Time-resolved effects, quantum beats

As compared with high field, in zero and weak Hq magnetic fields 

Formally, a pattern of quantum beats can be characterized by the parameters such as the set of oscillation fi-equencies, oscillation decay time, the phase shift of oscillations, and, finally, their amplitude. Each parameter contains useful information about the processes in radiation spurs. The oscillation frequencies correspond to the splittings in the ESR spectrum of radical ions. The decay of oscillations contains information about spin relaxation times. The phase shift reflects the time delay of pair formation from its precursor. Finally, the amplitude of oscillating component is determined by the fraction of spin correlated pairs. 

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Ishii, K., Takeuchi, S., Tahara, T., Pronounced Non Condon Effect as the Origin of the Quantum Beat Observed in the Time resolved Absorption Signal from Excited state cis Stilbene, J. Phys. Chem. A 2008, 112, 2219 2227. 

The second major section (Section III), comprising the bulk of the chapter, pertains to the studies of IVR from this laboratory, studies utilizing either time- and frequency-resolved fluorescence or picosecond pump-probe methods. Specifically, the interest is to review (1) the theoretical picture of IVR as a quantum coherence effect that can be manifest in time-resolved fluorescence as quantum beat modulated decays, (2) the principal picosecond-beam experimental results on IVR and how they fit (or do not fit) the theoretical picture, (3) conclusions that emerge from the experimental results pertaining to the characteristics of IVR (e.g., time scales, coupling matrix elements, coupling selectivity), in a number of systems, and (4) experimental and theoretical work on the influence of molecular rotations in time-resolved studies of IVR. Finally, in Section IV we provide some concluding remarks. 

BittI R, van der Est A, Kamlowski A, Lubitz W and Stehlik D 1994 Time-resolved EPR of the radical pair PJ f. Qjln bacterial reaction centers. Observation of transient nutations, quantum beats and envelope modulation effects Chem. Phys. Lett. 226 249-58... 

Because of the special properties of the exponential function the light decays with the same time constant r as the population decay. The light decay can be followed by a fast detector connected to fast, time-resolving electronics. If the excited state has a substructure, e.g. because of the Zeeman effect or hyperfine structure, and an abrupt, coherent excitation is made, oscillations (quantum beats) in the light intensity will be recorded. The oscillation frequencies correspond to the energy level separations and can be used for structure determinations. We will first discuss the generation of short optical pulses and measurement techniques for fast optical transients. 

While the previous chapter emphasized the high speotral resolution achievable with different sub-Doppler techniques, this chapter concentrates on some methods which allow high time resolution. The generation of extremely short and intense laser pulses has opened the way for the study of fast transient phenomena, such as molecular relaxation processes in gases or liquids due to spontaneous or collision-induced transitions. A new field of laser spectroscopy is the time-resolved detection of coherence and interference effects such as quantum beats or coherent transients monitored with pulse Fowoiev transform spectroscopy. 

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