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Introduction to photoelectron spectroscopy

AP.Chem:
SAP‑1 (EU)
,
SAP‑1.B (LO)
,
SAP‑1.B.1 (EK)
In the analytical technique of photoelectron spectroscopy (PES), a sample is ionized using high-energy radiation, and the kinetic energies of the ejected electrons (called photoelectrons) are measured. From this, we can determine the binding or ionization energy of each electron in the atom or molecule. The results are presented as a PES spectrum, which shows the photoelectron count on the y-axis and binding energy on the x-axis. Created by Sal Khan.

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  • female robot ada style avatar for user SarahLDeMonia
    Beginning at , what is "electron binding energy"?
    (8 votes)
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    • leaf red style avatar for user Richard
      It's essentially the energy keeping an electron bound to an atom. If we wanted to remove that electron we would have to expand that binding energy on the atom. Another term used is the ionization energy. Electron binding energy is different from nuclear binding energy. Hope this helps.
      (9 votes)
  • male robot hal style avatar for user Luke Medcalf
    Can Someone explain the concept of nuclear charge to me simply? I am having some trouble comprehending it
    (3 votes)
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    • leaf red style avatar for user Richard
      In an atom, the center is known as the nucleus which holds an atom's protons (which are positive in charge) and its neutrons (Which are neutral in charge). Surrounding this nucleus are the electrons (which are negative in charge) who orbit the nucleus. Opposite charges attract each other so the positive protons in the nucleus and the negative electrons in orbit feel a force of attraction between them. Atoms of different elements have different numbers of protons and therefore different degrees of attraction between the nucleus and the electrons. The number of protons tells us the quantity of positive charge in the nucleus and is therefore known as nuclear charge. A related and more helpful concept is known as effective nuclear charge. The electrons orbiting the atom's nucleus are of course negative in charge. Similar charges repel so any one electron feels a force of repulsion to the other electrons as they orbit. Another thing we have to understand is that electrons orbit the nucleus in layers (or shells). As you go down the rows (or periods) of the periodic table you add more shells of electrons to atoms. The outermost shell of electrons are known as the valance electrons, while all the inner shells of electrons are known as core electrons. The valance electrons still feel a force of attraction to the positive charge of the nucleus but they also have to contend with the negative charges of all the cumulative core electrons below repelling them. This difference between the force of attraction to the nucleus and the force of repulsion to core electrons that valance electrons feel is the effective nuclear charge. The formula for calculating the effective nuclear charge of an element is Zeff = Z - C. Where Zeff is the effective nuclear charge, Z is the atomic number (number of protons), and C is the number of core electrons. For example the Zeff of lithium is calculated by using Z = 3 and C = 2, so Zeff = 3 - 2 = +1. The reason we care so much about the valance electrons is because they are the electrons that do all the exciting reactions and bonding with other atoms. Knowing how tightly they are being held by the effective nuclear charge of the nucleus of an atom gives us an insight into the reactivity of elements. Hope this helps.
      (7 votes)
  • female robot amelia style avatar for user Luca Valenti
    If the PES was done by stripping electrons with X-Rays, wouldn't the binding energy already account for the rearranging of the forces in the atoms? That is, aren't the electrons going to get "stripped" from outer to inner? The last statement that Sal made at would make sense only if all the electrons are stripped at the same time. Am I correct?
    (3 votes)
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  • duskpin ultimate style avatar for user Isabel
    So in this video, Sal compares Ca and K. The more right and up an element on the periodic table is, the more electronegative it is. So Ca is more towards the right compared to K, so Ca is more electronegative, which is shown in the Photoelectron Spectrum, right? What if we compare Ca and Mg? Mg is higher, so it should be more electronegative, however, Sal did say in this video the more protons you have, the higher the pull, so Ca should attract electrons more? But Mg is the more electronegative one... can anyone help explain this to me?
    (2 votes)
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    • leaf red style avatar for user Richard
      So electronegativity is usually described as amount of attraction the protons of the nucleus have for the valence electrons. Two factors will effect this; the number of protons and the distance from the valence electrons to the nucleus. And this reason for this comes from Coulomb's Law.

      As you go from left to right in a period you increase the number of protons and thus the electronegativity of elements increases as you said.

      But as you down a group you increase the distance from the valence shell to the nucleus and the electronegativity decreases. If we're comparing calcium to the magnesium, calcium's valence shell is the 4s subshell while magnesium's valence shell is the 3s subshell. The larger the electron shell number the farther it is from the nucleus of the atom.

      Hope that helps.
      (2 votes)
  • leafers seedling style avatar for user lelaj.07
    I noticed that the number of electrons in a shell are also lined up with the relative number electron part of the diagram, will that be something that I can use as a tell when completing questions?
    (1 vote)
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    • leaf red style avatar for user Richard
      Yes and no, it depends. Yes in the sense that a photoelectron spectrum can tell you which subshell the electrons were from ( s,p,d, or f) by comparing the heights. No in the sense that they may not provide such a clean spectrum where you can easily tell the differences in height. Here we can see that the small peaks are 1/3 the height of the tall peaks. And so we can infer that the small peaks are s electrons while the tall peaks are p electrons because s subshell hold 1/3 the number of electrons as p subshells. But most real life spectra is much more messy and less clear so it may not be possible to visually identify the relative heights of the peaks. So it depends on the quality of the spectrum which is provided to you.

      Hope that helps.
      (2 votes)
  • blobby purple style avatar for user John Smith
    Is binding energy similar to the nth ionization energy?
    (1 vote)
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    • leaf red style avatar for user Richard
      Well binding energy is a general term which is used in various contexts in physics and chemistry. In it's most basic form, binding energy is the amount of energy required to liberate an individual particle from a collection of particles. We get different types of binding energies by specifying which particles we're discussing.

      We could be talking about nuclear binding energy with nuclear reactions where the particles are protons or neutrons being ejected from the nucleus of an atom. Bond dissociation energy is a type of binding energy where we remove atoms from a covalent bond. In this instance we're removing an electron from an atom which is exactly what ionization energy is. So ionization energy is one type of binding energy.

      Hope that helps.
      (2 votes)
  • male robot johnny style avatar for user Fraser Daniel
    Are Raman and FTIR considered to be photoelectron spectroscopy?
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    • leaf red style avatar for user Richard
      In short, no.

      Photoelectron spectroscopy works by supplying enough ionizing radiation to a sample in order to ionize it and liberate an electron. Ionizing radiation being high energy light. We would use this type of spectroscopy to measure ionization energies.

      Raman and FTIR spectroscopy use nonionizing radiation to make chemical bonds stretch and compress, or cause them to vibrate. These methods use lower energy light such as infrared light which do not cause electrons to be liberated. We use this type of spectroscopy to measure the strengths of chemical bonds.

      Hope that helps.
      (2 votes)
  • blobby purple style avatar for user John Smith
    If an atom has one more electron than another atom, but that electron is put into a partially filled orbital, can the binding energy go down instead of up?
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    • leaf red style avatar for user Richard
      So there's quite a lot of ambiguity with your question so it's hard to give a single answer.

      For example, an two atoms which differ by a single electron could mean two atoms of the same element where at least one is an anion. Or it could mean two neutral atoms of different elements which differ by one atomic number.

      Partially filled simply means not completely filled and could change the answer. Having a p subshell partially filled means we could have anywhere from one electron to five electrons in it (since a filled p subshell holds six electrons). But a half-filled subshell creates a certain degree of stability. So a discussion with a partially filled p subshell with two electrons versus three electrons will require discussion about that stability, and so it's not clear if you simply mean partially filled and specific type of partial filling.

      Like I've said before binding energy in this instance means ionization energy. But there are different types of ionization energies we can compare (1st, 2d, 3rd, etc.). And again it's not clear which ones you want to compare.

      You need more specifics in this question to get a proper answer.

      Hope that helps.
      (1 vote)
  • blobby green style avatar for user vivian Xia
    Why is the x-axis decrease as you go the right
    (1 vote)
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    • leaf red style avatar for user Richard
      The binding energy is inversely proportional the kinetic energy of the removed electrons. So while the binding energy increases as you go from right to left, the kinetic energy of the electrons decreases if you also go from right to left. So considering you would usually read a graph like this from left to right, scientists are prioritizing the kinetic energy of the electrons more so than the binding energy.

      Hope that helps.
      (1 vote)

Video transcript

- [Instructor] In this video, we're going to introduce ourselves to the idea of photoelectron spectroscopy. It's a way of analyzing the electron configuration of a sample of a certain type of atom. And so what you'll often see and you might see something like this on an exam, is a photoelectron spectrum that looks something like this. And so the first question is, well, what's even going on? How is this generated? Well, I'm not gonna go into the details, but the big picture is the analysis will be done by taking a stream of that atom, and so that atom, there's an atom stream going in one direction, and then the other direction, let me label this, so that's the atoms that we're trying to analyze, and then the other direction, you send high-energy photons that are going to bombard those atoms, photons. Now these photons are high enough energy, in fact, they're typically x-ray photons so that when they collide, the photons are high enough energy to overcome the binding energy of even the core electrons and as those electrons get knocked out, they move away and they enter into a magnetic field that will deflect those electrons and then make them hit a detector. And so you can imagine the electrons that are closer to the nucleus, those have the highest binding energy, and so more of that energy from the photon is going to be used to knock it off so less of it is going to be there for the kinetic energy, so those closer electrons aren't going to get as far and the outer electrons, those have the lowest electron binding energy. They're the easiest to knock off and so you have more of the photon's energy is going to be transferred into kinetic energy. And so they're going to get further away and they're going to hit the detector at a further point. And so one way to view the photoelectron spectrum is it gives you a sense of roughly how many electrons have various binding energies. And you can see that the binding energy increases as we go to the left. Now the reason why this makes sense, the binding energy is inversely proportional to how much kinetic energy these electrons have as they actually get knocked off. And so this spike on our spectrum at the extreme left, these are the innermost electrons, and then these would be electrons further out with the next lower binding energy, and then lower binding energy after that. And so we can analyze this to actually come up with the electron configuration of this mystery element right over here. What do you think that would be? Pause this video and try to think about that. Well as I mentioned, this spike right over here would correspond to detecting the innermost electrons, and so the innermost electrons are the one S electrons, and we know that those aren't the only electrons 'cause there's electrons that have lower binding energies, and so we know that would have filled up that innermost shell and so we know that they have two one S electrons and then we can then think that this next spike, that's going to be the two S electrons and we have more electrons than that so we must have filled up the two S sub shell and then this next spike, this looks like two P. And the reason why this really makes a lot of sense is notice the detector is detecting more electrons there, and we also have more electrons, and so that must have been filled and that makes sense, and actually the way this was constructed, it's not always going to be this perfect, but you can see you have roughly three times as many two P electrons as two S electrons, which makes sense. The two P sub shell can fit six electrons. Two S sub shell fits two. So this next spike is going to be the next highest energy shell, which is going to have a lower binding energy. It's easy to knock the, it's easier to knock those electrons off. And so this looks like it's going to be the three S two and then this next spike, this looks like three P six and then that one gets completely filled and we have one more spike after that and that spike seems to get roughly the same number of electrons as all of the other S sub shells and we know from the Aufbau principle that the next we fill is four S and it looks like there's two electrons there because this spike is about the same as the other filled S sub shells. And so just like that, we're able to use a photoelectron spectrum to come up with the electron configuration of this mystery element. Its electron configuration is one S two, two S two, two P six, three S two, three P six, four S two. And what element has this electron configuration? Well, we've worked on it in other videos, but I can get my periodic table of elements out, and we can see, let's see. One S two gets us to helium, then you have two S two, two P six gets us to neon. Three S two, three P six gets us to argon, and then four S two gets us to calcium. So our mystery element is calcium, and if someone were to ask about valence electrons, that would be this outermost spike right over here. The spike of electrons with the lowest binding energy. They have the lowest binding energy because they're the furthest out there. They are the easiest to knock off, and because they're the easiest to knock off, most of that photon energy is leftover after overcoming the binding energy that gets converted into kinetic energy. So those electrons get deflected further. And the base of what we see here are the photoelectron spectrum of calcium. What would we expect the photoelectron spectrum of potassium be? And just as a reminder, potassium has an atomic number of 19, so it has 19 protons in the nucleus, while calcium has 20 protons in the nucleus, and we're going to assume that we're talking a neutral potassium atom, so it's going to have 19 electrons, as well. Pause this video and think about how it might be different. When we think about potassium, it's going to have a very similar photoelectron spectrum as calcium, but because it only has 19 versus 20 protons, it has less positive charge in the nucleus, so it pulls a little bit less hard on our various shells. So in potassium, you're still going to have one S two, but it's going to have a slightly lower binding energy because it's not pulled into the nucleus as much. And I'm not drawing it perfectly. It might not be this much. Actually, you know what? It's probably more slight, probably. Something like this, but it's going to be a little bit to the right. Similarly, two S two is going to be a little bit to the right, and then two P six is going to be a little bit to the right, and once again, I'm not drawing it completely perfectly 'cause I don't have the exact data here. Three S two would be a little bit to the right. Once again, only 19 protons versus 20 for calcium, so we're pulling a little bit less inwards, so we have a lower binding energy for any given shell or sub shell, and three P six is going to be a little bit to the right, like this, and then what is the four S sub shell going to look like? Well, it doesn't have two electrons in the four S sub shell. It only has one, 'cause it only has 19 electrons and not 20. And so it's going to be a little bit to the right. It has a lower binding energy and it's only going to be half as high because you only have one electron, not two. So it's going to look something like that. That would be the photoelectron spectrum of potassium, roughly speaking. Now we've already talked about that your outermost shell shows where your valence electrons are. So if we're thinking about potassium, it would be right over there. Now that also tells us, when we're thinking about the binding energy over here, so this binding energy, that tells us how much energy do we need to remove an electron? And so when you're removing that first electron, that's your first ionization energy. Once you remove that first electron, because of all of the interactions between the electrons, your photoelectron spectrum would change so you can't think about your second or third ionization energies, but your first ionization energy, you just have to think about it's the binding energy of your outermost electrons.