Molecular Spectroscopy of Reactive Species

To date, more than 340 molecules have been detected in extraterrestrial environments. Because many of these regions exhibit extreme conditions—temperatures, pressures, and excitation mechanisms unlike those on Earth—laboratory studies are essential to reproduce and understand their chemistry. Such experiments require precisely controlled conditions, tailored to the needs of each molecular species.

A discharge experiment using the CASAC instrument. The colored glow is from the high-voltage discharge of a mixture of argon and hydrogen.

A discharge experiment using the CASAC instrument. The colored glow is from the high-voltage discharge of a mixture of argon and hydrogen.

At MPE, we use the CASAC (CAS Absorption Cell) spectrometer to study light molecular ions and radicals. The instrument's core is a long-pathlength glass flow cell with high-density polyethylene windows transparent to long-wavelength radiation. A 2-meter discharge region, enclosed in a copper solenoid, enables the formation of a magnetically confined, cooled plasma from selected gas mixtures. Spectroscopic measurements are performed across 70–1600 GHz using Schottky-based multiplier chains and Schottky diode or hot-electron bolometer detectors.I n addition to the discharge setup, CASAC also includes a static absorption cell (2-meter length), used for high-sensitivity spectroscopy of interstellar complex organic molecules (iCOMs) and other stable species, as well as a pyrolysis cell (3-meter length) connected to an oven reaching ~1500 °C. This latter configuration enables the generation of non-commercial, non-synthesizable, and semi-stable species through controlled thermal decomposition.

Together, these systems provide a versatile platform for high-resolution laboratory spectroscopy of molecules relevant to astrochemistry and the interstellar medium.

Fine and hyperfine structure of the N = 1 - 0 rotational transition of 15NH in its ground vibrational state. The bottom plot illustrates a schematic representation of the fine structure splitting. The top plot shows the recordings of the J = 0 - 1, J = 2 - 1, and J = 1 - 1 fine-structure components. The blue traces are the experimental spectra recorded with the time constant RC = 3 ms and accumulation times of ca. 350 s. In both plots, the red bars indicate the line positions and the relative intensities computed from the best-fit parameters. — Fine and hyperfine structure of the N = 1 - 0 rotational transition of ¹⁵NH in its ground vibrational state. The bottom plot illustrates a schematic representation of the fine structure splitting. The top plot shows the recordings of the J = 0 - 1, J = 2 - 1, and J = 1 - 1 fine-structure components. The blue traces are the experimental spectra recorded with the time constant RC = 3 ms and accumulation times of ca. 350 s. In both plots, the red bars indicate the line positions and the relative intensities computed from the best-fit parameters.

Fine and hyperfine structure of the N = 1 - 0 rotational transition of ¹⁵NH in its ground vibrational state. The bottom plot illustrates a schematic representation of the fine structure splitting. The top plot shows the recordings of the J = 0 - 1, J = 2 - 1, and J = 1 - 1 fine-structure components. The blue traces are the experimental spectra recorded with the time constant RC = 3 ms and accumulation times of ca. 350 s. In both plots, the red bars indicate the line positions and the relative intensities computed from the best-fit parameters.

Recording (blue trace) of a portion of the J = 2–1 fine-structure transition of (15)NH showing the four strongest components F₁,F= 5/2,2 The red dotted trace plots the modelled spectrum computed with the proFFit code using a modulated Voigt profile. The green trace plots the difference between the observed and the calculated spectrum.

A free-jet millimetre and sub-millimetre-wave spectrometer (CASJet) has also been developed to study unstable and transient species under conditions relevant to the interstellar medium. The system is based on a pulsed valve that can be operated in two interchangeable configurations. The first is a standard nozzle, consisting of a valve coupled with a discharge nozzle, used to generate reactive species directly in the gas expansion. The second is a heated nozzle assembly, comprising a heating reservoir, valve, and discharge nozzle, capable of reaching ~250 °C. This setup allows the efficient vaporization of low-vapor-pressure liquids and solids prior to expansion, extending the range of precursors that can be studied.In both configurations, the molecules expand through a supersonic free jet, creating an environment that reproduces the very low temperatures characteristic of interstellar space. The rapid and isolating nature of the expansion enables the observation of short-lived or reactive intermediates by effectively "freezing" them before they can undergo further reactions or relax to stable forms. This chamber can be coupled either with our continuous-wave (CW) frequency-modulation (FM) spectrometer—using the same frequency coverage (70–1600 GHz) and detection scheme as the CASAC instrument—or with our chirped-pulse Fourier transform spectrometer (CP-FTS), providing complementary capabilities for broadband, high-resolution studies of transient molecular species.

Laboratory spectrum of the HSCO⁺ J_Ka,Kc = 4_1,4 − 3_0,3 rotational transition around 318 GHz, acquired with the free–unit jet experiment. The integration time is ~ 15 mins with 30 μs time constant. The red dashed line represents the best fit to a speed-dependent Voigt profile. Each rotational transition has a double–peaked line shape, the result of the Doppler shift of the supersonic molecular beam relative to the two travelling waves that compose the radiation beam.