Reports on Applications for Lead Salt Lasers

Part I:

High resolution spectroscopy of unstable molecules.
University of Basel / Laser Center Vrije Universiteit Amsterdam / Observatory Leiden

Unstable molecules (ions, cluster ions, and radicals) play an important role as reactive precursors in plasmas, for example in combustion processes both in the Earth’s atmosphere and interstellar space (where the variety of exotic molecules is amazing). Both small ions such as HCO⁺ and long, linear carbon chains such as HC-CCCCCCCCCCN have been found in interstellar space. This finding was the result of detailed laboratory testing of transient molecules in the gas phase. During these tests interstellar conditions were simulated in ultrasound expansion plasmas. Detection of these molecules is done using sensitive modulation techniques.

The interaction of unstable molecules can also be tested in this manner. Is N₂-H⁺-N₂ stable? What happens when the proton is replaced by an Ar ion (N₂-Ar⁺-N₂) or when you combine a nitrogen molecule with an Ar ion ([Ar-N₂]+)? Experiments with an infrared diode laser spectrometer have shown that all of these molecules indeed exist and can be characterized spectroscopically with the highest precision.In the case of the charge transfer complex [Ar-N₂]⁺, it was possible, for example, to measure 100 rovibrational transitions at approximately 2272 cm^-1 (i.e. 4.4 µm) and assign them with an absolute precision of 0.001 cm^-1 (approx. 30 MHz) [1]. With the high number of proven absorption lines, the linear bond structure could be confirmed. In addition, both a strong bond energy of 1.2 eV and a shift of charge induced by complexation were proven. In combination with the newest calculations it was possible to understand the chemical bond of this complex perfectly for the first time [2].

In the next few years, infrared satellites will deliver a myriad of new spectroscopic data from interstellar space. At the moment, it is important to make sure that the greatest number of spectra of astrophysically relevant radicals becomes known.

References:
[1]  H. Linnartz, D.Verdes und J.P. Maier, Science 297, 1166 (2002).
[2]  H. Verbraak et al., J. Chem. Phys. 123, 144305 (2005).