Dephasieg-induced phenomena are a novel class of effects that are ordinarily forbidden because the two or more relevant quantum-mechanical amplitudes add coherently and cancel out.1 When dephasing is present, however, these amplitudes no longer cancel, and previously unseen effects can be observed. Indeed, a number of novel dephasing-induced phenomena have been seen.2-5 Of particular interest, both for fundamental reasons and for diagnostic purposes, are (third-order) dephasing-induced resonances between equally populated levels of a ground electronic state,6,7 specifically, Zeeman and hyperfine coherences. Working with a sodium-seeded flame, we8 have observed these latter effects and, at higher intensities, subharmonics of them.
Using four-, six-, and eight-wave-mixing geometries, we observe subharmonic resonances in collision-enhanced spectra in a sodium-seeded hydrogen-air flame. Appearing at high intensity, these resonances occur at +/-1/2, +/-1/3, +/-1/4, and +/-1/5 times the frequency of the ground-state hyperfine splitting omega(hfs). We argue that these resonances result from higher-order processes, specifically chi((n)), where n >/= 5. In particular, to describe the resonances at +/-omega(hfs)/5, at least chi((13)) is required. These high-order collision-enhanced effects are true perturbative nonlinearities and not sequential four-wave mixing processes.
Optical four-wave mixing resonances in sodium vapor induced by collisions with inert buffer gases (He, Ar, Xe,
N2) are characterized at high resolution ( 5 MHz) using lasers tuned near the
D lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. The roles of homogeneous and inhomogeneous contributions to the resonance line shapes are clarified. The line shapes exhibit structure sharper than the predicted linewidth, that characteristic of the
3P-to-
3S transition rate. We show that this is caused by collision-induced optical pumping which introduces the ground-state lifetime into the grating relaxation physics. These long-lived gratings are exemplified dramatically by experiments in the presence of
N2 which collisionally quenches the Na
3P states. Similar results are obtained in Xe where it is argued that excimer formation with the
3P states plays a role in the population dynamics. Model calculations of the effects of sodium level degeneracy on the grating dynamics are presented to support these conclusions. Finally we present data illustrating interference between the collision-induced population grating resonances detailed here and collision-induced coherent Raman resonances between Zeeman sublevels of the ground state.