Hole-Burning and Ion-Dip Double-Resonance Spectroscopy

Instead of keeping the pump laser at a fixed wavelength and tuning the probe-laser wavelength Ag one may also fix the probe transition i) k), 

If the absorption of the probe laser is monitored via two-photon ionization (Fig. 10.35b), the ion signal shows a dip when the tunable pump beam burns a hole into the common lower level population Nj ion-dip spectroscopy). This opens another way to detect stimulated-emission pumping. Now three lasers are employed (Fig. 10.35b). The first laser LI is kept 

L2 unnecessary. A third tunable laser L3 induces downward transitions k) m), 

---