1.10atomic spectra

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LIGHT INTERACTION WITH ATOMS AND MOLECULES

Atomic spectra


Atomic spectraThe simplest atomic

spectrum is that obtained by

examining the light emission from a

low-pressure hydrogen arc by means of a visual

spectrometer.

A characteristic series ofcoloured lines (the Balmer series) is

observed in Figure


Balmer seriesthese arise from the fall of electrons down the quantum levels of the hydrogen atom, each level being adequatelycharacterized for the present discussion by the relevant principal quantum number (n).

The electrons are initially promoted to the excited levels (n > 1) by the electrical discharge,and the Balmer series of lines is produced by spontaneous emission of light energyof very characteristic frequencies or wavelengths as the electrons return from thehigher excited states to the second energy state (n = 2).


Lymen series ( UV ) – Paschen (IR ) – Pfund SeriesObservations of the emissionsoutside the visible range show other line series in the UV (the Lyman series) and in the near-IR (Paschen series)

and far-IR (Pfund series).

The energy transitions giving rise tothese spectral emissions are also illustrated in Figure


atomic emission spectra of more complex atoms such as sodium andmercuryTo explain the atomic emission spectra of more complex atoms such as sodium andmercury it is necessary to label the states using symbols representative of three of the four quantum numbers which characterise the electrons in an atom.


Thus the inclusionof the secondary quantum number l defines s, p, d and f electrons (l = 0, 1, 2 and 3 respectively)

while the inclusion of the spin quantum number s (= ±1/2) gives theoverall resultant spin

indicated by the superscripts in the term symbols

used to definethe ground and excited states of the atom.


These concepts are incorporated in theatomic energy level diagrams for sodium and mercury (Figure 1.35),

in which thewavelengths of the characteristic lines in the emission spectra of these atoms

The ground state of the sodium atom (electronic configuration 2, 8, 1) arises from the electron in the outer 3s atomic orbital,

whilst that of mercury (electronic configuration 2, 8, 18, 32, 18, 2) arises from the spin-pairedelectrons in the outer 6s atomic orbital.


Atomic absorption spectroscopy results from the reverse transitions in atoms,

inwhich the absorption of a quantum of radiation

absorbed

results in the promotion of the electron in the atom

from the ground-state energy level

to an upper energy level.


Sodium ATOMIC SPECTRUM Thus atomic sodium shows strong absorption at 589.3 nm due to the reverse 3s to 3p transition (and at 330 nm due to 3s to 4p transition).

Atomic absorption spectroscopyhas become one of the major analytical tools for determining trace amounts of metals in solution.

Atomic absorption is also responsible for the dark lines (the Fraunhofer lines) seen in the spectrum of the sun.

The sodium atomic absorption line was thefourth in the dominant series of lines first observed by Fraunhofer and was labelled asline D; to this day the orange-yellow 589.3 nm line of sodium (actually a pair of lines at 589.0 and 589.6 nm due to electron spin differences) is known as the sodium D line.


Electronic transitions in the He–Ne laser

The principles involved in laser action were described in section 1.5.5,

the important characteristic being the formation of a relatively long-lived excited state (the metastable state),

which allows stimulated emission to be generated before spontaneousemission takes place.


HELLIUM-NEON METASTABLEIn the He–Ne laser electrical excitation ‘pumps’ one of

the 1s outer electrons in the helium atom to the higher-energy 1s 2s excited state,

which then transfers the energy(by collision) to the approximately equi-energy metastable He (2p 5s) state

From which the characteristic red 632.8 nm laser radiation is produced by the transitionshown in Figure 1.36.


HELLIUM-NEON METASTABLEFast deactivation processes from the terminal 3p level of thelaser transition ensures that

sufficient helium atoms are restored to the ground state ready to undergo excitation by energy transfer

and hence maintain the laser beam togive a continuous output (possible with this particular type of laser).

Other transitions are possible with the neon atom, but the design of the laser cavityensures that only the 632.8 nm radiation appears in the output beam (through one ofthe end mirrors, which is partially transmitting to the extent of about 1%).


UV absorption in simple molecules

In the hydrogen molecule, the simplest of all molecules,

the two atoms are held togetherby a single bond

formed by the two atomic electrons combining (with their spins paired)

to form a ground-state s molecular orbital.

The promotion of one of theelectrons into the nearest excited state can be induced

by absorption of radiation Very low down in the vacuum UV,

at about 108 nm

(Figure 1.37).


The absorption occurs so low in the UV because of the significant energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

To obtain absorption in a more accessible region of theUV (i.e. above 200 nm) it is necessary to use

organic molecules with double bonds or containing heteroatoms such as oxygen, nitrogen or sulphur.

For example, ethene with its single double bond absorbs at about 180 nm, but 1,3-butadiene and 1,3,5- hexatriene absorb at longer wavelengths with increasing strength of absorption as indicated by the values of their molar absorptivities, emax (Table

1.7).


Molecular orbitals for 1,3-butadiene involving the p-electron double bonds areshown in Figure 1.38, along with a simple energy diagram of the possible electronictransitions that produce absorption in the UV.


The HOMO to LUMO (p ® p*) transition leads to the longest-wavelength absorption band for butadiene quoted inÊ Table1.7. Extension of the conjugated (alternate single- and double-bonded) system to four double bonds leads to absorption just above 400 nm and a yellow colour;

β-carotene, with eleven conjugated double bonds, is the major orange component in carrotsand other vegetables, and one of the most important of the carotenoid plant pigments.

Lycopene, which gives tomatoes their red colour, is another example of a natural carotenoid colouring matter.


The UV absorption characteristics of methanal (formaldehyde) illustrates the important influence of the oxygen heteroatom.

In the methanal molecule bonding and nonbonding electrons are both involved in the ground state (Figure 1.39), with the lowest-energy transition arising from a weak absorption band at about 270 nm due toexcitation of one of the nonbonding electrons into an antibonding p* orbital.


The schematic UV absorption spectrum shows two bands of significantly different absorption intensities (note the logarithmic absorptivity scale),

which is typical of simple carbonylcompounds.

In the vapour phase or in solution in a nonpolar solvent, the 270 nm band of methanal shows

sub-band fine structure which is due to the simultaneouschanges in electronic and vibrational

structure.

Such vibrational structure in UVand visible absorption bands can be

represented schematically in energy level diagrams(Figure 1.40).


Absorption spectra of aromatic compounds and simple colorants

The structure of benzene is often represented as three pairs of conjugated p-bonds in the hexagonal ring structure, with three of the six p-orbital states available being occupied in the ground state by spin-paired electrons.

The UV spectrum of benzene showsan intense absorption band near 200 nm with a weaker but characteristic band near 255 nm.


This ‘benzenoid’ absorption band shows

highly characteristic vibrationalstructure,

but this is absent in the phenol spectrum, in which the band appears atlonger wavelengths (bathochromic shift)

and is of greater intensity.

This effect is enhancedif the phenol is made alkaline so that the OH group ionises to O –

(Figure1.41).


The bathochromic shift and enhanced intensity has been attributed to

the electrondonatingcapabilities of the OH and O– groups.

Such electron-donating effects of so calledauxochromic groups

have long been used in the synthesis of dye and pigmentmolecules,

which by definition have to absorb strongly in the visible region.


Azobenzene absorbs weakly just below 400 nm,

but substitution with an electrondonatingOH or NH2 group in the para position gives a simple disperse dye.

Incorporationof both electron-donating and electron-accepting groups (NO2 groups, for instance)

at opposite ends of the azobenzene structure gives an intense orange dispersedye.

The principle of incorporating donor–acceptor groups

in the synthesis of dyes andpigments is widely applied and is well illustrated in the anthraquinone series


Interaction with radiation during photon absorption causes

electron movement andcreates excited states with significantly higher dipoles than those in the ground-statemolecule.

It is presumed that the donor–acceptor groups in dye and pigment molecules

help to stabilise the formation of the polar excited states and hence result in stronglight absorption.

1.10atomic spectra

Education

s state

s excited state

s atomic orbital

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groundstate energy level