Light emission

Question 1

Describe the reasons for light emission.

Answer 1

thermal / blackbody / heat radiation:
E of the radiation from the vibrational and rotational E_kin of the particles forming the body (dependent on T)

luminescence, fluorescence, phosphorescence:
E of the radiation provided by the transition of e^- from the excited state to the ground state

Question 2

What is an emission spectrum?

Differentiate.

Answer 2

emitted light intensity (J) vs. wavelength (λ)

• continuous spectrum
• band spectrum
• line spectrum

Question 3

When can a continuous emission spectrum be observed?

Answer 3

can be observed for every material having T ≠ 0K due to emission of thermal radiation

• 0 - 900 K → infrared light
• 900 K + → emitted visible light

Question 4

When can a line emission spectrum be observed?

Answer 4

can be observed for excited atoms moving independently (e.g. low pressure gases)

→ E differences btw E levels correspond to E of emitted photons
_{(photon emitted from outer shell higher E than photon emitted from inner shell)}

Question 5

When can a band emission spectrum be observed?

Answer 5

can be observed for excited molecules (e.g. high pressure gases)

→ band spectrum = sum of slightly shifted individual spectral lines due to rotational, vibrational and electronic transitions

Question 6

How can atoms and molecules be excited?

:)

Answer 6

• heating → thermally excited (e.g. flame photometer)
• electric field (e.g. Hg vapor lamps)
• radiation (e.g. UV light)

Question 7

What is a flame photometer?

What is it used for?

Answer 7

device, in which the emission spectrum is produced after thermal excitation (flame)

→ used for the determination of alkali metal conc. of solutions

Question 8

What is a monochromator?

On what does the light intensity that is measured later by a detector depend?

Answer 8

an element, which selects a narrow wavelength range from the incident light using either

• dispersion on a prism
• diffraction on an optical grating

⇒ light intensity depends on slit width → proportional current on detector generated

Question 9

What is a spectrometer?

Answer 9

device that registers emission spectra by plotting the intensity vs. wavelength graph

Question 10

What is a spectroscope?

Answer 10

device to view emission spectra (by eye, CCD camera)

Question 11

Differentiate btw spectral analysis.

Answer 11

• qualitative: which components present in the sample
• quantitative: relative intensities investigated → concentration determination