A mathematical simulation of intramolecular proton phototransfer

A system of equations for the matrix elements of the density operator of a seven-level model molecule interacting with a light pulse was solved numerically to determine the time dependences of the populations of molecule states at various radiation pulse parameters and parameters characterizing radiative and nonradiative spontaneous molecule transitions and reversible transitions between some of its states. The results were used to characterize the photoisomerization of molecules between states with different positions of the proton of the intramolecular H-bond (the keto and enol forms). Examples of oscillating molecular state population modulation in isomer-isomer tunnel proton shifts are given. Changes in the development of photoionization in time as molecular parameters and radiation pulse width and intensity changed were considered. An analysis of the results obtained is an example of the use of mathematical simulation of intramolecular dynamics for increasing the effectiveness of using spectral-time data in the determination of the mechanism of proton phototransfer in molecules with intramolecular H-bonds     

Quantum-classical simulation of the photoisomerization during the transformation of ultrashort light pulses by a molecule

The dynamics of photoisomerization of a model molecule during its transformation of ultrashort (with a duration much shorter than the lifetime of the resonant excited electronic state) light pulses is simulated numerically. The two-level electronic subsystem of the molecule is described using the quantum theory, while the nuclear subsystem (taking into account the two isomeric states of the molecule) and the radiation field are described using the classical theory. The ranges of the carrier frequency, the peak intensity, and the durations of nπ sinusoidal pulses (n = 1–10) irradiation with which results in the photoisomerization of molecules of the type under study (for example, cyanine dyes) are determined from the analysis of solutions to self-consistent equations that describe the motion of the “isomerization oscillator” and the time evolution of the population amplitude of the resonant electronic state of the molecule. Each of these non-overlapping ranges corresponds to a particular value of n. Bifurcation values of the above parameters of the light pulse are boundaries of these ranges.     

Mathematical modeling of photoisomerization with intramolecular proton transfer

A system of equations describing time changes in the matrix elements of the density operator of a seven-level model of a molecule interacting with a light pulse taking into account spontaneous (including collective) decays of molecule excited states is suggested. Model parameters were selected to allow us to perform modeling of the photoisomerization of a molecule with two isomeric states with different stable proton positions on an intramolecular H-bond by numerically solving the suggested system of equations for density operator matrix elements. An analysis of the characteristic time dependences of the population of states of the model under consideration showed that proton phototransfer in the collective decay of various isomeric states of a molecule in an excited electronic state can be one of effective mechanisms of the photoisomerization of molecules whose structure is described by the model.     

Dynamics of localized wave packets of rotational states of a molecule in a strong laser field

Numerical simulations are performed to examine the rotational dynamics of a molecule in a strong laser field when the molecular axis is initially oriented in a certain direction. The results obtained by solving the quantum-mechanical problem are compared with those computed in the framework of classical mechanics. It is found that certain characteristics of rotational motion cannot be described by classical theory, particularly for light molecules. It is demonstrated that the axis of a heteronuclear molecule can be reversed by tunneling.     

Numerical simulation of intramolecular dynamics during dual fluorescence

The dynamics of transformation of a light pulse by a five-level model molecule whose secondary emission spectrum can contain two fluorescence bands is simulated. The system of equations that determine the time behavior of the matrix elements of the statistical operator of the molecule interacting with the light pulse is numerically solved. From this solution, the time dependences of the populations of the molecular states are determined for different values of the parameters of the irradiation pulse, which is described in terms of the classical theory, and of the parameters that characterize the rates of radiative and nonradiative spontaneous transitions of the molecule. Based on particular examples of the choice of these parameters, it is demonstrated that the mechanism by which dual fluorescence occurs in molecules with intramolecular hydrogen bonds can be efficiently established from the numerically simulated intramolecular dynamics.     

Detailed mechanism of trans–cis photoisomerization of azobenzene studied by semiclassical dynamics simulation

A realistic dynamics simulation study is reported for the trans–cis photoisomerization of azobenzene. The simulation follows both nπ* and ππ* excitations and each excitation is induced by a 50 fs (FWHM) laser pulse. The simulation results show that, for both excitations, the reaction path is predominated by the rotation coordinate of the NN bond. The simulation finds that the CNN inversion angles expand as soon as the rotation starts. The expansion of the CNN bond angles permits the molecule to rotate efficiently. It is therefore suggested that the photoisomerization of trans-azobenzene follows an inversion-assisted rotation path. These simulation results are significant for understanding the mechanism of this important process.     

Peculiarities of the rotational dynamics of an ensemble of diatomic homonuclear molecules in a strong laser field

The rotational dynamics of an ensemble of molecules in a laser field has been studied. The results obtained within the framework of the quantum-mechanical and classical approaches have been compared. The features of the rotational dynamics connected with the non-classical character of the system have been described. It was taken into account that before the interaction with a laser pulse the ensemble was in a thermodynamically equilibrium mixed state. The case of an ultrashort laser pulse has been considered as well.     

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.     

Polarization-dependence of optical spatial solitons in photoisomerization material

Experimental observation of spatial solitons in azobenzene-doped organic polymer is demonstrated in dye-doped polymer bulk material. Solitons cannot only be formed in this material with linearly polarized light, but also with circularly polarized light. An interesting phenomenon is revealed that the soliton is polarization-dependent. The solitons with the same intensity but with different polarizations have different widths. The experimental results are further theoretically explained with the dynamics model based on a photochemical process, namely photoisomerization.     

Anisotropy of ensembles of free polyatomic photoisomers

In order to describe the polarization response of an ensemble of molecules undergoing structural transformations (photoisomerization) under collisionless conditions, we have calculated the orientational correlation functions. We assume that changes in molecular structure can be considered as instantaneous on the molecular rotation scale. We have obtained general expressions for the anisotropy when the original molecule and the photoisomer are asymmetric tops. We have performed anisotropy calculations for steady-state experimental conditions and a number of limiting situations, when the characteristic times of the photoreaction are much shorter or much longer than the molecular reorientation times and when the original molecule and the photoisomer are planar tops. We have shown that detecting the polarization response allows us to estimate the characteristic times of the photoreaction and to determine the intramolecular orientation of the transition dipole moments for transitions with absorption and emission of light. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 582–587, September–October, 2006.     

On nonlinear propagation of extremely short pulses in optically uniaxial media

Nonlinear wave equations describing the propagation of optical pulses of duration up to a period of electromagnetic oscillations in transparent media with uniaxial optical anisotropy are derived on the basis of a quantum-mechanical model of material response. The electron and electron-vibrational nonlinearities, electron and ion dispersion, and diffraction are taken into account. It is shown that the inclusion of the electron response alone leads to a system of two constitutive equations for the ordinary and extraordinary polarization components. When a pulse propagates across the optical axis, this system is reduced to an inhomogeneous model of the Henon-Heiles type and, hence, generalizes the Lorentz classical electron model. In order to take into account stimulated Raman scattering (SRS) processes, an anisotropic analog of the Bloembergen-Shen quantum-mechanical model taking into account the population dynamics of SRS sublevels is obtained. The generation of an extraordinary wave video pulse with the help of the high-frequency ordinary component in the Zakharov-Benney resonance mode is investigated. Some analytic soliton-like solutions in the form of propagating bound states of ordinary and extraordinary video pulses corresponding to different birefringence modes are considered and their stability to self-focusing is analyzed.     

Controllable snail-paced light in biological bacteriorhodopsin thin film

We observe that the group velocity of light is reduced to an extremely low value of 0.091 mm/s in a biological thin film of bacteriorhodopsin at room temperature. By exploiting unique features of a flexible photoisomerization process for coherent population oscillation, the velocity is all-optically controlled over an enormous span, from snail-paced to normal light speed, with no need of modifying the characteristics of the incident pulse. Because of the large quantum yield for the photoreaction in this biochemical system, the ultraslow light is observed even at low light levels of microwatts, indicating high energy efficiency.     

Formation of nuclear molecules in cluster radioactivity on interpretation of the cluster radioactivity mechanism

The basis for cluster radioactivity is the property of nuclei of light isotopes of elements heavier than lead to spontaneously form clusters—nuclei of light elements—from valence nucleons, which gives rise to asymmetric nuclear molecules. The cluster formation proceeds through successive excitation-free transfer of valence nucleons to the α particle and to subsequent light nuclei. Nuclear molecule formation is accompanied by a considerable amount of released energy, which allows quantum-mechanical penetration of the cluster through the exit Coulomb barrier.     

Autler-Townes splitting in the multiphoton resonance ionization spectrum of molecules produced by ultrashort laser pulses

We report for the first time the proper conditions to observe Autler-Townes splitting (ac-Stark splitting) from vibrationally coherent states belonging to the different electronic terms of a diatomic molecule. Wave packet dynamics simulations demonstrate that such a process is feasible by multiphoton resonance ionization of the molecule Na2 with a single ultrashort intense laser pulse. With the ultrahigh time resolution of a femtosecond laser pulse, one can directly measure the absolute value of the transition dipole moment between any kinds of molecular states by this kind of Autler-Townes splitting, which is a function of the internuclear distance R.     

A theory of magneto-optical rotation in diamagnetic molecules of low symmetry

A quantum-mechanical theory for the Faraday effect in polyatomic molecules is developed along the lines of the general theory on natural optical rotation presented by Rosenfeld, Condon, et al.

The stationary perturbation of a magnetic field and the time-dependent perturbation of a light wave are treated simultaneously by means of a second-order time-dependent perturbation theory. The treatment is restricted to molecules having non-degenerate wavefunctions and zero spin. The moment of a single molecule, in the presence of a magnetic field and a light wave, is calculated in Part I. In Part II the average electric and magnetic moments per molecule are used in macroscopic optical equations to determine the Verdet constant, which is shown to be non-vanishing for the molecules under consideration. A discussion of the results will be given subsequently, in Part III of this work.     

Saturable absorption dynamics of DODCI

The saturable absorption dynamics of DODCI in a femtosecond dye laser is studied theoretically. The N-isomer and P-isomer photoisomerization dynamics is included. The wavelength region between 570 nm and 650 nm is considered where the N-isomer and P-isomer absorption dynamics changes from short-wavelength to long-wavelength s₀-s₁ excitation. The slow saturable absorber DODCI shortens a circulating pulse in a laser oscillator down to femtosecond duration if the gain medium compensates the absorber losses. Fast local relaxation in the S₁ -state in the case of short-wavelength excitation and fast level refilling in the S₀ -state in the case of long-wavelength excitation facilitate the pulse shortening and the background signal suppression.     

Magnetic trapping of strongly-magnetized Rydberg atoms

Effective magnetic moments of drift Rydberg atoms in strong magnetic fields are obtained for different energy and angular-momentum states. Classical two-body trajectory calculations and quantum-mechanical one-body calculations are employed. For heavy atoms such as rubidium, the trapping dynamics can largely be explained by the net magnetic moment due to the cyclotron and the magnetron motion of the Rydberg electron. In light Rydberg atoms such as hydrogen, the intrinsic two-body nature of the dynamics becomes manifest in that the ionic motion significantly contributes to the effective magnetic moment. Also, light drift Rydberg atoms exhibit an anisotropic response to field-inhomogeneities parallel and transverse to the magnetic-field lines. The results are relevant to magnetic trapping of Rydberg atoms in strong-magnetic-field atom traps.