Molecules In Laser Fields Research Articles

Laser-induced electron Fresnel diffraction by XUV pulses at extreme intensity

Ionization of atoms and molecules in laser fields can lead to various interesting interference structures in the photoelectron spectrum. For the case of a superintense extreme ultraviolet laser pulse, we identify a novel petal-like interference structure in the electron momentum distribution along the direction of the laser field propagation. We show that this structure is quite general and can be attributed to the Fresnel diffraction of the electronic wave packet by the nucleus. Our results are demonstrated by numerically solving the time-dependent Schr\"odinger equation of the atomic hydrogen beyond the dipole approximation. By building an analytical model, we find that the electron displacement determines the aforementioned interference pattern. In addition, we establish the physical picture of laser-induced electron Fresnel diffraction which is reinforced by both quantum and semiclassical models.

Nonstationarity and related measures for time-dependent hartree-fock and multiconfigurational models

Based on an earlier article (Eberly and Singh, Phys. Rev. D 1973, 7, 359) and related works on short-time evolution, this article proposes a many-electron formulation for the nonstationarity degree which can be assigned to quantum system at each time point. The key measure introduced, , is a nonstationarity index that can be thought of as an inverse nominal lifetime at each instance of time. The index is directly computed from the time derivative of one-electron density matrix and is a size-consistent quantity. In this article, the approach is developed for the time-dependent Hartree–Fock (TDHF), single-excitation (TDCIS), and time-dependent full configuration interaction (TDFCI) models. As a rule, nonstationarity effects are more pronounced in correlated electron systems, and a joint analysis of and the multiconfigurational character of wave functions apparently provide a deeper insight into dynamical molecular processes. The performed calculations on small molecules in laser fields show a preference for the TDCIS model when comparing TDCIS and TDHF with the “exact” TDFCI model. © 2013 Wiley Periodicals, Inc.

水分子在激光场中的密度泛函理论研究

In the framework of the time-dependent density functional theory (TDDFT) coupled non-adiabatically to molecular dynamics, the ionization and dynamic behaviors of the water molecule in laser fields with the different pulse lengths are investigaed. It is found that when the laser intensity is in the mediate region, the laser field with the longer pulse length can result in the stronger ionizion of the water molecule. With the same pulse length, the higher the laser frequency the stronger the ionization takes place. Furthermore, it is found that when the laser frequency is in the resonant region, the long pulse length can result in the dissociation of the water molecule and the distribution of residual electrons is more concentrated. When the laser frequency is out of the resonant region, although the long pulse length enhances the ionization of the water molecule, it effects less on the dynamics of ions.

Non-Born–Oppenheimer quantum chemistry on the fly with continuous path branching due to nonadiabatic and intense optical interactions

We extend our formerly proposed theory for non-Born-Oppenheimer electronic and nuclear wavepacket dynamics within on-the-fly scheme [T. Yonehara, S. Takahashi, and K. Takatsuka, J. Chem. Phys. 130, 214113 (2009)] to a case of nonadiabatic dynamics under an intense laser field: electron wavepacket in a molecule is propagated in attosecond time-scale along non-Born-Oppenheimer nuclear paths that smoothly branch due to nonadiabatic coupling and/or optical interactions. Such branching paths are determined consistently with the motion of the electron wavepackets. Furthermore, these nuclear paths are quantized in terms of Gaussian wavepackets (action decomposed function), which can be applied to nonclassical paths. Both electronic wavepacket dynamics and quantization of non-Born-Oppenheimer paths are generalized so as to include the direct effects of the classical vector potential of electromagnetic fields. In the second half of this paper, we perform numerical studies to explore nonadiabatic dynamics in a laser field by examining two cases: one is a two-state model system having an avoided crossing, and the other is two-state dynamics in HF molecule on the two low lying ab initio potential curves. Both are placed in laser fields. With the former system, we survey some basic properties of the coupling of nonadiabatic dynamics and laser interaction varying the relevant coupling parameters such as the laser timing with respect to the incident of nonadiabatic transition. This investigation will set a foundation for the future studies of control of electronic states in realistic multidimensional molecular systems. Application to the latter system shows that non-Born-Oppenheimer quantum chemistry in laser fields is indeed useful in the study of dynamics in ab initio level. Through the comparison with full quantum data, we verify that the formalism and methodology developed here work accurately. Furthermore, we attain some basic insight about the characteristics of molecules in laser fields.

Quantum and classical dynamics of a diatomic molecule in laser fields with frequency in the region producing maximum dissociation

By solving nonperturbatively the equations of Schr\odinger and of Hamilton, we have studied the time-dependent multiphoton dissociation of a diatomic molecule induced by a moderate low-frequency laser field. The photodissociation probabilities are calculated and analyzed as a function of the laser frequency, the intensity, and the pulse shape. A well-established quantum and classical result is that for laser intensities of the order of ${10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ the dissociation probability presents a maximum at the frequency ${\ensuremath{\omega}}_{\mathit{max}}\ensuremath{\sim}(0.80--0.90){\ensuremath{\omega}}_{01}$, where ${\ensuremath{\omega}}_{01}$ is the transition frequency from the ground to the first excited vibrational state (redshift phenomenon). In this work, we go further and explore the quantum and classical effects of radiation at the optimum frequency ${\ensuremath{\omega}}_{\mathit{max}}$ on the overall vibrational excitation and dissociation dynamics. First, it is shown that both quantum and classical results predict that ${\ensuremath{\omega}}_{\mathit{max}}$ continues to be the optimum frequency for photodissociation even at the order of $10\phantom{\rule{0.3em}{0ex}}\mathrm{TW}∕{\mathrm{cm}}^{2}$, i.e., low intensities. The quantum results show a multipeak structure versus laser frequency, which is attributed to resonant multiphoton transitions, while the classical results show a smooth curve with a broad maximum at ${\ensuremath{\omega}}_{\mathit{max}}$ which is explained by phase space arguments. Second, it is found that in both quantum and classical approaches ${\ensuremath{\omega}}_{\mathit{max}}$ marks a transition in the effects of turn-on time of the pulse shape on dissociation probability: for $\ensuremath{\omega}l{\ensuremath{\omega}}_{\mathit{max}}$ the gradual turn-on of the field leads to a noticeable reduction of the photodissociation probability, while for $\ensuremath{\omega}g{\ensuremath{\omega}}_{\mathit{max}}$ this effect is of minor importance. A classical interpretation of this finding is given, which is based on stickiness effects in phase space. Finally, the crucial role of ${\ensuremath{\omega}}_{\mathit{max}}$ is further demonstrated in the time dependence of bound-state occupation probabilities. The total survival probability decreases faster with time for $\ensuremath{\omega}l{\ensuremath{\omega}}_{\mathit{max}}$ rather than for $\ensuremath{\omega}g{\ensuremath{\omega}}_{\mathit{max}}$. Further, for $\ensuremath{\omega}g{\ensuremath{\omega}}_{\mathit{max}}$ the bound-state occupation probabilities exhibit multiphoton Rabi-type oscillations where more than two vibrational states are involved. These phenomena are predicted by both quantum and classical dynamics, although there are secondary differences which are revealed and discussed.

Molecular photoelectron spectrum in ultrashort laser fields: Autler-Townes splitting under rotational and aligned effects

The molecular dynamics of Autler-Townes (AT) splitting in a multiphoton resonance ionization spectrum is numerically simulated using the time-dependent wave packet method for a rotating ${\mathrm{Na}}_{2}$ molecule with a linearly polarized laser pulse. Employing the Fourier grid method and Legendre polynomials as basis functions, the initial wave packet is directly propagated using the split operator method. Wave packet dynamics simulations demonstrate that the AT splitting results from rapid Rabi oscillations induced by an ultrashort intense laser pulse in rovibrational coherent states that belong to different electronic terms. But Rabi oscillations will be damped because of the effects of rotation and alignment of the molecule in laser fields. The AT splitting in a photoelectron spectrum could be observed for a well-aligned molecule with sufficiently rapid Rabi oscillations. Only an adequately short laser pulse can induce AT splitting in a photoelectron spectrum. The effect of initial rotational temperature on the molecular dynamics is also studied.

Multiphoton ionization of the hydrogen moleculeH2

Generalized cross sections for two-, three-, and four-photonionization of H2 by linearly polarized laser light have beencalculated. Three different situations are considered: the laserpolarization being parallel or perpendicular to the internuclearaxis and the case of an isotropical distribution of H2molecules. The main features of the spectra are discussed. Inorder to assess the reliability of the results, two completelydifferent ab initio methods have been developed and usedfor the parallel orientation; good agreement is found betweenthem. The impact of the present results on the question ofalignment of molecules in laser fields is discussed.

A numerical ab initio study of harmonic generation from a ring-shaped model molecule in laser fields

When a laser pulse impinges on a molecule which is invariant under certain symmetry operations selection rules for harmonic generation (HG) arise. In other words, symmetry controls which channels are open for the deposition and emission of laser energy—with the possible application of filtering or amplification. We review the derivation of HG selection rules and study numerically the interaction of laser pulses with an effectively one-dimensional ring-shaped model molecule. The harmonic yields obtained from that model and their dependence on laser frequency and intensity are discussed. In a real experiment obvious candidates for such molecules are benzene, other aromatic compounds, or even nanotubes.

Multiphoton excitation of an ammonia molecule in laser fields

Multiphoton excitation of an ammonia molecule in laser fields

Three-dimensional Cartesian finite element method for the time dependent Schrödinger equation of molecules in laser fields

A finite element (FE) method in three-dimensional Cartesian coordinates is described to solve the time dependent Schrödinger equation for H+2, H2, and H+3 in the presence of time dependent electromagnetic fields. The ionization rates and harmonic generation spectra have been calculated for these molecules for field directions parallel or perpendicular to the molecular axis. Nonlinear optical susceptibilities of H+2 have been also obtained for different laser field directions. The time dependent Hartree–Fock results are compared to frozen core calculations for H2. Comparisons of present FE numerical results with previously published calculations show the FE method reproduces perturbative results and can also treat nonperturbatively the effect of intense short laser pulses as the method includes both bound and continuum electronic states.

Molecules in laser fields

Molecules in laser fields