INTRO TO SPECTROSCOPIC METHODSINTRO TO SPECTROSCOPIC METHODS(Chapter 6)(Chapter 6)

NATURE OF LIGHT AND INTERACTION WITH MATTERNATURE OF LIGHT AND INTERACTION WITH MATTER

Electromagnetic Radiation (i.e., “light”)Electromagnetic Radiation (i.e., “light”)

–Wave-particle dualityWave-particle duality

Particle properties (photons or quanta)Particle properties (photons or quanta)

Wave properties (sinusoidal waves)Wave properties (sinusoidal waves)

Plane-polarized em radiation of wavelength Plane-polarized em radiation of wavelength λλ

SHC, 6e, Fig. 6-1

A (in watts)

Frequency (ν) in Hz or s-1

ν = c/λ where c = 3.00 x 108 m/s

Electromagnetic spectrum showing representative molecular processes

Analogous to SHC, 6e, Fig. 6-3 and Table 6-1

MATH DESCRIPTION OF A WAVEMATH DESCRIPTION OF A WAVE

Equation of a sine wave:Equation of a sine wave:

Where the angular velocity:Where the angular velocity:

By substitution: By substitution:

1) 1) SUPERPOSITION OF WAVESSUPERPOSITION OF WAVES

Two or more waves interact either through:Two or more waves interact either through:

Constructive interference orConstructive interference or

Destructive InterferenceDestructive Interference

)sin( tAy

)2sin( tAy

2

SHC, 6e, Fig. 6-4: Constructive and destructive interference.

0° 90° 180°

Fourier Transform (FT): a math operation that Fourier Transform (FT): a math operation that reduces a complex wave to the sum of simple sine reduces a complex wave to the sum of simple sine and cosine terms.and cosine terms.

Used in signal analysis (e.g., FT-IR, FT-NMR, etc)Used in signal analysis (e.g., FT-IR, FT-NMR, etc)

2) 2) DIFFRACTIONDIFFRACTION

Consequence of interference (SHC, 6e, Fig. 6-8)Consequence of interference (SHC, 6e, Fig. 6-8)

3) 3) COHERENCECOHERENCE

Source output has the same Source output has the same νν’s and ’s and λλ’s’s Phase relationships remain constant in timePhase relationships remain constant in time

4) 4) TRANSMISSION OF RADIATIONTRANSMISSION OF RADIATION

c = 3.00 x 10c = 3.00 x 1088 m/s m/s in a vacuumin a vacuum

Light slows in other mediaLight slows in other media

Index of refraction:Index of refraction:

for liquids = 1.3 – 1.8; for solids = 1.3 – 2.5 for liquids = 1.3 – 1.8; for solids = 1.3 – 2.5 or higheror higher

5) 5) REFRACTION OF RADIATIONREFRACTION OF RADIATION

The bending of light as it passes from one The bending of light as it passes from one medium to anothermedium to another

ii v

c

SHC, 6e, Fig. 6-10 Refraction of Light

M1

M2

Snell’s Law:

2211 sinsin

less dense less dense more dense medium more dense medium bent bent towardstowards normal to interface normal to interface

more dense more dense less dense medium less dense medium bent bent away away fromfrom normal to interface normal to interface

6) 6) REFLECTION OF RADIATIONREFLECTION OF RADIATION

Two types:Two types:

Specular (from a smooth surface)Specular (from a smooth surface)

Diffuse (from a rough surface)Diffuse (from a rough surface)

Optical Fiber Construction and Principle of Operation

} glass or plastic

Any ray entering within the cone of acceptance

will be totally internally reflected

7) 7) SCATTERING OF RADIATIONSCATTERING OF RADIATION

– RayleighRayleigh from very small particles with diameters < from very small particles with diameters < λλ intensity ~ 1/intensity ~ 1/λλ44

– Tyndall effectTyndall effect from colloidal sized particlesfrom colloidal sized particles can be observed with naked eyecan be observed with naked eye

– RamanRaman radiation undergoes frequency changesradiation undergoes frequency changes decreased frequency (Stokes)decreased frequency (Stokes) increased frequency (anti-Stokes)increased frequency (anti-Stokes)

8) POLARIZATION OF RADIATION

– Plane-polarized:(end-on-view)

– Result:

polarizer

vertically polarizedelectric vector

SHC, 6e, Fig. 6-11 & 6-12 Polarization of Light