Magnesium argide
The magnesium argide ion, MgAr+ is an ion composed of one ionised magnesium atom, Mg+ and an argon atom. It is important in inductively coupled plasma mass spectrometry and in the study of the field around the magnesium ion.[1]
Spectrum
The spectrum of MgAr+ can be observed. It resembles that of Mg+, however some lines are blue shifted and others red shifted. In Mg+ the ground state is termed 2S. A first excited state has a 3s electron moved to the 3p orbital and the state is termed 2P. But because of spin-orbit coupling it is actually split into 2P1/2 and 2P3⁄2 with energy 35,669 and 35,761 cm−1.[1] In comparison the ionic molecule has a ground state called 2Σ+. The corresponding excited state is significantly split into two depending on whether the p orbital of the magnesium is pointing to the argon or is perpendicular. When the electron in the p orbital is perpendicular to the Mg-Ar axis, the argon sees a greater electrostatic force from the magnesium atom and is more tightly bound. This lowers the energy level of what is called the 2Π level. This too is split into 2Π1/2 and 2Π3⁄2. When the excited electron is in line with the argon the state is called 2Σ+ and corresponds only to 2P3⁄2 and so is not split.[1]
The MgAr+ spectrum shows bands, with the first one at 31,396 cm−1, which is redshifted 4300 cm−1 from Mg+. The band is blue degraded. The band consists of a series of doublets. The two lines in the doublet are separated by 75 cm−1, and from one pair to the next one is 270 cm−1. This band is due to A2Π ← X2Σ+.[1]
Properties
In the ground state the binding energy or MgAr+ is 1281 cm−1 and in the A2Π1/2 state is 5554 cm−1 (3.66 kcal/mol).[1] The A2Π1/2 state has a stronger bond because a p electron overlaps the argon atom less, and thus has less repulsion.[2]
The bond length is 2.854 Å for the ground state, and 2.406 Å for the excited state. The 2Π state is predicted to have a radiative lifetime of about 6 nanoseconds.[2]
Application
MgAr+ can interfere with determination of copper or zinc isotopes when using inductively coupled plasma mass spectrometry, particularly when using a desolvated plasma. When analysing mineral specimins, magnesium is a common element found in rock matrix. It can react with the argon ions present in the plasma.[3]
References
- ^ a b c d e Pilgrim, J. S.; Yeh, C. S.; Berry, K. R.; Duncan, M. A. (1994). "Photodissociation spectroscopy of Mg+–rare gas complexes". The Journal of Chemical Physics. 100 (11): 7945. doi:10.1063/1.466840.
- ^ a b Bauschlicher, Charles W.; Partridge, Harry (June 1995). "A study of the X 2Σ+ and A 2Π states of MgAr+ and MgKr+". Chemical Physics Letters. 239 (4–6): 241–245. doi:10.1016/0009-2614(95)00449-E.
- ^ Mason, Thomas F. D.; Weiss, Dominik J.; Horstwood, Matthew; Parrish, Randall R.; Russell, Sara S.; Mullane, Eta; Coles, Barry J. (2004). "High-precision Cu and Zn isotope analysis by plasma source mass spectrometry". Journal of Analytical Atomic Spectrometry. 19 (2): 209. doi:10.1039/b306958c.