Newbie spectroscopy physics query

[Tempus]

I wonder if someone can sort my physics understanding on this one...   

Imagine one were to heat a block of material (eg iron). As the temperature rose to say 500C, then 1000C, then 1500C etc, presumably it would radiate as a "black body" and produce a "continuous" spectrum whose precise peak would depend on the absolute temperature reached.

But where do emission lines come into this? If the material were iron (as above), does it radiate energy only in the iron emission line spectrum?? Or as a continuum as per a black body of that temperature? Or both??

I feel I am getting some pretty basic physics confused here, but just can't figure it out .

Any comments most welcome! 

[Merlin66]

As you heat it up yes, it will radiate as a Black body.....

push the temperatures up to the >5,000 deg mark then the Fe atoms will start to show spectra....you need enough energy to move electrons from one shell to the other.

Quantum Mechanics is what it's all about.

I'd recommend (for astronomical spectra) to start with Keith Robinson's " Spectrocopy : The key to the stars" He covers very well the basic's of Quantum Mechanics and how it relates to the astronomical spectra we observe.

[Tempus]

As you heat it up yes, it will radiate as a Black body.....

push the temperatures up to the >5,000 deg mark then the Fe atoms will start to show spectra....you need enough energy to move electrons from one shell to the other.

Quantum Mechanics is what it's all about.

I'd recommend (for astronomical spectra) to start with Keith Robinson's " Spectrocopy : The key to the stars" He covers very well the basic's of Quantum Mechanics and how it relates to the astronomical spectra we observe.

Ah... thanks for that Merlin66   

So, basically any emission spectra will sit on top of the underlying black body radiation. But there will be a temperature threshold - dependent on atomic structure - above which emission line photons will start to be (additionally) generated...

I'll see if I can find a second hand copy of that book by Robinson. Cheers!

[robin_astro]

Actually this is a very good question. It is not a mater of temperature. Solids in fact just show a black body curve regardless of how hot  you get them. You need a gas to produce emission lines. The reason is that in a solid the atoms are too close together which  smears out the quantum steps. There are too many interactions between the atoms  which reduces the amount of time before atoms interact. This allows many more ways to lose energy and produces a continuous spectrum.  In a gas the atoms are well separated so you get a simple discrete quantum steps that the electron jumps between before encountering another atom, producing sharp emission or absorption lines 

Cheers

Robin

[robin_astro]

Note though that you still need quantum mechanics  to explain the shape of the black body curve as classical physics predicts an "ultraviolet catastrophe"

http://en.wikipedia.org/wiki/Ultraviolet_catastrophe

you just get a continuum of possible quantum steps in a solid compared with specific discrete steps in a gas

Robin

[Merlin66]

Robin's answer is more complete....

The atoms need to be separated from their neighbours such that they can be seen to radiate as individuals - ie a spark spectrum produces emission lines from the gas plasma arising from the vaporized electrode material.

When most materials are heated beyond 3000 deg K they are effectively "evaporated" into gases.....

http://www.engineeringtoolbox.com/melting-boiling-temperatures-d_390.html

[Tempus]

Thanks for the thorough replies to my query, guys. Much appreciated. I definitely feel I've got a much clearer understanding of this basic point now.

FYI, I've just located & placed an order for that suggested spectroscopy book by Robinson. It does indeed sound like a excellent intro...!

Cheers

[robin_astro]

Even in gasses at  high  pressure, the proximity and degree of interaction between the atoms means they behave as  black bodies.  This is where the continuum we see in stellar spectra come from.  The lines we see superimposed on this black body continuum spectrum come from the less dense outer layers (photosphere and chromosphere)  The temperature in these regions does not need to be high to produce absorption  lines. Absorption lines in the visible spectrum can be seen even at temperatures below 1000k

The dramatic effect of pressure on spectra can be seen in  the spectra of low and high pressure sodium street lamps.  in the LP lamp the D lines are very narrow (<0.1A) but in the HP lamp the line is ~1000A wide giving a very different quality light

Robin