Photoatomic effect: Light-induced ejection of Na andNa2from polydimethylsiloxane surfaces

The photoejection of Na and ${\mathrm{Na}}_2$ from surfaces prepared by adsorption of sodium vapor on solid films of polydimethylsiloxane (PDMS) of various molecular weights and structures is found to be dependent on light intensity and frequency. At low power levels the desorption yields of Na and ${\mathrm{Na}}_2$ are linear functions of desorbing light power and exhibit frequency thresholds for photoejection analogous to the photoelectric effect. The frequency thresholds are 9500 ${\mathrm{cm}}^{\mathrm{-}1}$ for Na and 11 500 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{Na}}_2$, and the 2000-${\mathrm{cm}}^{\mathrm{-}1}$ difference can be accounted for with a model in which the alkali-metal species form charge-transfer complexes ${\mathrm{Na}}^{+}$ ${\mathrm{PDMS}}^{\mathrm{-}}$ and ${\mathrm{Na}}_2^{+}$ ${\mathrm{PDMS}}^{\mathrm{-}}$, with the cations stabilized by interaction with the oxygen atoms of the PDMS polymer backbone. The ejection mechanism takes place by a reverse charge transfer to form a less polar complex with enough energy to eject Na or ${\mathrm{Na}}_2$ from the surface. © 1996 The American Physical Society.