Our study of B←X absorption of molecular iodine (I2) isolated in a low-temperature crystalline xenon has revealed an exceptionally long-lived electronic coherence in condensed phase conditions. The visible absorption spectrum shows prominent vibronic structure in the form of zero-phonon lines (ZPLs) and phonon side bands (PSBs). The resolved spectrum implies weak interaction of the chromophore to the lattice degrees of freedom. The coherence extends past the vibrational period of the excited state molecule, unlike that observed in any condensed phase environment for I2 so far. The ZP transitions from the relaxing B-state populations were resolved in the hot luminescence when the 532 nm laser was used for excitation.
We report observation of multi-THz quantum beats in the amplitude of superfluorescence in pure argon gas, using a time delayed 780-nm probe beam that interrogates the relevant population densities. The superfluorescence is induced by three-photon absorption of an ultrashort 260.5-nm laser pulse that creates coherent and coupled populations in multiple closely spaced
3d and
5s Ar levels. Argon pressure and pump laser focusing parameters are seen to modify the frequency of the quantum beats, which is attributed to the absorption of the self-generated, nonresonant third harmonic of the pump from Ar near the zero of the real part of the overlapping linear susceptibilities of the closely spaced
3d and
5s excited states. The self-absorption of the phase-matched THG photons creates a coherent population of atoms which acts like an atomic excited state ($3\ensuremath{\omega}$-induced state). We have also observed a new two-step superfluorescent cascade that results from the population redistribution enabled by the 780-nm probe pulse between the $3{d}^{\ensuremath{'}}{[5/2]}_{3}$ and the $3\ensuremath{\omega}$-induced state and gives rise to a previously unknown transition that emits on 1587 nm.
We have examined the luminescence from highly excited self-trapped one-center states, the relaxation processes of the band excitations, and the mechanisms of formation of highly excited quasiatomic centers in low-temperature crystals of the light rare gases and their solid solutions. The fundamental channel of population of highly excited emitting centers involves hole self-trapping with formation of two-center R2+ states; dissociative recombination of these forms one-center np5(n + 1)p excitations. We discuss the effect of self-trapping of highly excited states on the population of the lowest emitting centers in neon and argon crystals.