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. The ionization potential of magnesium is lower than the first excitation state of argon, so the positive charge in MgAr⁺ will reside on the magnesium atom. Neutral MgAr molecules can exist in an excited state.

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 dissociation energy of the ground state ion is 1295 cm⁻¹ (15 kJ/mol).

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.

Neutral molecule

Unionized MgAr can also exist as a van der Waals molecule or temporarily in an excited state termed a Rydberg molecule. The neutral molecule can be formed by evaporating magnesium metal using a laser into argon gas, and then expanding it through a supersonic jet. MgAr is mainly held together with dispersion forces which vary as the inverse sixth power of the separation. The ground state MgAr has electron configuration Mg(3s3s ¹S₀)Ar ¹Σ⁺. The triplet states with one excited electron include Mg(3s3pπ ³P₀)Ar ³Π₀₊, Mg(3s4s ³S₁)Ar ³Σ⁺, Mg(3s3dδ ³D_J)Ar ³Δ, and Mg(3s4pπ ³P_J)Ar ³Π₀₊. A singlet single excited electron state is Mg(3s3pπ ¹P)Ar ¹Π.

A doubly excited state, where two electrons on the magnesium atom are boosted to 3p sub-orbitals, has a strong binding energy, even higher than in MgAr⁺.

state	electron state	Mg excitation energy cm−1	MgAr excitation energy cm−1	bond length Å	ωe	dissociation energy cm−1
ground	Mg(3s3s 1S0)Ar 1Σ+	0	0	4.56		small
singlet	Mg(3s3pπ 1P)Ar 1Π	34770	34770	3.31		175
triplet	Mg(3s3pπ 3P0)Ar 3Π0+	21850–21911	21760	3.66
	Mg(3s4s 3S1)Ar 3Σ+	41197	40317	2.84
	Mg(3s3dδ 3DJ)Ar 3Δ	47957	46885	2.90
	Mg(3s4pπ 3PJ)Ar 3Π0+	47847–47851	46663	2.84
double	Mg(3p3pπ 3PJ)Ar 3Π0+	57812–57873		2.41		2960

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 specimens, magnesium is a common element found in rock matrix. It can react with the argon ions present in the plasma.