brief description of Atomic Flourescence transitions.

1. Types of
atomic fluorescence transitions
1

2. Types of atomic fluorescence transitions:
Fluorescence emission occur through different pathways
based upon the wavelength of emitted radiation when
compared to absorbed radiation
The six types of atomic fluorescence transitions are as
given below.
i. Resonance fluorescence
ii. Stokes direct line fluorescence
iii. Stepwise line fluorescence
iv. Two step excitation or double resonance fluorescence
v. Thermal fluorescence
vi. Sensitized fluorescence
2

3. Most useful type because it generates the most intense
fluorescence
Used for most analytical determinations
Occurs when the atom re-emits a spectral line having the
same wavelength as that used for excitation
The wavelength of emitted radiation is equal to the
wavelength of absorbed radiation
Mathematically;
λE = λA
λE represents the wavelength of emitted radiation
λA represents the wavelength of absorbed radiation
1. Resonance fluorescence:3

4. (a) Energy transitions involved in resonance fluorescence spectral line
(b) Grotrian diagram of magnesium atom showing the origin of resonance fluorescence line
4

5. When magnesium atoms are exposed to an ultraviolet
source, a radiation of 285.2 nm is absorbed leading to the
excitation of 3s electrons to 3p level, then this excited
electrons emits a resonance fluorescence radiation at the
same wavelength which can be used for analysis.
Drawback:
Scattering of incident radiation by the particles in the flame
poses a serious drawback in this method. This is so because
the scattered radiation has the same wavelength as that of
fluorescence emission; therefore, false high values are
observed.
Example:
5

6. 2. Stokes direct line fluorescence:
Stokes direct line fluorescence is observed when an electron
in an atom excited to higher energy state by absorption of
radiation and then, goes to lower energy level (intermediate
level) which is above ground state by emission of radiation.
From this intermediate level, electron returns to ground
state by a radiation-less process.
So, the wavelength of absorption is shorter than wavelength
of emission or the wavelength of the emitted radiation is
longer than the wavelength of the absorbed radiation.
It is also called as Stokes fluorescence.
6

7. Mathematically;
λE > λA
λE represents the wavelength of emitted radiation
λA represents the wavelength of absorbed radiation
 Direct line fluorescence will always occur at a higher
wavelength than that of the resonance line which
excites it.
2. Stokes direct line fluorescence:
7

8. (a) Energy transitions involved in direct line fluorescence spectral line and
(b) Grotrian diagram of thallium atom showing the origin of direct line fluorescence
8

9. When thallium atom is excited by a radiation having a
wavelength of 377.6 nm, then thallium atom returns to the
ground state in two steps producing a fluorescence emission
line at 535.0 nm then followed by radiation-less deactivation.
Advantage:
The advantage of using direct line fluorescence is that if
appropriate filters are used, it eliminates interference due to
scattered radiation (during excitation) which is encountered
in resonance fluorescence.
Example:9

10. 3. Stepwise line fluorescence:
In this type of fluorescence an atom initially excited to
a higher energy state by absorption of radiation,
undergoes a partial deactivation by a collision or a
radiation-less process to a lower excited state
(intermediate level) which is above ground state. Then
from this (intermediate level) transition occurs atom
emit radiation (fluorescence) to return to the ground
state. It is also a type of Stokes fluorescence.
10

11. Mathematically;
λE > λA
λE represents the wavelength of emitted radiation
λA represents the wavelength of absorbed radiation
Example:
In sodium atom a 3s level electron is excited to 4p level by a 330.2
nm radiation. This electron then relaxes down to an intermediate
level 3p in a radiation-less process. The further relaxation of
electron from intermediate level 3p to ground state 3s level is
radiative. It is the fluorescence emission at 589.0 nm.
3. Stepwise line fluorescence:
11

12. (a) Energy transitions involved in stepwise line fluorescence
(b) Grotrian diagram of sodium atom showing the origin of stepwise fluorescence line
12

13. 4. Two-step excitation or double resonance
fluorescence:
In case of the two-step excitation, the population of atoms is excited
step by step from ground state into an excited state via an intermediate
state.
The double resonance fluorescence involves a two-step excitation
process using two dye lasers. The first laser excites the analyte from
ground state to an excited state from where it further gets excited to
another higher excited state with the help of a second laser. The de-
excitation of this higher excited state to a lower energy state is
accompanied by fluorescence emission. This is called as double
resonance fluorescence.
13

14. 14

15. 5. Thermal fluorescence:
Thermally assisted fluorescence is the converse of stepwise
line fluorescence.
It occurs when the electron from ground state is excited to a
higher energy level (intermediate level) by absorption of
radiation (radiative excitation).
Then electron from intermediate level gets further excited
to higher energy level (above the intermediate level) with
the help of thermal energy in a radiation less process.
The fluorescence emission occurs from both the excited
energy levels.
15

16. In this case part of the emitted radiation has shorter
wavelength (higher energy) as compared to the exciting
radiation.
Mathematically;
λE < λA
λE represents the wavelength of emitted radiation
λA represents the wavelength of absorbed radiation
It is also called as “anti-Stokes fluorescence”.
This type of fluorescence is weak, tough and has found few
analytical applications.
5. Thermal fluorescence:16

17. 17

18. 6. Sensitized fluorescence:
In sensitized fluorescence, the energy of the excited atom
(called the donor) is transferred to another atom (called the
acceptor) which gets excited and then relaxes back to
ground state accompanied by fluorescence emission.
The process of sensitized fluorescence emission can be
represented as follows:
S* + A S + A* (excitation energy transfer)
A* A + h ν (Fluorescence)
However, the thermally assisted fluorescence and
sensitized fluorescence generally are not employed for
analytical purposes.
18