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 Post subject: Ruby Excitation Wavelengths
PostPosted: Sat Oct 30, 2010 6:55 pm 
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Hello all,

This evening I have a rather straightforward question that generates more noise than data for me on a web search. I am hoping you can help.

I need to know which VISIBLE wavelengths (nm) of light DO NOT excite ruby to fluoresce.

Thank you,
Keith


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 Post subject: Re: Ruby Excitation Wavelengths
PostPosted: Sat Oct 30, 2010 7:40 pm 
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Hm, I don't remember exactly, and have no possibility to check it at the moment, but you might want to try looking in a book on general physics or chemistry. I remember reading it somewhere in my school days, they took ruby for example to describe the excitation of an atom that result in electrons at different energy levels, following a certain scale as they fall towards neutral and emit visible light (which is what we perceive as fluorescence). If you can find that, then it is just to calculate which wavelenghts that hold enough energy to excitate the atom to this level, anything below this shouldn't give any fluorescence. I don't remember the name of the book, but I got the impression that it was one of those staple examples you use in teaching, so it should be in just about any book on physics or chemistry.

It is at times like this I miss Doos, he could always answer. Hope someone else can help you get to an answer quicker than my way!

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 Post subject: Re: Ruby Excitation Wavelengths
PostPosted: Sat Oct 30, 2010 9:22 pm 
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Keith.Conrey wrote:
I need to know which VISIBLE wavelengths (nm) of light DO NOT excite ruby to fluoresce.


As a first approximation, I would say those wavelengths longer than the fluorescence wavelengths of ruby—because they don't have enough energy.

As for shorter wavelengths, that could be determined by anyone having a monochrometer, but first, YOU would have to define just exactly what YOU mean by fluorescence—how much is enough. If you simply heat the stone, you can supply energy and kick some of the electrons into higher energy orbitals. Then, as it cools, the stone will fluoresce as the electrons return. However, it might not appear as visible light because your detector is not sensitive enough. So, how do you know?

How is that for confusing the isssue? :?: :?: :D :D


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 Post subject: Re: Ruby Excitation Wavelengths
PostPosted: Sun Oct 31, 2010 8:30 am 
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Don't know an exact wavelength number but Brian did some experiments with ruby fluorescence so maybe he'll chip in. As far as I know any wavelength from green upwards will excite ruby...which exact nm value of green I'm not sure...Brian?


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 Post subject: Re: Ruby Excitation Wavelengths
PostPosted: Sun Oct 31, 2010 12:07 pm 
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I am with Dr. Bill on this one. Any shorter-wavelength light has some probability for absorption and subsequent fluorescence. Thus it just becomes a question of how bright is your light source and how sensitive is your fluorescence measurement.

Let's look at it another way. Ruby has a local minimum of absorption around 470 nm, but I can still make rubies fluoresce brilliantly with a 470 nm laser.


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 Post subject: Re: Ruby Excitation Wavelengths
PostPosted: Thu Nov 04, 2010 8:44 am 
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I bought a really cheap ($1!) UV LED flashlight with 9 LEDs off ebay that was rated 390-400 nm. It causes my synthetic ruby to absolutely glow bright pink/red and my Tajik pink spinel to glow a light pink.

-Allan

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 Post subject: Re: Ruby Excitation Wavelengths
PostPosted: Thu Nov 04, 2010 6:16 pm 
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Thank you all for your responses. I did have some flashlights (with relatively short bandpass) and lasers of known wavelengths, specifically laser Violet 405nm, LED Blue 470, laser Green 532, and LED Yellow 592. Using a 645 nm longpass filter, I could clearly see the fluorescence at each of these wavelengths. (Of course, red can't be tested by this method!) That's why I was interested to know which visible wavelengths ruby fluoresced to, and how they compared in strength.
I was sent some interesting information from a university in England related to the subject which I'm looking at. The answer is not as straightforward as I thought. For example, ruby fluoresces differently to SW and LW UV, which are typically 112 nm apart, so diffferences in the visible spectrum ccould be expected as well. Ruby does appear to fluoresce across the 400 - 700 nm range, and there appears to be a type of sinusoidal relationship between that fluorescence and wavelength, but most of the details haven't been published yet and so weren't provided. Still searching....Keith


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 Post subject: Re: Ruby Excitation Wavelengths
PostPosted: Thu Nov 04, 2010 7:54 pm 
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Keith.Conrey wrote:
For example, ruby fluoresces differently to SW and LW UV, which are typically 112 nm apart, so diffferences in the visible spectrum ccould be expected as well. Ruby does appear to fluoresce across the 400 - 700 nm range, and there appears to be a type of sinusoidal relationship between that fluorescence and wavelength, but most of the details haven't been published yet and so weren't provided. Still searching....Keith


Asking how ruby fluorescence compares at different wavelengths is quite a different question than asking where it doesn't fluoresce.

Firstly, I'm not convinced that ruby fluoresces under 250nm light. A mercury-based SW lamp has a non-negligible component of light at the LW wavelength 360nm, and it may be that the ruby reaction is due only to that weaker component of the SW lamp. I have some students working on little projects next spring, and demonstrating whether or not narrow-band 250nm light causes ruby to fluoresce will be one of their tasks.

As for the visible range, to first order, fluorescence is going to be proportional to absorption (the more light absorbed, the more light that can be converted). As seen in transmission spectra posted here, strongest absorption occurs at 400nm and at 550nm, so I'd expect maximum fluorescence at these wavelengths, and decreasing fluorescence as you move away from them. Indeed, this is why a 400nm LED flashlight makes a ruby react like crazy (as mentioned by Allan - gembug in the post above), whereas a LW lamp might only make the ruby glow perceptibly.


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