Wavepacket synchronization

Figure 10. Coherent wavepacket propagation initiated by absorption of ultrashort pulse. Absorption into Sj leads to coherent excited-state oscillations. Absorption into Sj or S3 may lead to synchronized (i.e., coherent) photodissociation.

Depending on the nature of the excited-state potential, wavepacket propagation following excitation may not be oscillatory. For example, in uni-molecular photodissociation from an unstable excited state (such as S2 or S3 in Figure 10), the wavepacket may simply move in one direction and never return. If the excitation pulse duration is short compared to the time required for photodissociation, then the photodissociation events in different excited molecules will be synchronized and wavepacket propagation can still be considered coherent. Experiments of this type will be discussed in the next section. 

In the IR regime, the field amplitude varies typically over the same time scale as molecular vibrational motions so that a synchronization between the nuclear motion and the laser oscillations is possible [10, 11]. This requires the definition of a time to (or a related phase S) when the field amplitude first reaches its maximum after the promotion of the initial wavepacket onto the excited electronic state. 

Due to the NAC, population is switched between the intersecting electronic states. Thereby, a superposition state and hence an electronic wavepacket is formed. In the vicinity of Coins, the time scales of the electron and nuclear dynamics are well synchronized. The energy difference between the coupled electronic states becomes very small slowing down the dynamics of the usually faster electrons to the time scale of the nuclear dynamics and below. The motion of the electronic density in the vicinity of a Coin is visualized in Fig. 8.14 for CoIn-1 of the cyclohexadiene/all-d -hexatriene system (Fig. 8.2). The underlying electronic wavepacket is created as the normalized superposition of the CASSCF-wavefunctions of ground and tirst excited state, keeping the nuclear geometry fixed. To describe the temporal evolution we take into account the time-dependent phase of both components. 

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