The superposition of vibrational states produced in gaseous C13H3F by 9.6-m infrared laser radiation results in anomalous echo-envelope and free precession-beat effects. The effects arise when pulsed Stark electric fields alter the state mixing among nearly degenerate spin-rotation states. The initial eigenstates are governed by the internal spin-rotation interaction and by applied dc bias Stark electric and Zeeman magnetic fields. Unresolved modulation of the photon echo envelope caused by sudden changes of state mixtures is confirmed by analytic theory. Separate measurement of ground- and excited-state spinrotation interactions is possible for future investigations with increase in apparatus resolution. A new anomalous coherent Raman beat also caused by altering the state mixtures is investigated. The reduction of state mixing by local collisional electric fields which increase with gas pressure is qualitatively confirmed. © 1982 The American Physical Society.