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Course: AP®︎/College Chemistry > Unit 1
Lesson 8: Photoelectron spectroscopyPhotoelectron spectroscopy
Key points
- Photoelectron spectroscopy (PES) is an experimental technique used to determine the relative energies of electrons in atoms and molecules.
- Photoelectron spectrometers work by ionizing samples using high-energy radiation (such as UV or x-rays) and then measuring the kinetic energies (
- Given the energy of the incident radiation (
- A PES spectrum is a graph of photoelectron count vs. binding energy.
- The peaks in a PES spectrum correspond to electrons in different subshells of an atom. The peaks with the lowest binding energies correspond to valence electrons, while the peaks with higher binding energies correspond to core electrons.
Introduction: What is photoelectron spectroscopy?
Photoelectron spectroscopy (PES) is an experimental technique that measures the relative energies of electrons in atoms and molecules. Scientists often use PES to study the elemental composition of materials or to characterize bonding in molecules. In this article, however, we’re going to use PES to deepen our understanding of atomic structure: by looking at PES data for pure samples of elements, we’ll see how photoelectron spectroscopy provides direct support for the concepts of electron shells and subshells, electron configurations, and more.
The basics of photoelectron spectroscopy
Photoelectron spectroscopy is based on the photoelectric effect, a physical phenomenon first characterized by Albert Einstein in 1905. The photoelectric effect is as follows: when electrons in a metal are exposed to light of sufficient radiation, the electrons are ejected from the metal surface. If we know the kinetic energy of the ejected electrons (known as photoelectrons) and the energy of the incident radiation, we can calculate the energy of the electrons in the solid metal. (For more details, check out this article on the photoelectric effect.)
Photoelectron spectroscopy simply applies the photoelectric effect to free atoms or molecules instead of metals. In PES, a sample is bombarded with high-energy radiation, usually UV or X-ray, which causes electrons to be ejected from the sample. The ejected electrons travel from the sample to an energy analyzer, where their kinetic energies are recorded, and then to a detector, which counts the number of photoelectrons at various kinetic energies. A simplified diagram of this process is shown below.
The energy required to eject an electron from the sample is known as the electron’s ionization energy or binding energy. We know the energy of the radiation (
The binding energy of an electron in an atom depends on its location around the nucleus. Electrons in the outermost shell (valence electrons) are more shielded and farther from the nucleus on average, so they have the lowest binding energies of all of the electrons in an atom. In contrast, electrons in inner shells (core electrons) are less shielded and closer to the nucleus on average, so they have higher binding energies. As we’ll see in the next section, understanding the relationship between an electron's binding energy and its location is essential for the interpretation of PES data.
Analyzing PES spectra
Data from PES experiments is obtained as plots of photoelectron count vs. binding energy, with binding energy usually expressed in units of electron volts (
A typical PES spectrum features peaks at different binding energies. Because electrons in a particular subshell of an atom have the same binding energy, each of these peaks corresponds to electrons in a different subshell. The binding energy of a peak tells us how much energy is required to remove an electron from the subshell, and the intensity of the peak tells us the relative number of electrons in the subshell.
To illustrate, let's look at some PES data. Real PES data is messy and often difficult to interpret by non-experts, so we're going to be examining idealized spectra in which the data has been simplified so as to be more understandable.
PES spectrum of lithium
Let's start with the idealized PES spectrum for lithium,
The PES spectrum shows two peaks, which represent electrons in the
This also makes sense in terms of binding energies: We know that the electrons in the
Note that the binding energy of lithium's
PES spectrum of oxygen
Next, let's consider an element with more electrons. Below is the idealized PES spectrum for oxygen,
In this spectrum, there are three peaks, each representing electrons in a different subshell of oxygen (either
We can double-check our peak assignments by considering the intensities of the three peaks: Oxygen's
Finally, notice that the peaks corresponding to the
Concept check: How many peaks would you expect in the PES spectrum of neutral calcium?
Identifying an element based on its PES spectrum
A pure sample of an unknown element was analyzed using a photoelectron spectrometer, producing the spectrum shown below. What is the identity of our mystery element?
The PES spectrum shows five peaks, which we would expect to correspond to electrons in the five subshells that are closest to the nucleus:
Which element has only one electron in its
As we already know, there are five peaks in the spectrum, which nicely corresponds to the five occupied subshells indicated in aluminum's electron configuration. The intensities of the peaks are consistent with the fact that the
Summary
- Photoelectron spectroscopy (PES) is an experimental technique used to determine the relative energies of electrons in atoms and molecules.
- Photoelectron spectrometers work by ionizing samples using high-energy radiation (such as UV or x-rays) and then measuring the kinetic energies (
- Given the energy of the incident radiation (
- A PES spectrum is a graph of photoelectron count vs. binding energy.
- The peaks in a PES spectrum correspond to electrons in different subshells of an atom. The peaks with the lowest binding energies correspond to valence electrons, while the peaks with higher binding energies correspond to core electrons.
Want to join the conversation?
Is there a substantive reason for the distinction in terminology between the work function and the binding energy? Does the difference simply exist to dinstinguish between contexts?(22 votes)
The distinction between work function and binding energy exists primarily to distinguish between different contexts and conceptual frameworks.
The work function refers specifically to the minimum energy required to emit an electron from the surface of a material into vacuum. It is considered in the context of electrons interacting with a material surface.
Binding energy, on the other hand, refers more generally to the energy required to separate constituents that are bound together. It is used in a variety of contexts beyond just electrons and material surfaces.
So the work function deals specifically with the energy required to remove an electron from the surface of a material, while binding energy is a more general term that can refer to the energy required to separate any kind of bound particles or entities.
Though numerically these values may be similar for a given system, the work function and binding energy terminology serve to clarify the conceptual framework being considered:
• Work function - focusing on electrons interacting with a material surface
• Binding energy - considering a more general interaction between any type of bound particles
So in summary, while the numerical values may coincide in some cases, the work function and binding energy concepts originate from different frameworks and serve different purposes. The distinction in terminology helps clarify whether we are focusing specifically on electron emission from a material surface or considering a more general separation of bound particles. The difference is primarily one of context and emphasis rather than substantive numerical difference.(5 votes)
In the third paragraph, how the energy analyzer "count" number of electrons correspond with one determined ionization energy ? Are there any proportion of energy to that number !(7 votes)
I don't think there can be a simple proportion of ionization energies from element to element. Remember, the Bohr model [E=(1/n^2)*13.6eV] fails beyond H and He+ so the ionization tables that this article refers to are empirical.(9 votes)
- When analyzing the binding energy of an element, does it matter what type of ionizing energy is first used to eject the electron(s) from a sample?
For example if you first bombarded a sample with XPS (according to the description under the subtitle "The basics of photoelectron spectroscopy" this would in theory eject the core electrons) would this alter the binding energy data? If the core electrons were ejected prior to the valence electrons, wouldn't shielding effects be attenuated and consequently, the binding energy of the outer-shell electrons would be increased?(5 votes)
Yes, your thinking is correct. However, we can quite safely assume that you would not hit the same atom twice during the experiment. Hence, you should not see this effect in the spectra.(5 votes)
The PES spectra show all electrons in a given shell to have the same binding energy. But that leads me to two questions on the same topic. If it were because other electrons were blocking it, than wouldn't further ionizing an already ionized sample make it easier? Or, if it had to do with the fact that protons are positive and electrons are negative, and therefore it takes more energy to remove more electrons to counteract the unequal charge, wouldn't the positive charge increase even more the moment one electron in a shell is removed, therefore increasing the binding energy for every electron rather than for every shell?(5 votes)- You are correct that after the first ionization event, the energy of the orbitals changes. However, in PES we can safely assume that you will not ionize the same atom twice. Hence, all peaks correspond to the ionization of the neutral atom.(5 votes)
I thought an electron has a huge amount of kinetic energy in the atom, I heard that it is moving at a third of the speed of light. Why is this not taken into account?(5 votes)- Electrons do have a significant amount of kinetic energy in atoms due to their high velocity. However, this kinetic energy is generally not taken into account when calculating work functions and binding energies for a few reasons:
The kinetic energy of electrons in atoms is not well-defined. Due to the Heisenberg uncertainty principle, we cannot precisely know both the position and momentum of an electron at the same time. So while we know electrons move fast, we can't pinpoint their exact location and velocity.
The important energy for work function and binding energy calculations is the potential energy, not the kinetic energy. The potential energy determines how strongly the electron is bound to the atom and how much energy is required to remove it. The kinetic energy, while large, does not reflect the strength of this binding.
When an electron is removed from an atom, its initial kinetic energy is lost and it gains new kinetic energy based on the work function or binding energy. So its final kinetic energy depends on the difference in potential energies, not its initial kinetic energy within the atom.
Work functions and binding energies are measured experimentally by seeing how much energy is required to remove an electron. This measures the change in potential energy, not any initial kinetic energy the electron may have had.
So in summary, while electrons do have significant kinetic energy due to their fast motion within atoms, this kinetic energy is generally ignored when calculating work functions and binding energies. The important factor is the change in potential energy, which determines how strongly the electron is bound and how much energy is required to remove it from the atom. The initial kinetic energy is lost during this process.(3 votes)
can you have the PES graph of an ion like Li+? If so, how would you be bale to tell the difference from the graph of an element that looks the same like He in this case?(4 votes)
I don't know the answer but from what I have read so for in this article once you eject an electron from one of its shell the binding energy changes. Lets say you would eject an the valence electron in this case the 2s electron from Li. Since the nucleus of Li has three protons and Helium has two 1s^2 electrons the binding energy for the remaining 1s^2 for Li+ would be different compared to the binding energy of the 1s^2 from He. I would belive this to be the case since the effective charge of the nucleus is not the same as for He. So I would believe if one could have a PES graph for ion like Li+ it would be different from the graph of He.
Could it be so that the the binding energy would by shifted by som factor for the two peaks compared with the peaks of He? Lets say He has it 1s^2 peak at 100 MJ/mol would it be the case then that Li+ (assuming that the 2s electron was ejected) would have it lets say a PES graph of peak of 130 MJ/mol. The 130 is just arbitrary I'm thinkin of som factor a>1 => 100*a is the new peak.(4 votes)
I'm curious - is there a way that a travelling electron can further increase its kinetic energy? For instance, if it was hit by another photon or traveled next to a negative charge. Or will it maintain the energy once got knocked off, until affected by other charges..(3 votes)
You can increase the kinetic energy of charged subatomic particles by accelerating them through an electric field. This is the basis of particle accelerators.
I don't think a travelling electron can absorb energy from a photon - at least, I've never heard of that happening but it might be a question better answered by a physicist.(3 votes)
- I have a question that's been bothering me for a while...
We know that ionization energy on each peak is the energy needed to remove an electron from a particular subshell. Bearing this in mind, do all electrons on a subshell eject when you exposed it to the ionization energy? For example, would all 4 of Oxygen's valence electrons eject if you expose it to 1.31 MJ/mol?(4 votes) - It states that, "However, the binding energy of the
1s peak is not equal to the second ionization energy of lithium. Once the first electron is removed from lithium, the 1s, electrons will be held even more tightly by the nucleus, increasing the binding energy of these electrons."
So it's saying that the second ionization energy is not equal to the second binding energy, in the sense that when you ionize it initially it changes the electron configuration making the electrons closer to the nucleus less shielded by the outer electrons. However, I don't see the difference between sequentially removing an electron from an atom and measuring the electron ionization energy to photoelectron spectroscopy's binding energy.
Both of those methods remove electrons sequentially and measure their energies, so why can't you determine the second ionization energy from a photoelectron spectroscopy chart?(2 votes)- at first glance, it may seem like photoelectron spectroscopy binding energies should correspond directly to ionization energies. However, there are a few key differences:
Ionization energies measure the energy required to remove an electron from a neutral atom, leaving an ion. Binding energies, on the other hand, measure the energy of electrons within an already ionized atom.
As you noted, removing the first electron changes the electron configuration, pulling the remaining electrons closer to the nucleus and increasing their binding energy. This effect is not accounted for in ionization energy measurements, which consider the removal of each electron independently from the neutral atom.
Ionization energies refer specifically to the process of electron removal - ejecting an electron into vacuum. Binding energies, however, reflect the energy of the remaining electrons within the ionized atom, not just the energy required to remove them.
Experimental measurements of ionization energies and binding energies involve different processes. Ionization energies are measured by exposing atoms to radiation or electric fields and detecting the ejected electrons. Binding energies are determined from the kinetic energies of photoelectrons ejected from ionized atoms.(4 votes)
- what is first ionization energy and second ionization energy ?(1 vote)
Ionization energy is energy required to remove the outer most electron from an atom. The first ionization energy would be the energy required to remove the first electron, the second ionization energy would be the energy to remove the second electron, and so on. The reason it's called ionization is that when you remove an electron from a neutral atom, it becomes a positively charged ion, or cation, having lost a negatively charged electron. Hope that helps.(4 votes)