1- Text Book, 1- Text Book, Fundamental of Molecular Fundamental of Molecular Spectroscopy, C. N. Banwell, 4 th Spectroscopy, C. N. Banwell, 4 th ed.,1995ed.,1995

2- Internet website2- Internet websiteWWW.hyperphysics.phy-WWW.hyperphysics.phy-astr.gsu.eduastr.gsu.edu

ResourcesResources

Molecular spectroscopy

may be defined as the study of the interaction

of electromagneti

c Waves and matter

We shall be concerned We shall be concerned withwith What spectroscopy can tell us of What spectroscopy can tell us of

the structure of matterthe structure of matter

Nature of electromagnetic radiationNature of electromagnetic radiation

How the interaction of How the interaction of electromagnetic and matter is electromagnetic and matter is occurred occurred

• Frequencies:Frequencies: are always expressed are always expressed in Hz (after Hertz) with the unit in Hz (after Hertz) with the unit 1/second ("cycles" is derived from 1/second ("cycles" is derived from Hz and only used for cyclic Hz and only used for cyclic movements. In one second how movements. In one second how many cycle is travelled.many cycle is travelled.

EnergiesEnergies are expressed in different are expressed in different units. eV (electron volts) for units. eV (electron volts) for atomic spectroscopy, photoelectron atomic spectroscopy, photoelectron spectroscopy, mass spectroscopy, 1 spectroscopy, mass spectroscopy, 1 eV is the energy that an electron eV is the energy that an electron acquires after passing through a acquires after passing through a voltage difference of one volt. voltage difference of one volt.

• WavelengthsWavelengths are expressed in different are expressed in different units. (The distance travelled during a units. (The distance travelled during a

complete cycle) Å (Ångstrom, 10-10 m) is complete cycle) Å (Ångstrom, 10-10 m) is common for short wavelengths: g-rays, X-common for short wavelengths: g-rays, X-rays, up short wavelength UV ("vacuum rays, up short wavelength UV ("vacuum

UV") nm (10-9 m) is common for UV-Vis UV") nm (10-9 m) is common for UV-Vis (350 nm - 800 nm) cm are used for (350 nm - 800 nm) cm are used for

microwave spectroscopy.microwave spectroscopy.

Wave numbers (1/Wave numbers (1/))

E = hE = h and and = c/ = c/

>~ >~E = hc/E = hc/

The Quantization of Energy•The energy of an oscillator (absorbed or emitted) is not continuous

and that any change in its energy can occur only by means of a jump between two distinct energy states i.e., it is quantized.

A molecule in space can have many sorts of energy ;1-Rotational energy:- by virtue of bodily rotation about its center of

gravity.2-Vibrational energy:- due to the periodic displacement of its atoms

from their equilibrium positions.3-Electronic Energy:- due to transfer of electrons associated with its

atoms or bonds between available energy levels.4 -Transitional Energy:- due to free movement of the whole

molecule in the space.

All the above energies of the molecule are quantized except the Transitional Energy.

E = E2 - E1 = h (joules)Where

is the frequency of the light photon

absorbed

Antisymmetrical stretch, there is a periodic

alteration in the dipole moment, infra-red active**

i- Symmetric stretching vibration; The dipole moment remains zero

**infra-red inactiv **

HomeworkHomework

ExerciseExercise 1- What is the role of Spectroscopy?1- What is the role of Spectroscopy? 2- Define and\or characterize:- 2- Define and\or characterize:- Molecular energies, light, Quantized energy, IR-Molecular energies, light, Quantized energy, IR-

active molecule.active molecule. 3- Arrange the following spectral regions 3- Arrange the following spectral regions

according to increase in energy (start with according to increase in energy (start with smallest), Radio wave, x-rays, visible rays, Which smallest), Radio wave, x-rays, visible rays, Which region can only produce n.m.r spectra?region can only produce n.m.r spectra?

4- Show by diagrams only how a vibrating 4- Show by diagrams only how a vibrating molecule can interact with light molecule can interact with light

•Problems 1.1 and 1.2 page 39 (Banwell)Problems 1.1 and 1.2 page 39 (Banwell)

Representation of SpectraRepresentation of Spectra

Generally, the spectral band is characterized by position, width, and intensity .

The position of the spectral bands, or the frequencies at which the molecule absorb ,depends on its structural features ( Functional group) as well as on its environment.

**The two most important features of spectral lines are -:

their height (~ intensity) and their

width (n).

I- WidthI- Width There are two factors cause the There are two factors cause the width broadeningwidth broadening

1- 1- Factors related to an Factors related to an instrumental defectinstrumental defect a)a) Signal to noise ratio S/N (4/1)Signal to noise ratio S/N (4/1) In order to detect the real sample signal from the instrument noise, .In order to detect the real sample signal from the instrument noise, . the intensity of this signal should be Four times that of noise.the intensity of this signal should be Four times that of noise.

b) b) Shape and ResolutionShape and Resolution A crucial question that arises with any spectroscopic A crucial question that arises with any spectroscopic technique is the question of how well we will be abletechnique is the question of how well we will be able to resolve two different spectral linesto resolve two different spectral lines

2- 2- Natural Broadening ( Deformation of molecular orbitals)Natural Broadening ( Deformation of molecular orbitals) a) a) Collision Broadening Collision Broadening 1- The collision of molecules causes the excited state to revert to the ground state1- The collision of molecules causes the excited state to revert to the ground state

2- Collisions thus shorten the lifetimes of excited states and lead to the broadening of the associated spectral lines.2- Collisions thus shorten the lifetimes of excited states and lead to the broadening of the associated spectral lines. b) b) Doppler Broadening Doppler Broadening * The frequency that an object emits is modified by its speed relative to the observer (detector).* The frequency that an object emits is modified by its speed relative to the observer (detector).

** ** It is possible to eliminate Doppler broadening by investigating a molecular beam of theIt is possible to eliminate Doppler broadening by investigating a molecular beam of the atoms / molecules and placing the detector at a 90 deg angle to the direction of the beam.atoms / molecules and placing the detector at a 90 deg angle to the direction of the beam.

c) c) Heisenberg Uncertainty Principle (Heisenberg Uncertainty Principle ( E • E • t > h / 4 t > h / 4 10 10-34-34 J • s) J • s)

Heisenberg Uncertainty PrincipleHeisenberg Uncertainty Principle

II- IntensityII- Intensity 1 -Transition Probability -:

The precise calculation of transition probabilities is involved and requires quantum mechanics. However, it is often possible to predict if a transition is symmetry allowed or forbidden. We will discuss the respective Selection Rules for each spectroscopic technique individually.

2 -Population of States -: For thermal equilibrium, the probability of a state in thermal equilibrium is given by the Boltzmann Distribution:

Nupper / Nlower = exp(-ΔE/kT)

3- The Concentration or path length of the sample)Lambert-Beer's Law ( - log [ I/Io] = c • e • l

LASERLASER((LLight ight AAmplification by mplification by SStimulatedtimulated

EEmission of mission of RRadiationadiation))

Properties of Laser Light Laser light differs in many aspects from ordinary light (that is: the light generated by light bulbs or fluorescent tubes). Laser light is

1 -Monochromatic The range of frequencies emitted by a laser is very narrow,typically Dn / n = 10-5 to 10-8 .

2 -Low Divergence Laser beams can not only be focused to a smaller point than any other light source (ca. 1 l), but the also show very small divergence. Under optimum conditions, the divergence can reach the theoretical limit of f = l / d

3 -Coherent The maxima and minima of all emitted wavelets occur at the same position .

4 -Highly Intense Pulsed lasers can generate light pulses of extremely high intensity. For example, commercially available Nd:YAG lasers (l = 1064 nm) are capable of 10 ns pulses with an energy of 108 W. This corresponds to an energy of 1020W / m2 at the focal point - 1017 (!)

times more intense than the sunlight at the earth's surface .5 -Polarized The E-planes of the individual light waves are all parallel to

each other .6 -Precisely Timed The ability to create incredibly short laser pulses

(currently: 4 femtoseconds = 4 fs = 4 x 10-15 s) allows the spectroscopic resolution of extremely fast processes. A 4 fs pulse in the visible

corresponds to only 2 cycles of the corresponding sin wave .