to achieve a level of assignment that leads to reliable predictions. If researchers collect sufficient information in the lab, they can determine the parameters with a high degree of precision. They can then use the information to extrapolate the spectrum to other frequency ranges at any temperature.
The spectrum that is collected during telescope observations contains all the lines from every molecule in the source. When a molecule is identified, its spectral features are matched to those predicted from the laboratory spectral assignment. Matching requires knowledge of not only the spectrum for the molecule of interest but also the spectra for all the molecules in the source.
Astrochemists can then connect each line to a molecule and sort out any ambiguities by recognizing the patterns from known laboratory measurements. If enough spectral lines are observed that can be uniquely assigned to a given molecule and if their relative intensities match the expected physical parameters of the source, the molecule is said to be detected in space."
But again, the key question is the extent to which we can have confidence that such a process yields laboratory spectra that can be matched to actual astrophysics observations. And if not, do we really need laboratory spectra? Unfortunately, laboratory studies have been limited because of the challenges of filling the terahertz gap. As Weaver points out, historically, "researchers have lacked stable, high-powered, tunable light sources in the far infrared regime and also lacked sufficiently sensitive detectors to cover that part of the electromagnetic spectrum".
Infrared (IR) spectroscopy itself, covering 20–430 THz in frequency, or 15 μm to 700 nm in wavelength extent, is well established. After all, commercial spectrometers are a standard tool in nearly every chemistry laboratory. Microwave spectroscopy, covering 300 MHz to 300 GHz in frequency, or 1 m to 1 mm in wavelength, is not as widely used as an analytical tool. But it is just as powerful as IR spectroscopy and is a well-established field of research dating back to the development of radar during World War II.
Weaver's contention is that far infrared spectroscopy has not been widely pursued. Of all the high-resolution spectroscopy research laboratories in the world, only around 10 have spectral access from 300 GHz to 1 THz. The number of labs with high-resolution spectral access above 1 THz can be counted on one hand. But with the development of new FIR observatories has come new technological capabilities, and many of the historical limitations in the FIR range have been overcome. A deluge of astronomical spectra is now arriving from FIR telescopes. In Weaver's words:
"My research group and the handful of others who work at the millimeter-to-micrometer wavelengths are striving to develop laboratory techniques to keep up with the quantity of data."
Obtaining tetrahertz spectral data in the laboratory is straightforward, although indirect and cumbersome. Consider just your basic direct-absorption experiment. Light of suitable wavelength must be directed into a molecular sample and then a detector records the amount that passes through it. The input light is then referenced to the output light to determine how much the sample absorbed. By scanning the input light across multiple frequency steps - that are smaller than the width of a spectral line- the absorption spectrum can be pieced together.
But then there are many other complexities at hand, most of which arise from having to use 'proxy' spectral bands. Weaver admits, for example, the FIR methods have to draw from both the infrared and microwave regimes. Indeed, in the former realm, we are informed "light sources that include lasers and optics based on ground glass with mirrored coatings are used" while in the latter, "radiation is generated by crystal oscillators, like those used in a watch and car radio". For this reason we are told "the FIR is often called the quasi-optical regime because it draws methods and equipment from both approaches. Combining the two types of experiments into one system that works for all wavelengths in the FIR is complicated and technologically challenging."
Other complexities and challenges cited by Weaver (ibid.);
- Once a system is established for generating light and directing it into the sample, a detector sensitive enough to measure the signals is required. But anything above roughly 300 GHz requires a custom-built detector cooled with liquid helium. (Unless kept at extremely low temperatures, the detector elements that have the best response at FIR wavelengths are flooded with thermal background noise.)
- Helium boils at a temperature of 4.2 K, and keeping a detector at that temperature requires routine cryogen fills, which greatly complicates the logistics of experiments.
- The natural supply of helium on Earth is rapidly dwindling, and using helium-dependent devices is increasingly expensive (see Physics Today, April 2019, page 26, and “Helium shortage has ended, at least for now,” Physics Today online, 5 June 2020). (Detector manufacturers are now implementing closed-cycle cooling systems that use helium recirculation to better conserve the supply. But those detectors are not readily available to every spectroscopy laboratory.)
- Once a source and detector are set up, the next step is to deliver the molecular sample into the system....For molecules that are highly reactive or unstable, however, they need to devise ways to produce the molecule and keep it sufficiently isolated to avoid its reaction or decomposition while recording spectra. Most options require sources that continually flow or pulse the gas mixture through the system.
- Those sources introduce complications for gas handling because they require large pumps and vacuum fittings, windows, and other hardware to couple the spectrometer to the gas cell.
-Another challenge has to do with the nature of the molecules. In the low density of space, molecules react slowly because they take tens of thousands of years to collide.
Weaver acknowledges that using ions - say from lab plasmas - can speed up the process experimental considerably. Trouble is, ions are difficult to produce in the lab at sufficient quantities to study their spectra. While plasmas can be used to make the ions (and then the gas sample is expanded into a vacuum) there isn't a sufficient density to ensure success. Even with the most efficient ion sources, only 1 in every 10 000 molecules gets ionized. Once expanded into the vacuum, the gas sample is even further diluted.
It turns out that detecting some ions in the lab is more difficult than doing it in space which makes an outside observer wonder why bother? But Prof Weaver makes a decent case that - at least for the time being- and for the benefit of future astrochemistry in the tetrahertz range, we need to use proxy methods as an accompaniment to spectrometric space observations.
