1. Introduction to Spectrochemical Methods
Chapter 24
Introduction to Spectrochemical Methods

2. Spectroscopy deals with the interactions of radiation and matter.
Spectroscopic analytical methods are based on measuring the amount of radiation produced or absorbed by molecular or atomic species of interest.
Spectroscopic methods are classified according to the region of the electromagnetic spectrum used or produced such as:
G-ray, X-ray, ultraviolet (UV), visible, infrared (IR), microwave, and radio-frequency (RF).
Spectrochemical methods are the most widely used tools for the elucidation of molecular structure as well as the quantitative and qualitative determination of both inorganic and organic compounds.

3. 24A Properties of Electromagnetic Radiation
Electromagnetic radiation is a form of energy that is transmitted through space at enormous velocities.
Electromagnetic radiation can be described as a wave with properties of wavelength, frequency, velocity, and amplitude.
The wave model fails to account for phenomena associated with the absorption and emission of radiant energy.
For these processes, electromagnetic radiation can be treated as discrete packets of energy or particles called photons or quanta.

5. The amplitude of an electromagnetic wave is a vector quantity that provides a measure of the electric or magnetic field strength at a maximum in the wave.
The period of an electromagnetic wave is the time in seconds for successive maxima or minima to pass a point in space.
The frequency of an electromagnetic wave is the number of oscillations that occur in 1s.
The unit of frequency is the hertz (Hz), which corresponds to one cycle per second, that is, 1 Hz = 1 s-1.
The frequency of a beam of electromagnetic radiation does not change as it passes through different media.

6. The product of the frequency in waves per unit time and the wavelength in distance per wave is the velocity v of the wave in distance per unit time (cm s-1 or m s-1).
v = 
In a vacuum, light travels at its maximum velocity.
This velocity, c, is  108 m s-1.
The velocity of light in air is only about 0.03 % less than its velocity in vacuum.
c =  = 3.00  108 m s-1 = 3.00  1010 cm s-1

7. The wavenumber, , is another way to describe electromagnetic radiation.
It is defined as the number of waves per centimeter and is equal to 1/.
By definition, has the units of cm-1.
The wavenumber in cm-1 (Kayser) is most often used to describe radiation in the infrared region.

9. Radiant Power and Intensity
The radiant power, P, in watts (W) is the energy of a beam that reaches a given area per unit time.
The intensity is the radiant power-per-unit solid angle.
The Particle Nature of Light: Photons
The energy of a single photon is related to its wavelength, frequency, and wavenumber
E = h = hc/ = hc
where h is Planck’s constant (6.63  Js).

11. 24B Interaction of radiation and matter
The electromagnetic spectrum covers an enormous range of energies (frequencies)
and thus wavelengths.
Useful frequencies vary from >1019 Hz (g-ray) to 103 Hz (radio waves).

13. Spectrochemical methods that use not only visible but also ultraviolet and infrared
radiation are often called optical methods.
Spectroscopists use the interactions of radiation with matter to obtain information
about a sample.
Several chemical elements were discovered by spectroscopy.
The analyte is predominately in its lowest-energy or ground state.
The sample is then stimulated in some way by applying energy in the form of heat or electrical energy (emission spectroscopy) , light, particles, or a chemical reaction (chemiluminescence spectroscopy).
The stimulus causes some of the analyte species to undergo a transition to a higher-energy or excited state.
Information about the analyte is obtained by measuring the electromagnetic radiation emitted as it returns to the ground state.

17. In absorption spectroscopy, the amount of light absorbed as a function of wavelength is measured.
Absorption measurements can give both qualitative and quantitative information about the sample.
In photoluminescence spectroscopy, the emission of photons is measured following absorption.
The most important forms of photoluminescence for analytical purposes are fluorescence and phosphorescence spectroscopy.

18. 24 C Absorption of radiation
Every molecular species is capable of absorbing its own characteristic frequencies of
electromagnetic radiation.
This process transfers energy to the molecule and results in a decrease in the intensity of the incident electromagnetic radiation.
Absorption of the radiation thus attenuates the beam in accordance with the absorption law.
The absorption law, also known as the Beer-Lambert law describes quantitatively
how the amount of attenuation depends on the concentration of the absorbing molecules and the path length over which absorption occurs.

20. The transmittance T of the solution is the fraction of incident radiation transmitted by the solution.
Transmittance is often expressed as a percentage and called the percent transmittance.
T = P/P0
The absorbance, A, of a solution is related to the transmittance in a logarithmic manner.

22. Transmittance and absorbance usually cannot be measured because the solution
to be studied must be held in a container (cell or cuvette).
Reflection and scattering losses can occur at the cell walls.

23. To compensate for these effects, the power of the beam transmitted through a cell containing the analyte solution is compared with one that traverses an identical cell containing only the solvent, or a reagent blank.
An experimental absorbance that closely approximates the true absorbance for the solution is thus obtained, that is,
A = log P0/P  log Psolvent/Psolution

24. According to Beer’s law, absorbance is directly proportional to the concentration of the absorbing species, c, and to the path length, b, of the absorbing medium:
A = log(P0/P) = abc
a is a proportionality constant called the absorptivity.
Concentration expressed in moles per liter and b in cm, gives the proportionality constant, called the molar absorptivity, .
A = bc

27. Beer’s law also applies to solutions containing more than one kind of absorbing
substance.
Provided that there is no interaction among the various species, the total absorbance for a multicomponent system at a single wavelength is the sum of the individual absorbances.
Atotal = A1 + A2 + …+ An = 1bc1 + 2bc2 + …+ nbcn
where the subscripts refer to absorbing components 1, 2, , n.

28. An absorption spectrum is a plot of absorbance versus wavelength (or wavenumber or frequency).
A plot of molar absorptivity e as a function of wavelength is independent of concentration.
This type of spectral plot is characteristic for a given molecule and is sometimes used to aid in identifying or confirming the identity of a particular species.

30. The major atomic absorption transitions occur when the single outer electron of sodium is excited from its room temperature or ground state 3s orbital to the 3p, 4p, and 5p orbitals.
These excitations are brought on by absorption of
photons of radiation whose energies exactly match the
differences in energies between the excited states and
the 3s ground state.
Transitions between two different orbitals are termed
electronic transitions.

31. Molecules undergo three types of quantized transitions
when excited by ultraviolet, visible, and infrared radiation.
In addition to electronic transitions, molecules exhibit two
other types of radiation-induced transitions:
Vibrational transitions and rotational transitions.
Vibrational transitions occur because a molecule has a
multitude of quantized energy levels, or vibrational states,
associated with the bonds that hold the molecule together.

33. The total energy E associated with a molecule is
E = Eelectronic + Evibrational + Erotational
where Eelectronic is the energy associated with the electrons in the various outer orbitals of the molecule,
Evibrational is the energy of the molecule as a whole due to interatomic vibrations
and Erotational accounts for the energy associated with rotation of the molecule about its center of gravity.

35. There are few exceptions to the linear relationship between absorbance and path length at a fixed concentration.
Some of these deviations, called real deviations, are fundamental and represent real limitations to the law.
Others are a result of the method that we use to measure absorbance (instrumental deviations) or from chemical changes that occur when the concentration changes (chemical deviations).

36. Beer’s law describes the absorption behavior only of dilute solutions and in this sense is a limiting law.
At concentrations exceeding about 0.01 M, the average distances between ions or molecules of the absorbing species are diminished to the point where each particle affects the charge distribution and thus the extent
of absorption of its neighbors.
Because the extent of interaction depends on concentration, the occurrence of this phenomenon causes deviations from the linear relationship between absorbance and concentration.

43. 24 D Emission of electromagnetic radiation
Chemical species can be caused to emit light by (1) bombardment with electrons; (2) heating in a plasma, flame, or an electric arc; or (3) irradiation with a beam of light.

44. Radiation from a source is conveniently characterized by means of an emission
spectrum, which usually takes the form of a plot of the relative power of the emit-
ted radiation as a function of wavelength or frequency.
Three types of spectra are superimposed in the figure:
a line spectrum, a band spectrum, and a continuum spectrum.
The line spectrum consists of a series of sharp, well-defined spectral lines caused by excitation of individual atoms that are well separated, as in a gas.
The band spectrum, marked bands, is comprised of several groups of lines so closely spaced that they are not completely resolved.
The source of the bands is small molecules or radicals in the source flame.
Finally, the continuum spectrum is responsible for the increase in the background that appears above about 350 nm.

46. A spectral continuum of radiation is produced when solids such as carbon and tungsten are heated to incandescence.
Thermal radiation of this kind, which is called blackbody radiation, is more characteristic of the temperature of the emitting surface than of its surface material.
The radiant power P of a line or a band depends directly on the number of excited atoms or molecules, which in turn is proportional to the total concentration c of the species present in the source.
P = kc

48. When excitation and emission wavelengths are the same, the resulting emission is called resonance fluorescence.
Fluorescence is a photoluminescence process in which atoms or molecules are
excited by absorption of electromagnetic radiation.
Nonradiative relaxation and fluorescence emission are two mechanisms by which an excited atom or molecule to give up its excess energy and returns to its ground state.

49. There are two types of nonradiative relaxation:
Vibrational deactivation, or relaxation takes place during collisions between excited molecules and molecules of the solvent.
Internal conversion occurs when there is relaxation between the lowest vibrational level of an excited electronic state and the upper vibrational level of another electronic state can also occur.
The Stokes shift refers to fluorescence radiation that occurs at wavelengths that are longer than the wavelength of radiation used to excite the fluorescence.