0198506961.pdf

166 Doppler-free laser spectroscopy

Fig. 8.11(a) The hyperfine struc-
ture of the 1s and 2s configurations
of hydrogen (not to scale). The two-
photon transitions obey the selection
rule ∆F = 0. This allows the tran-
sitionsF =0toF′=0andF =1
toF′ = 1. (b) A two-photon spec-
trum of the 1s–2s transition in atomic
hydrogen. The recorded signal comes
from photons emitted from the gas (fol-
lowing the two-photon excitation) that
are detected by a photomultiplier, as
shown in Fig. 8.8. This signal arises in
a slightly different way to that shown in
Fig. 8.9(b): the 2s configuration in hy-
drogen decays very slowly since it has
no allowed transition to 1s but transfer
from 2s to 2p occurs by collisions with
atoms (or molecules) in the gas, and
the 2p configuration decays rapidly by
the emission of Lyman-αphotons. The
scale gives the (relative) frequency of
the ultraviolet radiation used to excite
the two-photon transition (Footet al.
1985). Copyright 1985 by the Ameri-
can Physical Society. Relative frequency of ultraviolet radiation (MHz)

(a)

(b)

Collisions

0 200 400 600 800

Intensity of Lyman- radiation

Lyman-

resolution. A laser with a pulse of durationτ=10nshasalower

limit to its bandwidth of

∆fL

1

τ

100 MHz. (8.22)

This Fourier transform limit assumes a perfectly shaped pulse and

in practice pulsed lasers typically have a bandwidth an order of

magnitude greater. Commercial continuous-wave dye lasers have

bandwidths of 1 MHz but researchers use sophisticated electronic

servo-control systems to reduce this. The best ion trap experiments