  We theoretically demonstrate the quenching effect in below-threshold high
harmonic generation (HHG) by using the time-dependent density-functional theory
(TDDFT) and solving the time-dependent Schr\"{o}dinger equation (TDSE). It is
shown that the HHG is substantially suppressed in particular harmonic orders in
the below-threshold region when multi-electron interaction comes into play. The
position of the suppression is determined by the energy gap between the highest
occupied orbital and the higher-lying orbital of the target. We show that the
quenching effect is due to a new class of multi-electron dynamics involving
electron-electron energy transfer, which is analog to the fluorescence
quenching owing to the energy transfer between molecules in fluorescent
material. This work reveals the important role of the multi-electron
interaction on HHG especially in the below-threshold region.
