Why do Eu doped phosphors show fluorescence property without singlet?

[pallab]

As it is mentioned fluorescence is a singlet to singlet transition and this is the reason that fluorescence is a fast process. now consider the Eu doped phosphor material where 5D0--->7F2 and other transitions show the prominent intensity peaks in down-conversion process. those are not singlet to singlet transition but yet Eu doped phosphor material shows fluorescence. Why?

 

[HAYAO]

There are some multiple stages of misunderstanding you have, which makes it hard for me to know where to start.

No, trivalent Eu (which is what I am assuming you are talking about, not divalent) does not show "fluorescence".

But it's kinda hard to say of lanthanide transition as "fluorescence" or "phosporescence". First of all, spin-orbit coupling of lanthanides are fairly strong, so the "spin-ness" and "orbital-ness" is muddled here since they are mixed. As such is not really easy to say fluorescence (transition between same spin states) or phosphorescence (transition between different spin states) in lanthanides. I generally say just "luminescence". I have seen reports saying "phosphorescence", but I have never seen anyone say of lanthanide luminescence as "fluorescence".

Also, I don't like how you use the word "down-conversion". In a broad sense, almost all luminescence is a down-conversion, but most people use the term when multiple emission occurs (e.g. sequential emission, quantum cutting).

Trivalent lanthanides primarily show luminescence based on 4f-4f transition. This is a Laporte forbidden transition, so it's not a fast process at all. ⁵D₀ --> ⁷F_J transition in total only has radiative rate constant of around 400 - 1000 s-1. To say that is fast is a massive understatement considering fluorescence has somewhere around 10⁹ s^-1.

 

[pallab]

There are some multiple stages of misunderstanding you have, which makes it hard for me to know where to start.

No, trivalent Eu (which is what I am assuming you are talking about, not divalent) does not show "fluorescence".

But it's kinda hard to say of lanthanide transition as "fluorescence" or "phosporescence". First of all, spin-orbit coupling of lanthanides are fairly strong, so the "spin-ness" and "orbital-ness" is muddled here since they are mixed. As such is not really easy to say fluorescence (transition between same spin states) or phosphorescence (transition between different spin states) in lanthanides. I generally say just "luminescence". I have seen reports saying "phosphorescence", but I have never seen anyone say of lanthanide luminescence as "fluorescence".

Also, I don't like how you use the word "down-conversion". In a broad sense, almost all luminescence is a down-conversion, but most people use the term when multiple emission occurs (e.g. sequential emission, quantum cutting).

Trivalent lanthanides primarily show luminescence based on 4f-4f transition. This is a Laporte forbidden transition, so it's not a fast process at all. ⁵D₀ --> ⁷F_J transition in total only has radiative rate constant of around 400 - 1000 s-1. To say that is fast is a massive understatement considering fluorescence has somewhere around 10⁹ s^-1.

Thanks for your reply

 

[pallab]

There are some multiple stages of misunderstanding you have, which makes it hard for me to know where to start.

No, trivalent Eu (which is what I am assuming you are talking about, not divalent) does not show "fluorescence".

But it's kinda hard to say of lanthanide transition as "fluorescence" or "phosporescence". First of all, spin-orbit coupling of lanthanides are fairly strong, so the "spin-ness" and "orbital-ness" is muddled here since they are mixed. As such is not really easy to say fluorescence (transition between same spin states) or phosphorescence (transition between different spin states) in lanthanides. I generally say just "luminescence". I have seen reports saying "phosphorescence", but I have never seen anyone say of lanthanide luminescence as "fluorescence".

Also, I don't like how you use the word "down-conversion". In a broad sense, almost all luminescence is a down-conversion, but most people use the term when multiple emission occurs (e.g. sequential emission, quantum cutting).

Trivalent lanthanides primarily show luminescence based on 4f-4f transition. This is a Laporte forbidden transition, so it's not a fast process at all. ⁵D₀ --> ⁷F_J transition in total only has radiative rate constant of around 400 - 1000 s-1. To say that is fast is a massive understatement considering fluorescence has somewhere around 10⁹ s^-1.

I have seen that Eu3+/Yb3+ doped phosphor emitting reddish light while excited with NIR laser. and it stopped emission immediately the laser turned off. So my thought was that it is a fluorescence. though for the 5D0--->7F2 transition it was excited with UV light, not with NIR laser. then I studied the singlet state-triplet state involvement and the question arose which has been posted.

 

[HAYAO]

What did you major in college and what is your major now? It will help me write an answer that best suits your level of knowledge on this subject.

 

[pallab]

What did you major in college and what is your major now? It will help me write an answer that best suits your level of knowledge on this subject.

physics

 

[HAYAO]

physics

Could you be more specific? You are doing your masters as far as I can see from your profile. You are in a graduate school, which means you are in a lab. What is your research area? It sounds like you are clearly clueless about luminescence so much that you don't seem to understand my answer. I want to know your background so that I can write an answer that you can comprehend.

I have seen that Eu3+/Yb3+ doped phosphor emitting reddish light while excited with NIR laser. and it stopped emission immediately the laser turned off. So my thought was that it is a fluorescence. though for the 5D0--->7F2 transition it was excited with UV light, not with NIR laser. then I studied the singlet state-triplet state involvement and the question arose which has been posted.

This is wrong in so many different stages. What is the typical emission lifetime of 5D0 state? A quick search will get you the answer (and my previous answer should've answer it as well), and that will answer why "emission stopped immediately", and no, that's not how we identify fluorescence and phosphorescence. Also, excitation with NIR laser means you attempted upconversion, while as for UV light you most likely excited the charge transfer band of Eu3+ (unless you use a laser, you usually can't visibly see an emission because of the small absorption cross-section of 4f-4f transition, so you instead excited the CT band. This is the principle of red portion of emission of the fluorescent lamps). Also, how is singlet and triplet involved here? 5D0 is quintet in purely LS terms and 7FJs are septet. And even that is irrelevant to lanthanides because of spin-orbit coupling.

 

[pallab]

Could you be more specific? You are doing your masters as far as I can see from your profile. You are in a graduate school, which means you are in a lab. What is your research area? It sounds like you are clearly clueless about luminescence so much that you don't seem to understand my answer. I want to know your background so that I can write an answer that you can comprehend.

This is wrong in so many different stages. What is the typical emission lifetime of 5D0 state? A quick search will get you the answer (and my previous answer should've answer it as well), and that will answer why "emission stopped immediately", and no, that's not how we identify fluorescence and phosphorescence. Also, excitation with NIR laser means you attempted upconversion, while as for UV light you most likely excited the charge transfer band of Eu3+ (unless you use a laser, you usually can't visibly see an emission because of the small absorption cross-section of 4f-4f transition, so you instead excited the CT band. This is the principle of red portion of emission of the fluorescent lamps). Also, how is singlet and triplet involved here? 5D0 is quintet in purely LS terms and 7FJs are septet. And even that is irrelevant to lanthanides because of spin-orbit coupling.

Project: spectroscopic studies on Eu3+/Yb3+ doped Y2O3 phosphors
Yes, I am clueless but I am trying to fathom out the physics behind this luminescence.
what I was considering as true for this luminescence, faced a challenge to the result of UV light excitation

 

[HAYAO]

Project: spectroscopic studies on Eu3+/Yb3+ doped Y2O3 phosphors
Yes, I am clueless but I am trying to fathom out the physics behind this luminescence.
what I was considering as true for this luminescence, faced a challenge to the result of UV light excitation

Y₂O₃. I see, so you are working on one of the most common lanthanide-based phosphors. They are used in fluorescent lamps by doping Eu³⁺ (red), Tb³⁺(green), and Ce³⁺(blue) to produce white light. These lanthanide ions absorb in the UV region by CTS band (Eu³⁺) and 4f5d band (Tb³⁺ and Ce³⁺). The emission is ⁵D₀ --> ⁷F₂ (Eu³⁺ red) and ⁵D₄ --> ⁷F₅ (Tb³⁺ green). But Ce³⁺ is an exception since its blue emission is also 4f5d transition (also, it's fluorescence unlike Eu³⁺ and Tb³⁺).

Human eyes only have detection frequency up to 60 frames per second, under the circumstances that you are looking at a flashing light source. That means no less than 16.7 millisecond of light intensity changes can be detected by the human eye (and even longer if you are talking about one single pulse of light). Emission lifetime of Eu3+ ( ⁵D₀ --> ⁷F_J transition ) in Y2O3 is somewhere around 1 - 3 ms (depending on how they are prepared). So if you turn off the excitation light, the emission of Eu3+ appears to human eye as if it instantaneously stops. In another words, you can't tell the difference between fluorescence and phosphorescence by human eyes (there are some minor exceptions where some organic phosphors show phosphorescence lifetime of over few seconds).

If you are working on Eu³⁺/Yb³⁺, my speculation is that you are working on its upconversion properties. By using NIR laser, you are exciting multiple Yb³⁺ ions in hope that some of them are going to sequentially transfer its energy to Eu³⁺ ion, and exciting it to higher energy level so that it will emit. Basically, you are converting NIR light (lower energy) into visible light (higher energy), hence the term "upconversion".

There are literally hundreds and thousands of upconversion articles out there, and even more about spectroscopic properties of Eu³⁺ and Yb³⁺ ions. Their quantum mechanical spectroscopic theory was developed by Judd (Phys. Rev. 1962) and Ofelt (J. Chem. Phys. 1962) separately, and they are widely known as the Judd-Ofelt theory. No one working on the optical properties of rare-Earth's should be oblivious to the existence of this theory, so I highly suggest you read them. A less technical but well summarized article of the theory is available by Hehlen (J. Lumin. 2013).

You also might want to check out textbooks for basic spectroscopy and photoluminescence. Also, you should grab an inorganic chemistry textbook and condensed matter physics textbook that covers crystal field theory. Crystal field theory is crucial in understanding lanthanide luminescence, and will also help you understand the Judd-Ofelt theory since that's what the theory bases itself on. Also, Judd-Ofelt theory is a semi-empirical quantum mechanical theory based on spherical tensor techniques. Therefore, you should also refer to any textbook that concerns atomic spectra. Since you are a physics major, a strong quantum mechanics background will greatly help you in understanding them.

Good luck.