Ultrashort Intense Laser Fields Research Articles

Multiple Explosion Pathways of the Deuterated Benzene Trication in 9-fs Intense Laser Fields

The fragmentation of deuterated benzene (C6D6) in ultrashort intense laser fields (9 fs, 1 x 10(15) W/cm2) is studied by the ion-coincidence momentum imaging technique. Five two-body and eight three-body Coulomb explosion pathways from the trication (C6D6(3+)), associated with the deprotonation and ring-opening reactions, are identified. It is found from the fragment momentum correlation that all the observed three-body explosion processes proceed sequentially via the two-body Coulomb explosion forming molecular dications, C(m)D(n)(2+), with (m,n) = (6,5), (5,5), (5,4), (4,4), (4,3), and (3,3), which further dissociate into pairs of monocations. The branching ratio of the fragmentation pathways estimated from the number of the observed coincidence events indicates that the fragmentation is nonstatistical.

Electron–ion coincidence momentum imaging of molecular dissociative ionization in intense laser fields: Application to CS 2

An electron–ion coincidence momentum imaging apparatus has been developed and applied to dissociative ionization of CS 2 in ultrashort intense laser fields. Photoelectron images of CS 2 in 35 fs intense laser fields (2 × 10 13 W/cm 2, 800 nm, linearly polarized) recorded in coincidence with the parent ion, CS 2 +, show clear concentric ring patterns due to the above-threshold ionization (ATI) process. On the other hand, broad structureless distributions elongated along the direction of the laser polarization are observed in the coincidence electron images for the CS + and S + fragment ions. The difference in the electron images indicates that the dissociative ionization does not proceed sequentially by the formation and photodissociation of CS 2 + in intense laser fields.

Publisher’s Note: “Efficient ejection of H3+ from hydrocarbon molecules induced by ultrashort intense laser fields” [J. Chem. Phys. 129, 104302 (2008)

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Efficient ejection of H3+ from hydrocarbon molecules induced by ultrashort intense laser fields

The ejection processes of hydrogen molecular ion H(3)(+) from 12 kinds of hydrocarbon molecular species, methanol, ethanol, 1-propanol, 2-propanol, acetone, acetaldehyde, methane, ethane, ethylene, allene, 1,3-butadiene, and cyclohexane, induced by intense laser fields (approximately 10(14) W/cm(2)) have been investigated by time-of-flight mass spectroscopy. The observation of the H(3)(+) production with the kinetic energy range of 3.5-5.0 eV from doubly ionized ethylene, allene, 1,3-butadiene, and cyclohexane, which have no methyl groups, showed the existence of the ultrafast hydrogen migration processes that enables three hydrogen atoms to come together to form H(3)(+) within a hydrocarbon molecule.

Acetylene-vinylidene isomerization in ultrashort intense laser fields studied by triple ion-coincidence momentum imaging

The isomerization of acetylene via hydrogen migration in intense laser fields (8 x 10(14) W/cm2) has been investigated by coincidence momentum imaging of the three-body Coulomb explosion process, C2H2 (3+)-->H+ + C+ + CH+. When ultrashort (9 fs) laser pulses are used, the angle between the momenta of C+ and H+ fragments exhibits a sharp distribution peaked at a small angle ( approximately 20 degrees ), showing that the hydrogen atom remains near the original carbon site in the acetylene configuration. On the other hand, a significantly broad distribution extending to larger momentum angles ( approximately 120 degrees ) is observed when the pulse duration is increased to 35 fs, indicating that the ultrafast isomerization to vinylidene is induced in the longer laser pulse.

Nonadiabatic rotational excitation of benzene by nonresonant intense femtosecond laser fields

We investigated the nonadiabatic rotational excitation of the oblate symmetric-top molecule, benzene, induced by nonresonant intense ultrashort laser fields. By employing a quantum-state resolved probe using ns laser pulses, rotational excitation up to J = 10 was observed for the irradiation of a molecular ensemble, initially cooled to 0.5 K in an adiabatic expansion, by the femtosecond laser pulse with the intensity of 2.2 TW/cm 2. The observed excitation was analyzed by the aid of quantum mechanical calculations. These calculations show the systematic change in the excitation pathways for different K, which is characteristic of nonadiabatic rotational excitation in symmetric-top molecules.

Dalitz plot analysis of Coulomb exploding O3 in ultrashort intense laser fields

The three-body Coulomb explosion of O3, O3(3+)-->O++O++O+, in ultrashort intense laser fields (2x10(15) W/cm2) is studied with two different pulse durations (9 and 40 fs) by the coincidence momentum imaging method. In addition to a decrease in the total kinetic energy release, a broadening in the Dalitz plot distribution [Philos. Mag. 44, 1068 (1953)] is observed when the pulse duration is increased from 9 to 40 fs. The analysis based on a simple Coulomb explosion model shows that the geometrical structure of O3 remains almost unchanged during the interaction with the few-cycle intense laser fields, while a significant structural deformation along all the three vibrational coordinates, including the antisymmetric stretching coordinate, is identified in the 40 fs intense laser fields. The observed nuclear dynamics are discussed in terms of the population transfer to the excited states of O3.

Publisher’s Note: Electronic and Nuclear Responses of Fixed-in-SpaceH2Sto Ultrashort Intense Laser Fields [Phys. Rev. Lett.97, 243002 (2006)

Publisher’s Note: Electronic and Nuclear Responses of Fixed-in-Space<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">S</mml:mi></mml:math>to Ultrashort Intense Laser Fields [Phys. Rev. Lett.<b>97</b>, 243002 (2006)

High‐order harmonic generation of heteronuclear diatomic molecules in intense ultrashort laser fields: An all‐electron TDDFT study

AbstractWe present a time‐dependent density functional theory (TDDFT), with proper asymptotic long‐range potential, for nonperturbative treatment of multiphoton processes of homonuclear and heteronuclear diatomic molecules in intense ultrashort laser fields. A time‐dependent two‐center generalized pseudospectral method is presented for accurate and efficient treatment of the TDDFT equations in space and time. The procedure allows nonuniform and optimal spatial grid discretization of the Hamiltonian in prolate spheroidal coordinates and a split‐operator scheme in the energy representation is used for the time propagation of the individual molecular spin‐orbital. The theory is applied to a detailed all‐electron study of multiphoton ionization (MPI) and high‐order harmonic generation (HHG) processes of N2 and CO molecules in intense laser pulses. The results reveal intriguing and substantially different nonlinear optical response behaviors for N2 and CO, despite the fact that CO has only a very small permanent dipole moment. In particular, we found that the MPI rate for CO is higher than that of N2. Furthermore, while laser excitation of the homonuclear N2 molecule can generate only odd harmonics, both even and odd harmonics can be produced from the heteronuclear CO molecule. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Electronic and Nuclear Responses of Fixed-in-SpaceH2Sto Ultrashort Intense Laser Fields

The Coulomb explosion dynamics of H2S, H2S3+-->H+ +S+ + H+, in ultrashort intense laser fields (12 fs, approximately 2 x 10(14) W/cm2) is studied by the coincidence momentum imaging of the three fragment ions. Different electronic and nuclear responses are identified depending on the direction of laser polarization epsilon in the molecular frame. The dependence can be interpreted in terms of the electronic and bonding characters of charge transfer states of H2S coupled to the electronic ground state.

Structure and dynamics of cluster plasmas created by ultrashort intense laser fields

Ultrashort, intense laser-cluster interaction and the resultant structure formation are investigated in detail by employing a particle-in-cell code including relaxation and ionization processes. In addition to a laser-induced polarization field, we have found that an ambipolar radial electric field and internal and external sheath fields play a key role in the explosion dynamics of the cluster. Electrons and ions show complex behavior leading to a prominent dip in the structure of the electron energy distribution, and a double-layered structure in ion density and energy distributions for higher charge states.

Electron correlation of one-dimensionalH2in intense laser fields: Time-dependent extended Hartree-Fock and time-dependent density-functional-theory approaches

Ionization and high-order harmonic generation of the one-dimensional (1D) ${\mathrm{H}}_{2}$ molecule in intense ultrashort laser fields are investigated using several current approximations for electron dynamics. Single- and double-ionization probabilities are compared with exact results. It is found that for the ground state $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$, time-dependent extended Hartree-Fock gives generally comparable results except in the plateau region. The adiabatic local density approximation and time-dependent optimized effective potential with self-interaction correction (TDKLI) methods underestimate the ionization probabilities with no plateau and knee for double ionization contrary to the exact results. For the triplet excited state $A\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\Sigma}_{u}^{+}$, where exchange is important, the TDKLI results agree well with the exact results. The exact double-ionization probabilities suggest the need for accurate pair-correlation functions.

On the mechanism of strong-field double photoionization in the helium atom

We perform time-dependent quantum calculations on the ionization dynamics of helium in intense ultrashort laser fields. A linear model of the atom is used to extract single- and double-ionization yields for a 780 nm excitation. For an intensity of 1015 W cm-2 clear evidence is found for a direct double-ionization process which is caused by rescattering of an electron from the core in the presence of the field. A momentum-space analysis of the wavefunctions shows that in this case both electrons are preferentially emitted in the same direction and with different momenta.

Enhanced ionization of in ultra-short intense laser fields

The ionization dependences of triatomic molecular ions exposed to ultra-short intense laser fields on the bond extension, the laser frequency, intensity and pulse envelope have been investigated numerically. We find that the ionization of is greatly enhanced when one of the two bonds stretches into a critical bond length range -6 bohr for the laser intensity . Moreover, the enhanced ionization is laser-frequency dependent, namely, the critical bond length shifts toward a small value for the short-wavelength excitation. In addition, the critical bond length is found to be variable with the laser intensity. For low-frequency excitations, the enhanced ionization (EI) effect can be explained by the field-induced over-the-barrier model. However, the EI effect is attributed to the resonance-enhanced multiphoton ionization mechanism for high-frequency (248 nm) excitation.