3.11.A.1 Types of Molecular Transitions and Photon Absorption/Emission:

1. Electromagnetic Spectrum Overview:

 i. Microwave Radiation:
Wavelength Range: 1 mm to 30 cm

Frequency Range: 1 GHz to 300 GHz
Energy: Low
Examples: Microwave ovens ,Radar, Satellite communications

Interactions with Matter:
Primarily absorbed by water molecules, causing them to vibrate and heat up.
Applications:
Cooking (microwave ovens)
Radar systems (weather radar, military, air traffic control)
Telecommunications (satellite communication, Wi-Fi)

ii. Infrared (IR) Radiation:

Wavelength Range : 700 nm to 1 mm
Frequency Range: 300 GHz to 430 THz
Energy: Moderate
Examples: Thermal radiation ,Remote controls ,Night vision technology
Interactions with Matter:
Absorbed by molecules, which, upon vibration, emit heat.
Applications: Thermal vision ,Infrared sensors (motion sensors), Night vision goggles ,Remote control electronics ,Heating (infrared heaters)

 iii. Visible Light:
Wavelength Range: 400 nm (violet) to 700 nm (red)
Frequency Range: 430 THz to 770 THz

Energy : Moderate-High
Examples: Sunlight, Artificial lighting ,Colors that we see
Interactions with Matter:
Detected by the human eye, which initiates photochemical reactions in the retina.
Applications:
Vision (what we see) ,Photography, Optical instruments (microscopes, telescopes) , Fiber optics (communication)

iv. Ultraviolet (UV) Radiation:
Wavelength Range: 10 nm to 400 nm
Frequency Range: 750 THz to 30 PHz
Energy: High
Example: Sunlight (UV rays) ,Black lights

Interactions with Matter:
Can break molecular bonds (e.g., DNA), causing sunburn, skin aging, and possible skin cancer.
Applications:
Sterilization (UV-C light kills bacteria and viruses)
Tanning beds
Forensic analysis (black light reveals hidden substances)
Vitamin D production in skin

2.Photon Energy and Transitions:

i. Microwave (Rotation): Rotational transitions are informative about the geometry of a molecule and bond lengths. These transitions can also provide information about the rotational constants, which are useful in identifying the isotopic composition of a molecule.

ii. Infrared (Vibration): IR spectroscopy is non-destructive and allows real-time monitoring of chemical reactions. It is used extensively in material science, environmental analysis, and forensic studies to detect specific compounds.

iii. UV/Vis (Electronic): Electronic transitions in conjugated systems have been studied with UV/Vis spectra, particularly where electron delocalization can affect absorption patterns. This becomes important in the fields of organic electronics and dye chemistry, wherein molecular design strongly influences optical properties.

iv. Microwave (Rotation): This region also helps to study rotational relaxation and energy transfer processes in gases. These studies are important to understand molecular collisions and atmospheric phenomena.

v. Infrared (Vibration): IR spectroscopy not only serves the purpose of determining functional groups, but it also plays a major role in determining molecular dynamics: anharmonicity and vibrational coupling, for example. It also applies to proteins and other biomolecules, enabling a probe of changes in conformation.

3.Relaxation:

After a molecule absorbs a photon, it often undergoes relaxation to return to a lower energy state. This process can involve emitting light in different forms:

i. Fluorescence:
a. Fast process (nanoseconds to microseconds).
b. The molecule absorbs a photon and quickly returns to a lower electronic state, emitting light in the visible or UV range.
c. Example: Fluorescent dyes or molecules such as GFP (Green Fluorescent Protein) which have the property to emit light on excitation.

ii. Phosphorescence:
Slow process; milliseconds to seconds, even to minutes or seconds.
a. The molecule takes in a photon, but immediately after, there is a spin flip to the triplet state for emission of the light. Typically, it emits at lower energy (long wavelength) and usually much longer after excitation.
b. Phosphorescence: materials from glow-in-the-dark products which are sometimes made from organic material or metal-based compounds.
c. Phosphorescence: Slower, at lower energy, typically longer wavelength, and it is a triplet state.

The relaxation process has been of paramount importance in fluorescence microscopy, spectroscopy, and photochemistry, where the light emitted gives vital information about the behavior and structure of molecules.