Principles

Asterix IV / PALS is a gas laser using neutral iodine atoms to produce near-infrared light at the wavelength of 1.315 µm. The iodine atom is obtained from a parent perfluoroisopropyl iodide molecule C3F7I in a photochemical process called photodissociation (or photolysis). In this reaction, UV radiation produced by powerful flashlamps is used to free the iodine atom from the chemical bonding. The iodine emerges from the photodissociation reaction as an electronically excited atom, which inherently provides population inversion with respect to the lower-lying iodine ground state, constituting thereby conditions for lasing action.

The lasing action occurs between fine-structure-split levels 2P1/2 and 2P3/2 of the fundamental configuration 5s25p5 of the neutral iodine atom. The electronic configuration of these two levels is described using a spectroscopic notation corresponding to so-called LS coupling which is applicable to atoms in which the spin-orbit interaction is small. The upper left index denotes the multiplicity of the electronic spin, the capital letter gives the orbital angular momentum, and the right lower index expresses the total value of the electronic angular momentum.

I(5s25p52P1/2) -> I(5s25p52P3/2)+ hν (1.315 μm)

The transition between the 2P1/2 and 2P3/2 levels in the iodine atom is of type called magnetic dipole transition. The decay of the upper level 2P1/2 down to the 2P3/2 level does not modify the character of the spatial distribution of the electronic charge, and thus the transition does not produce an oscillating electric-dipole moment. Instead, the transition generates an oscillating magnetic-dipole moment - the atom may be portrayed as a microscopic coil producing oscillating magnetic field.

The light emitted by the transition 2P1/2 to 2P3/2 is characterised by six very closely spaced spectral lines, in spectroscopic nomenclature called components. These components arise because the magnetic field of the iodine nucleus (nuclear spin 5/2) splits the upper and lower levels into 2 and 4 so-called hyperfine levels, while the selection rules limit the number of possible transitions between these levels to six. Among the individual spectral components the strongest one belongs to the transition F = 3 -> 4, where F labels the total angular momentum of the hyperfine level.

Emission spectrum for different buffer gas pressuresAlong with isopropyl C3F7I (labelled also as i-C3F7I), the active medium of the Asterix IV/PALS laser contains Ar which acts as a buffer gas. The mutual ratio of C3F7I and Ar in the mixture is different for individual laser modules, but Ar is always the largely predominant component and the typical sum pressure is on the order of 1 to 3 atmospheres (1 000 to 3 000 mbar). Ar does not participate in the lasing but has three other practical functions.

First it broadens, via atomic collisions, the spectral width of the emitted radiation. The individual spectral components thus “melt” into one spectral peak, as seen from the image. This increases the efficiency of the system (as all the spectral components participate in the lasing) and reduces the small-signal gain, inhibiting potential parasitic oscillations of the laser chain.

Secondly, the presence of Ar helps to improve the transverse uniformity of pumping, since the partial pressure of the active gas C3F7I is low despite the high total pressure (required for the spectral broadening).

Thirdly, Ar acts as a heat sink carrying away a part of the thermal energy dumped into the active medium by the flashlamps. This reduces the rate of formation of certain undesired products during the photolysis.

Upon photolysis reaction, free alkyl radicals C3F7 and free iodine atoms tend gradually to recombine. This recombination involves 3 main channels:

C3F7 + I -> C3F7I (repairing recombination)
C3F7 + C3F7 -> C6F14 (dimerisation)
I + I -> I2 (molecular formation)

While the dimerisation products (perfluorohexane or perfluoro-2-methylpentane) are benign to the ability of the gas mixture to work as a laser medium anew, iodine molecules are both quenchers of the population inversion and make the mixture slightly corrosive. The gas mixture is therefore, after each laser shot, restored using cryogenic units. These units store the C3F7I liquid at a temperature providing the saturated vapour pressure at which the given laser module operates. The iodine molecules are captured in the cool alkyliodide liquid and thus removed from the gas mixture, while fresh C3F7I molecules are supplied to it.

The choice of isopropyl i-C3F7I as the parent molecule bears on the ability of this molecule of regeneration. The other allotropic modification of the alkyl iodide, n-propyl (n-C3F7I) has less favourable properties to be restored using a cryogenic circuit, although its ability to produce excited iodine atom upon UV flash irradiation is slightly higher than that of isopropyl.