CEE 540 Spring Term 2012

1. CEE 540 Spring Term 2012 • go over syllabus and course requirements • project • spectra • lab demo: • parts of a spectrograph • image in focal plane • hg spectral emission lines • varying entrance slit width • pixelation of the detector • widths of spectral lines • integration time • spectral resolution

2. low pressure mercury

4. lab demo

5. we will spend a lot of time talking about spectra – the use of molecular spectral absorbance is a common theme in instrumentation designed to measure molecular concentrations – e.g. 515 EPA instrumentation. we will start out with a very brief introduction to molecular spectra and then get into the instrumentatioin

7. red shows radiation from a black body at 5700K for the Sun and 288K for Earth vertical scales for Sun and Earth are not the same ! how important is the 7% UV from the Sun? the solar and earth spectra

8. Solar spectrum at top of atmosphere and ground level why are TOA, BB, and sea level different? What are the bumps and wiggles in the real solar spectrum??

9. solar + Earth spectrum : measurement made from the ground through the atmosphere sun how do you distinguish spectral lines in the Sun from those in the atmosphere when observing the sky or direct Sun with a spectrometer? sun sun sun

10. history of spectra • physical basis of spectra – molecular and atomic • 2-level atom • spectral line profile – transitions should be monochromatic??

11. why does a spectral line have width? • lifetime • collisions • pressure broadening • temperature broadening - Doppler

12. Molecular transitions  emission/absorption molecules Electronic (A  B) Vibrational (n’’  n’) Rotational (J’’  J’) Radiative transitions to/from various energy levels

13. HCl rotational spectrum

14. rotational structure of HBr

15. spectrum of the O3 molecule at T = 0°C

16. lab testing of the OMI space instrument prior to launch in 2004 sky spectrum comparedto laboratory NO2 photoabsorption cross section the correlation of the observed spectrum to NO2 is clear

17. spectrum of the Sun + Earth as measured from ground at KPNO think about spectral resolution. The ability to distinguish colors with your instrument. What is the spectral resolution of your eye? Can you see spectral features in the Sun when you look at it? What happens to spectral resolution when you put on sunglasses? Newton experimented with prisms to disperse solar light into colors. The spectral features (lines) were first observations in the early 1800’s and were not understood. What would the spectral shown to the left here look like with an instrument that could barely separate colors? These data were taken with a very high resolution system at Kitt Peak. Blow up!

21. solar spectral lines are wide and Earth spectral lines are narrow – why?

22. difference between spectral SAMPLING and spectral RESOLUTION

23. absorbance

24. absorbance

25. absorbance

26. absorbance

27. absorbance

28. absorbance

29. another example – an atmospheric absorption spectrum sampling affects the definition of the spectrum, but not the spectral resolution what is spectral resolution?

34. now take a bunch of spectral lines which are viewed with a spectrograph of infinite resolving power – it can see the absorption spectrum in infinite detail

35. now look at this infinite resolution set of spectral features with a real spectrograph of spectral resolution 1 nm

36. inf 1 nm

37. 1 nm 8192 pixels 1 nm 2048 pixels

38. 1 nm 8192 pixels 1 nm 2048 pixels

39. 1 nm 8192 pixels 1 nm 512 pixels

40. not feasible 1 nm 8192 pix NASA MFDOAS 1 nm 2048 pix OMI 1 nm 512 pix

41. spectrum (cross section) of the NO2 molecule at T = +20°C

42. spectrum of the NO2 molecule at T = +20°C

43. spectrum of the NO2 molecule at T = +20°C

45. spectrometers and spectrographs

47. terms: • spectral range [nm] • spectral resolution [nm] • spectral coverage [nm] • spectral sampling [pixels] • angle of grating • focal length • f/ • grating blaze angle • s/n • throughput • expected signal level vs other parameters (e.g. spectral coverage) • polarization

49. basic components: • light source • slit • collimator mirror • disperser • camera mirror • focal plane detector