Optical spectroscopy of reactive molecules

The research activities of the group focus on the production and spectroscopic study of astrophysically relevant molecules in laboratory experiments. We use experimental techniques to generate molecules such as short-lived hydrocarbon radicals in the laboratory and to direct them into a molecular beam. This allows us to spectroscopically study these molecules under conditions similar to those in space. Such laboratory measurements form the basis for spectroscopically identifying these molecules in astronomical observations and thereby gaining insights into chemical processes in the interstellar medium.

We currently have an opening for a research employee to collaborate on research and work on an independent project as part of a doctoral program. We also offer the opportunity to participate in research as part of a bachelor’s or master’s thesis. We welcome inquiries!

The composition of cosmic matter is surprisingly diverse. Although hydrogen and helium make up by far the largest proportion, the combined abundance of carbon, nitrogen, and oxygen—at about 0.1%—is sufficient to enable complex chemistry. In addition to atoms and small molecules, the interstellar medium also contains larger organic molecules, such as polycyclic aromatic hydrocarbons (PAHs), which are produced during reactions in gas and dust clouds. Under extreme temperature and pressure conditions, many of these molecules exist as radicals or ions, and the formation of exotic species—such as chains of pure carbon—is favored. Astrochemistry thus differs fundamentally from the chemistry we know in Earth’s atmosphere. The reaction pathways leading to complex molecules are determined by the local conditions in the interstellar medium, and research into these mechanisms promises deep insights into astrochemistry.

Diffuse interstellar bands (DIBs) are one of the most fascinating mysteries in astrophysics, one that has puzzled generations of astronomers and astrophysicists for 100 years. DIBs are absorption bands in the near-ultraviolet, visible, and near-infrared spectral regions and are observed in the spectra of stars whose light passes through interstellar clouds along the line of sight to Earth. Since the discovery of the first DIBs in the early 20th century by M. L. Heger, the number of known DIBs had risen to over 100 by the end of the last century, and current catalogs list over 500 known DIBs in more than 100 line-of-sight directions. The so-called carriers of the DIBs—the sources of the absorption lines—remain unknown to date (see Fig. 1). The exception is the Buckminsterfullerene ^C₆₀+, to which a few of the DIBs have recently been attributed through elaborate laboratory measurements.

Electronic transitions of PAHs absorb in the optical spectral range. PAH radicals, in particular, possess low-lying electronic states, so their vibronic transitions lie in the visible spectral range—where DIBs also absorb. Another aspect that underscores the astrophysical relevance of PAHs is their presumed role as building blocks of fullerenes. The discovery of this class of large molecules in the interstellar medium fills a gap between small hydrocarbons and large fullerenes, which are thought to emerge as stable products from a chain of reactions involving smaller molecules. In particular, hydrogen-poor PAH radicals may be involved in the formation of these large molecules (see Fig. 2). Laboratory studies of these molecules thus form the basis for research into carbon chemistry in the interstellar medium.

Sources:
[1] Campbell et al., Nature 523, 322 (2015)
[2] Ehrenfreund and Foing, Nature 523, 296 (2015)
[3] Berné and Tielens, PNAS 109, 401 (2012)

To generate hydrocarbon radicals—which are important astrophysical molecules—under laboratory conditions, techniques are used to produce them in situ through reactions involving precursor molecules, since these radicals are short-lived and cannot be stored under normal conditions. We use electrical discharges in which stable precursor molecules are excited, ionized, and destroyed, allowing them to react further with one another and form new molecules, molecular ions, and radicals. After a brief interaction time in a reaction channel, these molecules are expanded into the vacuum together with an inert carrier gas, where they first cool down and then propagate through the vacuum chamber with virtually no further interactions. A skimmer is used to separate a portion of the expansion as a molecular beam, which is then passed through additional vacuum chambers for spectroscopic analysis of the generated molecules (see Fig. 3).

A dye laser emits tunable radiation in the near-ultraviolet, visible, and near-infrared spectral ranges. The laser beam is focused and superimposed on the molecular beam. If the sample molecules absorb sufficient energy through interaction with the light, electrons can be ejected and the molecules become ionized. The ions are accelerated by electric fields, and their mass-to-charge ratio is determined based on their time-of-flight. The time-of-flight mass spectrometer thus provides information on both the number of ions produced and their mass.

The REMPI scheme can be extended to include an infrared spectroscopic component. To do this, the absorption of an infrared (IR) photon via a vibrational transition is required in addition to the absorption of an ultraviolet (UV) photon in order to reach the electronically excited state from which ionization occurs. By detuning the IR frequency (while simultaneously maintaining the resonance condition), this technique allows the IR spectrum of the sample molecules to be measured in addition to the UV spectrum. In the mid-IR region, we achieve significantly higher spectral resolution using narrow-band optical parametric oscillators (OPOs) than in the UV region with the dye laser. With the OPOs, we achieve a total coverage of the wavelength range from 2.7 to 4.6 μm, which makes the C–H stretching vibrations accessible in addition to other vibrational modes. Thanks to the high spectral resolution of the OPOs, the vibrational spectra of the hydrocarbon radicals are fully rotationally resolved, which greatly facilitates the structural determination of these molecules.