Why does the effect linger with Glow in the dark?

[pinball1970]

TL;DR Summary

I would like a better understanding of the glow in the dark mechanism. Why are photons emitted slowly?

A question about Glow in the dark

Fluorescent chemicals can activated by UV, absorb energy at short wavelength then emit photons the human eye can see in the visible spectrum.

Stop the UV source, the visible light emission stop. You cannot see anything. Electron returns to non-excited state photon emitted eye detects it.

With Glow in the dark chemistry applied to textiles, stimulate with UV then stop the light source and the material glows for a few minutes.

My question is, as is with normal colour and fluorescence the effect stops immediately the light is switched off.

Why does the effect linger with Glow in the dark? Why are the emissions so much slower not instant the moment the light is switched off?

I have the read the wiki page regarding, forbidden states including,

An example is phosphorescent glow-in-the-dark materials,[2] which absorb light and form an excited state whose decay involves a spin flip, and is therefore forbidden by electric dipole transitions.

The result is emission of light slowly over minutes or hours.”

This transition state delays the release of a photon? Why does the material glow straight away? No delay? What is the difference between immediate photons and later photons?

I looked up “spin flip”

https://arxiv.org/abs/1603.02572

“Rashba effect due to a perpendicular electric field or a dielectric substrate, gives a negligible radiative decay rate (about 107 times slower than that of bright excitons).

Spin flip due to Zeeman effect in a sufficiently strong in-plane magnetic field can give a decay rate comparable to that due to the intrinsic interband spin-flip dipole.”

Is the sort of thing I should be looking at?

Intuitively in terms of a physical process is this like heating a material which releases the heat/energy slowly in line with the environment? Or is this more like radioactive decay?

 

[WernerQH]

This transition state delays the release of a photon? Why does the material glow straight away? No delay? What is the difference between immediate photons and later photons?

There is no delay. It's just exponential decay with a longer time constant.
Not the states are "forbidden", just the lines (the transitions between them).

 

[vela]

Summary: I would like a better understanding of the glow in the dark mechanism. Why are photons emitted slowly?

Intuitively in terms of a physical process is this like heating a material which releases the heat/energy slowly in line with the environment? Or is this more like radioactive decay?

It's similar to radioactive decay.

One point that's typically glossed over when first learning about quantum mechanics is that an electron in an energy eigenstate of an atom or molecule should stay there indefinitely. These are, after all, stationary states. The reason an excited atom returns to its ground state is because of interactions with the something outside the atom stimulates a transition from the excited state to a lower-energy state. If the stimulation is man-made, we call that stimulated emission. If it's just due to random effects from the environment, we call that spontaneous emission.

As with all things quantum mechanical, we can't predict when an emission will happen; we can only calculate the probability an emission will occur in a certain time interval. For glow-in-the-dark materials, the probability/time is low so light is emitted over an extended time after the light source is removed. For regular materials, the probability/time is relatively high, so the atoms and molecules seem to return to the ground state almost immediately after the external light source is turned off.

 

[Vanadium 50]

Stop the UV source, the visible light emission stop.

This is a quantitative difference, not anything fundamental. The light emission continues on for some time, but so long as it is less than a human reaction time, it appears instantaneous. But you wouldn't be able to tell a millisecond from ten milliseconds.

 

[pinball1970]

This is a quantitative difference, not anything fundamental. The light emission continues on for some time, but so long as it is less than a human reaction tiome, it appears instantaneous. But you wouldn't be able to tell a millisecond from ten milliseconds.

I have a question about that.

 

[berkeman]

I have a question about that.

Tease!

 

[pinball1970]

Tease!

Sorry. I wanted to acknowledge the comments and people taking time to get back but I need my work desk top. I'll post Monday.

 

[pinball1970]

The part I was not sure about, “intersystem crossing.”

This is the link, with that diagram below.

https://www.horiba.com/int/scientif...e-spectroscopy/what-is-the-jablonski-diagram/

“As shown in the figure, the final photo-emission transition can either occur through a fast singlet state (fluorescence) or through a slower triplet state (phosphorescence).

In conventional photoluminescence, photons are emitted at higher wavelengths (lower energy) than the wavelength of the absorbed photons.”

 

[DrClaude]

The part I was not sure about, “intersystem crossing.”

That's the key. The energy has to go into a mode that is metastable, where decay by emission of photons has a low probability, meaning a long lifetime.

Absorption and emission are symmetric processes. If it is easy for a molecule to absorb light at a given frequency, it will be easy for it to emit the light back. To get fluorescence at another wavelength or phosphorescence, there needs to be faster decay mechanisms that will change the state of the molecule before simple reemission takes place.

 

[Mister T]

My question is, as is with normal colour and fluorescence the effect stops immediately the light is switched off.

Haven't you ever seen glow in the dark watches? They've been around for about 100 years.

 

[pinball1970]

Haven't you ever seen glow in the dark watches? They've been around for about 100 years.

Yes. I wanted to know what the electrons were up to and more specifically the photons

 

[Mister T]

Yes. I wanted to know what the electrons were up to and more specifically the photons

I realize that, but you also said this ...

My question is, as is with normal colour and fluorescence the effect stops immediately the light is switched off.

My point is simply that it's not "normal" behavior for the effect to stop when the light is switched off. That's the purpose of glow-in-the-dark watches. They glow for hours once your eyes get adapted to the dark.

 

[pinball1970]

I realize that, but you also said this ...



My point is simply that it's not "normal" behavior for the effect to stop when the light is switched off. That's the purpose of glow-in-the-dark watches. They glow for hours once your eyes get adapted to the dark.

When I said normal colour I meant an apple looks red or green in daylight or artificial daylight D65 say in a light box. Turn the lights off and every thing looks grey or black, immediately.
Fluorescence dyes glow different colours under UV as do optical brightening agents for whites and they stop immediately the UV source is stopped.
GID lingers and I wanted to know the difference between the mechanisms in each.
Perhaps I should not have said 'normal' colour? I just mean the colour we see under daylight/one of the artificial daylights.

 

[Thadriel]

When I said normal colour I meant an apple looks red or green in daylight or artificial daylight D65 say in a light box. Turn the lights off and every thing looks grey or black, immediately.
Fluorescence dyes glow different colours under UV as do optical brightening agents for whites and they stop immediately the UV source is stopped.
GID lingers and I wanted to know the difference between the mechanisms in each.
Perhaps I should not have said 'normal' colour? I just mean the colour we see under daylight/one of the artificial daylights.

I think maybe what that person is getting at is that when discussing quantum mechanics, it is not good to use every day experience as the baseline for “normal.”

But apologies is that wasn’t the point. Obviously for materials that glow in the dark, glowing is quite normal behavior.

 

[pinball1970]

I think maybe what that person is getting at is that when discussing quantum mechanics, it is not good to use every day experience as the baseline for “normal.”

But apologies is that wasn’t the point. Obviously for materials that glow in the dark, glowing is quite normal behavior.

Reason I asked to see if there was any fundamental difference between the phenomena.

I took a few images to illustrate:

1/Top row D65. Coloured materials left. Cotton fabrics (white and cream) have been through the same bleach except RHS has optic.

2/Middle row UV. Fluorescent pigment and optic glow, the filter stops the effect

3/Bottom row No light. The GID application continues after stimulation.

 

[Vanadium 50]

This thread is kind of a random walk through all things fluorescent.

The original question is answered: this occurs on all sorts of timescales, including the very short and the very long.

The thread also discusses how practical long-lasting fluorescence can be achieved: you need three energy levels: low to high is fast, high to mid can be comparably fast, and mid to low is slow.

If there are more questions, it's best to be explicit about what you don't understand.

 

[berkeman]

Yeah, it looks like this is a good time to tie off this thread. Thanks for the interesting reponses everyone!