Excitation gratings in cross-beam filament wake channels in a dense argon gas: Formation, control, and Rabi sideband manifestation.

When two intense laser beams cross at a small angle, the interference in the crossing area results in a finite intensity grating. We consider femtosecond laser filamentation in such a grating, in a situation when the process is largely confined to the grating maxima and leads to formation of a structured filament wake channel. In a dense gas, electron impact processes during the laser pulse cause a copious excitation of neutral atoms, resulting in formation of a finite grating of the density of excited atoms. Numerically solving the equations of laser-driven kinetics, we obtain the properties of this grating, as depending on the characteristics of the interfering beams and especially on the interbeam phase delay. The excitation gratings thus formed give rise to a hallmark effect of Rabi sideband emission when probed by a picosecond 800 nm laser pulse, which couples with transitions in the excited states manifold. Spectral and spatial interference of the emitted radiation forms four-dimensional spatial-spectral fringe patterns accessible for observation on a remote screen. The patterns are indicative of the excitation grating structure; their sensitivity to the phase delay between the crossing pump pulses warrants experimental verification.

Control of the excited-to-ionized atoms ratio in a dense gas in the wake of an intense femtosecond laser pulse.

We demonstrate the sensitivity of the plasma composition in the filament wake channel in a dense gas to the temporal shape of the driving femtosecond laser pulse. During the pulse, the electrons released via strong-field ionization and driven by oscillating laser field are actively engaged in collisional processes with neutral neighbor atoms, including inverse Bremsstrahlung, impact ionization, and collisional excitation. By the end of the pulse, these collisional processes produce considerable numbers of additional free electrons (or ionized atoms) and excited atoms, and these contents of the filament wake channel determine its subsequent evolution dynamics. Addressing the case of high-pressure argon gas and using a kinetic model of these competing collisional processes, we explore the sensitivity of the resulting excited-to-ionized atoms number density ratio to the envelope shape of the driving laser pulse. By considering several families of pulses, we show that asymmetric pulse envelopes skewed toward the earlier time allow for efficient control of the ratio of excited atoms to ionized atoms. The pulse-shape control of the plasma composition in the immediate wake of the laser pulse projects into control of the wake channel evolution and of the associated transient electronic and optical nonlinearities.

Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations.

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D1 surface with little excess energy at the equilibrium neutral geometry. These D1 ions are preferably formed because in this case D1 and D0 switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D1 surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D1/D0 conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D0 becomes populated with molecules that are energized mainly along the cis-trans isomerization coordinate, with the kinetic energy above the cis-trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D0 surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole-lone pair interaction that drives the molecule along the cis-trans isomerization coordinate to facilitate the transition between the involved electronic states.

Filament-Assisted Impulsive Raman Spectroscopy of Ozone and Nitrogen Oxides.

The filament-assisted impulsive Raman spectra of ozone, nitric oxide, and nitrogen dioxide are presented. The Raman response as a function of ozone concentration scales as N(2), where N is the number of oscillators in the interaction region. The system described has a detection limit of ∼300 ppm for gas-phase ozone. Ozone produced via the strong field chemistry occurring within the filament pump was also detected. The measurements reveal spectral interference in the Raman features. Simulations show the spectral fringing results from interference of the Raman signal with pump-induced cross-phase modulation. The fringes are used to classify the symmetric mode of the low concentration filament-generated ozone.

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching--the constructive addition of x-ray waves from a large number of atoms--favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth-limited pulse trains of ~100 attoseconds.

Higher-order nonlinearity of refractive index: the case of argon.

The nonlinear coefficients, n4, of the time-dependent refractive index for argon are calculated in the non-resonant optical regime. Second-order polynomial fitting of DC-Kerr, γ((2))(-ω; ω, 0, 0), electric field induced second harmonic generation (ESHG), γ((2))(-2ω; ω, ω, 0), and static second-order hyperpolarizability, γ((2))(0; 0, 0, 0), is performed using an auxiliary electric field approach to obtain the corresponding fourth-order optical properties. A number of basis sets are investigated for the fourth-order hyperpolarizability processes at 800 nm at coupled cluster singles and doubles level of theory, starting with the t-aug-cc-pV5Z basis set and expanding that basis set by adding diffuse functions and polarization functions. Comparison shows that the results obtained with the t-aug-cc-pV5Z basis are in very good agreement with the results obtained using the q-aug-cc-pV5Z, t-aug-cc-pV6Z, and q-aug-cc-pV6Z basis sets. To calculate the nonlinear refractive index n4, an approximate formula is suggested which expresses the related degenerate six-wave mixing coefficient, γ((4))(-ω; ω, -ω, ω, -ω, ω), in terms of the DC-Kerr, γ((4))(-ω; ω, 0, 0, 0, 0), ESHG, γ((4))(-2ω; ω, ω, 0, 0, 0), and the static fourth-order hyperpolarizability coefficients. The higher-order nonlinear refractive index n4 is found to be positive over the wavelengths 300 nm-2000 nm. In the infrared spectral range, the obtained values of n4 are in qualitative agreement with the results of Kramers-Kronig-based calculations.

Strong Field Adiabatic Ionization Prepares a Launch State for Coherent Control.

We demonstrate that excitation of acetophenone with a strong field, near-infrared femtosecond pulse (1150-1500 nm) results in adiabatic ionization, producing acetophenone radical cation in the ground electronic state. The time-resolved transients of the parent and fragment ions probed with a weak 790 nm pulse reveal an order of magnitude enhancement of the peak-to-peak amplitude oscillations, ∼ 100 fs longer coherence time, and an order of magnitude increase in the ratio of parent to fragment ions in comparison with nonadiabatic ionization with a strong field 790 nm pulse. Equation of motion coupled cluster and classical wavepacket trajectory calculations support the mechanism wherein the probe pulse excites a wavepacket on the ground surface D0 to the excited D2 surface at a delay of 325 fs, resulting in dissociation to the benzoyl ion. Direct population transfer to the D2 state within the duration of a 1370 nm pump pulse eliminates wavepacket oscillation on the D0 state.

Measurement of ionic resonances in alkyl phenyl ketone cations via infrared strong field mass spectrometry.

Strong-field excitation of alkyl phenyl ketone molecules reveals an electronic resonance at 1370 nm in the radical cations upon measuring mass spectra as a function of excitation wavelength from 1240 to 1550 nm. The ratio of the benzoyl fragment ion to parent ion signal in acetophenone increases from 1:1.5 at 1240 nm excitation to 5:1 at 1370 nm (0.9 eV), and back to 1:1 at 1450 nm. Unlike acetophenone and propiophenone, the homologous molecules acetone and ethylbenzene exhibit no wavelength-dependent fragmentation patterns over the range from 1240 to 1550 nm, supporting the hypothesis that the electronic structure of the alkyl phenyl ketone cation enables the one-photon transition. Calculations on the acetophenone and propiophenone radical cations show the existence of a bright state, D2, 0.87 and 0.88 eV, respectively, above the ground-state D0 minimum. Calculations of the potential energy surfaces of the acetophenone radical cation suggest that a D2 → D0 radiationless transition precedes dissociation on D0. Upon population transfer to the D2 surface, the wavepacket motion is directed toward a three-state conical intersection (D0/D1/D2) that facilitates the photodissociation by converting electronic to vibrational energy on the D0 surface.

Spectral-to-temporal amplitude mapping polarization spectroscopy of rotational transients.

A new implementation of pump-probe polarization spectroscopy is presented where the revivals of an impulsively excited rotational wavepacket are mapped onto a broad-band, chirped continuum pulse to measure a long temporal window without the need for delay scanning. Experimental measurements and a theoretical framework for spectral-temporal amplitude mapping polarization spectroscopy (STAMPS) as applied to impulsive rotational motion are presented. In this technique, a femtosecond laser pulse is used to prepare a rotational wavepacket in a gas-phase sample at room temperature. The rotational revivals of the wavepacket are then mapped onto a chirped continuum (400-800 nm) pulse created by laser filamentation in argon. Nearly single-shot time-resolved rotational spectra are recorded over a 65 ps time window. The transient birefringence spectra are simulated by including terms for polarization rotation of the probe as well as cross-phase modulation. Measurements and simulations are presented for the cylindrically symmetric N2, O2, and CO2 molecules. The long time window of the method allows measurement of rotational spectra for asymmetric top molecules, and here we present measurements for ethylene and methanol.

Measurement of an Electronic Resonance in a Ground-State, Gas-Phase Acetophenone Cation via Strong-Field Mass Spectrometry.

A one-photon ionic resonance is measured in the strong-field regime in acetophenone by recording the mass spectra as a function of excitation wavelength from 800 to 1500 nm. The ratio of the benzoyl to parent ion signals in the mass spectrum varies significantly with excitation wavelength, where the highest ratio observed upon excitation at 1370 nm (0.90 eV) indicates the presence of a one-photon resonance. At the resonant wavelength, the ratio of the benzoyl to parent ion signals increases linearly with laser intensity over a range from 1.1 × 10(13) to 6.0 × 10(13) W cm(-2). The ratio increases by a factor of 5 at 1370 nm with increasing pulse duration from 60 to 100 fs. Calculations using the equation of motion coupled cluster method support the existence of a one-photon transition from the ground ionic to a dissociative excited ionic state (0.87 eV), where motion of the acetyl group from a planar to nonplanar structure within the pulse duration enables the otherwise forbidden transition.

Numerical bound state electron dynamics of carbon dioxide in the strong-field regime.

Time-dependent Hartree-Fock simulations for a linear triatomic molecule (CO(2)) interacting with a short IR (1.63 eV) three-cycle pulse reveal that the carrier-envelope shape and phase are the essential field parameters determining the bound state electron dynamics during and after the laser-molecule interaction. Analysis of the induced dipole oscillation reveals that the envelope shape (Gaussian or trapezoidal) controls the excited state population distribution. Varying the carrier envelope phase for each of the two pulse envelope shapes considerably changes the excited state populations. Increasing the electric field amplitude alters the relative populations of the excited states, generally exciting higher states. A windowed Fourier transform analysis of the dipole evolution during the laser pulse reveals the dynamics of state excitation and in particular state coupling as the laser intensity increases.

Observation of broadband time-dependent Rabi shifting in microplasmas.

Coherent broadband radiation in the form of Rabi sidebands is observed when a ps probe laser propagates through a weakly ionized, electronically excited microplasma generated in the focus of an intense pump beam. The sidebands arise from the interaction of the probe beam with pairs of excited states of a constituent neutral atom via the probe-induced Rabi oscillation. Sideband shifting of >90 meV from the probe carrier frequency results in an effective bandwidth of 200 meV. The sidebands are controlled by the intensity and temporal profile of the probe pulse; with amplitude and shift in agreement with the predictions of a time-dependent generalized Rabi cycling model.

Adaptive reshaping of objects in (multiparameter) Hilbert space for enhanced detection and classification: an application of receiver operating curve statistics to laser-based mass spectroscopy.

We propose a new approach to the classical detection problem of discrimination of a true signal of interest from an interferent signal, which may be applied to the area of chemical sensing. We show that the detection performance, as quantified by the receiver operating curve (ROC), can be substantially improved when the signal is represented by a multicomponent data set that is actively manipulated by means of a shaped laser probe pulse. In this case, the signal sought (agent) and the interfering signal (interferent) are visualized by vectors in a multidimensional detection space. Separation of these vectors can be achieved by adaptive modification of a probing laser pulse to actively manipulate the Hamiltonian of the agent and interferent. We demonstrate one implementation of the concept of adaptive rotation of signal vectors to chemical agent detection by means of strong-field time-of-flight mass spectrometry.

Numerical simulation of nonadiabatic electron excitation in the strong-field regime. 3. Polyacene neutrals and cations.

The electron optical response for a series of linear polyacenes and their molecular ions (mono and dications) in strong laser fields was studied using time-dependent Hartree-Fock theory. The interactions of benzene, naphthalene, anthracene, and tetracene with pulsed fields at a frequency of 1.55 eV and intensities of 8.77 x 10(13), 3.07 x 10(13), 1.23 x 10(13), and 2.75 x 10(12) W/cm2, respectively, were calculated using the 6-31G(d,p) basis set. Nonadiabatic processes, including nonadiabatic time evolution of the dipole moment, Löwden charges, and occupation numbers, were studied. The nonadiabatic response increased with the length of the molecule and was greatest for the molecular monocations. The only exception was tetracene, in which the very strong response of the dication was due to a near resonance with the applied field. The intensity and frequency dependence of the dipole moment response for the monocations of naphthalene and anthracene was also calculated. As the intensity increased, the population of higher-energy excited-states increased, and as the frequency increased, the excitation volume increased in good agreement with the Dykhne approximation.

Control goal selection through anticorrelation analysis in the detection space.

A statistical method is reported to determine the pairs of fragment ions in a mass spectrum that are most susceptible to control by adaptive optimization of the laser pulse shapes in the strong-field regime. The proposed method is based on covariance analysis of the mass spectral fragmentation patterns generated by a set of randomly shaped pulses. The pairs of fragment ions that have higher negative covariances possess a correspondingly higher degree of controllability in an adaptive control experiment, whereas the pairs that have higher positive covariances possess correspondingly lower controllability.

Rapid proton transfer mediated by a strong laser field.

Kinetic energy distributions of ejected from a polyatomic molecule, anthraquinone, subjected to 60 fs, 800 nm laser pulses of intensity between 0.2 and 4.0 x 10(14) W x cm(-2), reveal field-driven restructuring of the molecule prior to Coulomb explosion. Calculations demonstrate fast intramolecular proton migration into a field-dressed metastable potential energy minimum. The proton migration occurs in the direction perpendicular to the polarization of the laser field. Rapid field-mediated isomerization is an important new phenomenon in coupling of polyatomic molecules with intense lasers.

A numerical simulation of nonadiabatic electron excitation in the strong field regime: Linear polyenes.

Time-dependent Hartree-Fock theory has been used to study of the electronic optical response of a series of linear polyenes in strong laser fields. Ethylene, butadiene, and hexatriene have been calculated with 6-31G(d,p) in the presence of a field corresponding to 8.75 x 10(13) W/cm2 and 760 nm. Time evolution of the electron population indicates not only the pi electrons, but also lower lying valence electrons are involved in electronic response. When the field is aligned with the long axis of the molecule, Löwdin population analysis shows large charges at each end of the molecule. For ethylene, the instantaneous dipole moment followed the field adiabatically, but for hexatriene, nonadiabatic effects were very pronounced. For constant intensity, the nonadiabatic effects in the charge distribution, instantaneous dipole, and orbital populations increased nonlinearly with the length of the polyene. These calculations elucidate the mechanism of the strong field nonadiabatic electron excitation of polyatomic molecules leading to their eventual ionization and fragmentation. The described computational methods are a viable tool for studying the complex processes in multielectron atomic and molecular systems in strong laser fields.

Numerical simulation of nonadiabatic electron excitation in the strong field regime. 2. Linear polyene cations.

Time-dependent Hartree-Fock theory has been used to study the electronic optical response of a series of linear polyene cations (+1 and +2) in strong laser fields. The interaction of ethylene, butadiene, and hexatriene, with pulsed and CW fields corresponding to 8.75 x 10(13) W/cm(2) and 760 nm, have been calculated using the 6-31G(d,p) basis set. Nonadiabatic processes including nonlinear response of the dipole moment to the field and non-resonant energy deposition into excited states were more pronounced for the monocations in comparison with dications. For a given charge state and geometry, the nonadiabatic effects in the charge distribution and instantaneous dipole increased with the length of the polyene. For pulsed fields, the instantaneous dipole continued to oscillate after the field returned to zero and corresponded to a non-resonant electronic excitation involving primarily the lowest electronic transition. For a given molecule and fixed charge state, the degree of nonadiabatic coupling and excitation was greater for geometries with lower excitation energies.

A time-dependent Hartree-Fock approach for studying the electronic optical response of molecules in intense fields.

For molecules in high intensity oscillating electric fields, the time-dependent Hartree-Fock (TDHF) method is used to simulate the behavior of the electronic density prior to ionization. Since a perturbative approach is no longer valid at these intensities, the full TDHF equations are used to propagate the electronic density. A unitary transform approach is combined with the modified midpoint method to provide a stable and efficient algorithm to integrate these equations. The behavior of H2+ in an intense oscillating field computed using the TDHF method with a STO-3G basis set reproduces the analytic solution for the two-state coherent excitation model. For H2 with a 6-311++G(d,p) basis set, the TDHF results are nearly indistinguishable from calculations using the full time-dependent Schrödinger equation. In an oscillating field of 3.17 x 10(13) W cm(-2) and 456 nm, the molecular orbital energies, electron populations, and atomic charges of H2 follow the field adiabatically. As the field intensity is increased, the response becomes more complicated as a result of contributions from excited states. Simulations of N2 show even greater complexity, yet the average charge still follows the field adiabatically.

Coulomb explosion of large polyatomic molecules assisted by nonadiabatic charge localization.

The electron-nuclear dynamics of the Coulomb explosion of a large polyatomic molecule, anthracene, is probed using kinetic energy distributions of produced H+ ions. The kinetic energy release of ejected protons exceeds 30 eV for anthracene exposed to 10(14) W cm(-2), 800 nm pulses of 60 fs duration. We propose a strong-field charge localization model, based on nonadiabatic dynamics of charge distribution in a (multiply) ionized molecule; the charge localization lasts many laser periods and is sustained through successive ionizations of the molecular ion. The model explains quantitatively the dependence of the H+ kinetic energy on the laser intensity. Dissociative ionization of a polyatomic molecule enabled by long-lived charge localization is a new type of electron-nuclear dynamics and is essential for understanding the pathways of molecular or ionic fragmentation in strong fields.