Leak-out spectroscopy: a new, universal action spectroscopy method
Philipp C. Schmid1, Thomas Salomon1, Sven Thorwirth1, Carsten Czapczyk1, Oskar Asvany1, and Stephan Schlemmer1
1 Physikalisches Institut, Universität zu Köln, Germany,
schmid@ph1.uni-koeln.de
More than 200 molecular species are known in the interstellar medium to this day, indicating a complex underlying chemical network. In particular ions play an important role in the chemical evolution in the cold environment of the interstellar medium. Molecular spectra of the ions are thus required to identify the molecules. But as spectra of many ions of astrophysical interest are not available, astrochemistry is in need not only for precise quantum chemical calculations, but even more for detailed and accurate experimental measurements.
For such experiments action spectroscopy in cold ion traps is an important tool for recording the spectra of molecular ions. Thereby the spectrum is identified as a change of the ion mass by fragmentation upon photon absorption, pre-dissociation of tagged ions or via laser induced chemical reactions. Although all these techniques have advanced significantly, action spectroscopy cannot be applied to many ions as here the aforementioned methods fail to work. Here a new method of action spectroscopy in cold ion traps, leak-out spectroscopy (LOS), is presented. In comparison to the methods mentioned above, LOS does not rely on a change in the mass, but on the transfer of internal energy of an excited ion to its translation energy upon the collision with a neutral partner. By adjusting the ion trap potentials accordingly, these fast ions can leave the ion trap and thus the spectrum can be recorded. This overcomes the limitations of other action spectroscopy methods, thus making the new me thod suitable for virtually any ions.
In a recent measurement the advantage of LOS for recording of infrared spectra of ions was shown. In detail, the ro-vibrational resolved spectrum of the ν1 C-H stretching vibration of linear l-C3H+ was recorded. In addition to the vibrational fundamental mode, the associated ν1+ν5 - ν5 hot band, originating from the energetically lowest bending mode was observed. Both spectra are in good agreement with estimates based on previous quantum-chemical calculations and low-resolution measurements.
Apart from pure spectroscopy, LOS offers further experimental possibilities: If the trapped ion ensemble consists of different isomers, conformers or nuclear spin isomers, only a single component can be addressed by exciting a suitable transition. With the help of LOS, the now excited ions will leave the ion trap while the other species remains trapped. As a result, the isomer composition of the ion cloud can now be analyzed and LOS can be used to prepare an isomer clean sample. Pure samples of ortho-H3+ and para-H3+ were thus prepared in a recent experiment in Cologne by exciting a suitable ro-vibrational transition. Thereby the ortho/para ratio of the stored H3+ ions was determined, allowing now to study state-specific processes like the ortho to para transition in reactions like H3+ + H2.
References:
B. McGuire et al., Nature Reviews Physics 2 (2020) 402.
S. Brünken et al., J. Phys. Chem. A 123 (2019) 8053.
Schlemmer, S., patent pending (2021102213585000DE, 10 2021 127 556.3)