It is found that Fe ii emission contributes significantly to the optical and ultraviolet spectra of most active galactic nuclei .
The origin of the optical/UV Fe ii emission is still a question open to debate .
The variability of Fe ii would give clues to this origin .
Using 7.5 yr spectroscopic monitoring data of one Palomer-Green ( PG ) quasi-stellar object ( QSO ) , PG 1700+518 , with strong optical Fe ii emission , we obtain the light curves of the continuum f _ { \lambda } ( 5100 \AA ) , Fe ii , the broad component of H \beta , and the narrow component of H \beta by the spectral decomposition .
Through the interpolation cross-correlation method , we calculate the time lags for light curves of Fe ii , the total H \beta , the broad component of H \beta , and the narrow component of H \beta with respect to the continuum light curve .
We find that the Fe ii time lag in PG1700+518 is 209 ^ { +100 } _ { -147 } days , and the H \beta time lag can not be determined .
Assuming that Fe ii and H \beta emission regions follow the virial relation between the time lag and the FWHM for the H \beta and Fe ii emission lines , we can derive that the H \beta time lag is 148 ^ { +72 } _ { -104 } days .
The H \beta time lag calculated from the empirical luminosity–size relation is 222 days , which is consistent with our measured Fe ii time lag .
Considering the optical Fe ii contribution , PG 1700+518 shares the same characteristic on the spectral slope variability as other 15 PG QSOs in our previous work , i.e. , harder spectrum during brighter phase .