6 Electronic Spectroscopyv

Chemistry 3820 (Fall 2006)

© 2006 - Dr. Paul G. Hayes – University of Lethbridge

1

Chemistry 3820 – Electronic Spectroscopy of Transition Metal Complexes

PART 1: Qualitative Description of UV-Visible Spectra

The following figure shows the UV-Visible absorption spectra for aqueous solutions of first-row

transition metal complexes with the formula [M(H2O)6]x+ (x = 2 or 3).



2

Note that in the preceding absorption spectra, the y-axes show molar absorptivity (ε with units of

dm3 mol-1 cm-1) which is calculated from the measured absorbance (A) by the equation

log(Io/I) = A = εcl {Io = incident light intensity; I = emerging light intensity; c = concentration

(mol dm-3), l = pathlength through the test solution (cm)}.

From the UV-Visible absorption spectra on the previous page, it can be seen that certain complexes

have relatively simple spectra (e.g. d1, HS d4, HS d6 and d9 complexes), while others are far more

complex.

First, let us look at the simplest case; the aqueous Ti3+ ion (d1) which displays only one band in the

UV-Visible absorption spectrum. Considering the splitting of the d-orbitals in an octahedral

environment, this band can be assigned to the transition eg1 ß t2g

1 as shown below:

hv ∆o = hc/λmax

eg

t2g

For aqueous V3+ (d2), a similar representation would look like:

hv ∆o

eg

t2g

Since the t2g and eg* sets of orbitals are degenerate, we might expect to see only one absorption in the

spectrum. However, there are clearly two bands in the spectrum of [V(H 2O)6]3+ on page 1. Consider

the relative orientations of orbitals involved in the electronically excited states of V 3+:



3 Interelectronic repulsion makes excited state configurations such as those shown below

non-degenerate. As a consequence, more than one absorption band is observed and ∆o cannot be

obtained directly from the position of the maximum in the spectrum.

dz2 dx2-y2

dxy dxz dyz

dz2 dx2-y2

dxy dxz dyz

In order to understand the number and position of peaks in

UV-Visible absorption spectra of transition metal

complexes with more than one d-electron, we need to use

a more rigorous treatment that separates the different

electronic states into groups with the same energy. We

will look at this process in more detail in the next few

pages, but it is worth noting that the final outcome of this

process can be summarized in Tanabe -Sugano diagrams

(plots of the energy of different groups of degenerate

electronic states as a function of ligand field strength) for

each d-configuration with a particular geometry. The

Tanabe-Sugano diagram for an octahedral d2 complex is

shown on the right.

PART 2: Understanding Tanabe Sugano Diagrams

In the next few pages, we will discuss the fundamental principles behind this diagram.

To start with, we will consider the transition metal atom or ion of interest in the gas phase, rather

than as part of a complex. Note that it is only necessary to consider valence electrons in order to

understand the observed UV-Visible absorption spectra.

The following discussion is for a d2 ion in the gas phase :



4 A – What different combinations of ml and ms possible for two d-electrons, and how may the

electronic quantum numbers (ml and ms) be combined to give the atomic quantum numbers ML

and MS which describe atomic microstates?

Remember that an electron may be described using four quantum numbers (n = principal q.n.; l =

orbital angular momentum q.n.; ml = magnetic q.n.; ms = spin q.n. ).

For a gas-phase d1 ion, the lone d-electron can take values of ml = +2, +1, 0, -1, -2 and ms = +1/2,

-1/2, which gives a total of 10 possible arrangements of ml and ms. In the absence of an electric or

magnetic field, these 10 arrangements are degenerate (all of the same energy).

For a gas-phase d2 ion, one electron can in principle have ten different arrangements of ml and ms as

shown below , while the other can have nine different combinations (Pauli Exclusion Principle),

giving a total of 90 possible combinations . However, since the two d-electrons are indistinguishable

particles, the number of possible states is reduced to 45.

The two d-electrons are not independent of each other and their orbital angular momenta

(characterized by ml values) and spin angular momenta (characterized by ms values) interact in a

manner called Russell-Saunders coupling. These interactions produce states called microstates that

can be described by the new quantum numbers ML and MS:

Total Orbital Angular Momentum = ML = ? m l

(e.g. for a microstate with two electrons with ml = +2 and +1, ML = +3)

Total Spin Angular Momentum = MS = ? ms

(e.g. for a microstate with two electrons with ms = +1/2 and +1/2, MS = +1)

We need to determine how many possible combinations of ml and ms values there are for a d2

configuration. Once these combinations are known, we can determine the corresponding values of

ML and MS. For shorthand we will designate the ms value of each electron by a superscript + (for ms =

+1/2) or – (for ms = -1/2). For example, an electron having ml = +1 and ms = +1/2 will be written as

1+. One possible set of values for two d-electrons is shown below:

First electron:

Second electron:

ml = +2 and ms = +1/2

ml = 0 and ms = -1/2 Notation: 2+0-



5 We have seen that for a d1 configuration, there are 10 possible microstates, and for a d2 ion, there are

45 microstates. In general, for a subshell (of certain l) with n electrons, the equation below can be

used to calculate the total number of states:

Total number of microstates = [2(2l+1)]!

n![2(2l+1) – n]!

B - Tabulation of all the possible microstates.

When tabulating microstates, we must: (1) make sure that no two electrons have identical quantum

numbers (Pauli exclusion principal), and (2) count only unique mic rostates (e.g. 1+0- and 0-1+ are the

same so only one of these should be listed). The 45 microstates possible for a d2 configuration are

tabulated below according to their ML and MS values:

MS = +1 MS = 0 MS = -1

ML = +4 2+ 2-

ML = +3 2+ 1+ 2+ 1- 2- 1+

2- 1-

ML = +2 2+ 0+ 2+ 0- 2- 0+ 1+ 1-

2- 0-

ML = +1 2+ -1+ 1+ 0+

2+ -1- 2- -1+ 1+ 0- 1- 0+

2- -1- 1- 0-

ML = 0 2+ -2+ 1+ -1+

2+ -2- 2- -2+ 1+ -1- 1- -1+ 0+ 0-

2- -2- 1- -1-

ML = -1 -2+ 1+ -1+ 0+

-2+ 1- -2- 1+ -1+ 0- -1- 0+

-2- 1- -1- 0-

ML = -2 -2+ 0+ -2+ 0- -2- 0+ -1+ -1-

-2- 0-

ML = -3 -2+ -1+ -2+ -1- -2- -1+

-2- -1-

ML = -4 -2+ -2-

¦ = 1G

¦ = 3F

¦ = 1D

¦ = 3P

¦ = 1S



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C – Labeling a group of microstates using the quantum numbers L and S.

We have seen how electronic quantum numbers (ml and ms) may be combined into atomic quantum

numbers ML and MS which describe atomic microstates. While quantum numbers ML and MS describe

the microstates themselves, microstates can be organized into groups with a particular energy, and

these groups (called free ion terms) are labelled using symbols called term symbols which take the

form (2S+1)L, where L and S are the total angular momentum quantum number and total spin quantum

number respectively. These are defined below:

For a collection of microstates (we will learn how to group microstates into terms with the same

energy next), S and L are the largest positive values of ML and MS w ithin the collection. For example,

for the group of microstates; {2+ 2-}, {2+ 1-}, {2+ 0-}, {2+ -1-}, {2+ -2-}, {-2+ 1-}, {-2+ 0-}, {-2+ -1-},

{-2+ -2-}, L = 4 and S = 0. While the value of S is represented by a number, the value of L is

represented by a letter (see below). Therefore, for this collection of microstates, the term symbol

(defined by (2S+1)L , where 2S+1 is called the spin multiplicity) is 1G.

L = 0 à S term

L = 1 à P term

L = 2 à D term

L = 3 à F term

L = 4 à G term

L = 5 à H term

L = 6 à I term

The use of a letter (S, P, D , F, G , H, I …) in place of the

numerical value for L is similar to the use of letters to describe

orbitals (l = 0 à s-orbital; l = 1 à p-orbital; l = 2 à d-orbital;

l = 3 à f-orbital; l = 4 à g-orbital)

The number of microstates in a term = (2L+1)(2S+1).

D – Identification of the terms to which the microstates in a microstate table belong.

* Start with the microstate of largest ML in the table, {2+ 2-}. This microstate has ML = 4, so must be

the first microstate of a term with L = 4 (G). This term has MS = 0, and there is no term with ML = 4

and MS ? 0, so this microstate must belong to the 1G term. The 1G term must contain (2L+1)(2S+1) =

(2x4+1)(2x0+1) = 9 microstates. To account for the other 8 microstates, we can pick microstates

with MS =0 and ML = 3, 2, 1, 0, -1, -2, -3, -4. For example: {2+ 1-}, {2+ 0-}, {2+ -1-}, {2+ -2-}, {-2+ 1-},

{-2+ 0-}, {-2+ -1-} and {-2+ -2-}. It is worth noting that there is a rigorous method to identify exactly

which microstate belongs to which term, but this is outside the scope of Chem 3820, and a more

qualitative approach is sufficient in order to identify the terms for a particular d-configuration.



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* Now we consider the remaining microstates. Choose the one with the largest ML (and the largest

MS if there are several microstates with the same maximum ML) that has not yet been assigned: {2+

1+} in this case, which has ML = 3 and MS = 1. This must be the first microstate in the term 3F, which

will have (2L+1)(2S+1) = (2x3+1)(2x1+1) = 7x3 = 21 microstates. After assigning all of these 21

microstates, we are left with 15 microstates. Following the same method we can identify the other

terms of this configuration: 1D , 3P and 1S .

The same procedure can be used to determine the terms for different d-electron configurations, and

the resulting terms for each configuration are shown below:

d1 2D

d2 1S, 1D , 1G, 3P , 3F

d3 2D, 4P , 4F, 2P , 2D, 2F , 2G, 2H

d4 5D, 1S , 1D, 3P , 3F, 3P, 3D, 3F , 3G, 3H , 2S, 1D , 1F, 1G , 1I

d5 2D, 4P , 4F, 2P , 2D, 2F , 2G, 2H , 2S, 2D , 2F, 2G , 2I, 4D , 4G , 6S

d6 5D, 1S , 1D, 3P , 3F, 3P, 3D, 3F , 3G, 3H , 2S, 1D , 1F, 1G , 1I

d7 2D, 4P , 4F, 2P , 2D, 2F , 2G, 2H

d8 1S, 1D , 1G, 3P , 3F

d9 2D

E – Identification of the lowest energy term.

* Since all microstates belonging to the same term are degenerate, the electronic transitions that are

observed spectroscopically occur between terms.

It is important to identify the ground state term because this is the term from which electronic

transitions will occur. This can be done using two of Hund’s rules:

(1) The ground state term is the term with the highest value of S or (2S+1).

(2) If >1 term shares the highest value of S , the lowest energy term has the highest value of L

For the d2 configuration, the ground state term is 3F, and is located at the bottom left hand corner of

the Tanabe-Sugano diagram. With these rules we can identify the microstate with the highest ML and

MS in the ground state term for any electronic configuration (see below):



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Configuration m l ML MS Ground

2 1 0 -1 -2 Term

d1 ? 2 1/2 2D

d2 ? ? 3 1 3F

d3 ? ? ? 3 3/2 4F

d4 ? ? ? ? 2 2 5D

d5 ? ? ? ? ? 0 5/2 6S

d6 ?? ? ? ? ? 2 2 5D

d7 ?? ?? ? ? ? 3 3/2 4F

d8 ?? ?? ?? ? ? 3 1 3F

d9 ?? ?? ?? ?? ? 2 1/2 2D

Note that these simple rules can only be used to identify the ground state term. In order to find the

energy of the higher energy terms, Schrödinger’s equation must be solved, which involves the

evaluation of many “2-electron” integrals. Racah worked out all of the integrals for a d2

configuration, and found that they can be classified into three sets called A, B and C. Even better, he

found that the energy of all terms can be written as simple combinations of these integrals. The

values of A, B and C can be computed, but are more frequently matched to spectroscopic results, and

are thus used as “parameters”. Of the three parameters, A is not meaningful when dealing with

differences in energy between terms rather than actual energies. In addition, it is found that in many

cases C ˜ 4B. This permits fairly simple interpretation of spectra using only parameter B, which is

called the principal Racah parameter.

F – Splitting of the free ion terms in coordination complexes.

The previous discussion applies only to metal ions in the gas phase. To describe the situation in a

coordination complex, we must consider the effects of a reduction in symmetry.

Using either Crystal Field Theory or Ligand Field Theory, confinement of a metal ion in an

octahedral environment results in a splitting of the energy into the t2g and eg* sets. For the

monoelectronic d1 system, there is only one possible electronic transition eg1 ß t2g

1 (see below). The 2D term corresponds to a d2 atom or ion in the gas phase, while the two possible electronic

configurations in an octahedral environment, t2g and eg* result in two terms 2Eg

* and 2T2g. Notice the

distinction of electronic configuration and spectroscopic terms using a different letter case. The

difference in energy between 2Eg* and 2T 2g is also called ∆o, and as ligand field strength increases, ∆o

increases.



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2D

2Eg

2T2g

∆o0.6 ∆o

0.4 ∆o

Ion in the gas phase OhIncreasing Ligand

Field Strength

The figure shown above is called a correlation diagram, and indicates the relationship between the

free ion terms for a gas phase ion and an ion in an octahedra l environment (Oh). Correlation diagrams

depict the energy of the O h terms in relation to the parent free ion term.

For other dn configurations (n > 1), the reduction in symmetry also results in splitting of the terms.

This splitting is formally obtained using group theory, but for our purposes, it is enough to examine

the end result shown in the table below:

Free Ion Term Splitting under Oh symmetry

S à A1g

P à T1g

D à Eg + T2g

F à A2g + T1g + T2g

G à A1g + Eg + T1g + T2g

H à Eg + 2(T1g) + T2g

I à A1g + A2g + Eg + T1g + 2(T2g)



10 The correlation diagram below shows the splitting of the important triplet terms (3F and 3P) for the d2

configuration:

3F

3T2g

3T1g

Ion in the gas phase OhIncreasing Ligand

Field Strength

3P 3T1g

3A2g

∆o

∆o

∆o

approx∆o

t2g1eg

1

t2g2

eg2

Field strengths for actual complexes

Infinitely high field strength

The free d2 ion is shown on the left hand side of the diagram, while the right hand side of the

diagram shows the energy levels in an infinitely strong ligand field (not one that could be found in a

real complex). At this “strong field limit”, the effects of the ligands is so strong that it completely

overrides Russell-Saunders coupling, and we are left with the familiar electronic configurations of

t2g2, t2g

1eg1 and eg

2. At this strong field limit, ∆o can easily be marked on the diagram. In actual

compounds, the situation is intermediate between these two extremes. However, by looking at these

extremes, it can be seen that in the central region of the diagram, ∆o is represented by 3T2g ß 3T1g

and 3A2g ß 3T2g transitions. Note: the former of these transitions cannot be used reliably to find Do

because 3T1g does not vary linearly with ∆o (the line is curved – see later).

G - Tanabe-Sugano diagrams.

A Tanabe-Sugano diagram is a special type of correlation diagram that is particularly useful for the

interpretation of electronic spectra for transition metal complexes. In a Tanabe-Sugano diagram, the

ground state term is set as the base line and all other energies are referenced to this ground state. The

energy and ∆o are presented as multiples of the principal Racah parameter B, and energies are

calculated using a value of C (~ 4) which is reported in the diagram.



11 It is fortunate that we do not need to carry out detailed analysis for all cases. The Tanabe -Sugano

diagrams have been worked out for most dn configurations with common geometries (e.g. octahedral

and tetrahedral). Tanabe-Sugano diagrams for octahedral complexes are shown below.



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Tanabe-Sugano Diagrams for Octahedral dn Complexes

2



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Tetrahedral Complexes

The same octahedral Tanabe-Sugano diagrams can be used for tetrahedral complexes. However, for a

dn tetrahedral complex, the d(10-n) Tanabe-Sugano diagram must be used (i.e. for d8 Td, use the d2 Oh

T-S diagram). For more details, see Miessler and Tarr, Inorganic Chemistry, 3rd Edition, pg 406.



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Some important general observations from the Tanabe -Sugano diagrams are:

(1) Some lines are curved. This arises because whenever two states of the same symmetry (same

term symbol) are close in energy, they can be combined to yield one state of lower energy and

another of higher energy. In a Tanabe-Sugano diagram, this makes it appear that some lines bend

away from each other to avoid crossing. As a result, this phenomenon is referred to as the “non-

crossing rule”. Note that transitions involving these curved lines must be avoided when determining

∆o from Tanabe-Sugano diagrams.

(2) For some configurations, there is an abrupt change of slope and the ground state term also

changes. This happens when there is a transition from a High-Spin to a Low -Spin state.

H – Selection rules and molar absorptivity.

There are two selection rules which govern the relative intensities of electronic absorption bands:

Spin Selection Rule – Transitions between different spin multiplicities are forbidden. For example,

transitions between 4A2 and 4T1 states are spin-allowed, but between 4A2 and 2A2 are spin-forbidden.

Laporte (Parity) Selection Rule – Transitions between states of the same parity (symmetry with

respect to inversion) are forbidden. For example, transitions between d-orbitals are forbidden (these

are g à g transitions) but transitions between d and p orbitals are allowed (g à u).

These rules would seem to rule out many of the electronic transitions for metal complexes (see

Tanabe-Sugano diagrams). However, many transition metal complexes are famous for their bright

colours; a consequence of various mechanisms by which the above rules can be relaxed. The most

important relaxation mechanisms are:



15 * Vibronic Coupling (molecular vibrations ) temporarily changes molecular symmetry, and can result

in temporary loss of the centre of symmetry in an octahedral complex. This provides a way to relax

the Laporte selection rule. As a consequence, d-d transitions often have extinction coefficients (ε ) of

10 to 100 dm3 mol-1 cm-1 which is responsible for the bright colours of many transition metal

complexes.

* Tetrahedral C omplexes absorb more strongly than octahedral complexes. Metal-ligand σ-bonding

in transition metal complexes of Td symmetry, and the lack of a centre of symmetry makes

transitions between d-orbitals more allowed (relaxa tion of the Laporte selection rule ). As a result,

extinction coefficients of around ε = 500 dm3 mol-1 cm-1 are common. As a qualitative example,

[Ni(H 2O)6]2+ is pale green while [Ni(PPh3)4]2+ is deep blue.

* Spin-Orbit Coupling can provide a mechanism for relaxation of the spin selection rule . Such

absorption bands for first-row transition metal complexes are generally weak, with typical extinction

coefficients of around ε = 1 dm3 mol-1 cm-1. For example, octahedral Mn2+ complexes (d5) cannot

undergo any spin-allowed transitions from the 6A1 ground state, so are extremely pale coloured. By

contrast, Spin-Orbit coupling can be more important for 2nd and 3rd row transition metal complexes.

The molar absorptivity (ε , extinction coefficient) is a good indicator of the type of electronic

transition:

Type of transition ε (dm3 mol-1 cm-1) Example

Spin-forbidden < 1 Mn2+ (aq)

d-d Laporte-forbidden 10-100 Most 3d Oh complexes

d-d noncentrosymmetric system ~ 500 Td complexes

π*-π transition, symmetry allowed ~ 10,000 Organic chromophores

Note: while it is important to have an idea of what the different terms on a Tanabe-Sugano diagram

represent, it is most important to be able to use Tanabe-Sugano diagrams and molar absorptivity to

interpret and understand the electronic spectra of transition metal complexes (including finding ∆o or

∆t for a complex).

See annotations to TS diagrams made in lecture for finding ∆o spectroscopically.



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CHARGE TRANSFER ABSORPTIONS [Cr(NH3)6]3+: there is an intense absorption in the UV region of the spectrum. This type of intense absorption (ε ~ 104 dm3 cm-1 mol-1) can be seen more clearly in [CrCl(NH3)5]2+:

Note: The lowest energy dßd transition in [CrCl(NH3)5]2+ is significantly lower in energy than that

of [Cr(NH3)6]3+ due to a lower ∆o (Cl- is lower in the spectrochemical series than NH3). Note: For [CrCl(NH3)5]2+, there are shoulders on the dßd transitions due to reduced symme try

(C4v instead of Oh). Both peaks are examples of LMCT (Ligand to Metal Charge Transfer), which involves the transfer of electrons from molecular orbitals that are primarily ligand based to orbitals that are primarily metal based.

MLCT = Transfer of electrons from the eg and t2g ligand sets to low lying π* orbitals on the ligands. LMCT = Transfer of electrons from filled ligand based orbitals to the eg and t2g sets.

[CrCl(NH3)5]2+ [Cr(NH3)6]3+



17 Other examples of complexes with prominent LMCT absorptions are shown below (relative energies of the LMCT absorptions are also shown using < and >). These are all d0 complexes, so cannot undergo dßd transitions:

3d 4d 5d

MnO4- < TcO4

- < ReO4-

M7+

^ ^ ^ CrO 4

2- < MoO42- < WO4

2- M6+

^ ^ ^

VO43- < NbO4

3- < TaO43- M5+

In complexes containing ligands which have low-lying empty π*-orbitals, MLCT (Metal to Ligand Charge Transfer) can be observed. This is often found for diimine type ligands (bipyridine, phenanthrolene) as well as a variety of other soft ligands such as S2C2R2

x-, S2CNR2-, CO, CN- and

SCN-. See molecular orbital diagram above. Examples of molecules which show strong MLCT bands are [M(bipy)3]2+ (M = Fe or Ru), [W(CO)4(phen)] and [Fe(CO)3(bipy)]. [Cr(CO)6] shows both LMCT and MLCT absorptions. Both MLCT and LMCT results in a dramatic redistribution of electron density between the metal and the ligands (hence the name “charge transfer”). This redistribution of electron density results in differences in the solvation for the ground state and excited state molecules. As a result, the energy of charge transfer bands is highly dependent on solvent (SOLVATOCHROMISM).

----------------------------------------------------------------

higher oxidation state metal is easier to reduce

3d easier to reduce



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LUMINESCENCE A complex is luminescent if it emits radiation after it has been electronically excited by the absorption of radiation. Luminescence competes with non-radiative decay by thermal dissipation of energy (heat loss to the surroundings). Luminescence can be divided into two categories: (1) Fluorescence à Radiative decay from an excited state with the same multiplicity as the ground

state. This process is fast (nanoseconds). (2) Phosphorescence à Radiative decay from an excited state with different multiplicity from the

ground state. This process is spin-forbidden, so is usually slow (microseconds to seconds). The mechanism of phosphorescence involves intersystem crossing (the non-radiative conversion of the initial excited state into another excited state of different multiplicity). This second excited state acts as an energy reservoir because radiative decay to the ground state is spin forbidden.

Ruby is an important example of a phosphorescent compound. Ruby absorbs green and violet light so appears red. Red phosphorescence helps to enhance the red colour and lustre of ruby (see below). Ruby = Al2O3 with partial replacement of Al3+ by Cr3+ - the Cr3+ ions are “dilute” and are surrounded by an octahedral arrangement of oxide ligands.

Molecular examples of phosphorescent complexes include [Cr(bipy)3]3+ and [Ru(bipy)3]3+. The first laser (1960) was based on ruby.

----------------------------------------------------------------



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LASERS

• A synthetic ruby rod is irradiated using a powerful xenon lamp (see below).

• The long lifetime of the metastable 2Eg level (~ 4 ms) means that with suitably intense irradiation, more than half of the chromium ions can be excited to the 2Eg level at one time. This is called a population inversion.

• Decay of these excited state chromium ions results in a photon at 694 nm.

• The ruby rod is housed between 2 mirrors (one fully reflecting and one only partially reflecting).

• Therefore, if the 694 nm photon is emitted in the correct direction, it bounces back and forward between the two mirrors. When it encounters another excited state Cr3+ ion, it can cause decay by ‘stimulated emission’. This stimulated emission results in production of a photon that relative to the photon that stimulated the emission, is at exactly the same wavelength, is in phase and is travelling in the same direction (i.e. the light produced is monochromatic, coherent and collimated).

• The two photons now move back and forwards between the two mirrors in the resonant cavity, which results in more stimulated emission. As a result, the intensity of the beam of photons increases with each pass through the cavity until it reaches a ste ady state (see figure below).

• Remember that one of the mirrors at the end of the resonant cavity is only partially reflecting. A portion of the coherent, collimated and monochromatic light is therefore able to escape through this output mirror, resulting in a laser beam (see below).

The figure on the left is an illustration of the gain or amplification that occurs with increased path length in the resonant cavity due to the mirrors at each end. Figure 1(a) shows the beginning of stimulated emission, which is amplified in Figure 1(b) through Figure 1(g) as the light is reflected from the mirrors positioned at the cavity ends. A portion of light passes through the partially reflected mirror on the right-hand side of the cavity (Figures 1(b, d, and f)) during each pass. Finally, at the equilibrium state (Figure 1(h)), the cavity is saturated with stimulated emission.

• Other common types of laser include: other solid state lasers (e.g. Yttrium Aluminium Garnet

(YAG) doped with Nd3+, Ho3+ or Er3+; YAG = Y3Al5O12), dye lasers (e.g. a dilute solution of rhodamine or fluorescein ), gas lasers (e.g. He/Ne, Ar, Ar/Kr, CO2) and semiconductor lasers (e.g. InGa/AlP). Solid state and dye lasers are generally excited using optical pumping. Gas lasers and semiconductor lasers generally use electrical pumping.

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