Overlapping resonances in the coherent control of radiationless transitions: internal conversion in pyrazine

Coherent control of radiationless transitions is developed and applied to internal conversion. Conditions for active versus passive control are described and overlapping resonances are shown necessary for the phase control of radiationless transitions in molecular systems. Applications to pyrazine show the possibility of extensive control via optimized state preparation, as well as the significant role of overlapping resonances, even in the evolution of single vibrational states in S2.     

Preparation of Long-Lived States in a Multi-Spin System by Using an Optimal Control Method

The lifetime Ts of a long-lived nuclear spin state (LLS) could be much longer than the longitudinal order T₁. Many spin systems were used to produce long-lived states, including two or more homonuclear spins that couple to each other. For multiple homonuclear spins with rather small chemical shift difference, normally it is difficult to selectively control the spins and then to prepare a LLS. Herein, we present a scheme that prepares different spin orders in a multi-spin system by using optimal control and numerical calculation. By experimentally measuring the lifetime of the states, we find that for a three-spin physical system, although there are many forms of state combinations with different spin orders, each component has its own lifetime.     

Enhancement of the degree of control of photofragment distributions by laser phase modulation

The possibility of enhancing the degree of control of a transient photofragment distribution achieved by means of laser phase modulation is explored. Such a control is exerted through interference between overlapping resonances, which are populated in a superposition by the laser field. It is found that by varying the central frequency of the pump laser across the energy range of the resonance superposition created, the effects of laser phase modulation on the resonance interference mechanism change remarkably, causing significant variations on the photofragment distribution. Knowledge of these variations can be used to increase and optimize the control effects of the scheme applied. The results presented here show that the intensity of the phase modulation control effect on the fragment distribution can be increased by more than double. Thus, varying the laser carrier frequency as an additional control parameter makes the control scheme more efficient and flexible.     

Quantum control of internal conversion in 24-vibrational-mode pyrazine

Quantum control of the S(2)-->S(1) internal conversion in a complete 24-mode dimensionality model of pyrazine is demonstrated. The fully quantum mechanical study makes use of the recently developed "QP algorithm" for performing accurate computations of projected quantum dynamics and the role of overlapping resonances in control. The results are extremely encouraging, demonstrating active control over internal conversion so as to almost completely suppress the process over time scales of approximately 50-100 fs [well in excess of the natural internal conversion times (approximately 20 fs)] or to accelerate it to complete internal conversion in less than 5 fs. A number of new diagnostics are introduced to demonstrate the significance of overlapping-resonance contributions to control. Control is far better than for a reduced dimensionality model of pyrazine, presumably because of the increased degree of overlap between bound state resonances existing in the full dimensionality case.     

Evidence for the preparation of a nonstationary state from the optical double resonance spectrum of NO2

In addition to the narrow resonances from the hyperfine structure of the NO₂ excited state, a broad resonance appears in the optical radio frequency double resonance spectrum. It has a pressure-independent linewidth, corresponding to a lifetime of 0.4 ± 0.1 μs, which is attributed to the preparation to the preparation of a nonstationary state by the excitation source.     

Multichannel quantization for molecular systems. III. Complex rotation of coordinates

Multichannel quantization is performed with a complex rotation of the coordinates for both bound states and resonances. Two examples are considered: (a models corresponding to overlapping resonances with two harmonic potentials and a linear potential, for which analytical solutions can be formulated; ( photodissociation of a linear triatomic system. The purpose of these studies is two-fold (i) illustrating that the localization of the resonance wavefu upon rotation allows for the use of boundary conditions identical to those prevailing for a bound state. Our numerical experiments show that complex ro does provide a simple solution for the initialization for large interparticle distance and that closely lying resonant energies can be accurately deter (ii) examining two versions of the complex rotation method when there is more than one coordinate. One may either rotate all coordinates in the startin hamiltonian and derive coupled equations from this transformed hamiltonian, or rotate the coordinate in the coupled equations derived from the untransf hamiltonian. We observe, and demonstrate numerically, that the first version, in the case of a bound state, can only provide a real energy after conver results are obtained. The photodissociation rates obtained in the two versions of the method are shown to agree well with those obtained with a coupled approach with real coordinates. However, the occurrence of spurious widths due to truncation in the number of channels or inaccuracies in the numerical may prevent the study of narrow resonances.     

Remote bond breaking by interacting temporary anion states

The cross section for bond breaking at the site of a dissociative temporary negative ion state through the dissociative electron attachment process can be considerably enhanced by the presence of a second longer-lived temporary negative ion state elsewhere in the molecule, even one quite remote from the first. In a series of chloroalkenes possessing both C-Cl and C==C bonds separated by various distances, we show that the cross sections are determined by the lifetime of the lower anion state created by the mixing of the anion states of these two moieties, with the wave function's coefficients giving the probability that the electron is located at the dissociative site. Furthermore, the lifetime of the composite anion state can be expressed in terms of these same coefficients and the lifetimes of the unmixed resonances. We also discuss how these results may give insight into the means by which strand breaks are induced in DNA by the attachment of slow electrons.     

Intramolecular vibrational redistribution in Ne-Br2: the signature of intermediate resonances in the excitation spectrum

Quantum-mechanical simulations of the Ne-Br(2)(B,v') excitation spectra produced after vibrational predissociation in the v'=20-35 range are reported. The aim is to investigate the signature in the excitation spectra of intermediate resonances lying in the lower v相似文献   

Excitation Frequency Dependence of Ultrafast Photoinduced Charge Transfer Dynamics

The dependence of the photoinduced charge transfer rate constant on the pump pulse carrier frequency is shown to be strong, and it is considerably affected by the value of the reorganization energy of low‐frequency modes at the stage of excitation. In the area of small values of the reorganization energy, the dependence of the charge transfer rate constant on the pump pulse carrier frequency is strongly nonmonotonic that is caused by vibrational resonances and variation of the initial position of the wave packet on the term of the locally excited state. Increasing the reorganization energy smoothes the dependence. The smoothing is caused by the broadening of the vibrational resonances and their overlapping. The high‐frequency vibrational mode excitation typically accelerates the charge transfer in both areas of high and low exergonicity and decelerates it in the vicinity of the Marcus barrierless region.     

Quantitative spectroscopic and theoretical study of the optical absorption spectra of H2O, HOD, and D2O in the 125-145 nm region

The room temperature absorption spectra of water and its isotopomers D2O and HOD have been determined in absolute cross section units in the 125 to 145 nm wavelength region using synchrotron radiation. The experimental results for these B band spectra are compared with results from quantum mechanical calculations using accurate diabatic ab initio potentials. A Monte Carlo sampling over the initial rotational states of the molecules is applied in order to calculate the cross sections at a temperature of 300 K. The overall rotation of the water molecule is treated exactly. Both for the experimental and for the theoretical spectrum an analysis is made in terms of a component attributed to rapid direct dissociation processes and a component attributed to longer-lived resonances. The agreement between the results from experiment and theory is excellent for H2O and D2O. In the case of HOD in the results of theory two more resonances are found at low energy. It is demonstrated that the width of the resonances of 0.04 eV is the result of overlapping and somewhat narrower resonances in the spectra of molecules differing in rotational ground state.     

Ultrafast relaxation dynamics observed through time-resolved photoelectron angular distributions

Time-resolved photoelectron imaging of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion is presented. Photoelectron angular distributions (PADs) are qualitatively analyzed in terms of the simple s-p model that is based on symmetry arguments. The internal conversion dynamics from the first excited state (1(2)B(3u)) to the ground state ((2)B(2g)) may be observed through temporal changes in the PADs of the spectrally overlapping photoelectron features arising from photodetachment of the ground state and the excited state. A formulism for extracting the population dynamics from the β(2) anisotropy parameter of overlapping spectroscopic features is presented. This is used to extract the lifetime of the first excited state, which is in good agreement with that observed in the time-resolved photoelectron spectra.     

Overlapping Resonances,Multiple Time Regime Evolution Laws and the Sampling of Phase Space in Unimolecular Processes

Unimolecular processes can be described as the decay of an ensemble of N excited resonances coupled to K decay channels. Resonances are metastable states characterized by a complex energy whose real part is the position of the state along the energy axis while the imaginary part gives the individual decay rate of the state. Resonances usually overlap in the RRKM regime. The degree of overlap is measured by the parameter R = <I>/dE where <I> is the average of the individual decay rates of the excited resonances and dE is the average spacing between their position. In the exact degeneracy limit, that is, for an infinite value of R, (N-K) resonances have a zero width, so that a fraction of the initial excitation remains permanently trapped in the bound subspace. This trapping effect subsists in the non degenerate case but is not complete. We use a random coupling effective Hamiltonian model to discuss the effect of the degree of overlapping R, and of the number of resonances N and decay channels K, on the temporal evolution laws of the bound subspace and of the fragments. The decay law of the bound subspace and the temporal evolution of the yields in fragments exhibit several time regimes. This is due to the fact that after the diagonalization of the effective Hamiltonian, the decay widths of the resonances cluster into one group of K large widths and one group of (N-K) small ones. The trapping effect is due to the (N-K) small widths. The amount of trapping depends on the value of the degree of overlapping R, and for a given value of R, on the ratio N/K: large values of R and of N/K correspond to a large amount of trapping in the bound subspace for times long when compared to ?/<I>. The temporal evolution laws of the yields in fragment are also strongly affected by the degree of overlapping and the value of the ratio N/K. Due to the reorganization of the partial widths which follows the diagonalization of the effective Hamiltonian, we show that the nature of the dominant product can change while increasing the value of R and N/K. We also discuss the time evolution of the sampling of phase space for a specific preparation in terms of these two parameters. The volume sampled is computed using an entropic measure. When the resonances overlap, there is not enough time to completely sample phase space prior to dissociation. The fraction sampled decreases as the amount of trapping in the bound phase space increases.     

Overlapping resonances interference-induced transparency: The S(0) → S(2)∕S(1) photoexcitation spectrum of pyrazine

The phenomenon of "overlapping resonances interference-induced transparency" (ORIT) is introduced and studied in detail for the S(0) → S(2)∕S(1) photoexcitation of cold pyrazine (C(4)H(4)N(2)). In ORIT, a molecule becomes transparent at specific wavelengths due to interferences between envelopes of spectral lines displaying overlapping resonances. An example is the S(2)???S(1) internal conversion in pyrazine where destructive interference between overlapping resonances causes the S(0) → S(2)∕S(1) light absorption to disappear at certain wavelengths. ORIT may be of practical importance in multi-component mixtures where it would allow for the selective excitation of some molecules in preference to others. Interference-induced cross section enhancement is also shown.     

Direct calculation of resonant states in reactive scattering. Application to linear triatomic systems

A direct method for calculating resonant states in reactive scattering is suggested, permitting us to obtain the characteristics of multichannel resonances (partial width amplitudes). The method is based on the construction of a Laurent expansion of the scattering matrix S(? ?iΓ/2) in the complex plane. The position of the poles of the S matrix are derived by solving the dynamical problem with complex energy values. The residue at the pole gives all the information concerning the partial widths. The method is applied to a linear triatomic reactive scattering problem. The properties of the resonant states in the H + H₂ system are calculated as an example. Two broad resonances are found which have not been reported in previous calculations. The interference of overlapping resonances is shown to have a profound effect on the energy dependence of the transition probabilities.     

Heterogeneous anionic coordinated polymerization of propylene oxide: A quantitative 13c NMR analysis of the regio- and stereo-selectivity of the catalyst via microstructure analysis of polymers

Poly(propylene oxide)s synthesized with an heterogeneous catalytic system have been analyzed by ¹³C NMR spectroscopy by using DEPT experiments and sub-spectrum editing techniques. The results have been compared with those obtained from commercial products, in order to reach a greater insight into the selectivity of the catalytic system. A quantitative and reliable determination of the inversion ratio is proposed, which implies each type of carbon as well. This inversion ratio was found to be much higher than that observed in commercial PPO. It is shown that when the inversion ratio is no longer negligeable, its accurate determination is a prerequisite to a reliable approach of the tacticity distribution in regular sequences. So, a quantitative determination of the configurational diad and triad distribution is proposed which takes into account the fraction of “defect” resonances (consequence of inverted addition) overlapping with “normal” resonances (unaffected by the inverted addition) in the different sub-spectra. The main features of the catalyst activity are outlined and compared with results obtained with commercial PPO.     

Stark and field-born resonances of an open square well in a static external electric field

The resonance positions, widths (inverse lifetimes), and wave functions of a square-potential well in the presence of a static electric field are calculated by using the outgoing boundary conditions. Our study concentrates on the field-born states that, unlike the well-known Stark resonances, are not associated with the field-free bound states. The effect of a lower cutoff of the static field on the field-born resonance phenomena is studied. The feasibility of experiments, where the isolated long-lived and overlapping short-lived field-born resonances can be explored, is discussed.     

弯曲振动引致的过渡态的混沌

Molecular vibration is correlated to the motion of a pendulum and the lowly excited states are corresponding to that of the pendulum around its stable fixed point while the highly excited states are to the unstable fixed point. Specifically,the transitional state due to internal rotation is also corresponding to the unstable fixed point of the pendulum. As for a perturbed pendulum,chaos occurs first around its unstable fixed point,which is a reasonable consequence that the highly excited state and the transitional state are full with intrinsic chaotic motion. With this conjecture,HCN,its isomer HNC and the excited delocalized transitional state due to the internal rotation of H-C around the skeleton of C-N are interpreted with Morse oscillators in resonance. It is stressed that the delocalized transitional state is in multiple resonances between the H - C stretch and the bending due to that the classical bending frequency is lowered as the transitional state is approached. Multiple resonances,or the overlapping of resonances,lead to chaos as noted by Chirikov. Hence,the delocalzed transitional state can be in a chaotic state. Besides,the internal rotational state of HCP due to H atom is analyzed by this physical picture. For this purpose,an algebraic Hamiltonian for HCN,its isomer HNC and the delocalized transitional state is proposed with its coefficients elucidated by fitting with the quantal levels adopted from the literature by the quantum mechanical algorithm. The result shows that both the transitional state due to the internal rotation of H atom and the highly excited states of HCN and HNC are full of multiple resonances. Therefore,chaos is expected for these systems. Finally,all these ideas are compounded by a proposed model for unfolding the characteristics of chaos in the molecular system.     

Lifetime and predissociation yield of 14N2 b 1Piu(v=1)

The lifetime of the b 1Piu(v=1) state in 14N2 has been determined experimentally using a laser-based pump-probe scheme and an exceptionally long lifetime of 2.61 ns was found. Semiempirical close-coupling calculations of the radiative lifetime, which include Rydberg-valence interactions in the singlet manifold, are consistent with this large value, giving a value of 3.61 ns and suggesting a predissociation yield of approximately 28% for this level of the b state.     

Solid-state carbon-13 NMR Studies of vulcanized elastomers. V. Observation of new structures in sulfur-vulcanized natural rubber

The complementary application of the NMR inversion recovery measurements and the computer fitting of the overlapping spectral region is found to be a useful method for structural analysis of vulcanized natural rubber in the solid state. Since the linewidths in ¹³C-NMR spectra of solids are relatively broad compared with the differences between chemical shifts, some weak signals are completely obscured in the resulting spectra. If the resonances have sufficiently different relaxation times, such as methyl and methylene carbons, it is possible to detect neighboring heavily overlapped signals by using the inversion recovery delay τ value at which the interfering strong resonance has null intensity. The 20 resonances observed in the spectra of crosslinked rubbers are tentatively assigned to the structural units formed during the vulcanization process. It is found that vulcanizates containing smaller amounts of sulfur (1 and 3%) show insignificant changes in the NMR spectra for curing times of 30 and 90 min. Structural modifications in rubbers cured with 10% sulfur continuously increase with the increasing curing time up to 120 min, indicating a significant loss in double bonds in the later stages of the reaction.     

Resonances of CH2(a 1A1) and their roles in unimolecular and bimolecular reactions

Low-lying resonances of the CH2(a 1A1) system (J=0) in an accurate ab initio potential energy surface are studied using a filter-diagonalization method. The width of these resonances fluctuates by more than two orders of magnitude and on average increases with the energy. Analysis of the resonance states concludes that the unimolecular decay of the excited molecular system near the dissociation threshold is neither mode specific nor statistical state specific. This is apparently due to remnant regularity embedded in the largely chaotic classical phase space, as evidenced by periodic orbit analysis. As a result, the Rice-Ramsperger-Kassel-Marcus and statistical adiabatic channel models overestimate the average unimolecular decay rate. The implications of the resonances for the bimolecular C(1D)+H2 reaction are also discussed.