摘要:

Quantum control theory has the potential for great success in the ability to control the dynamics of gas phase molecules. Beyond 3 or 4 atoms, however, we are at present unable to compute the quantum dynamics needed to solve the control equations. An alternative applicable to larger systems, including polyatomics, clusters, surfaces, and condensed phases, is to approximate the quantum dynamics either classically or semiclassically. In this work, we present and illustrate a classical mechanical implementation of the weak field density matrix control theory. We consider I2wavepacket focusing in the gas phase and in Ar solutions, comparing with full quantum calculations when possible. The classical calculations give results that are qualitatively similar to the full quantum calculations, and thereby show that the basic phenomenology of the control theory is accessible for more complex systems in a straightforward manner, in cases when purely quantum effects, such as tunneling and interference, are not dominant. We present calcualtions of the focusing of the I2phase space distribution in Ar solution and discuss how this type of control may allow us to measure the influence of the solvent on a reacting system.

ISSN:0094-243X    

DOI:10.1063/1.45395

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We discuss recent studies of the photodissociation, internal conversion, and vibrational relaxation dynamics of I−2in ethanol [see J. Chem. Phys.98, 5375 (1993) for further details]. I−2was photoexcited at 770 nm (1.6 eV) and probed by ultrafast transient‐absorption spectroscopy at 15 wavelengths between 580 and 950 nm. Using a two‐state spectroscopic model to analyze the effect of vibrational excitation on the I−2absorption spectrum, we conclude that internal conversion and vibrational relaxation at the top of the well are extremely rapid (<0.3 ps), with loss of the final 0.3 eV of energy (v≤20) occurring on a timescale of ∼4 ps. A simple kinetic scheme for the vibrational relaxation is able to qualitatively account for the observed behavior of the transient‐absorption signals. We find good qualitative agreement with recent molecular‐dynamics simulations of I−2vibrational relaxation [I. Benjamin and R. M. Whitnell, Chem. Phys. Lett.204, 45 (1993)]. In contrast to experiment, however, the calculated vibrational relaxation rate is independent of vibrational energy; the calculated rate is too slow at the top of the well and too rapid near the bottom.

ISSN:0094-243X    

DOI:10.1063/1.45405

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Preliminary experiments on the photoinduced geminate recombination reaction of methyl‐iodide are described. The picosecond dynamics are dominated by vibrational relaxation in the methyl radical. Geminate recombination only occurs from the2I3/2electronic ground state of the iodine atom. Electronic quenching of the2I1/2excited iodine atom is too slow to allow cage recombination.

ISSN:0094-243X    

DOI:10.1063/1.45392

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A computer simulation model of a photosynthetic reaction center due to Marchi, Gehlen, Chandler, and Newton (MGCN) provides information about the mechanism for the primary electron transfer in photosynthesis. According to the model, a two step mechanism for the primary electron transfer is not plausible, and the process must occur through a super‐exchange pathway. Nevertheless, the kinetics can be nontrivial. In particular, time correlation functions obtained from the MGCN model for some of the dynamical variable pertinent to the primary process exhibit complex relaxation on the same time scale as the primary transfer.

ISSN:0094-243X    

DOI:10.1063/1.45411

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Electron transfer from Fe2+to Fe3+in acidic aqueous solutions is analyzed theoretically. Quantum chemical estimates of the tunneling rate as function of Fe2+–Fe3+distance, together with estimates of the probability distribution of this distance, suggests that the usually assumed mechanism for the electron transfer is hard to reconcile with experimental reaction rates. A new mechanism is proposed, based on recently obtained H2O–Fe ionG(r), and an assumed deprotonation of a water molecule bound to the Fe3+ionafterthe reaction barrier has been passed.

ISSN:0094-243X    

DOI:10.1063/1.45416

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Diffusive encounters of H and D atoms with O2in water are investigated with the time‐domain EPR free induction decay attenuation technique. Given the paramagnetic triplet ground state of the O2molecule, it is expected that all H–O2encounters will contribute to spin dephasing, regardless of whether reaction to form HO2occurs. In H2O the second‐order spin‐spin dephasing rate of H in the presence of O2is 2.0×1010M−1sec−1at 25 °C, with an activation energy of 14.1±0.6 kJ/mole between 8 and 80 °C. In a mixture of 90% D2O and 10% H2O, H atom dephasing is somewhat slower in the same temperature range, with a smaller activation energy. Dephasing of D atoms in 90% D2O is ca. 5%–10% slower than H, indicating that diffusion of D is slower than H. The results are analyzed in terms of other available data concerning H and O2diffusion and the reaction rate in water. It seems clear that neither the Stokes Einstein hydrodynamic theory nor classical activated rate theory applies to the diffusion of light hydrophobic gases in water. We tentatively conclude that H–O2spin exchange is slightly less efficient than can be predicted in the theoretical diffusion limit.

ISSN:0094-243X    

DOI:10.1063/1.45419

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Recent advances in density functional theory have extended the use of this method of electronic structure calculations to intermolecular interactions in hydrogen bonded systems. Computational costs have also been reduced. These new techniques have been implemented in anabinitiomolecular dynamics code. A simulation of a small liquid water sample (32 molecules) with eight valance electrons per molecule has been performed. The various structural and dynamical properties investigated are in good agreement with experiment. The feature of the electronic structure that is most interesting for the study of ultrafast reaction dynamics in water is the LUMO of the liquid. The structure of this state as determined by simulation will be compared to the properties of excess electrons in the delocalized conducting state.

ISSN:0094-243X    

DOI:10.1063/1.45381

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Femtosecond photochemical investigations of an aqueous halide (Cl−) allow to discriminate primary events triggered by an energy deposition with the subsequent photoejection of an electron and ultrafast geminate recombination. Time‐resolved absorption spectroscopy of ultrafast charge transfer in this aqueous ionic solution permits to identify the existence of multiple electronic states whose relaxation processes involve either an electron solvation phenomena or an early electron‐atom reaction. A short‐lived electronic state has been discriminated in the near infrared and assigned to an electron‐atom contact pair {e−:Cl}n’H2O. This specific solvent cage effect relaxes with a characteristic time of 770 fs in light water and does not contribute to the formation of a hydrated electron in ground state. The ultrafast electron‐atom reaction occurs in a nondiffusive regime and would represent a specific mode of deactivation of excited CTTS states towards the Cl−ground state.

ISSN:0094-243X    

DOI:10.1063/1.45400

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The development of new algorithms for computer simulation of quantum systems in solution coupled with a rapid increase in computational resources is allowing the direct observation of electronic dynamics of solutes at a molecular level on the same timescale as that probed by ultrafast transient spectroscopy. Here, we describe some of our recent theoretical approaches to the analysis of electronic spectroscopy and relaxation dynamics in solution, and outline some of the recent results obtained for the cases of energetic excess electrons in liquid water and for an aqueous halide ion. Emphasis is placed on the use of simulation results to clarify aspects of the interpretation of experimental data.

ISSN:0094-243X    

DOI:10.1063/1.45401

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Non‐adiabatic dynamical simulation of electron hydration in pure water provides a detailed source of data by which one can test the validity and improve the generality of kinetic analyses of corresponding experimental measurements. Based on a careful analysis of simulation results of a system consisting of flexible water molecules evolving via classical dynamics, the electron being described by the time‐dependent Schro¨dinger equation in the solvent field, a new mechanism is proposed to describe the hydration of electrons in pure water. According to this mechanism, a thermalization via spontaneous deexcitations across a manifold of delocalized excited states is followed by a branching between a two‐step hydration and a trapping path leading directly to the ground state of the hydrated electron. The purely two‐state mechanism with only two absorbing species is only a small subprocess of this mechanism. The analysis of two different sets of simulations, with the injected electron having about 2 eV and 2.5 eV excess energy, respectively, showed that thermalization times, branching ratios, and characteristic times for the solvation steps all depend on the initial excess energy of the electron. The comparison of experimental to the simulation results, based on the calculated spectra of each electronic species, shows that experimental data can be interpreted in terms of the mechanism proposed.

ISSN:0094-243X    

DOI:10.1063/1.45402

出版商:AIP    

年代:1994

数据来源: AIP