Now how

How are scientists able to observe a single atom?

Who?

Aneta Stodolna (FOM Institute for Atomic and Molecular Physics, Netherlands) and colleagues

What?image of hydrogen atom2

A “quantum microscope” sounds like an impossible device developed by some brilliant but possibly insane scientist who is working on a secret government project to inspect recently-discovered infinitesimal evidence from a decades-old conspiracy. Putting the science fiction aside, though, in May 2013, physicists were able to develop a quantum microscope that directly observed the electron orbitals of a hydrogen atom, a revolutionary step in quantum theory.

Why?

The world of quantum physics deals with matter on the atomic and subatomic level, fittingly named because the word “quantum” is generally used to indicate “small” (or in this case, as small as you can possibly get). Quantum theory is a branch of theoretical physics that was initially developed in the first few decades of the 20th century in order to understand the fundamental properties of matter. It was a collaborative effort between some of the most prominent physicists of the time, including Niels Bohr, Erwin Schrödinger, Werner Heisenberg, and Max Planck. When observing matter from a quantum viewpoint, though, the standard atomic model of the time required numerous adjustments.

British physicist Ernest Rutherford performed several experiments to determine that the atom has a small, dense nucleus around which electrons orbit. Niels Bohr refined Rutherford’s model by introducing various separate orbits in which electrons circle the nucleus. Bohr also argued that each electron has a fixed amount of energy, which corresponds to its fixed orbit. As a result, when an electron absorbs energy and no longer corresponds to the energy of its initial orbit, it jumps to the next higher orbit (rather than moving continuously between orbits).

This proposal of electrons having fixed discrete energy levels, known as quanta, completes the quantum theory of the atom. Bohr’s model of the atom was successful in describing the behavior of the electron in a hydrogen atom. When the hydrogen electron was excited, it jumped into a higher orbital according to its new energy levels exactly as predicted. However, hydrogen is the simplest of all elements, and the model failed when attempting to explain the behavior of more complex atoms.

This revelation introduced the idea that perhaps, similar to light, subatomic particles might have a dual nature, acting as both a wave and a particle. Electrons orbiting around a nucleus interact with each other not through collisions, as would be typical for particles, but rather through interference patterns, similar to those of wave interference.  This concept leads to the notion that if we consider the velocity of an electron to be its wavelength, then the crests of neighboring electron waves either amplify each other or cancel each other out, creating a pattern that corresponds to Bohr’s orbits.

Because the behavior of an electron is so strange, it is impossible to determine an electron’s location at any point in time. At two points in time, an electron can jump from one location to another without any understandable causal relationship between the two. At best, we can only determine the probability an electron has for being at any particular place (hence quantum theory). At least, this was the best that could be done before a quantum microscope was able to directly observe the orbital structure of an excited hydrogen atom.

How?

The problem with the wave function component of an electron’s behavior is that it is incredibly difficult to observe. Previously, any direct observation of the wave function would destroy it before it could be fully observed. The quantum microscope, though, can directly magnify the microscopic state of a quantum particle to the laboratory scale in such a way that some quantum properties can be observed.

The quantum microscope utilizes photoionization microscopy to acquire the nodal structure of the electronic orbital of a hydrogen atom. This was performed by Aneta Stodolna, of the FOM Institute for Atomic and Molecular Physics in the Netherlands, with Marc Vrakking at the Max-Born-Institute in Berlin, Germany and other colleagues in Europe and the US.

In this experiment, a hydrogen atom is placed in an electric field and is excited by laser pulses. The excited electron then escapes from the atom and follows a trajectory to a dual microchannel plate detector perpendicular to the electric field. Since there are numerous trajectories that may reach that same point on the detector, the interference patterns created by the phase differences between these trajectories were observed, which were then magnified by more than 20,000x using an electrostatic zoom lens that would not disrupt the quantum coherence. The interference pattern observed revealed the structure of the wave function (Commissariat).

So?

After all that scientific jargon, the question arises “What’s the point of doing this? Why do we need to know more about atoms?” The invention of the quantum microscope, though, has an incredible wealth of potential for the development of atomic and molecular-scale technologies in quantum mechanics. Quantum mechanics has applications in energy conversion at the molecular level, advancements in lasers – which are used in everything from CDs to missiles, ultraprecision in objects such as thermometers and atomic clocks, and instantaneous communication (Atteberry). By increasing our knowledge of objects at the atomic and subatomic level, these real-world applications can be explored to enhance our current technologies to their maximum potential.

 Written by Constance Kaita

Works Referenced 

Image courtesy of physicsworld.com – a website from the Institute of Physics

Atteberry, Jonathan. “10 Real-world Applications of Quantum Mechanics.” Discovery.com. Discovery Channel. n.d. Web. 11 June 2013.

Commissariat, Tushna. “‘Quantum microscope’ peers into the hydrogen atom.” physicsworld.com. Institute of Physics. 23 May 2013. Web. 11 June 2013.

“Quantum Theory.” TheBigView.com. The Big View: Space Time. n.d. Web. 11 June 2013.

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