Question

A molecule can be excited to only the next higher rotational level by (a) absorption of energy (b) release of energy

Short answer

(a) Absorption of energy can excite a molecule to the next higher rotational level. (b) Release of energy makes a molecule fall back to a lower rotational level.

Step-by-step solution

Understanding Molecular Excitation

Energy is required for excitation of a molecule from one rotational level to the next. As energy is absorbed by the molecule, the energy level increases, leading to the excitation of the molecule.

Analysing Molecular Energy Absorption

When a molecule absorbs energy, it rises to a higher rotational level. This absorbed energy corresponds with the difference between the original energy state and the excited state. Therefore, the molecule can only reach the next higher rotational level if it absorbs the precise amount of energy required.

Analysing Molecular Energy Release

When a molecule releases energy, it falls back to a lower rotational level. The released energy corresponds to the difference between the original high energy state and the lower energy state. This results in a drop in the rotation level of the molecule.

Key concepts

Rotational Levels

In the world of molecular physics, rotational levels refer to the distinct energy states that a molecule can exist within while rotating. These are akin to the 'rungs' on a ladder, where each rung represents a new level of energy. When a molecule rotates, it doesn't do so at a single, continuous energy level but rather in specific quantized states.

• A molecule can 'step up' from one rung to another, but it needs to absorb energy to do so.
• Conversely, it can 'step down' and release energy.

Each movement between these levels involves a change in energy, which is typically observable in spectroscopic experiments. Understanding these rotational levels is key to comprehending how molecules behave in different environments, particularly those involving heat or light, which can cause them to absorb or release energy.

Energy Absorption

Energy absorption in molecules involves a fascinating process where energy, often in the form of light or heat, is taken in by a molecule. This leads to an increase in its energy state called excitation. For a molecule to move to a higher rotational level, it must absorb exactly the right amount of energy required for that transition.

• The absorbed energy changes the rotational kinetic energy of the molecule.
• Different types of molecules have varying energy gaps between levels, which means they absorb specific wavelengths of light.

This selective absorption property is instrumental in many applications, such as gas identification through spectroscopy. It’s also a critical process in contexts such as photosynthesis, where energy absorption by chlorophyll occurs.

Energy Release

Energy release is the process by which a molecule at a higher rotational level loses energy, transitioning to a lower energy state. This is similar to a person descending down a ladder after reaching a higher step. As the molecule releases energy, usually in the form of light or heat, it results in a decrease in its energy state, often observed as emission in spectroscopy.

• The amount of energy released matches the gap between the higher and lower rotational levels.
• This is a crucial process in phenomena such as fluorescence, where substances emit light upon energy release.

Understanding energy release is important in various scientific and industrial fields, including those involving chemical reactions and lighting technologies. It provides insights into how molecules dissipate energy and stabilize at lower energy states.