Abstract

Laser-induced collisional energy transfer (LICET)¹ is a process that involves two dissimilar atoms, one of which is excited, colliding in the presence of a monochromatic laser field. If the radiation field is properly tuned to an interatomic resonance, the initially excited system undergoes a transition to its ground state, while its partner gains both the excitation energy and that of a photon. The transfer cannot occur unless both collisional and radiative interactions are present. For a weak laser field, the LICET cross section, peaked at the interatomic resonance frequency, shows a strongly asymmetric shape with an extended wing related to the van der Waals shift of the atomic levels. At increasing laser intensity, as a result of saturation and Stark dynamic effect, theoretical models predict a narrowing and symmetrization of the line shape with a frequency shift of the peak. At the present time, the excellent agreement between theory and recent measurements² over the full line shape allows a full understanding of the process in the weak-field regime. On the contrary, theoretical predictions of strong-field models appear to lack experimental confirmation,³ raising the question whether the basic assumptions used in weak-field models are still valid at increasing laser intensities or the strong-field effects have not been observed because they are masked by concomitant effects induced by the high laser intensity required. To overcome these problems, we have recently proposed a LICET experiment in a two-laser configuration⁴ and made specific calculations for the Eu-Sr system.⁵ A diagram of the relevant energy levels involved in the process is shown in Fig. 1. The Eu atom, prepared in the (6s6p)⁸P_9/2 excited state, undergoes a collision with the Sr atom in the (5s²)¹S₀ ground state in the presence of a weak monochromatic laser field (of frequency ω_p), nearly resonant with the interatomic transition Eu(6s6p)⁸P_9/2 → Sr(5p²)¹D₂ (of frequency ω₃₁), and of an intense monochromatic laser field (of frequency ω), nearly resonant with the Sr transition (5s5p)¹P₁ → (5p²)¹D₂ (of frequency ω₂₁). The use of a weak field, to induce the interatomic transition, and of a strong field, to modify the energy-level position, combines the advantages of weak-field analysis under strong-field conditions. In particular, since the dressing field can be resonant with the Sr transition (5s5p)¹P₁ → (5p²)¹D₂, a significant Stark dynamic effect can be induced by a moderate laser intensity, thereby reducing most of the problems of conventional strong-field LICET experiments. From a theoretical viewpoint, the different role played by the two laser fields allows a perturbative treatment of the equations of motion in a dressed-state basis, providing an explicit solution for the excitation spectrum.⁵ In Fig. 2, the excitation spectrum, calculated for strong-laser detuning Δ = ω₂₁ − ω = 1 cm⁻¹ and intensity I = 25 MW/cm², is reported, showing a remarkable splitting of the resonance peak, easily detectable. Experimental work is in progress at LENS.

© 1994 IEEE