Atomic, molecular, and optical physics facts for kids

Atomic, molecular, and optical physics (often called AMO) is a science field that studies how tiny particles like atoms and molecules interact with each other and with light. It looks at things on a very small scale, like individual atoms, and at energy levels around a few electron volts.

These three areas – atomic, molecular, and optical physics – are very closely connected. Scientists in AMO physics use ideas from classical physics (like everyday physics) and quantum physics (which deals with the super tiny world). They study how atoms and molecules give off light (emission), soak up light (absorption), and scatter light. They also look at how lasers work and the general light-related properties of different materials.

Understanding Atoms and Molecules

Atomic physics focuses on atoms by themselves. It treats an atom as a central core (the nucleus) with electrons orbiting around it.

Molecular physics studies the physical properties of molecules. Molecules are groups of two or more atoms joined together.

It's important to know that "atomic physics" is different from "nuclear physics". Atomic physics looks at the whole atom, including its electrons. Nuclear physics, on the other hand, only studies the atom's central core, the nucleus.

Both atomic and molecular physics are mainly interested in how electrons are arranged in atoms and molecules (their electronic structure). They also study how these arrangements change. Scientists often use quantum mechanics to understand these changes. For molecules, this is sometimes called quantum chemistry.

How Molecules Move and Change

In molecular physics, the idea of atomic orbitals (where electrons are in atoms) expands to molecular orbitals (where electrons are in molecules). Molecules can do more than just change their electron arrangements. They can also rotate and vibrate. These rotations and vibrations have specific, fixed energy levels, just like electron levels.

• The smallest energy differences are for rotations. So, pure rotational light patterns are found in the far infrared part of the electromagnetic spectrum.
• Vibrational light patterns are in the near infrared.
• Light patterns from electron changes are usually in the visible and ultraviolet light regions.

By measuring these light patterns, scientists can figure out things about molecules, like the distance between their atoms.

The Physics of Light

Optical physics is about how electromagnetic radiation (light) is made, what its properties are, and how it interacts with matter. It's especially interested in controlling and manipulating light.

Optical physics is different from general optics or optical engineering. Optical physics focuses on discovering new light-related phenomena and how they can be used. However, all these fields are very connected. New discoveries in optical physics often lead to new devices in optical engineering, and those devices help with more research.

Scientists in optical physics create and use light sources that cover the entire electromagnetic spectrum, from microwaves to X-rays. This field includes making and detecting light, studying how light behaves in different ways, and using spectroscopy (studying light patterns).

Lasers and laser spectroscopy have greatly changed optical science. Important research areas also include quantum optics (how light behaves at the quantum level) and femtosecond optics (studying extremely short light pulses). Optical physics also helps with new ways to measure things at the tiny nano scale, like using diffractive optics and interferometry.

The discoveries in optical physics lead to many useful things in our lives. They help improve communications, medicine, manufacturing, and even entertainment.

A Brief History of AMO Physics

The Bohr model of the Hydrogen atom shows electrons orbiting the nucleus in specific paths.

One of the first big steps in atomic physics was realizing that all matter is made of tiny particles called atoms. John Dalton developed this idea in the 18th century. At first, scientists didn't know exactly what atoms were like inside. But they could describe and sort them based on how they behaved in large groups, which led to the creation of the periodic table by Dmitri Mendeleyev in the 19th century.

Later, scientists like Joseph von Fraunhofer noticed that light from the sun had dark lines in its spectrum. This showed a connection between atoms and light.

From the late 1800s to the 1920s, physicists tried to explain these spectral lines and how hot objects give off light (blackbody radiation). The Bohr atom model, proposed by Niels Bohr in 1913, was an early attempt to explain the hydrogen atom's light patterns. Bohr suggested that electrons orbit the nucleus in specific, fixed paths and don't lose energy while in these paths. When an electron jumps from one orbit to another, it either gives off or absorbs light.

Many experiments, like the photoelectric effect (where light knocks electrons off a metal) and the Compton effect (where light changes energy when it hits an electron), showed that the old physics ideas couldn't fully explain how light and matter interact. These discoveries, along with the limitations of the Bohr model, led to a completely new way of thinking about matter and light: quantum mechanics.

In 1900, Max Planck came up with a formula to describe light inside a hot box, suggesting that energy comes in tiny packets, or "quanta." In 1905, Albert Einstein used Planck's idea to explain the photoelectric effect, saying that light is made of tiny particles called photons. In 1911, Ernest Rutherford discovered that atoms have a tiny, dense center called a nucleus. These ideas paved the way for quantum mechanics.

Modern Ways of Studying AMO

Today, scientists use quantum mechanics to study atoms and molecules. Important steps in this field were made by Werner Heisenberg and Erwin Schrödinger, who developed mathematical ways to describe the quantum world.

Sometimes, scientists use a "semi-classical" approach. This means they treat some parts of the problem using quantum mechanics and other parts using classical physics. This can make complex calculations much easier. For example, when a laser shines on an atom, the atom might be treated using quantum mechanics, while the laser light itself is treated using classical physics.

Atoms and Molecules on Their Own

In AMO physics, scientists often study atoms and molecules as if they are completely by themselves, not interacting with other atoms or molecules. For example, they might look at a single atom with its nucleus and electrons, or a simple molecule like hydrogen.

They study processes like ionization (when an electron is removed from an atom) or excitation (when an electron gains energy and moves to a higher energy level). This can happen when atoms absorb light or collide with other particles.

Even though atoms and molecules are rarely truly isolated in the real world, this way of studying them is very useful. In a gas or plasma, the time between interactions between molecules is much longer than the time it takes for changes to happen within a single atom or molecule. So, for most of the time, individual atoms and molecules can be thought of as being isolated. This is why atomic and molecular physics provides the basic ideas for fields like plasma physics and atmospheric physics, even though these fields deal with huge numbers of particles.

How Electrons Are Arranged

Electrons orbit the nucleus in specific "shells" or energy levels. Normally, electrons are in their lowest possible energy levels, called the ground state. But they can get extra energy from light (photons), magnetic fields, or collisions with other particles. When they get this energy, they can jump to a higher energy level, becoming an excited electron.

The energy needed to completely remove an electron from its shell is called the binding energy. If an electron absorbs more energy than its binding energy, the extra energy becomes kinetic energy, and the electron flies away. This process is called ionization.

If an electron absorbs less energy than its binding energy, it might move to a higher energy level. After a short time, an excited electron will usually drop back down to a lower energy level. When it does this, it gives off the extra energy as a photon (a particle of light). The energy of this photon matches the difference in energy between the two levels.

Sometimes, if an electron drops to an inner shell, the energy released can be transferred to another electron, kicking that second electron out of the atom. This is called the Auger effect, and it can cause an atom to lose more than one electron from a single event.

There are specific "selection rules" that determine which electron energy levels can be reached by absorbing light. However, these rules don't apply when electrons are excited by collisions with other particles.

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