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.

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 ²P_⁠1/2⁠ and ²P_3⁄2 with energy 35,669 and 35,761 cm⁻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 ²Π_⁠1/2⁠ and ²Π_3⁄2. When the excited electron is in line with the argon the state is called ²Σ⁺ and corresponds only to ²P_3⁄2 and so is not split.

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⁺.

Properties

In the ground state the binding energy or MgAr₊ is 1281 cm⁻¹ and in the A²Π_⁠1/2⁠ state is 5554 cm⁻¹ (3.66 kcal/mol). The A²Π_⁠1/2⁠ state has a stronger bond because a p electron overlaps the argon atom less, and thus has less repulsion.

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.

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.