bei48482_FM

Atomic Structure 149

As in Fig. 4.28, the laser transition from the metastable state ends at an unstable in-

termediate state rather than at the ground state. Because the intermediate state decays

rapidly to the ground state, very few atoms are in the intermediate state. Hence even

a modest amount of pumping is enough to populate the metastable state to a greater

extent than the intermediate state, as required for laser amplification.

Practical Lasers

The first successful laser, the ruby laser,is based on the three energy levels in the

chromium ion Cr^3 shown in Fig. 4.27. A ruby is a crystal of aluminum oxide, Al 2 O 3 ,

Figure 4.28A four-level laser.

Intermediate state

Laser transition

Metastable state

Initial transition

Pumped state

E 3

E 2

E 1

Pumping

transition

E 0

Ground state

A robot arm carries a laser for cutting fabric in a clothing factory.

bei48482_ch04.qxd 1/14/02 12:21 AM Page 149