1.10atomic spectra

LIGHT INTERACTION WITH
ATOMS AND MOLECULES

Atomic spectra

COMPILED BY TANVEER AHMED 1

Atomic spectra
The 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 of
coloured lines
(the Balmer series) is observed
in Figure


Balmer series
these arise from the fall of electrons
down the quantum levels of the
hydrogen atom,
each level being adequately
characterized 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
energy
of very characteristic frequencies or
wavelengths
as the electrons return from the
higher excited states to the second
energy state (n = 2).


Lymen series ( UV ) – Paschen (IR ) –
Pfund Series
Observations of the emissions
outside 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 to
these spectral emissions are also
illustrated in Figure


atomic emission spectra of
more complex atoms such as
sodium and
mercury
To explain the atomic emission spectra of more complex atoms such as
sodium and
mercury
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 inclusion
of 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 the
overall resultant spin

indicated by the superscripts in the term
symbols

used to define
the ground and excited states of the atom.


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

in which the
wavelengths 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-paired
electrons in the outer 6s atomic orbital.


Atomic absorption spectroscopy results from the
reverse transitions in atoms,

in
which 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 spectroscopy
has 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 the
fourth in the dominant series of lines first observed by Fraunhofer
and was labelled as
line 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 spontaneous
emission takes place. COMPILED BY TANVEER AHMED 10

HELLIUM-NEON METASTABLE
In 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 transition
shown in Figure 1.36.


HELLIUM-NEON METASTABLE
Fast deactivation processes from the
terminal 3p level of the
laser 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 to
give a continuous output (possible with
this particular type of laser).

Other transitions are possible with the
neon atom,
but the design of the laser cavity
ensures that only the 632.8 nm radiation
appears in the output beam (through
one of
the 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 together
by 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 the
electrons 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 the
UV (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 are
shown in Figure 1.38,
along with a simple energy diagram of the possible electronic
transitions that produce absorption in the UV.


The HOMO to LUMO (p ® p*) transition leads to
the longest-wavelength absorption band for butadiene quoted inÊ Table
1.7.
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 carrots
and 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 to
excitation 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 carbonyl
compounds.

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 simultaneous
changes in electronic and vibrational
structure.

Such vibrational structure in UV
and 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 shows
an intense absorption band near 200 nm
with a weaker but characteristic band near 255 nm.


This ‘benzenoid’ absorption band
shows

highly characteristic vibrational
structure,

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

and is of greater intensity.

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

(Figure
1.41).


The bathochromic shift
and enhanced intensity has been
attributed to

the electrondonating
capabilities of the OH and O– groups.

Such electron-donating effects of so called
auxochromic groups

have long been used in the synthesis of
dye and pigment
molecules,

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


Azobenzene absorbs weakly just
below 400 nm,

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

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

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

The principle of incorporating donor–
acceptor groups

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

Interaction with radiation during
photon absorption causes

electron movement
and
creates excited states
with significantly higher dipoles
than those in the ground-state
molecule.

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 strong
light absorption.


1.10atomic spectra

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1.10atomic spectra