Quantum beats in the phosphorescence of a photo-excited triplet state

Abstract

We have observed quantum beats in the phosphorescence of tetramethylpyrazine-d12 in a durene host crystal where the two upper spin components (Tz, 7v) decay via a common radiative channel. The initiol phase coherence required to make the beats observable was created by a 89 microwave pulse resonant with the "rz-'r v transition at 857 MHz. When, after a few microseconds, the phase coherence is lost it can be restored by a second microwave pulse and the beats then appear as an echo signal : from this experiment one obtains a dephasing time T2 = 1"2/~s.

Spin coherence and quantum beats in the phosphorescence of a photo-excited triplet state

“…The explanation of this remarkable behaviour on a molecular level has previously been given [1] ; its essence is as follows. In TMP the phosphorescence originates from a 3~.…”

“…Equation (10), together with the relations (9 and 12), from the basis for the discussion of coherent transient experiments in zero-field. Taking the system TMP in durene as an example one may, as in our previous experiment [1 ], first prepare the molecules in the ensemble in the stationary state T u, i.e. ri o = r2 ~ 0, rs~ I.…”

“…In the earlier experiments this evolution was followed by observing the magnetic dipole transition between the Ty and T~ spin levels, with energy Y and Z respectively, with the aid of a microwave detection system tuned to too = I Z-YIh -1 [10] or, indirectly, by monitoring the relative populations of the two spin levels via the intensity of the phosphorescence seen by a photomultiplier system having a time constant ~to0 -1 [8,9]. In the quantum beat experiments [1], on the contrary, one observes the non-stationary nature of the emitting state in the electric dipole radiation to the ground state by looking at the phosphorescence with a detection system capable of following the recurrence frequency tO O .…”

“…In a recent paper [1] we have reported the observation of quantum beats in the phosphorescence of tetramethylpyrazine-d12 (TMP-d12), present as a guest in a single crystal of durene. In this experiment the coherence in the triplet system was created via the coupling with a pulsed microwave field resonant with a pair of spin states of the phosphorescent triplet state.…”

EPR of photo-excited triplet states: A personal account

“…Whereas the principle of quantum interference is simple, its experimental realization in a macroscopic ensemble is less straightforward: one has to prepare all molecules in the system in a coherent manner in order to get an observable signal. In the first experiment of this kind [51] a coherent superposition of the T, Ty zero-field states was created by a brief microwave pulse resonant with the Tx-Ty transition, and quantum beats indeed could be observed in the phosphorescence. One might say that this experiment, although neat, merely proved the correctness of the quantummechanical description -a rather superfluous exercise.…”