The behavior of the photon echo produced by an inhomogeneously broadened spectral line is analyzed, with particular emphasis on the effect of spatial degeneracy of the molecular energy levels. The theoretical results are used to aid in the identification of the states involved in the echoes produced from gaseous SF6 excited by pulses of CO2 laser radiation near 10.6 μ. The analysis demonstrates that for the |Δj|=1 transitions (P and R branches) the echo induced by two linearly polarized pulses propagating along the same direction, with an angle ψ between their electric vectors, is polarized at an angle greater than ψ with respect to the first pulse, and the echo intensity decreases to a minimum, but not zero, as ψ approaches π2. An exception is the j=1j=0 transition, for which the echo is always polarized along the second pulse and the intensity varies as cos2ψ. For Δj=0 transitions (Q branch) the echo polarization generally lies at an angle smaller than ψ, i.e., between the first two pulses, and its intensity also decreases as |ψ| approaches π2. The exceptions for the Δj=0 transitions are the cases j=12j=12 and j=1j=1. In the former case, the echo is polarized at an angle 2ψ with respect to the electric field vector of the first pulse, and the echo intensity is independent of ψ; whereas in the latter case, the echo behaves like that for j=1j=0. The dependence of the echo behavior on the angular momentum of states involved in the transition can serve to identify the transitions when measurements of the echo polarization and intensity are possible. © 1969 The American Physical Society.
