Franck-Condon pumping

R. W. Field Each acetylene polyad contains zero-order states that are easily accessible via plausible direct or multiple-resonance A XAU — X Franck-Condon pumping schemes. Each Vib 16,000... 

In deriving (3.29) we make use of the fact that ni oc ip . One of the important parameters affecting the magnitude of is the rate of decay Tjijii which is proportional to the Franck-Condon factor for the radiative transition J — J". As a matter of fact, this is where the term Franck-Condon pumping comes from (the term is used occasionally for denoting fluorescent population processes). 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Excitation of the coupled A2, Bi states results in the decay rate designated X3 which appears to be nearly independent of cluster size. A small increase in the value of x3 appears to occur for (S02)m clusters from the monomer (0.6 ps) to the dimer (0.9 ps), but remains constant at about 1 ps for larger cluster sizes. A likely interpretation of the observed decay process can be found in a detailed computational study [6] which reports that following the initial vertical excitation of the 1 B state, the excited state wave packet travels from the Bi state into the double wells that result from the crossing of the 1A2 and Bi states. The transition of the excited state population into the double wells of the A2 and B states is believed to lead to the decay observed in the pump-probe experiment because the potential energy well minima of both of these states are outside of Franck-Condon region for the absorption of the probe laser pulse. Therefore, ion signal is not observed once the transition has occurred. The primary discrepancy between the computational results of Ref. [6] and the... 

Fig. 2. Pump and probe scheme within a tiers picture (schematic). The zeroth order bright state which is not Franck-Condon (FC) active in the electronic transition is excited via the near IR- laser pulse. FC-active modes m later tiers having no population at t=0 are probed and their time dependent population is a measure for IVR (Vj being matrix elements connecting zeroth order states) in the molecule giving rise to an enhancement of the electronic absorption.

The pump and probe spectrum S(t) is then proportional to the overlap of the density pB(Qs, r,Franck-Condon window WFc(Qs,r,probe laser pulses, respectively ... 

Figure 2. Franck-Condon windows lVpc(Gi, r, v5) for the Na3(X) - N83(B) and for the Na3(B) Na3+ (X) + e transitions, X = 621 nm. The FC windows are evaluated as rather small areas of the lobes of vibrational wavefunctions that are transferred from one electronic state to the other. The vertical arrows indicate these regions in statu nascendi subsequently, the nascent lobes of the wavepackets move coherently to other domains of the potential-energy surfaces, yielding, e.g., the situation at t = 653 fs, which is illustrated in the figure. The snapshots of three-dimensional (3d) ab initio densities are superimposed on equicontours of the ab initio potential-energy surfaces of Na3(X), Na3(B), and Na3+ (X), adapted from Ref. 5 and projected in the pseudorotational coordinate space Qx r cos

R. D. Levine As emphasized by the speakers on femtosecond pumping schemes, an important point is that the initial excitation is localized within the Franck-Condon regime. The question is whether the sheer localization can be used to advantage to induce laser-selective chemistry (K. L. Kompa and R. D. Levine, Acc. Chem. Res. 27, 91 (1994)]. As we understand better the topography of potential-energy surfaces for polyatomic molecules, it may be possible to launch the system with such initial conditions that it will, of its own accord, proceed to cross a particular transition state and so exit toward a particular set of products. 

Although the presented numerical approach to the coupled master equations has shown that a turnover feature can be seen in vibronic dynamics appearing in the calculated pump-probe stimulated emission spectra as a function of the energy gap between the two relevant vibronic states. It is found that vibronic quantum beats cannot be observed when the energy gap becomes larger in which situation it leads to smaller Franck-Condon overlaps between the energy conserved levels. 

What is not too surprising is that the one-photon LIF spectrum (Fig. 2.13a) and two-photon OODR spectra (Fig. 2.13b) are similar, since these spectra sample the same Si level structure. The major differences between these panels lies in the intensity relationships of the bands within the progressions. These differences can be understood by recognizing that the OODR is a sequential process where a substantial time delay is introduced between the pump and the probe photons. Thus the Franck-Condon factors for the S2<—Si <— So process is a... 

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