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We have studied the temporal profile of photon-echo signals generated by combined gated cw and pulsed dye-laser excitation of the inhomogeneously broadened, 555.6-nm absorption line of (174)Yb vapor. We find that the echo profile is, after time reversal, essentially identical with that of the first excitation pulse. We give a new analysis of this effect. Since time-reversed pulse reproduction should also occur in inhomogeneously broadened solid samples, and since we observe time-reversed reproduced pulses up to 4% as intense as the input pulse, this effect may have important applications in optical signal-processing systems.

Polarization spectroscopy makes use of the polarization dependence of the nonlinear interaction between two laser beams in a gaseous medium.The laser-induced optical anisotropy is calculated using a rate equation approach, and the effect of this anisotropy on a polarized probe beam is derived.The method is useful for Doppler -free spectroscopy, for similification of molecular spectra, and for relaxation studies.A comparison with other Doppler -free saturation spectroscopy methods shows an advantage in signal -to -noise for polarization spectroscopy.Recent high resolution experiments with hydrogen, molecular sodium, and nitrogen dioxide are presented.

Two-step polarization-labeling spectroscopy of diatomic molecules is described. Formulas for relative transition intensities are derived, and their use in determining angular momenta of molecular states is discussed. The experimental apparatus and procedure used to identify twenty-four new 'S,+, 'Il, , and '3, states in Na, are deScribed. The states are identified as low-lying members, n = 3 to 14, of several molecular Rydberg series, and the dependence of their properties on principal quantum numbers is shown. These results are extrapolated to yield constants for the ground state of Na, +.

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We show that an inhomogeneously broadened sample, excited by a series of three resonant optical pulses, emits an optical signal whose electric field envelope closely approximates the convolution or cross correlation of the field envelopes belonging to two of the three excitation pulses. The convolution (cross correlation) function is obtained when the first (second) pulse in the excitation sequence is short compared to the temporal structure of the other two excitation pulses.

Ten new excited *A g states and five new excited *ng states of Na 2 have been identified with the use of the two-step polarization labeling technique. The electronic energies of these states form a molecular Rydberg series. By extrapolating the dissociation energies, vibrational constants, and rotational constants of these Rydberg states it is possible to obtain good estimates of the same quantities for the -^22g ground state of Na 2 + .PACS 35.20.Gs, 35.20.Pa In recent years, the Rydberg states of the atoms have been the subject of many experiments. However, over the last fifty years the Rydberg states of only a few diatomic molecules have been studied. 1 The vibrational-rotational structure of molecules complicates the electronic absorption spectra considerably. However, the absorption spectrum of a molecule can be greatly simplified by use of polarization labeling because transitions from only one selected vibrationalrotational level are observed.The technique of two-step polarization labeling 2 has been demonstrated to be an extremely effective tool for the study of excited states in diatomic molecules. As in polarization spectroscopy, 3 a resonant polarized pulsed pump laser induces an optical anisotropy in the levels of a single vibrational-rotational transition. A broadband linearly polarized pulsed probe laser will experience a change in polarization for frequencies resonant with transitions which have a common level with the pump-laser transition. If the probe is passed through a crossed polarizer, only those frequencies which interacted with a level of the pumped transition will be transmitted. A spectrograph can then be used to record the probe spectrum photographically. The absorption spectrum is greatly simplified because probe signals can only occur from the two levels of the pump transition. Therefore, P-, (?-, and R-branch transitions can be readily identified.Probe signals corresponding to transitions from the upper level of the pump transition to higher excited states can be distinguished from absorption of oriented ground-state molecules by the absence of two-step signals on a delayed probe spectrum. 2 Alternatively, dipole selection rules for the oriented molecules, and the different vibrational-rotational constants, provide ways to recognize new states. 4 The use of a broadband probe makes it possible to cover large regions of the optical part of the spectrum on a single photographic plate. This is very useful when searching for new excited states, since several different vibrational bands of an excited state will appear on a single plate. Such vibrational sequences yield immediate information about the vibrational-rotational structure of the excited state.In this experiment, single vibrational-rotational transitions in the X-B band of Nag (Ref. 5) were labeled by the pulsed pump laser. A series of broadband probes which covered the region from 7000 to 5500 A were used to probe transitions to higher excited states. In Fig. 1, a typical probe spectrum is shown. The long bright lines FIG. 1...

We have performed a photon-echo experiment on the 555.6-nm intercombination line of atomic Yb vapor which demonstrates that an intense laser field acts to inhibit the dephasing resulting from velocity-changing collisions. This finding implies that the optical Bloch equations, which assume that relaxation is independent of the strength of the driving field, cannot correctly describe the behavior of gas-phase samples in all regimes.