Page 57 - Tunable Lasers Handbook

38 R. C. Sze and D. G. Harris

there are features in the spectrum due to the absorption of molecular oxygen
(Schurnann-Runge band) within the resonator cavity. Interest in line narrowing
and tuning of ArF has grown as applications for shorter wavelength sources
developed in the area of microfabrication. Ochi et al. [28] has built an oscillator
with a 1.6-pm linewidth at 350 Hz with 7.4 mJ per pulse.

2. 7.2 KrF (248 nrn)
Much research has been done on KrF lasers because of their use as high-
power lasers for laser fusion research as well as their use in the microelectronics
industry. The KrF spectrum is a broad continuum (Fig. 2), which is considered to
be homogeneously broadened owing to its repulsive ground state. Narrow absorp-
tion lines have been observed that are attributed to the excited states of rare gas
ions. Spectral tuning has been observed over a continuous range of 355 cm-1.

2.7.3 XeF (BEXJ
The structure of the XeF molecule is significantly different from that of the
other rare gas halides and consequently its spectral properties also differ. The X
state is bound by 1065 cm-1 and therefore has vibrational levels. Additionally,
the C state lies about 700 cm-1 below the B state. The spectra of the B+X tran-
sition show emissions at 353 and 351 nm [30-331. Early investigators also noted
that as the temperature was increased, the lasing efficiency of the B+X transi-
tion improved significantly [35.36] (Fig. 3). Several explanations exist to explain
this improved efficiency: (1) increased vibrational relaxation of the B state, (2)
increased dissociation of the X state, and (3) decreased narrowband absorption
at 351 nm. The complexity of the molecular structure implies that energy is not

IIIIIIIIIII II’IIIIII I I I I I I I ’ I I I
260 250 240 230
Wavelength (nm)

FIGURE 2 Fluorescent spectrum from the B’E,,2-X2Z,:2 transition in KrF (from Brau and
Ewing [29]).