Picosecond Tri-Transition and Two-Color Photon Echoes in a Doped Molecular Solid

Picosecond tri-level photon echoes are generated among vibronic transitions of pentacene doped into a naphthalene host. The echoes are generated with three excitation pulses of which the first one, at ω1, always excites a vibronic transition in the pentacene molecule. With the second excitation pulse at ω2 and the third at ω1, a tri-transition echo (TTE) is formed. With the time ordering of the second and third pulse reversed, a connected two-color stimulated echo (C2CSE) is generated. It is shown that, for small pulse angles, the low-temperature decay of both echo effects is identical and that a smooth transition of one echo effect into the other occurs at the overlap in time between the second and third excitation pulse. Observation of these echoes further indicates that the inhomogeneous broadening at the selected transitions is strongly correlated