Chem 524

Chem 524-- Outline (Sect. 6) - 2005

FOR A PDF VERSION OF THIS SET OF NOTESWITH EMBEDDED FIGURES CLICK HERE (this may not be available for download until Oct.??.)

IV. Wavelength discriminators (text Ch. 3.5 )

A. Monochromators work by dispersing wavelength, l, in space

1. Prism - dispersion of wavelengths due to refractive index, n, dispersion,

· material dependent--all index, n, values increase as l go to uv, each has different penetration of uv,

· very non-linear — fast change in uv, slow in IR (poor l separation) - not a simple function of l

· monochromator — collimate beam in, focus output (f is focal length) onto dispersed detector (film)

· or rotate prism to focus different l on slit, whose width DS gives Dl or resolution--bandpass

· angular dispersion: linear separation:

· linear disperson:

· uses

—for uv (higher throughput/good dispersion--e.g. CD spectrometer)

--inexpensive predisperser

--Laser — low loss, freq. select l (Pellin-Brocca) with no beam angle change

2. Grating transmission or reflection-- diffraction cause interference for different l : (picture)

d (sin a + sin b) = ml , m = 0, ±1, ±2, … (order) –this is critical equation
orders need to be sorted out, reason for prism predisperser or filter system:
free spectral range, extent of wavelength range before interfere with a higher order --
zero order (top fig) à a = -b here no solution or all solutions, grating acts like mirror
Blaze — maximum l efficiency: l_b = d sinb (defined for a = 0, Littrow condition)

-- most useful (2/3 l_Bà 3/2 l_B) , cut-off l>2d no diffraction

-- polarized, ^ to grooves more to red, || more to blue,

--annomalies occur as function of l blaze, extreme polarization

Dispersion

-- closer spaced grooves (small d) — more dispersion

-- higher order, larger |m|, more dispersion, also small diffraction angles better (cosb)

-- linear dispersion:

-- Sine bar drive (e.g. Czerny Turner, f-fixed, l~sinq): d (sina + sinb) = 2d (sinq sinf) = ml

i.e.: l ~ sin q --normal design, turn screw move nut (linear motion) coupled to arm that rotates (q) grating, so motion creates sinq and is proportional to l

--practical: reciprocal linear dispersion:

--spectral band pass S_g = R_dW -- W = slit width

-- move image of entrance slit across the exit slit (slide rectangles over each other and results will be a triangle representing the amount of open area vs. the distance moved, i.e.

--triangular slit function Dl = s_g = RdW, full width at half height

--normal conditions, get instrument limited triangle shape for spectral line narrow compared to S (e.g. atomic line); for molecule, get broad bandshape

ideal resolution, separate line (bands) to baseline, Dl = 2s_g = 2R_dW
realistic, separate bands to distinguish
common statement of resolution, FWHH ~ s_g
very high resolution —atomic line, can get Rayleigh disperison ,

--Resolving power (theoretical): --

-- Depends on order, M, and # of grooves, N – consider, more grooves more interfering wave differences so more selection between wavelengths to be in phase

Throughput — aperture diameter ,,--,, solid angle
Limiting aperture normally is the grating—most expensive component – effective size reduced by angle q
Broadband output: -- varies like W²or F = HWBT_opR_dW² - so increased resolution costs light throughput
Stray radiation an important consideration in design (multiple mono better, but cost throughput) hard to quantify, usually given as F_SR/F_l_o
Examples — monochromator problems to learn to solve
Models/designs (not all links work, see text for old designs, see links below for newer ones): Czerny Turner, Ebert, Littrow, Roland Circle, Echelle, Concave gratings, transmission gratings, multiple grating , double monochromators (subtractive and additive dispersion)--often used additively for Raman spectra to reduce scattered light. and increase resolution in visible

Homework – read Chap. 3-5,6. That is a minimal start. Read from the Richardson book, the web sites below by JY and/or winkipedia

Problems: Chap 3: 3 (previous classes have assumed f=7^o), 15-16-17, 20-21-22 (these groupings of 3 re very similar, maybe I will ask you hand in one set of them), 30

Again, those not mentioned are good for testing

Link to grating manufacturers

Richardson Grating Lab, formerly Bausch and Lomb, now apparently part of Newport Optical

Historically they have produced a very useful book on grating use and design, worth reading, download at:

http://www.gratinglab.com/Diffraction%20Gratings/1/productmain.aspx

Jobin-Yvon (French) now with Horiba (Japanese)

http://www.jobinyvon.com/usadivisions/gratings/gratings.htm

Check out their explanatory grating tutorial page

http://www.jobinyvon.com/usadivisions/OOS/oos1.htm

Gratingworks, smaller sizes

http://www.gratingworks.com/

Grating Calculator