Quantum transient

Quantum transients are temporary features that appear in the time evolution of matter waves before they reach a stationary regime. They usually arise as a result of a sudden switch interaction that modifies the confinement of particles in a spatial region or after the preparation of a decaying sfafe [48]. The archetypical quantum transient phenomena is diffraction in time which consists of the sudden opening of a shutter to release a semi-infinite beam producing temporal and spatial oscillations of the time evolving wave [49]. A common feature in the mathematical description of quantum transient phenomena is the Moshinsky function, which as we have seen is closely related to the Faddeyeva function. Since in a recent review [48] there appears a discussion on transient phenomena for the dynamics of tunneling based on the present resonant state formalism [54, 66-76], here we restrict the discussion to the time evolution of quantum decay. 

A. del Campo, G. Garcia-Calderon, J.G. Muga, Quantum transients, Phys. Rep. 476 (2009) 1. 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

Kothe G, Weber S, BittI R, Ohmes E, Thurnauer M and Norris J 1991 Transient EPR of light-induced radical pairs in plant photosystem I observation of quantum beats Chem. Rhys. Lett. 186 474-80... 

BittI R, van der Est A, Kamlowski A, Lubitz W and Stehlik D 1994 Time-resolved EPR of the radical pair bacterial reaction centers. Observation of transient nutations, quantum beats and... 

Nonradiative reiaxation and quenching processes wiii aiso affect the quantum yieid of fluorescence, ( )p = /cj /(/cj + Rsiative measurements of fluorescence quantum yieid at different quencher concentrations are easiiy made in steady state measurements absoiute measurements (to detemrine /cpjj ) are most easiiy obtained by comparisons of steady state fluorescence intensity with a fluorescence standard. The usefuiness of this situation for transient studies... 

Delaire et al. [124] have reported that laser photolysis of an acidic solution (pH 2.8) containing PMAvDPA (23) and MV2 + allows the formation of surprisingly long-lived MV + - and DPA cation radicals with a very high charge escape quantum yield. The content of the DPA chromophores in PMAvDPA is as low as less than 1/1000 in the molar ratio DPA/MAA. Figure 20 shows a decay profile of the transient absorption due to MV + monitored at 610 nm [124]. The absorption persists for several milliseconds. As depicted in Fig. 20, the decay obeys second-order kinetics, which yields kb = 3.5 x 10s M 1 s. From the initial optical density measured at 610 nm, the quantum yield for charge escape was estimated to be 0.72 at 0.2 M MV2 +. ... 

Hyun et al. [345] prepared PbS Q-dots in a suspension and tethered them to Ti02 nanoparticles with a bifunctional thiol-carboxyl linker molecule. Strong size dependence due to quantum confinement was inferred from cyclic voltammetry measurements, for the electron affinity and ionization potential of the attached Q-dots. On the basis of the measured energy levels, the authors claimed that pho-toexcited electrons should transfer efficiently from PbS into T1O2 only for dot diameters below 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of the PbS/Ti02 assembly were consistent with electron transfer from small Q-dots. The measured charge transfer time was surprisingly slow ( 100 ns). Implications of this fact for future photovoltaics were discussed, while initial results from as-fabricated sensitized solar cells were presented. 

E. Broclawik, J. Haber, Quantum chemical study of the reaction of ammonia with transient oxygen species, J. Mol. Catal., 1993, 82, 353. 

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

Finally a few sentences are deserved for the vast area of DNA photochemistry. Thymine dimerization is the most common photochemical reaction with the quantum yield of formation in isolated DNA of all-thymine oligodeoxynucleotides 2-3% [3], Furthermore, a recent study based on femtosecond time-resolved transient absorption spectroscopy showed that thymine dimers are formed in less than 1 ps when the strand has an appropriate conformation [258], The low quantum yield of the reaction in regular DNA is suggested to be due to the infrequency of these appropriate reactive conformations. 

The F + H2 — HF + FI reaction is one of the most studied chemical reactions in science, and interest in this reaction dates back to the discovery of the chemical laser.79 In the early 1970s, a collinear quantum scattering treatment of the reaction predicted the existence of isolated resonances.80 Subsequent theoretical investigations, using various dynamical approximations on several different potential energy surfaces (PESs), essentially all confirmed this prediction. The term resonance in this context refers to a transient metastable species produced as the reaction occurs. Transient intermediates are well known in many kinds of atomic and molecular processes, as well as in nuclear and particle physics.81 What makes reactive resonances unique is that they are not necessarily associated with trapping... 

Other speculative mechanisms [26] may be proposed based on the presence of singlet oxygen and C = C in oxidized polymer. The reaction of the latter may lead to the transient formation of dioxetanes, the decomposition of which has an even higher quantum yield of luminescence than CIEEL mechanism [27],... 

The spectrometer supports phase cycling, asynchronous sequence implementation, and parameter-array experiments. Thus, most standard solid-state NMR experiments are feasible, including CPMAS, multiple-pulse H decoupling such as TPPM, 2D experiments, multiple-quantum NMR, and so on. In addition, the focus of development is on its extension of, or modification to, the hardware and/or the software, in the spirit of enabling the users to put their own new ideas into practice. In this paper, several examples of such have been described. They include the compact NMR and MRI systems, active compensation of RF pulse transients, implementation of a network analyzer, dynamic receiver-gain increment,31 and so on. 

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

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