Lecture 2- Spectroscopy

Transmittance

The ratio of the transmitted power 𝑃 to the incident power 𝑃0

Beer’s Law

- Quantifies the amount of radiation absorbed by a sample.

- It describes how light is reduced based on the concentration of absorbing molecules and the distance it travels through the sample

Photomultiplier tube (PMT)

a highly sensitive device that detects light. It operates by

converting incoming photons into an electrical signal through a series of processes

Monochromator

- an optical device used in spectrophotometers to isolate and select a specific wavelength of light from a polychromatic source.

- The monochromator is typically composed of several key components:

➢ Lenses or mirrors

➢ Entrance and exit slits

➢ Dispersing element

Molecular Spectroscopy

study of how matter interacts with electromagnetic radiation, providing information about the energy levels, composition, and structure of molecules.

Prism

a prism monochromator uses a prism to disperse light.

Wavelength (λ)

The linear distance between successive maxima or minima of a wave

Absorption

-the energy of a photon is transferred to the electrons of an atom or molecule,

- the electrons move to higher energy levels & results in the atom or molecule entering an excited state.

Mid IR - Wavelength range

2.5-50μm

Frequency (V)

The number of cycles passing a fixed point per unit time. The units of the frequency

are Hz or S–1

Fluorescence

- process in which a molecule absorbs light and experiences electronic transitions. 

- molecule absorbs energy, electron is excited from the ground singlet state (S₀) to an excited singlet state (S₁). 

- electron may relax to a lower vibrational level within the excited singlet state before returning to the ground state (S₀).

- During transition back to the ground state, molecule emits a photon of light, resulting in fluorescence

Electromagnetic radiation

A form of energy with properties that can be described in terms of waves or, alternatively, as particles, depending on the method of observation.

Absorbance

- The opposite of transmittance, but the relationship is not linear. - The absorbance of a solution is related to the transmittance of a logarithmic function.

𝐴 = − log 𝑇 = − log 𝑃 /𝑃0= log 𝑃0/𝑃

Visible Wavelength range

380-780nm

Spectroscopy 

A general term used to describe techniques based on the measurement of absorption, emission, or luminescence of electromagnetic radiation.

Excited states

higher energy levels

UV Wavelength range

180-380nm

Wavenumber(𝛎̅ )

The reciprocal of the wavelength and is the number of

waves in a unit length or distance per cycle (typically cm–1)

Emission

- an excited molecule or atom ejects a photon of energy

- decreases its energy causing the molecule or atom to be demoted to a lower energy level. 

Near - IR Wavelength range

0.78-2.5nm

Diffraction grating

- A diffraction grating consists of a large number of parallel lines (or grooves) ruled on a highly polished surface such as aluminum. 

- The grooves on the grating act as scattering centers for the incident light beam.

- works by causing constructive interference for specific wavelengths of light depending on the angle it reflects off the grating.