Separation of Electronic States and Isomers for Atomic and Molecular Ionsin Cooled Gases by Ion Mobility Measurements
Tanuma, H.,**presenter
tanuma-hajime@tmu.ac.jp, Tokyo Metropolitan University, Japan
The interaction potentials between the ions and the neutral gases determine the drift velocities
of ions in gases. Because the interaction depends on the electronic state and geometricalmolecular structure, the drift velocity can separate ion species beyond the mass analysis. This
technique is applied for chemical analysis and is named ion mobility spectrometry as describedin technical books 1 ,2,3. On the other hand, the fundamental physics of ion mobility wasdeveloped in the 20th century and described in detail by pioneers of theoretical and experimental
investigations4.
We have developed an apparatus and measured the drift velocity vd of various atomic andmolecular ions in cooled helium gas at 77 and 4.3 K with liquid nitrogen and helium,
respectively5,6,7. As we used an electron-impact ion source, metastable ions were produced inmany cases, e.g., C+(2s2p2 4P) from CH4 gas, and the separation from the ground state in thearrival time spectra was observed. Not only atomic ions but also molecular ions, e.g., NH+ andNH3+, have two peaks corresponding to different electronic states in the spectra.
Recently, we performed experiments with hydrogen gas instead of helium gas. With theinjection of H2+ into the drift tube filled with cooled H2 gas, we have observed H3+ producedby the following ion-molecule reaction:
H2+ + H2 → H3+ + H.
+The arrival time spectra of H3 surprisingly split two components in weak electric fields. As
the mobility of molecular ions depends on the molecular structure, we consider that two peaks
correspond to the triangle and linear structures. The ground state of H3+ has a triangle
structure confirmed with spectroscopic observation8, and theoretical calculation proposes thefirst excited triplet state of linear structure. However, the energy level of this triplet state is
too high to be produced in the low-energy collision of H2+ with H2. Then, the meta-stablelinear structure in the electrically ground state will be the candidate9. If this interpretation iscorrect, this result might be the first observation of linear H3+ in the laboratory.
With Li+ injection at 77 K, we observed the formation of cluster ions, Li+H2, and measured thearrival time spectra. Here, the splitting into two peaks also indicates the unexpected existenceof isomers, namely triangle and linear types.
Eiceman, G.A.; Karpas, Z.; Hill, Jr., H.H. Ion Mobility Spectrometry, 3rd ed., CRC Press, FL, 2014.
Shvartsburg, A.A. Differential Ion Mobility Spectrometry, CRC Press, Boca Raton, FL, 2008.
3Trimpin, S.; Wilkins, C.L. Ion Mobility Spectrometry-Mass Spectrometry, Taylar & Francis, FL, 2010.
4Mason, E.A; McDaniel, E.W. Transport Properties of Ions in Gases, John Wiley & Sons, New York, 1988.
5Tanuma, H., et al. Very low temperature drift tube mass spectrometer, Rev. Sci. Instr. 2000, 71, 2019.
6
Tanuma, H.; Fujimatsu, H.; Kobayashi, N. Ion mobility measurements and thermal transpiration effects inhelium gas at 4.3 K, J. Chem. Phys. 2000, 113, 1738.
7Matoba, S; Tanuma, H.; Ohtsuki, K. Mobility of the metastable C+ ion in cooled He gas at 77 and 4.3 K, J.
Phys. B 2008, 41, 145205.
8Oka, T. Observation of the infrared spectrum of H3+, Phys. Rev. Lett. 1980, 45, 531.
9Aguado, A. et al. Three states global fittings with improved long range: singlet and triplet stated of H3+, Phys.
Chem. Chem. Phys. 2021, 23, 7735.
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