Monochromators

Monochromators

1. Monochromators

2. Function of Monochromator : To isolate narrow bands of radiant energy High efficiency than filters Dispersing Media: 1)Quartz Prism 2) Diffraction Grating Types: 1) Prism monochromator 2) Diffraction grating monochromator

3. Prism Monochromator : based on refraction of light and fact that different wavelengths of radiation have different values of refractive index in a medium .

4. Entrance slit allows source radiation to illuminate the first lens which collimates the light spreading it across the face of the prism. Prism disperses radiation into component wavelengths and the second lens focuses the spectrum at the focal plane. An exit slit selects the band of radiation to reach the detector. Dispersing element can be a prism or a diffraction grating. Focusing elements can be lenses or mirrors.

5. • Desired wavelength can be selected by rotating the prism. • In visible region – can use glass down to 350 nm • In the UV region – quartz is material of choice • In the IR region – NaCl , KBr , etc. Kinds of Prisms Littrow Prism & Mounting – compact design

6. Problem with quartz prisms is that quartz is optically active (optically anisotropic). With the Littrow prism or any reflecting prism, the light travels essentially the same path in both directions and this effect is eliminated.

7. h 1 sin q 1 = h 2 sin q 2 Recall Snell’s Law of Refraction Also, remember that no refraction occurs if light at normal or q 1 = 0 So, light must hit prism at an angle. Most common is a 60 o prism (glass or quartz).

8. Gratings – based on diffraction & interference Transmission Gratings & Reflection Gratings consist of a series of grooves in glass or quartz or a mirror (usual kind)

9. Reflection Grating: most commonly used Grating Monochromator : based on diffraction of light (constructive and deconstructive interference) - grooved surface with reflective coating (Al, Au, Pt) Now, spacing of slits (d) is distance from one groove to next. Typically have 300-2000 grooves/mm. Constructive and deconstructive interference occurs because light travels different distances when reflected from each grating Angle at which constructive interference occurs is now given by: n l = d(sin i + sin r) n l = d(sin i + sin r)

10. Comparison of Gratings vs. Prisms: – increase size of either prism or grating will give better dispersion. – stray light can be removed with filters. – glass absorbs light l ≤ 350 nm Type of Dispersion Size Stray Light l range of use Grating uniform dispersion vs. l smaller Higher stray light unlimited Prism shorter l better separated larger Less of problem Limited ( l ≤ 350 nm)