Fluorescence of ruby

The characteristic red fluorescence of ruby, a variety of the mineral corundum (Al₂O₃), is a fascinating phenomenon that is due to the special properties of its crystal lattice and the chromium(III) ions (Cr³⁺) incorporated in it.

When ruby is irradiated with UV light or visible light in the blue to green spectral range, the chromium ions absorb this energy. After excitation, the chromium ions emit the energy in the form of light in the red range, typically at 694 nm. Foreign ions or lattice defects can weaken the fluorescence.

Recording a fluorescence spectrum

A ruby zoisite (also known as anyolite) was used to record the spectrum shown here. This is a rock that consists of a combination of red ruby, green zoisite (a calcium aluminum silicate (Ca₂Al₃(SiO₄)(Si₂O₇)O(OH))) and often black speckles of hornblende (a dark, iron-rich amphibole mineral).

For excitation, we used a 375 nm LED to illuminate the ruby zoisite at an angle of 45 degrees. The emitted light was recorded with a light guide at an angle of 45 degrees and fed to the spectrometer for analysis.

A DIY spectrometer in Czerny-Turner design with a focal length of 150 mm, a slit size of 10 µm, a grating with 300 lp/mm and a CCD detector with 3648 pixels (line scan camera e9u-LSMD-TCD1304-STD) was used.

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With an integration time of just 300 ms, the characteristic, relatively narrow-band main emission of the ruby appeared at 694 nm. This corresponds to the R line, which is produced by the transition from the excited states ²E to the ground state ⁴A₂.

Higher integration times, such as in the following spectrum of 1500 ms, also show vibronic transitions in the range between 660 nm and 720 nm, while the main maximum at 694 nm already overdrives the detector due to saturation because of its intensity.

Such vibronic transitions are combined transitions in which energy from electronic state changes is coupled with lattice vibrations (phonons) in the crystal. The chromium ions in the ruby crystal lattice are embedded in an environment that allows such vibrations. These vibrations couple to the electron transitions and lead to the additional emission lines.

These lines usually appear asymmetrically around the main maximum and are typically much weaker. The position and intensity of such vibronic peaks can be influenced by disturbances in the crystal lattice, such as impurities or lattice defects. Higher temperatures promote vibronic transitions as thermal vibrations in the lattice increase.

In the range between 400 nm and 425 nm, remnants of the light from the exciting UV LED can still be seen. This signal therefore does not originate from the fluorescence of the ruby.

Interesting facts about ruby

Ruby crystals are not only used as jewelry, but also in lasers in which this fluorescence at 694 nm is amplified by optical pumping and emitted coherently.

The intensity of the fluorescence of ruby is temperature-dependent and is influenced by the effect of thermal energy on the chromium ions in the crystal lattice of the corundum. The fluorescence of ruby is more intense at low temperatures. This is due to the fact that thermal vibrations are minimal, which means that less energy is lost in the form of heat and electron transitions remain efficient. At higher temperatures, the fluorescence intensity decreases noticeably as the thermal vibrations in the crystal lattice increase. These vibrations disrupt the electronic transitions of the chromium ions and lead to an effect known as thermal quenching.

Due to the temperature dependence of its fluorescence, ruby is also used as a temperature meter in extreme environments, such as in high-pressure physics or in cryogenic experiments. Fluorescence is an important feature for the identification of natural ruby. Syntheses and counterfeits often fluoresce differently or not at all.