Read UV-Vis Spectra in Your Browser: λmax, Beer–Lambert, Colour and Tauc Band Gaps

Try the UV-Vis viewer here

Try it here

Why use a browser viewer for UV-Vis data?

UV-Vis is one of the quickest measurements in a lab. The slow part comes after: you have a file and need λmax, a molar absorptivity, a figure or a band gap. In a spreadsheet that means converting units, finding maxima by eye and building plots by hand. The viewer does these steps on one page, with nothing to install.

The science in brief

λmax. Molecules absorb UV and visible light when an electron moves to a higher orbital, as in π→π* or n→π* transitions. λmax is the wavelength where a band absorbs most. Longer conjugation moves it to longer wavelengths. β-carotene, with eleven conjugated C=C bonds, absorbs in the blue part of the visible range.

To get a rough idea of where a compound should absorb before you measure it, predict UV-Vis bands from a drawn structure with the Spectroscopy plugin.

Beer–Lambert law. Absorbance is A = εcl, where ε is the molar absorptivity (L mol⁻¹ cm⁻¹), c the concentration (mol/L) and l the path length (cm). Also, A = −log₁₀T. The law holds best for dilute solutions.

Colour. A sample looks the colour complementary to the light it absorbs. A band in the blue makes a solution look yellow. A band in the green, as in permanganate, makes it look purple.

Tauc plot. For a semiconductor, plot (αhν)² against photon energy hν for a direct allowed transition, or (αhν)^1/2 for an indirect allowed one. Extend the straight part of the absorption edge to zero. The crossing point estimates the optical band gap, Eg.

Files you can open

• JCAMP-DX (.jdx, .dx). The viewer reads the x and y units from the file.
• CSV or TXT with two columns. The x column can be in nm, µm, cm⁻¹ or eV. The y column can be absorbance or transmittance. Without headers, the viewer guesses from the numbers.

Pick several files at once to overlay them. To practise, use the simulated examples or download the sample .jdx.

How to analyse a spectrum, step by step

1. Press Open UV-Vis file(s) or drop files on the box. You can also press an example: β-Carotene, KMnO4, Dye series (5 conc.), ZnO band gap or Sample .jdx file.
2. Choose Absorbance or %T for the y axis, and nm, eV or cm⁻¹ for the x axis.
3. Read the λmax peaks table. It gives each peak in nm, eV and cm⁻¹ with its absorbance and colour. Move the Peak prominence slider if a shoulder is missing or small bumps show up.
4. In the Beer-Lambert box, enter Path l (cm) and a Concentration in mol/L, mmol/L or µmol/L to get ε and log ε. Or enter ε to get the concentration. Tap a table row or the plot to use another wavelength. Use λmax again switches back.
5. For a band gap, open Tauc plot and band gap, choose the Transition, and press Auto fit or type your own Fit from (eV) and to values.
6. Press Export PNG, Export CSV or Tauc PNG.

What the viewer does

Ways to use it in class and in the lab

• Conjugation and colour: load β-Carotene, then add KMnO4 with Add to the plot instead of replacing. Compare where each absorbs and why one looks yellow and the other purple.
• Checking Beer–Lambert: load the dye series. Select each spectrum by its name and enter its concentration. If the law holds, ε should come out about the same each time.
• Unknown concentration: enter ε from a calibration or the literature to get c.
• Semiconductor films: use the ZnO example to learn the Tauc method, then open your own film spectrum. For an organic semiconductor, compare that optical gap with the electrochemical gap from HOMO and LUMO estimates in the cyclic voltammetry viewer.
• Lab reports: export a PNG for the figure and a CSV for the data.

Tips and limits

• Record a solvent blank on the spectrometer. The viewer does not subtract one.
• A weak shoulder on a large band may not appear as a peak.
• The Tauc plot uses absorbance in place of the absorption coefficient α. This is fine for finding where the line meets zero. For powders measured by diffuse reflectance, convert to the Kubelka–Munk function before you open the file.
• Check the shaded fit range and R² before you report Eg. Another range can give another answer.
• The colour is a guide. Real colour also depends on band width, concentration and the light source.
• The CSV puts all visible spectra on the x values of the first one, so spectra with different grids are interpolated.

More free data viewers