are 12% and 5%, respectively. Some of the discrepancy can be understood to be due to the finite-range effect 9 ; the difference may well be within the range expected for shell effects in highly deformed nuclei. 10 The relative importance of multiple-chance fission contributions to the total fission excitation function calculated with parameter set (c) is exhibited in Fig. 2. The principal multiple-chance fission components are displayed. In the threshold region, multiple-chance fission is dominant; and at 169 MeV, the contributions due to firstthrough fourth-chance fission are 2.5, 1.4, 3.8, and 2.5 mb, respectively. The relative importance of multiple-chance fission decreases with increasing excitation energy, although at 337 MeV the multiple-chance contribution is still 33% of the total.To summarize: (1) It is necessary to fit fission and evaporation-residue excitation functions simultaneously over a broad excitation-energy range and to use experimental Z crtt values in order to obtain unambiguous model parameters.(2) The assumption that only first-chance fission need be considered in such analyses is a poor one for the parametrization considered herein. (3) A reduction in the liquid-drop barriers on the order of 35-45% is necessary to reproduce the experimental excitation functions in the mass-150 region.The optical analog of pulse Fourier-trans form NMR spectroscopy is demonstrated. Doppler-free infrared spectra are obtained for a set of closely spaced 13 CH 3 F transitions at 9.66 jum from two-pulse echo and free-induction decay transients. The effects of elastic or inelastic collisions on the decay rate are obtained for each transition, and the long-range force laws are deduced from the dependence of the scattering cross section on molecular velocity.The method of Fourier transforming transient phenomena from the time to the frequency domain has proven to be an extremely versatile technique in pulsed nuclear-magnetic-resonance (NMR) spectroscopy. 1 With it, ultrahigh-resolution NMR spectroscopy can be performed quickly and with high sensitivity in a set of densely spaced lines. 2 Since the NMR signals display coherent transient behavior, dynamic information about nuclear spin interactions can be derived in a selective manner for each transition as well. 3 This Letter reports an initial demonstration of this Fourier-transform technique in the optical region. By employing suitable coherent optical transient effects in a sample of 13 CH 3 F, we are able to resolve Doppler-free spectra in a set of closely spaced lines (Fig. 1). The decay characteristics for each transition are obtained also and 275

Photon echoes, observed recently in ruby and in NH, D gas by two-pulse laser excitation, exhibit a modulation behavior as the pulse delay time is varied. The effect, which arises in a three-level quantum system, is due to the interference of two optically coupled transitions that share a common level. In this article, the three-level echo problem is treated exactly. Wave-function solutions are incorporated that are handled conveniently by a Laplace-transform technique. We, therefore, extend the earlier theory of echo modulation by Lambert et al. by removing their restriction of on-resonance pulse excitation. This procedure allows us to examine the effect of inhomogeneous broadening on echo amplitude explicitly, and we treat the problem by numerical and also by analytical means for the case of a Doppler-broadened gas. Analytic expressions are derived either for a high or a low laser power regime corresponding to excitation of either a large or a small fraction of the Doppler linewidth. The former case is analogous to pulsed nuclear magnetic resonance and is well known, whereas the latter result is new and applies to current photon echo measurements.

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