Probe-Interaction-Signal

Flower
In memory of Dr. Alex Punnoose
1968-2016

Dr. Alex Punnoose was one of my favorite teachers at Boise State. Tragically, he passed away in 2016. However, his memory lives on in his students and what he taught us about the universe and our roles as scientists.

Dr. Punnoose’s “Probe-Interaction-Signal” method of classifying material characterization techniques is one of the most useful lessons he taught me. I do not know if he invented this approach to categorizing characterization techniques, but his implementation of Probe-Interaction-Signal was very intuitive, and I still use it as a memory tool and when writing methods sections in my papers.

Probe-Interaction-Signal

In essence, most material characterization methods can be understood by breaking the process into 3 steps: Probe, Interaction, and Signal.

  • Probe: Whatever you use to interrogate the specimen: light, radiation, physical contact with a tool, bombardment with particles, etc.
  • Interaction: The physics-based principle which describes how the probe and specimen interact. Usually, this incorporates some material properties of the specimen.
  • Signal: What you measure to understand the material: emitted particles, reflected or emitted radiation, reaction forces, etc.

This framework is essentially the same as classifying the components of an experiment as independent factors, theory, and dependent response variables. The Probe and Signal are the components of the experiment which a scientist can measure and record. The Interaction involves the theory and equations which we can use to gain insight into the specimen we are interrogating.

Let’s start with a really simple, silly example.

Poking Something with a Stick

So suppose you are a child walking along the beach, and you see a crab sitting on the sand. It’s not moving. What do you do? Well, you can design an experiment within the Probe-Interaction-Signal framework.

  • Probe: Poke it with a stick.
  • Interaction: If the crab is alive, it will feel you poking it, and it will move. If it is dead, it will not react.
  • Signal: You look at the crab to see if it moves or does not move.

So here we have a simple example of a characterization method based on the physical principle that living things react to being poked while nonliving things do not. The Interaction theory tells us how to draw a conclusion based on the Signal we measure after applying a Probe.

X-ray Photoelectron Spectroscopy (XPS)

XPS is an excellent choice of name for a characterization technique since the name describes almost exactly how XPS works.

  • Probe: Monochromatic X-rays of known energy
  • Interaction: Electrons are released with an energy equal to the incoming X-rays minus the binding energy that held them to the specimen
  • Signal: The kinetic energy of the electrons released from the material

In many cases, the Interaction portion of the process is based on an underlying physical principle which we can describe mathematically. XPS is based on measuring binding energies which are characteristic of the atoms which emitted the photoelectron.

$$ E_{binding} = E_{X-ray} – (E_{kinetic} + \phi) $$

We controlled the X-ray energy and measured the kinetic energy. The φ part of the equation is just a constant representing the work function of the electron detector. (Some versions of the equation will leave φ off, presuming that it is accounted for in the kinetic energy term.) So this equation lets us compute the binding energy of the atoms which emitted the, which in turn can be matched to the specimen composition.

Understanding the Interaction also helps us anticipate the limitations of a characterization technique. Since XPS measures the energy of emitted electrons, and electrons’ ability to escape a material diminishes with depth below the surface, we know that XPS measures surface composition. Anything that would interfere with the electrons’ path to the detector could interfere with the measurement, so XPS is best performed in a vacuum.

Let’s take a quick look at a few other techniques through the Probe-Interaction-Signal lens.

X-ray Fluorescence Spectroscopy (XRF)

  • Probe: X-rays from a source (can be monochromatic or not)
  • Interaction: Electron transitions induced by the probe X-rays result in the emission of “secondary” X-rays which are characteristic of the atoms which produce them
  • Signal: The secondary X-rays

XRF is another spectroscopic technique because the Signal is X-rays which are characteristic of the element which produced them. XRF is also very similar to XPF because they share the same Probe. However, they measure different Signals and require different, but related, Interaction theories to interpret their results.

Optical Microscopy

  • Probe: Light from a source
  • Interaction: Light is scattered by features of the specimen surface
  • Signal: Scattered light is magnified and directed to a human eye or a camera

Microscopy is intuitive because, like human vision, it lets us measure the spatial locations of features on the surface of a specimen. However, the Probe-Interaction-Signal framework helps us understand how modifications to optical microscopic techniques could be made to extract additional information.

For example, we could modify the probe and signal by adding filters, making the interaction more complex and providing us with new information. By adding polarizing filters, we could deliberately exclude light which changed polarization upon interaction with the specimen. This can help determine the orientation of crystals of birefringent materials, such as an aluminum oxide or barium titanate.

  • Probe: Polarized light from a source
  • Interaction: Light is scattered by features of the specimen surface and changes polarization angle depending on the orientation of birefringent surface features
  • Signal: Scattered light which is filtered by a polarizer, magnified, and directed to a human eye or a camera

Transmission Electron Microscopy (TEM)

  • Probe: Monochromatic electron beam
  • Interaction: Electrons are transmitted through the specimen and scattered by its atoms
  • Signal: Transmitted electrons form an image in the image plane and a diffraction pattern in the back focal plane

TEM is a far more powerful and sophisticated technique than my description makes it sound. My goal is to emphasize one important point: the spatial information from TEM images comes primarily from the Signal part of the technique. Let’s compare this to how scanning electron microscopy works.

Scanning Electron Microscopy (SEM)

  • Probe: “Primary” electron beam directed to pre-determined locations on a grid
  • Interaction: “Secondary” electrons are knocked loose from the specimen surface
  • Signal: Secondary electrons are collected by a detector

Notice how the location information for SEM comes from the Probe part of SEM rather than directly from the Signal like TEM and optical microscopy. It can be tempting to focus on the information carried in the Signal part of an experiment. However, the Signal-Interaction-Probe framework helps illustrate how important it is to interpret the dependent variables of an experiment in the context of the independent variables.

A Useful Model

We have only scratched the surface of material characterization techniques out there, but the Probe-Interaction-Signal concept can be applied to practically all of them. I find it especially handy when learning a new technique for the first time or training someone else. It’s also very useful in troubleshooting misbehaving instruments.

I hope you find Probe-Interaction-Signal useful, and I suspect it will not be the last of Dr. Punnoose’s lessons I transcribe into a blog post.

Author: Wade

Materials scientist, lab manager, and educator based in the Seattle area.

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