Rotational Spectroscopy of Glycolaldehyde Dimer

We used the CP-FTMW COMPACT spectrometer in Hamburg (13 , 40 ) to collect the rotational spectra of a mixture of commercially available Gly (2,5-dihydroxy-1,4-dioxane) sample and water. Glycolaldehyde dimer was purchased from Sigma-Aldrich as a crystalline powder (98% purity) and used without further purification. The sample was placed in an internal reservoir located close to the valve orifice and heated to 80^°C to sufficiently vaporize the sample. For obtaining the spectra of the clusters, pure neon gas was flowed over an external reservoir of deionized water at a total pressure of 3 bar. This water-enriched gas was then passed through the internal reservoir where it picked up the Gly vapor. The carrier gas mixture seeded with Gly and water was then supersonically expanded into vacuum (ca. 10⁻⁶ mbar) through a 1-mm pinhole with gas pulses of 800 μs. At this point, the collisional formation of clusters takes place. The supersonically cooled clusters were probed by a series of microwave chirped pulses. In our experiment, a train of eight excitation pulses is broadcast per gas pulse at a repetition rate of 8 Hz, yielding an effective repetition rate of 64 Hz. After each excitation, the molecular emission is collected as a free-induction-decay (FID) in the time-domain, for 40 μs, on a fast oscilloscope after amplification by a low-noise amplifier. The final dataset represents a time-domain average of 5 Million FIDs, which was subsequently Fourier transformed into the frequency domain after applying a Kaiser–Bessel window function to improve the baseline resolution. For the second measurement, a 6:1 (H₂¹⁶O:H₂¹⁸O) mixture prepared from a commercially available 97% H₂¹⁸O sample was used. The H₂¹⁸O-enriched rotational spectrum was then recorded under the same experimental conditions, and 5 Million FIDs were once again collected. In order to obtain the rotational spectra presented here (2 to 12 GHz), we expanded the bandwidth of our 2- to 8-GHz spectrometer to allow for the collection of the entire 2- to 12-GHz frequency range in a single acquisition. In short, two dual-polarization horn antennae were used for both the excitation and detection. A 2-μs chirped-pulse spanning 2- to 8-GHz was broadcast horizontally, while the vertical polarization was used to excite the molecules in the 8- to 12-GHz range with another 2-μs chirped-pulse. The molecular emission was then collected by the second dual-polarization horn and combined into a single channel using a power combiner, amplified and collected on a fast oscilloscope. This setup reduces the acquisition time by half without large signal loss, see SI Appendix for additional details. The structural analysis as well as the theoretical methodologies are also explained in detail in SI Appendix.