Understanding Spectroscopy: Answers to Common Questions | Lab Experiments"

[mrjeffy321]

I have two Spectroscopy questions from a "lab" I need to answer.

1. When do Mercury [gas] lights appear blue even though Mercuy also emmits yellow and green wavelengths of light?

From what I know, Mercury doesn't just emmit blue, yellow, and green light, but also a very significant proportion of ultraviolet light. Since you can't see ultraviloet light, they pressurize the bulbs of Hg gas so it shifts the favorable frequency of light emmited to blue. Does that explain for the yellow and green wavelengths too? And/or the blue light is emmited more intensly, and thus is easier to see since it carries more energy?

2. Why does the color emmited from ions in flame tests differ? {from normal atoms I suppose, the question is unclear}

This could be because ions have either gained or lost electrons (thats how they became ions), and electrons are what causes the emmition of light [ie. electrons absorb energy, then re-emmit it], if there are less/more electrons to absorb energy, the frequencies of the photons emmited might change from a normal atom.

 

[GCT]

i'm studying spectroscopy at the moment, but you'll be better off asking this question in one of the physics subforums

 

[TRCSF]

1. I'm not sure about mercury lights, but I think they're blackbody emitters. i.e. the blueish/white light is because it's emitting in all colors, especially the blue wavelengths, because it's very hot. The same reason heated metal goes from red to yellow than white as it gets hotter.

2. I think what this question is asking is why different elements emit different colors while undergoing flame ionization. It's got to do with the kernel electron energies being different in different elements.

 

[mrjeffy321]

Well I guess I will find out tomorrow.

2. I think what this question is asking is why different elements emit different colors while undergoing flame ionization. It's got to do with the kernel electron energies being different in different elements.

That is another possible interpretation of the poorly worded question that probably makes more sense.

 

[Gokul43201]

1. In regular fluorescent bulbs, mercury is at a low pressure. At these pressures, the bulk of the emitted intensity is in the UV-blue-green region rather than the yellow-orange-red region. But more importantly, the color that you see coming out of a fluorescent bulb is a function of the fluorescent material that is used to coat the bulb wall. This coating happens to have an emission peak in the blue wavelengths for a Hg vapor lamp.

Fluorescent lamps are NOT blackbody emitters. Blackbody radiation (or close to it) can be got, however, from incandescent lamps.

2. TRCSF is correct about this one.

The frequency of light emitted by the transition metal ions (in a flame test) is typically a characteristic of the energy difference between the degenerate energy (in an octahedral crystal field, which is most common, these are the $e_g$ and $t_{2g}$) levels of the valence d-orbitals. It is the electronic transitions between these levels that produces most of the visible emitted light. Naturally, the energy difference ($\Delta _0$), is a function of the electronic configuration (a strong function of the oxidation state and period number), and hence changes from one cation to another. Note that in Ba(2+), Ca(2+), K+, (and some other non-transition metal ions) there are are no such d-orbitals and in these ions the light emitted is from other electronic transitions.

Nevertheless, the moral is that the frequency of light emitted depends on the cationic energy levels (which are characteristic to the ion involved).