Laser induced fluorescence spectroscopy of the Ca deposited on Helium and Mixed Helium/Argon Clusters
Résumé
We study the laser induced fluorescence spectroscopy of the calcium dimer deposited on helium and mixed helium∕argon clusters. In the wavelength range between 365 and 385 nm, the Ca dimer is excited from its ground state up to two excited electronic states leading to its photodissociation in Ca(1P)+Ca(1S): this process is monitored by recording the Ca(1P) fluorescence about 422.7 nm. These electronic excited states of Ca2 are respectively a diexcited one correlating to the Ca(4s 4p 3P)+Ca(4s 3d 3D) and a repulsive one correlating diabatically to the Ca(4s 4p 1P)+Ca(4s2 1S) asymptote, accounting for the dissociation of Ca2 and the observation of the subsequent Ca(1P) emission. On pure helium clusters, the fluorescence consists of the calcium atomic resonance line Ca(1S←1P) at 422.7 nm (23652 cm−1) assigned to ejected calcium, and a narrow red sided band corresponding to calcium that remains solvated on the helium cluster. Interestingly, the branching ratio to the ejection of Ca(1P) increases along with the wavelength of the excitation photon, i.e. when its energy decreases. This is contrary to what is intuitively expected and to what is really found on pure argon clusters. On mixed helium∕argon clusters, a new spectral band appears on the red side of the calcium resonance line; the intensity and the red shift of this component increase along with the argon quantity deposited on the helium cluster: it is assigned to the emission of Ca(1P) associated with the small argon aggregate embedded inside the helium cluster.