Optically prepared state

The time-frequency bandwidth relationships underlie the above pulse width considerations but also appear in another light if the nature of the resonant molecule laser interaction is carefully considered. A 1 ps visible pulse carries "3 cm spectral width, and thus it is pertinent to enquire, from the molecular perspectivey how many optically prepared states are to be found within that bandwidth This leads to the need to tailor the experiment so that the molecular dynamics probed are indeed intrinsic to the natural... 

Neither the bright nor the dark states are stationary in the sense that they are not sharp energy eigenstates of the Hamiltonian. To determine the eigenstates we need to allow for the weak residual coupling between the bright and dark states. We take the coupling to be weak because if it were not we would not be able to prepare the optically prepared state as it would couple to other states while our laser pulse was still on. 

This is the operative technical term. It is equivalent to our earlier condition that there is a definite initial state and not an ensemble. The optically prepared state is then a pure state in the quantum mechanical sense, namely, it can be expressed as a superposition of states, each with its own phase, hence coherent. ... 

V can bridge the energy gap between adjacent states, E/Z) > 1, for the decay of the optically prepared state to be faster than the rate of revival. ... 

The expression for the nonradiative decay rate of the optically prepared state sm ) now becomes... 

The Rydberg state which is optically prepared in a typical ZEKE experiment is usually directly coupled to the continuum [45c, 57]. Other considerations being absent, it should decay promptly, possibly with a stable, trapped component. The point is that the initially prepared state is also directly coupled to many other states, due both to external perturbations [37] and to intramolecular coupling [3b]. The conclusion that the initial state has two components, one that decays promptly and one that is trapped, is thus only valid in zero order (so-called golden rule limit). One needs to allow for the coupling terms represented by V and U. 

Prompt and delayed ionization is familiar for very energy rich molecules. The special feature of high Rydberg states is the initial state that is optically prepared, a state directly coupled to the continuum on the one hand and to a very dense bound manifold on the other. The dynamical theory necessary to describe such states has been reviewed, with special reference to the extremely long-time decay. It is suggested that this resilience to decay is due... 

Photodissociation can be simplified into the three steps of optical preparation or excitation, evolution through the transition state, and production of final products, respectively. In the optical preparation stage... 

It is sufficient to determine the quantity Rxk in the second order with respect to the CC photon interaction. We further assume that the optical preparation of the excited state by the applied field E is short compared to the emission process and, finally, we neglect anti-resonant contributions. When calculating F(u> t) we also have to perform a summation with respect to the transversal polarization and a solid angle integration. Introducing dm = dmem where eTO is the unit vector pointing in the direction of the transition dipole moment one gets... 

Figure 8. Time-resolved photoelectron spectra revealing vibrational and electronic dynamics during internal conversion in DT. (a) Level scheme in DT for one-photon probe ionization. The pump laser prepares the optically bright state S2. Due to ultrafast internal conversion, this state converts to the lower lying state Si with 0.7 eV of vibrational energy. The expected ionization propensity rules are shown S2 —> Do + e (ei) and Si —> D + (b) Femtosecond time-...

In Section III.A.l we did not discuss the way the surface emission is excited. The radiative behavior of the surface shows that emission (normal to the surface) is observed as soon as the K = 0 state is prepared. This state may be prepared either by a short ( 0.2ps) resonant pulse, or by relaxation from higher, optically prepared excited states. It is obvious that the quantum yield of the surface emission will critically depend on the excitation, owing to intrasurface relaxation accelerated by various types of fission processes (see Fig. 2.8) and in competition with fast irreversible transfer to the bulk (3.30), which is also a surface relaxation, at least at very low temperatures. Thus, the surface excitation spectra provide key information both on the upper, optically accessible surface states and on the relaxation mechanisms to the emitting surface state K = 0. 

ISC from the optically prepared singlet state populates one or two low-lying A" triplet states in a few hundreds of femtoseconds, see Sect. 3. Triplet states are initially populated hot, that is nonequilibrated both in terms of the molecular structure and the medium. Relaxation processes, which occur on the timescale of picoseconds to nanoseconds (depending on the medium), will be discussed in Sect. 5. Herein, we will deal with thermally equilibrated (relaxed) lowest triplet states and their theoretical as well as experimental characterization. 

Phosphorescence of s-triazine has been observed by Ohta et al. following excitation of the 6o band of the Si — So transition. Values for the phosphorescence lifetime and quantum yield were reported. The effects of rotational excitation on the yields and decays of the fast and slow components of Si state s-triazine fluorescence have been studied. Excitation along the rotational contours of the 6j and 6o bands revealed that the fast component showed little rotational level dependence in contrast to the slow component. This behaviour was interpreted in terms of an increase in the number of triplet levels coupled to the optically prepared singlet levels with increasing angular momentum quantum number, J. A broad emission feature present in addition to narrowline fluorescence from rovibronic levels of 6 or 6 in S, s-triazine has been observed and the rotational level dependence of its quantum yield and decay over a range of pressures reported... 

Typical electron injection times are faster than, or comparable with, relaxation of the optically prepared Franck-Condon MLCT excited states. Hence, the electron injection can actually occur directly from the Franck-Condon state. This is the case of [Fe(4,4 -(COOH)2-bpy)2(CN)2] which reacts from its optically prepared MLCT state [304], before its deactivation through the lower-lying LF states can occur. The relaxation time of the MLCT state of the actual [Ru(4,4 -(COOH)2-bpy)2(NCS)2] sensitizer was determined as <75 fs [82], only a little slower than electron injection itself. Hence, it is possible that electron transfer occurs from both the optically prepared MLCT and relaxed MLCT states. 

The nature of the initially prepared state is of paramount importance in determining (1) the subsequent IVR dynamics of a species and (2) the way in which the dynamics is manifest in time-resolved and time-integrated fluorescence. The theoretical picture, reviewed in Section III B of the manifestations of IVR in time-resolved fluorescence relies on the assumption that single zero-order states act as doorway states in optical transitions from and to... 

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