Ch. 19 Spectrophotometry II

Lecture 16

Diffraction grating

separates wavelengths by angle

monochromator

selects the wavelength

How does the size of diffraction grating slits affect light and resolution

wider slit = more light, lower/worse resolution.

narrow slit = less light, higher/better resolution

What is a detectors role

to turn light into electrical signals

Photomultiplier Tube (PMT)

extremely sensitive, but strong light detector. electrons multiply through dynodes to give a larger signal

Charge-Coupled Device (CCD), sensitivity? purpose?

array of many detector pixels that can store/transfer charge. converts light to electrical signals. sensitive like PMT, more sensitive than photodiode array. can measure many wavelengths at once

Linear Photodiode Array

similar to CCD, no storage on the chip. detects light at different positions/wavelengths

Linear Array Spectrometer

measures many/all wavelengths at once. fast and cheap, but lower resolution.

Fluorescence, how does it work, what happens to energy and wavelength

instant re-emission of the absorbed energy. molecule absorbs light/photon → gets excited → loses a little energy (non-radiative decay) → gives off light/releases photon with less energy than initial, absorbed photon. Less energy = longer wavelength ex: highlighter glowing under UV light (immediately stops glowing when light is removed)

Phosphorescence, what is it, how does it work,

delayed re-emission of the absorbed energy, molecule absorbs light → e- gets excited → e- eventually falls back down → light is emitted. excited e- gets stuck in different spin state (triplet state) and has to do a spin flip to return to the ground ex: glow in the dark stars on the ceiling (slowly dies down)

Non-Radiative Decay

energy transfer to other molecules, usually in vibrational states

Which wavelength is longer, λemission or λexcitation

λexcitation < λemission

Single Beam Spectrometer

light goes through blank first to find P0 → sample second to find P. Lght source could fluctuate, making errors/uncertainties

Dual Beam Spectrometer

sample and blank compared at the same time

What is used as the light source for Spectrometers

Tungsten lamp and Deuterium lamp.

Destructive Interference

waves cancel each other out, light decreases/disappears (up + down = 0)

Constructive Interference

waves added together if the crests line up, wave gets bigger, brighter (up + up = extra up)