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

low pressure mercury

lab demo

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

the solar and earth spectra 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?

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

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

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

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

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

HCl rotational spectrum

rotational structure of HBr

spectrum of the O 3 molecule at T = 0°C

sky spectrum compared to laboratory NO 2 photoabsorption cross section the correlation of the observed spectrum to NO 2 is clear lab testing of the OMI space instrument prior to launch in 2004

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!

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

difference between spectral SAMPLING and spectral RESOLUTION

absorbance

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

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

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

inf 1 nm

8192 pixels 1 nm 2048 pixels

1 nm 8192 pixels 1 nm 2048 pixels

1 nm 8192 pixels 1 nm 512 pixels

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

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

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

spectrometers and spectrographs

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

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

diffraction gratings

diffraction grating camera mirror collimator mirror exit slit or detector pixel entrance slit

Acton Research Corp. spectrograph, model 300i, s/n , cost ~$10,000 slit: why? width variable from 10 µm to 2 mm height – 5 mm collimator mirror/lens size – 60 mm diameter mirror focal length F = 300 mm f/number = F/a = f/5 disperser grating/prism – diffraction grating lines/mm = 1800 size 68 mm x 68 mm blaze – 320 nm = 32.7° how are these made included angle  = 13.7° detector film eye photodiode photodiode array channel plate CCD – xx pixel x yy pixel

basic parameters for a spectrometer/spectrograph dispersion [nm/mm] – will compute below spectral sampling [number of detector resolution elements/slit width or for an array detector, number of detector pixels/FWHM of the spectral line profile resolution [nm] what is the importance of spectral resolution what is the effect on your instrument of increasing or decreasing spectral resolution throughput [fraction] = efficiency of the unit [photons detected/photon in] polarization [percent] – comes mainly from the diffraction grating can be a major problem when looking at polarized light what are examples of light sources that are polarized? free spectral range [nm] bandwidth overlapping orders – will talk about more when we discuss grating details grating blaze angle

other types of spectrographs

Ebert-Fastie Mounting

Rowland circle mounting

Dutch OMI instrument – launched 2004 and still operational on NASA/AURA

NASA JPL Orbiting Carbon Observatory launch April 2013

diffraction gratings

diffraction gratings and spectrographs grating equation: n = a [sin  + sin  ] where,  = angle of incidence relative to grating normal [°]  = angle of diffraction relative to the grating normal [°] n = diffraction order = think of the multiple slit problem, a grating is just a multiple slit used in reflectance, not transmission a = line spacing on the grating [mm] For a Czerny-Turner spectrograph like the one we will use in the lab:  –   where  is the angle the grating is rotated [°] from the mirror angle and  is the half angle of the angle between the center of the grating and the 2 mirrors

substituting into the grating equation and doing the arithmetic n = 2a sin  cos  [for Czerny-Turner/Ebert types only – never ever use this eq. on another type of spectrograph – go back to the basic grating equation] dispersion [number of wavelength units per physical dimension at the focal plane] n = a [sin  + sin  ] simple differentiation   /  = [a cos  ]/n   =  x / F   /  x = [a cos  ] / (Fn) which is now the linear dispersion in the focal plane [nm/mm] xx  F

for the Acton spectrograph: a = [1/1800] mm = 556 nm actual separation of the grating rulings = 400 nm   = 35.4° from the grating equation n = 2a sin  cos  F = 300 mm n = 1   /  x = 1.51 nm/mm e.g. compute spectrograph entrance slit size for 0.1 nm spectral resolution at 400 nm wavelength in the Acton spectrograph focal plane: 1.51 nm/mm dispersion  0.15 mm slit size = 150 µm

what spectrograph parameters determine the resolution of the instrument  /  x = [a cos  ] / (Fn) size of entrance slit slit on our NASA instrument is about 100µm = 0.82 nm spectral can go to perhaps 10µm as smallest easily achievable size remember the amount of light going to the detector changes linearly with size. Small slit  low light  low s/n big slits  poor spectral resolution big time tradeoff between these two items focal length of camera mirror longer focal length  higher dispersion, higher resolution, lower s/n Acton is 300mm, a nice size the OH spectrograph is 2m focal length – huge – with required spectral resolution nm grating spectral order increasing “n” gives more resolution, but overlapping orders are problem  (e.g.) 1 x 400 nm is the same grating angle as 2 x 200 nm so both 200 nm radiation and 400nm radiation is falling simultaneously onto detector n = 1 gives no overlapping orders, but also the lowest resolution story about Elmo Brunner and OSO

difraction angle – can’t make too big and fit in box grating groove density finest gratings are about 5000 g/mm fine gratings are extremely expensive normal gratings are inexpensively available with groove densities of ~ g/mm The SCIENCE determines spectral resolution needed. Figure it out and see if you can do the science with a real instrument. Higher spectral resolution  fewer nm onto your detector  lose information lower spectral resolution  more nm onto your detector, but poorer ability to distinguish spectral features

types of detectors

silicon photodiod