Low-energy quadrupole modes in deformed clusters

Properties of low-energy (infra-red) quadrupole modes (LEQM) of multipolarity $+AFw-lambda+AFw-mu =$20, 21 and 22 in deformed sodium clusters are studied within the Kohn-Sham LDA RPA approach. Possible manifestations of LEQM in stimulated Raman adiabatic passage (STIRAP) reaction are discussed. It is shown that, in free light clusters, where the low-energy spectrum is delute, LEQM can be unambiguously identified as particular electron-hole pairs. This gives a chance to reconstruct the mean field level scheme near the Fermi surface. Moreover, due to the connection with electric $+AFw-lambda+AFw-mu =$21 mode, the scissors mode can be detected. In heavy (supported) oblate clusters, LEQM are in general rather involved. Nevertheless, some interesting $+AFw-lambda+AFw-mu =$21 and 22 structures determined by specific deformation effects can be resolved. The origin of the structures is discussed in detail.