6 Spectra of HE and alkaline metals

Contents:

1.     Spectra of Helium (He) Atom

2.     Spectra of two valence electron systems

The students will be able to lean spectra of Helium and alkaline metals and in general spectra of two electron system including their energy levels and the fine structure

1. Spectra of Helium (He) Atom:

The Helium (He) atom has electronic configuration of 1s². These two electrons in the configuration are responsible for the valency of two and hence in deciding the general characteristics of the optical spectrum. The ground state Helium has two 1s electrons. Both the electrons have l = 0 and as n, l and ml are same; therefore, according to the Pauli Exclusion Principle their spins are antiparallel.

Thus for 1s², S and L are zero and hence J = 0 and the ground state is ¹S0. Now assuming that one of the two electrons is excited to the 2 p orbits yielding a configuration 1s2p due to n =1 for the s electron and n = 2 for the p electron. Pauli Exclusion Principle does not apply, as the two electrons are non-equivalent. For the s and p electrons, l = 0 and 1, respectively and thus L = 1.The state is represented by the symbol P. The spins are either parallel or antiparallel giving S =1 and 0, respectively. As the multiplicity is 2S+1, one gets triplet and singlet and for S = 0 the). The J values for triplet are 2, 1, 0 and for singlet, it is 1. The possible terms are therefore, ³P₂₁₀ and ¹P₁.

In case, one of the two electrons is promoted to various excited configuration, then the resulting states are singlet and triplet (The ground state configuration leads only to singlet) and according to Hund’s rule, the triplet states lie lower than the corresponding singlet states.

The energy level diagram of Helium is shown in the figure. It depicts the energies of the ground state and first few excited states of helium atom obtained from an analysis of the optical line spectra. The energy of the term increases in the order of nS, nP, nD, nF,…….., for the same n. The lowest triplet state is 2 ³S, and the lowest singlet state is 1¹S. The state 1³S is not allowed because of Pauli’s Exclusion Principle. The energy difference between the 1¹S and 2³S is quite large ~19.72 eV reflecting the tight binding closed shell electrons.

Helium atoms in singlet states (antiparallel spins) constitute parahelium and those in triplet states (parallel spins) constitute orthohelium. The lowest state of orthohelium, 2³S does not combine with the ground state 1¹S. Those terms, that cannot go to a lower state with the emission of radiation and correspondingly cannot be reached from a lower state by absorption, are called metastable states. The 2¹S state is also metastable, as the selection rule ∆l = ± 1 does not allow any transition to 11S. The metastability of the 2³S state is however, stronger than that of 2¹S, as the transition 2³S →1¹S would contradict the prohibition, of an ortho-para transition as well as ∆l = ± 1. However, 2³S → 1¹S transitions can be caused by collision with other atoms or with the wall of a containing vessel that distorts the electronic structure of the atom, and allow a change of spin to occur.

The transition from one energy state to another is governed by the selection rule ∆L = ± 1, ∆S = 0, ∆J =0, ±1. Allowed transitions between different energy levels gives rise to a large number of lines that can be put into two distinct system of lines: a system of closely spaced triplets and a system of singlets. In the emission spectrum of helium atom, lines of singlet and triplet system may be grouped into four series each:

(i) Principle series: Two principle series for singlet are observed one is from higher P state to normal 1¹S0 state and member of this series lie in far ultraviolet region. This series also exists in absorption. Besides this there exists another principle series whose spectral lines arise from transitions from higher ¹P states to the 2¹S state. This principle series lie from infrared to near ultraviolet region. The members of the triplet principle series are due to transitions from higher ³P state to 2³S state. The triplet principal series lie from infrared to ultraviolet region.

(ii) Sharp series: The sharp series arise due to transitions from S states to the lowest P state. The lines of sharp series of singlet and triplet system lie in visible and ultraviolet region.

(iii) Diffuse series: The diffuse series arise from transition between higher D states to the lowest P state. The lines of this series for both singlet and triplet system lie in visible and ultraviolet region.

(iv) Fundamental series: This series arise from transitions between higher F states to the lowest D state. The lines of this series for singlet and triplet system lie in infrared region.

Under high resolution, the lines of parahelium system are single and those of orhtohelium show closely spaced triplet structure, that is very difficult to resolve. The triple fine structure is due to spin-orbit interaction. The figure shows ³P→³S transition.

³S state has one component ³S1 while ³P has three components ³P₂, ³P₁ and ³P₀. Three lines are observed according to selection rules. ∆L = ± 1,∆J = 0, ±1 and ∆S = 0, between these states.

The adjacent figure shows the fine structure of triplet diffuse series.

For triplet D we have S = 1, L = 2 and hence J = 3, 2, 1. According to allowed selection rules, six transitions are possible. The complete spectrum should consist of six lines. Normally, the very close spacing is not resolved and only three lines are observed. For this reasons the spectrum is referred to as compound triplet.

For He, the component separations ³P0-³P1 = 0.996 cm-1 and ³P1-³P2 = 0.078 cm1. The terms of 2³P of He are inverted with ³P0 component lying highest in energy and ³P2 the lowest. Further there is breakdown of Lande interval rule. This is due spin-spin interaction.

2.    Spectra of two valence electron systems

The electronic configuration of neutral atoms of group II elements (alkaline elements) are given as

The last two equivalent electrons in the group II elements are responsible for the general characteristics of the optical spectra. These elements in their ground state have two equivalent s electrons in the n^th (n = 2, 3, 4, 5 and 6) orbit and therefore the ground state is n¹S₀. On exciting one of these two electrons to higher s, p, d or f orbit one obtains a system of two non-equivalent electrons. These systems gives singlet and triplet S, P, D and F state as described in the case of helium.

The ground state configuration leads only to singlet states whereas each excited configuration arising from the promotion if an electron gives rise to singlet and triplet states with later lying lower than the corresponding singlet state.The figure depicts the energies of the ground states and first few excited states of Sr atom obtained from the analysis of the optical line spectra. This figure also shows some of the transitions allowed by selection rules. (The fine structure splitting is not shown).

Allowed transitions between different energy levels give rise to a large number of lines that can arise from transitions

(a)  between closely spaced triplets and

(b)   between a systems of singlet.

In the spectra of alkaline earth metals, the lines of tripled system and of singlet system can be further classified into four chief series. These are:

(i) Principal series: This series arise from transition from higher P states to the lowest S state.

(ii) Sharp series: This series arise from transition from higher S states to the lowest P state.

(iii)Diffuse series: This series arise from transition from excited D states to the lowest P state.

(iv) Fundamental series: This series arise because of transitions from excited F states to lowest D state.

The regions in which the lines of these four series of singlet and triplet system lie for the Sr element

Each series converge towards shorter wavelength to fit the wavenumber of the spectral line to hydrogen like formula

are successful only if non integral quantum number are used. For each spectral series the term T’’ = RZ2/ ′′ 2 is constant and is the wavenumber of series limit. ne = n – µ, is the effective quantum number where n is an integer and µ is a fraction called quantum defect that is zero for non-penetrating orbits. (As in case of alkali atoms, the penetration of various possible electron orbits into the atom core is measured by the deviation of the term values from those of hydrogen). Two methods are used to measure the penetration

(i)   by modifying the nuclear charger in the term value expression for hydrogen,

(ii)   modifying the principal quantum number.

These leads to term values for (i) as

Where 6 is a screening constant, Z0 is the effective nuclear charge when the electron is well outside the core. From the spectrum, ignoring fine structure, it is observed that:

(i) Triplet sharp and diffuse series have one common limit and singlet sharp and diffuse another.

(ii)The frequency difference between common limit of triplet sharp and diffuse series and triplet principal series is equal to the frequency of the first member of the principal series (Rydberg-Schuster law).

(iii) The frequency difference between the common limit of the triplet sharp and diffuse series and the triplet fundamental series is equal to the frequency of the first member of the triplet diffuse series (Runge law).

(iv)  The same is true even if fine structure is taken into account.

(v)   (i), (ii), and (iii) holds for singlet series also.

Fine structure

The examination of alkaline spectra reveals that lines of various triplets’ series have a structure known as fine structure. It is observed that:

(i) All members of the principal series are composed of three lines with decreasing separation with increasing frequency, and approach a single limit.

(ii)   All member of sharp series are composed of three lines with the same separation, and approach a triple limit.

(iii) All members of diffuse and fundamental series contain six lines, three strong and three satellites and approach a triple limit.

The fine structure in triplet series is due to splitting of various energy levels by spin-orbit interaction as described previously in case of alkali atoms. In case of alkaline metals, in triplet levels we have S = 1 and L can take values 0, 1, 2 and 3 for S, P, D and F states, respectively. For S state, S = 1 L = 0, therefore, J = 1. For 3S1 there is no splitting of the level. For P state, L = S =1, therefore J = 2, 1, 0 and P state is split into three components with different J values. Similarly, D and F states, represented by ³D₃₂₁ and ³F₄₃₂, respectively splits into three components each.

In the triplet principal series transitions take place from ³P₂₁₀ to lowest ³S₁ level as shown in the figure and according to selection rules

∆S = 0, ∆L = ±1, ∆J = 0,±1

three transitions take place.

The spacing between the transitions depends on the splitting of P level which decreases with increasing n as per the equation.

For large value of n, the splitting of the P level is negligible and as a result, one observes a single transition in series limit.

Fig. shows the fine structure transition for sharp series that take place from different excited triplet S states to lowest triplet P state.

Figure shows the Fine structure transitions for sharp series

As a result of allowed transitions, three lines are observed. The spacing between the lines of triplet is governed by the splitting of the lowest triplet P state. As the sharp series triplet separation is produced by the splitting of the lower ³P₂₁₀ level on which all the transitions terminates, therefore it remains constant.

Figure shows the fine structure transitions for the triplet diffuse series that take place from different excited ³D levels to lowest ³P level. Each of ³P and ³D level splits into three levels. As a result of allowed selection rules, six transitions are possible. The spacing between the transitions depends on the splitting of ³P and ³D levels. For large value of n, the spacing between ³D₃₂₁ levels diminishes and therefore ³D₃₂₁ levels are not resolved. As a result of this only three transitions are observed whose spacing depends on the splitting of ³P₂₁₀ levels. Same situation exists for the fundamental series. However, now for large value of n, the spacing between the transitions depends on the splitting of ³D₃₂₁ levels, which is constant.

As for sharp and diffuse series the transitions terminate at lowest ³P level therefore, for large value of n, the spacing between transitions is same for both sharp and diffuse series.

Qualitatively, the relative intensity can be obtained by rules given for alkali atom spectra. Let us apply those rules for the members of diffuse series. For transitions

(a) ∆J = + 1 with J = 3,

(b)         ∆J = 0, J = 2

(c)         ∆J = + 1, J = 2

(d)          ∆J = -1, J = 1

(e)          ∆J = 0, J = 1 and

(f)         ∆J = +1, J = 1.

For all these transitions ∆L has the same value. Thus transition ‘a’ is the strongest and ‘d’ is the weakest. The intensities of other transitions are intermediate between the intensities of ‘a’ and ‘d’.

The transitions in order of decreasing intensities are a >c >f >b >e >d.

Assignment 1.

Calculate the Rydberg denomionators for the first term value of the Principle series of sodium. The wavenumber of the transition is 16973.7 cm-1. The R for Na is 109734 cm^-1

Write down the spectral term

T= R/n_e²

So n_e²=109734/16973.7=254

Assignment 2.

The first ionization potential of sodium is two fifth that of  hydrogen. Calculate the effective nuclear charge of the sodium atom as far as the 3s electron is concerned.

For hydrogen

Z = 1, n = 1.

Therefore using En=-RZ²/n² E1 = -R cm^-1 = – 13.6 eV. The ionization energy of hydrogen is 13.6eV.

For Na, n = 3 and for hydrogen Z_eff = 1. Putting (I_Na/I_H) = 2/5 in the above equation

Z² eff =1.897

Assignment 3.

The principal and sharp series for the Li atom converge to continua at 43487 and 28583 cm^-1 respectively. Calculate the quantum defect for the common term in each of the series (RLi = 109729 cm^-1).

The common term for sharp and principal series is 2²P term. The wavenumber position of this term is 28535 cm^-1 and we know T= R/ne²

Therefore ne²=R/T= 109729/28583 =3.83896

So, n_e= 1.9593

Μ= n- n_e= 2-1.9593=.0407

Assignment 4

The principal and sharp series for the sodium atom converge to continua at 41450 and 24477 cm^-1, respectively. Calculate the ionization potential of Na.

The wavenumber corresponding to series limit of principal series give the position of normal ground state 32S of Na. Hence ionization energy corresponds to 41450cm^-1 as 1 eV= 8065 cm^-1. Thus 41450 cm^-1 is equal (41450/8065) eV = 5.139eV.

Summary:

• A two-electron atom or helium-like ion is a quantum mechanical system consisting of one nucleus with a charge of Ze and two electrons. This is the first case of many-electron systems where the Pauli Exclusion Principle plays a central role. It is considered as a three body problem.
• The important two-electron atoms are:

Hydrogen  anion  (Z=1)   Helium   atom (Z=1) Lithium ion (Z=1) Beryllium ion(Z=1) Boron ion (Z=1)

The optical spectrum of the two electron atom has two systems of lines: A para system of single lines, and an ortho system of triplets (closely spaced group of three lines).

• The energy levels in the atom for the single lines are indicated by ¹S0 ¹P1 ¹D2 ¹F3 etc., and for the triplets, some energy levels are splitted: ³S1 ³P2 ³P1 ³P0 ³D3 ³D2 ³D1 ³F4 ³F3 ³F2

Alkaline earths and Mercury also have spectra with similar features, due to the two outer valence electron.