Absorption and Emission Spectra

Introduction

When white light passes through a sample, some wavelengths may be removed; when a sample is heated or excited, it may give out light of particular wavelengths. These two situations produce absorption and emission spectra. Both carry the same information about energy gaps, but they are recorded in different ways. Understanding how a spectrometer builds up a spectrum helps you read spectra critically and spot where errors can creep in.

Core explanation

Absorption spectra. Radiation containing a continuous range of wavelengths passes through the sample. Molecules take in photons whose energies match their energy gaps, moving from lower to higher levels. The detector registers less radiation at those wavelengths. Plotted as absorbance, these appear as peaks; plotted as transmittance, as dips.

Emission spectra. Atoms or molecules are first given extra energy — by heating in a flame, by an electrical discharge or by absorbing light. As they return to lower levels, they release photons with energies equal to the gaps. The detector records bright signals on a dark background. Flame tests and the line spectrum of hydrogen are classic emission examples.

The same gaps, two views. For a given atom, the lines in an emission spectrum appear at the same wavelengths as the corresponding lines in its absorption spectrum, because both correspond to the same energy differences.

Components of an absorption spectrometer.

Component Function --- --- Source Produces radiation over the required range (for example a deuterium lamp for UV, a tungsten lamp for visible, a heated ceramic for IR) Sample holder Holds the sample in a cell made of a material transparent to the radiation (quartz for UV, glass or plastic for visible, salt plates such as NaCl for IR) Wavelength selector A monochromator (diffraction grating) or, in IR, an interferometer Detector Converts radiation into an electrical signal (photodiode or photomultiplier for UV-visible, thermal or semiconductor detector for IR) Computer Processes the signal and plots the spectrum

The reference. The solvent, the cell walls and even the air absorb some radiation. A blank — the same cell containing only solvent — is measured first, or simultaneously in a double-beam instrument. The instrument then reports only the absorption due to the analyte.

Modern IR instruments. Most IR spectrometers are Fourier-transform (FTIR) instruments. Instead of scanning one wavelength at a time, they pass all wavelengths through the sample at once, record an interference pattern and use a mathematical transformation to convert it into a spectrum. This is faster and gives a better signal-to-noise ratio.

Step-by-step reasoning

How a double-beam UV-visible spectrometer records a spectrum:

1. The source emits a broad range of wavelengths. 2. The monochromator selects one narrow band. 3. The beam is split: one half passes through the sample, the other through the reference. 4. Detectors compare the two intensities. 5. The wavelength is changed step by step, and the computer plots absorbance against wavelength.

Visual explanation

Picture a strip of rainbow. For an absorption spectrum, the strip is bright with a few dark lines where light has been removed. For an emission spectrum, the strip is black with a few bright coloured lines. For the same element, the dark lines and bright lines occupy identical positions.

Real-world analogy

A spectrometer is like a careful shopkeeper counting stock. The blank is the stock count before any customer arrives; the sample measurement is the count afterwards. Only the difference tells you what the customer — the analyte — actually took.

Real-world example

Atomic absorption spectrometers measure trace metals such as lead in drinking water. A lamp emits light characteristic of lead, and the amount absorbed by the vaporised sample reveals how much lead is present, down to parts per billion.

Why?

Why must UV cells be made of quartz rather than ordinary glass? Ordinary glass strongly absorbs ultraviolet radiation below roughly 300 nm, so it would hide the sample's absorption. Quartz remains transparent down to about 200 nm.

Common misconception

"Emission and absorption lines of an element occur at different wavelengths." For transitions between the same pair of levels, the energies are identical, so the wavelengths are the same. Emission spectra may simply show extra lines from transitions between higher levels.

Worked example

Question: An absorption spectrum of a solution is run without a blank. The solvent absorbs slightly at 280 nm. What effect does this have on the recorded absorbance at 280 nm?

Reasoning: Without a blank, the instrument cannot subtract the solvent's contribution. The measured absorbance is the sum of analyte and solvent absorbances.

Answer: The absorbance at 280 nm is too high, giving an overestimate of the analyte.

Quick check

1. In an absorption spectrum, what happens to molecules at the wavelengths where signals appear? Answer: They absorb photons of those energies and are promoted from lower to higher energy levels.

Exam focus

Be able to name spectrometer components and state their functions. Examiners frequently ask why a blank is used and why quartz cells are needed in the UV region. Distinguish clearly between absorption (dark lines on bright background) and emission (bright lines on dark background).

Advanced insight

Fluorescence is a special kind of emission: a molecule absorbs a UV photon, loses a little energy as vibration, then emits a photon of slightly longer wavelength. Because the emitted light differs from the excitation light, fluorescence detection is extremely sensitive and is used to track single labelled molecules in cells.

Summary

Absorption spectra show which wavelengths a sample removes from a continuous source; emission spectra show which wavelengths excited species give out. Both correspond to the same energy gaps. A spectrometer consists of a source, sample cell, wavelength selector, detector and computer. A blank corrects for solvent and cell absorption, and modern IR uses the Fourier-transform method.

Practice questions

1. State one difference between an absorption spectrum and an emission spectrum. Answer: An absorption spectrum shows dark lines or peaks where radiation is removed; an emission spectrum shows bright lines where radiation is given out. 2. What is the function of a monochromator? Answer: It selects a narrow band of wavelengths from the broad output of the source. 3. Why are sodium chloride plates used to hold samples for IR spectroscopy? Answer: Sodium chloride does not absorb significantly across most of the IR range, so it does not interfere with the sample's spectrum. 4. Give one advantage of a Fourier-transform IR spectrometer over a scanning instrument. Answer: It measures all wavelengths at once, so spectra are recorded faster and with better signal-to-noise ratio.