H2 molecules were ionized by Ti:sapphire (45 fs, 800 nm) and Nd-doped yttrium aluminum garnet lasers (6 ns, 1064 nm). The relative populations of the vibrational levels of the H+2 ions were determined and found to be concentrated in the lowest vibrational levels. Tunneling ionization calculations with exact field-modified potential curves reproduce the experimental results. The reason for the departure from conventional Franck-Condon-like distributions is the rapid variation of the ionization rate with internuclear distance.

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We show that Coherent Population Oscillations effect allows to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an Erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits to achieve a group velocity as low as 2.7 m/s with a transmission of 40 %. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40 % to 90 %, and the light group velocity from 2.7 m/s to 100 m/s. [5,6,7,8]. In addition to the strangeness of producing light propagating at speeds as low as few m/s, Slow Light Propagation (SLP) is at the very heart of new fundamental and applied fields of research in nonlinear and quantum optics. From the nonlinear optical side, SLP allows to strongly enhance the lightmatter interaction time. Moreover, this interaction time can be continuously tuned to produce optical buffers and variable delay lines for optical networks. From the quantum optical point of view, SLP should allow, under specific conditions, classical and quantum properties of an electromagnetic field to be mapped into an atomic system [10]. The fundamental physical idea at the origin of SLP is the creation of a very narrow spectral hole in the homogeneous absorption profile. As stipulated by Kramers-Krönig relations, this narrow spectral hole is accompanied by a strong dispersion of the index of refraction inducing a low group velocity and an increase of the transmission. These two aspects are crucial in the choice of the atomic system and the coherent interaction inducing SLP.The first direct demonstration of SLP [1,2,3,4,5] was achieved via Electromagnetically Induced Transparency (EIT) [9]. It was originally implemented by applying a secondary control field to eliminate the linear absorption of a resonant probe field through an otherwise absorbing medium. The standard scheme for EIT is a three-level Λ system, where the probe field drives the system from one of the ground states and the control field from the second ground state.