IV. Wavelength discriminators (text Ch. 3.5 )

A. Monochromators work by dispersing l in space

1. Prism dispersion due to index dispersion,

• material dependent--all increase as go to uv, different penetration,
• very non-linear — fast change in uv, slow in IR (poor l separation)
• monochromator — collimate beam in, focus output onto dispersed detector (film) or rotate prism to focus different l on slit width DS gives Dl
• angular dispersion: separation:
• linear disperson:
• uses

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

--inexpensive predisperser

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

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

• d (sin a + sin b) = ml , m = 0, ±1, ±2, … (order)
• orders need sort: free spectral range --
• Blaze — maximum l efficiency: l_b = d sinb (a = 0)

  -- most useful , cut-off l>2d

  -- polarized, ^ to grooves more to red, || more to blue, annomalies occur as function of l blaze, extreme polarization

• Dispersion

-- linear dispersion:

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

--practical: reciprocal:

--spectral bandpass S_g = R_dW --W = slit width

-- triangular slit function Dl = 2s_g = 2RdW, normal conditions, get instrument triangle for line narrow compared to S; molecule, get broad bandshape

• very high res —atomic line, can get Rayleigh ,

  --Resolving power (theoretical): --

  -- Depends on order, M, and # of grooves, N

• Throughput — aperture diameter ,,--,, solid angle
• Output: -- like W² or F = HWBT_opR_dW²
• Examples — problems
• Models: 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