[10] Technische Universität Ilmenau, Praktikum Physikalische Chemie II: Physikalische Chemie/Mikroreaktionstechnik, Versuch Fluoreszenz-Quenching(in German), tu-ilmenau.de. (accessed January 27, 2017)

Generally, all kinds of luminescence are based on so-called photo-physical processes[4]. Usually, molecules themselves are described as fluorescent. This is the case with fluorescent dyes like fluorescein or curcumin [1,5]. However, to explain photo-physical processes, one has to take a closer look at an even smaller level than the molecular one.

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I was wondering before I purchase “firefly” earrings that have phosphorescent particles which are inside the glass beads would be safe for my granddaughter who is 7 to wear? I think she would like them but I was concerned about the phosphorescent particles.

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There are a couple of significant advantages to using a prime lenses. First of all, prime lenses typically deliver much sharper, clearer images than comparable zoom lenses. The reason for this improved performance is simply that prime lenses contain fewer glass elements than zoom lenses. Every glass element has an effect on the light passing through it, and so the less elements the better. For example, the Nikon AF-S Nikkor 50mm f/1.8G lens contains just 7 elements, whereas the AF-S DX Nikkor 18-200mm f/3.5-5.6G lens contains 16 elements. And indeed, the sharpness score for the 50mm lens is much higher, while distortion and chromatic aberration scores are lower.

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Brilliant presentation. I collect fluorescent minerals from Franklin, New Jersey, and I’ve always wondered the mechanism of fluorescence. This article explain it well. But how do activators, like manganese, impart fluorescence?

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Different energetic states of an atom or molecule are known as “energy levels”. Depending on the molecule and atom, the electrons can only occupy discrete energy levels since the energy is quantized, which means, energy can only be absorbed and emitted in certain amounts [8]. The difference between two levels can be calculated with equation 1 (where E2 is the higher energy level and E1 the lower one).

Such electronically excited states are unstable. Electrons drop back to their ground states. At the same time, the excitation energy is released again. One distinguishes between radiative and non-radiative decay processes. Most of the time, the decay is non-radiative, for example through vibrational relaxation, quenching with surrounding molecules, or internal conversion (IC) [6,7,10]. These processes will be explained in detail later.

A Polarizer filter corrects this problem producing deep, dramatically blue skies. It also removes glare from non-metallic surfaces, such as windows and water.

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Even though this text focuses on photoluminescence, the photo-physical processes are the same for all types of luminescence [4].

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The bokeh effect is actually caused by a shallow depth-of-field—a narrowing of the space within which things are sharp and in focus. The bokeh effect is probably the number one reason that photographers choose to use prime lenses. Portrait and wedding photographers in particular love the artistic quality that a shallow depth of field provides, and so they regularly choose prime lenses as their go-to. The downside of using a shallow depth of field is that the images are usually less sharp than those taken at narrower apertures. Also, it can be more difficult to nail your focus when the depth of field is narrow so you might find that you get less usable shots overall from your shoot.

Excitation happens in the same way as in fluorescence, namely through electromagnetic radiation. The release of energy through vibrational relaxation and internal conversion while maintaining the same spin is the same here, as well, but only until the S1 state is reached.

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To compare the photo-physical processes behind both phenomena, there are some facts about electrons that are helpful for understanding: Electrons are particles that have a so-called spin and a spin quantum number. This can have two different values, namely either +1/2 or –1/2 [6]. This number is a property that we actually cannot imagine or describe easily. It is often compared with a spinning top, either spinning in a clockwise or anti-clockwise direction. However, this description is neither mathematically nor physically quite correct. Two electrons in a single orbital of an atom have antiparallel spin, which is noted as (↑↓) [6,12].

The spin-allowed and -forbidden processes serve as explanations for an immediately ceasing glow of fluorescence and for the afterglow of phosphorescence. Phosphorescence usually occurs only with “heavier” molecules since the spin has to be reversed with the help of spin-orbit-coupling. Whether electromagnetic radiation is emitted at all, and with which wavelength, depends on how much energy can be released beforehand by non-radiative decay [6,7]. It also depends on the properties of so-called quenchers that are surrounding molecules and are able to take up larger amounts of energy.

A prime lens is a lens that has a single, fixed focal length, as opposed to a range of focal lengths like a zoom lens. For example, many photographers like to use a 50mm lens as a good general-purpose lens. Street photographers often favor a 35mm lens as it allows them a wider view. Portrait photographers often use an 85mm lens which is flattering to the human face. While there are a number of advantages to prime lenses, the downside is that you have to physically move yourself closer to, or further from your subject in order to reframe your shot. Many people prefer to use zoom lenses because they are useful in a wide variety of scenarios.

The plastic spider and the compound in the small tube are examples for phosphorescence. Strontium aluminate, which is contained in the tube, is initially excited by UV radiation and eventually emits green light. The cause for this is a doping with elements like europium, which makes the compound usable as a luminescent pigment [15].

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In general, the glow that occurs in all the aforementioned phenomena is called luminescence. Luminescence is energy released by a substance in the form of light [1,2]. Several types of luminescence can be differentiated:

Photons, particles of which electromagnetic radiation or light consists, have to have a certain energy value to be able to excite electrons. The energy of a photon can be calculated with equation 2, where h is the Planck constant and ν is the frequency of the light.

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All processes that can lead to an inhibition of radiative decays can cause fluorescence quenching. Examples are non-radiative decay processes, but also the destruction of the fluorescent molecule [10]. The quantum efficiency describes the efficiency of the process and is defined as the ratio of absorbed and emitted photons [13]. This property is different for each substance.

Bending the glow stick (at the right-hand side) initiates a chemical reaction between hydrogen peroxide and a dye and phenyl oxalate. Chemiluminescence can be observed.

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Have you ever wondered why glow sticks (see Fig. 1) are glowing after you have bent them? Or why those plastic stars are still glowing at night after the lights have been switched off? You may not have. However, whether you have asked yourself those questions or not, the following text will hopefully provide you with answers and explanations of the phenomena of glowing bracelets, shining jellyfish, black light, and many more.

The electrons are lifted from the ground state S0, for example, to an energy level of the second excited state S2, when excited by electromagnetic radiation. After excitation stops, the electrons only stay in that excited state for a short period of time (ca. 10–15 s) and then immediately start falling back down into the ground state [6]. In doing so, energy initially can be released to the surroundings by vibrational relaxation. That means thermal energy is released by the motion of the atom or molecule until the lowest level of the second excited state is reached.

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[1] K. Arnold et al., Chemie Oberstufe (in German), Cornelsen Schulverlage, Berlin, 2015, 496-497. ISBN: 978-3-06-011179-4

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Theoretically, the electrons could relax even further in a non-radiative way until they eventually reach the ground state again. However, it can be the case that the last amount of energy is too large to be released to the surroundings because the surrounding molecules cannot absorb this much energy. Then, fluorescence occurs, which leads to an emission of photons possessing a certain wavelength. The emission lasts only until the electrons are back in the ground state. Since during all those transitions the electron spin is kept the same, they are described as spin-allowed [6,7,10].

Both fluorescence and phosphorescence are spontaneous emissions of electromagnetic radiation. The difference is that the glow of fluorescence stops right after the source of excitatory radiation is switched off, whereas for phosphorescence, an afterglow with durations of fractions of a second up to hours can occur [6,7].

In addition to products like glow sticks, fluorescence and phosphorescence are used in many other ways. Further examples are guideposts leading to an emergency exit that need no electric supply but glow at night due to phosphorescence. Even plants can be made fluorescent: Spinach can be modified with the help of nanotechnology so that it can detect traces of explosive substances in the groundwater. The leaves contain carbon nanotubes to which nitroaromatics can bond. If they do, a fluorescent signal is released by the plant and can be detected with infrared cameras [14].

Sometimes, a radiative decay can occur in form of fluorescence and phosphorescence. The energy is emitted as electromagnetic radiation or photons. The emitted light has a longer wavelength and a lower energy than the absorbed light because a part of the energy has already been released in a non-radiative decay process [10]. This is the reason that an emission in the visible spectrum can be achieved by excitation with non-visible UV-radiation. This shift towards a longer wavelength is called Stokes shift [11].

Alongside the singlet states, a triplet state exists and so-called intersystem crossing (ISC) can occur since the T1 state is energetically more favorable than the S1 state. This crossing, like internal conversion, is an electronic transition between two excited states. But contrary to internal conversion, ISC is associated with a spin reversal from singlet to triplet. Electrons in the triplet state have parallel spins, which is noted as (↑↑) [6]. This ISC process is described as “spin-forbidden”. It is not completely impossible – due to a phenomenon called “spin-orbit coupling” – however, it is rather unlikely [7].

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[6] P. W. Atkins, J. de Paula, Kurzlehrbuch Physikalische Chemie (in German), Wiley-VCH, Weinheim, 2008, 853ff., 921ff. ISBN: 978-3-527-31807-0

The necessary excitation energy for the electrons equals the difference between the energy levels. Only light with a certain energy, and accordingly with a certain frequency and wavelength, is capable of exciting electrons [3]. By equalizing equations 1 and 2, and with the help of equation 3 (where c stands for the speed of light), the necessary frequency and wavelength can be calculated (see eq. 4) [9]. In many cases, UV-radiation is used for excitation.

In the T1 state, non-radiative decay is possible as well. However, a transition between the lowest energy level of the triplet state and the S0 state is not readily possible, because that transition is spin-forbidden, too. Still, it can happen anyway with a small possibility. It causes a rather weak emission of photons because the electron spin has to be reversed again. The energy is trapped in this state for a while and can only be released slowly [6]. After all energy has been released, the electrons are back in the ground state [6,7,10].

Atoms of different elements have a different number of electrons distributed into several shells and orbitals. Electrons are a type of elementary particle. Electronic transitions are responsible for luminescence [6,7]. When the system absorbs energy, electrons are excited and are lifted into a higher energetic state. Before excitation, in the ground state, some of the electrons are in the so-called HOMO (Highest Occupied Molecular Orbital). After they reach an excited state, they are in the LUMO (Lowest Unoccupied Molecular Orbital) [1] (see Fig. 2). How this works exactly will be explained using photoluminescence as a specific example.

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In the Jablonski diagram for fluorescence (see Fig. 3), the singlet spin state S0 is the ground state of the electrons, and S1 and S2 are singlet excited states (the states are only used as an example in this text and do not necessarily apply to certain atoms, molecules, etc.). Within those states, there are several energy levels. The higher the level is, the more energy an electron possesses when being in that level. In the case of singlet states, the electrons have antiparallel spins.

The video demonstrates different types of luminescence. On the left-hand side, it shows the fluorescence of the dye curcumin, which is contained in the spice turmeric, under UV light [5,15]. Curcumin is dissolved in alcohol to make the fluorescence visible.

The bigger gap between the second and first excited state is overcome by internal conversion. That describes an electronic transition between two states while the spin of electrons is maintained. Now, the electrons can relax further due to more vibrational relaxation until they reach the lowest energy level of the S1 state.

[3] D. Wiechoczek, Wenn Mineralien selber leuchten – Phosphoreszenz, Fluoreszenz und Lumineszenz (in German), chemieunterricht.de 2010. (accessed January 25, 2017)

For phosphorescence, things are a bit different (see Fig. 4). There are again an S0 ground state and the two excited states, S1 and S2. Additionally, there is an excited triplet T1 state which lies energetically between the S0 and S1 state. The electrons again have antiparallel spins in the ground state.

The second major advantage of a prime lens over a zoom lens is that a prime lens usually has a much wider maximum aperture than a comparable zoom lens. Looking at the Nikkor lenses mentioned above, you can see that the 50mm has a maximum aperture of f/1.8. The 18-200mm lens has a maximum aperture between f/3.5 and f/5.6. The significance of a larger maximum aperture is two-fold. Firstly, it gives you improved low-light performance, as that wider aperture essentially allows more light into the camera. Secondly, it allows you to create the so-called ‘bokeh effect’ which blurs out the background in your images and makes your subject really pop.

[14] Spinat erkennt Sprengstoff im Grundwasser (in German), Spiegel Online, November 1, 2016. (accessed January 27, 2017)