Class: Mineral, Corundum (Al2O3)
Notable Properties: Gemstone, Pulse Laser, Phosphorescence
The precious gemstone ruby is structurally identical to sapphire, and aluminum oxide (mineral name: corundum). Corundum itself is of great technological utility. It is an excellent optical glass over a large range of wavelengths; it is extremely hard and is one of the most common polishing and grinding media, and it is a chemically inert even at very high temperatures serving as a classic refractory oxide. The precious gemstones are just alumina with small quantities of transition metals (< 5 mol%) that provide the light coloring. The sapphires, therefore, have the general formula TMxAl2-xO3, TM = V, Cr, Ti, and Fe. Ruby’s molecular formula is CrxAl2-xO3, x ~ 0.05, and is nearly identical to pink sapphire, but with a slightly higher concentration of chromium(III) yielding the darker blood red hue.
A representation of the ruby crystal structure is shown above. The chromium dopants form a solid solution with alumina to yield a statistical distribution of Cr3+ throughout the crystal structure. The chromium centers are octahedral d3 ions in an oxide ligand field. The color and absorption spectrum is therefore nicely described by a Tanabe-Sugano diagram. We expect to see at least two spin-allowed transitions in the visible region of the electromagnetic spectrum which correspond to a single electron being excited from the approximately non-bonding t2g manifold to the antibonding eg orbitals.
Interestingly, the absorption spectrum also reveals the 2E spin forbidden transition as an extremely weak absorption at 1.8 eV, which can be visualized as a spin-flip solely within the t2g manifold. The opposite process (de-excitation from 2E to 4A2 and emission of a photon) is the same photon emitting transition that is used in the ruby laser. Note the complementary photoemission spectrum shows a phosphorescent transition at the same energy. In addition to the absorption spectrum of ruby, that of Cr(H2O)63+ is also shown (see Miessler and Tarr). Note the ruby spectrum is nearly duplicated but the absorption energies are red shifted because water is a weaker ligand than oxide and the d-d energy splitting is, therefore, less than in ruby (see Jablonski diagram below).
The first laser was constructed using ruby as the lasing medium by Theodore Maiman. Ruby can be used to make a 3-level pulse laser. The diagram below shows the critical electron pathway required in order for lasing to occur. (1) Electrons must be excited from the ground state configuration to a high energy excited state via photon absorption. This is colloquially referred to as “pumping”. (2) The excited chromium ion rapidly relaxes to the vibrational ground state of the electronically excited state through non-radiative decay. At this point, the electron can either fluoresce, decay back to the electronic ground state non-radiatively or participate in, (3), intersystem-crossing. In brief, intersystem-crossing is a radiationless spin-flip (via tunneling) that can occur when two different spin states are equienergetic and there is significant vibrational overlap. (4) The electron again vibrationally relaxes via non-radiative decay (i.e. releases heat). The electron is now in a long-lived excited state (on the order of milliseconds) since radiative decay is now spin-forbidden. If a photon of identical energy to ΔE, propagates through the ruby crystal within the vicinity of the excited electron, it can induce “stimulated emission” of a second photon perfectly in phase with the stimulating photon passing by. The electron is then de-excited back to the starting electronic ground state.
Alternatively, a photon of identical energy to ΔE can be emitted by spontaneous emission but, in this case, the phasing will be essentially random. This process competes with the lasing phenomena described above and dominates when the density of stimulating photons is too low. The final, essential ingredient that is needed for lasing to occur is what is known as a population inversion. That is, there need to be more electrons in the 2E manifold than in the ground state. Otherwise, all of the lasing photons will just be absorbed. This is why the 4T1 and 4T2 states in ruby can’t be used to make a yellow or green laser. These excited states decay too quickly and ruby’s absorption coefficient is too high for these transitions to be useful.
Unlike laser pointers, a ruby laser cannot produce laser light continuously. This is, impart, because photon absorption is used as the pump and becaue of the instrinsically inefficient nature of the 3-level laser compare to 4-level systems like the ubiquitous He-Ne laser. The key mechanistic problem is that the lasing photons of ruby are produced via de-excitation to the electronic ground state, such that a population inversion can only be maintained for a very short period of time, and can only be generated using an extremely intense light source as the pump. This takes the form of a flash tube, similar to that used in cameras but much more intense.
In the video below, Ben Krasnow, built his own ruby laser in his garage! Note the really intense (and somewhat terrifying) high voltage flash tube required to pump the laser to a state of population inversion and the time delay between pumping and the laser pulse. Even though the laser pulse is extremely short, it still packs enough power to ablate stainless steel.