Robert J. Levis

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 Since a perturbative approach is no longer valid at these intensities, the full TDHF equations are used to propagate the electronic density.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.

Near‐infrared femtosecond photoionization/dissociation of cyclic aromatic hydrocarbons

Pulses of 780 nm light of duration 170 fs and power densities up to 3.8×1013 W cm−2 are used to study the photoionization/dissociation processes in the series of gas phase, cyclic aromatic hydrocarbons including benzene, naphthalene, phenanthrene, and anthracene. The near‐infrared ionization process leads to the production of intact molecular ions for all of the molecules studied. Measurements of the ion intensity as a function of laser fluence revealed the order of the ultrafast ionization process to be 8.0±0.1 for anthracene, 6.9±0.1 for phenanthrene, 8.5±0.1 for naphthalene, and 8.1±0.1 for benzene. The relative femtosecond photoionization cross section decreased from 1.0 for anthracene to 0.2 for phenanthrene to 0.1 for naphthalene to ∼0.005 for benzene. The relative order and cross section of the femtosecond ionization processes suggest that a field ionization mechanism is operative.

Mass spectrometry of intact neutral macromolecules using intense non-resonant femtosecond laser vaporization with electrospray post-ionization

Intact, nonvolatile, biological macromolecules can be transferred directly from the solid state into the gas phase, in ambient air, for subsequent mass spectral analysis using non-resonant femtosecond (fs) laser desorption combined with electrospray ionization (ESI). Mass spectral measurements for neat samples, including a dipeptide, protoporphyrin IX and vitamin B12 adsorbed on a glass insulating surface, were obtained using an 800 nm, 70 fs laser with an intensity of 10(13) W cm(-2). No appreciable signal was detected when atmospheric matrix-assisted or neat (matrix-free) fs laser desorption was performed without ESI, indicating neutral desorption.

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

Short wavelengths birth shorter ones The shortest laser pulses—with durations measured in attoseconds—arise from a process termed high-harmonic generation (HHG). Essentially, a longer, “driving” pulse draws electrons out of gaseous atoms like a slingshot, and, when they ricochet back, light emerges at shorter wavelengths. Most HHG has been carried out using light near the visible/infrared boundary for the driving pulse. Popmintchev et al. used an ultraviolet driving pulse instead, which yielded an unexpectedly efficient outcome. These results could presage a more generally efficient means of creating x-ray pulses for fundamental dynamics studies as well as technological applications. Science, this issue p. 1225 Ultraviolet pulses show unexpected efficiency in generating the higher-frequency emission underlying attosecond spectroscopy. 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.

Photoionization/dissociation of alkyl substituted benzene molecules using intense near-infrared radiation

Abstract The photoionization products for benzene, toluene, ethylbenzene and n-propylbenzene are measured using time-of-flight methods upon interaction with 780 nm radiation pulses of duration 170 fs and intensity 10 13-13.6 W cm −2 . The relative ionization yields scale as 1, 1.2, 1.4, and 0.35 for benzene, toluene, ethylbenzene and n-propylbenzene at maximum laser intensity. Limited dissociation was observed for benzene and toluene at all laser power densities. A significant (>5%) dissociation/ionization yield was observed for ethylbenzene and n-propylbenzene at the lowest laser power densities. The dissociation increased as 11th order and 9th order processes for ethylbenzene and n-propylbenzene, respectively to a maximum of 70% dissociation/ionization for n-propylbenzene at 3.8 × 10 13 W cm −2 . At 3.8 × 10 13 W cm −2 the dissociation yield increases quadratically with alkyl chain length for the series. The observed ionization/dissociation trends are consistent with a model which incorporates both field ionization and energy redistribution concepts.

Determination of inorganic improvised explosive device signatures using laser electrospray mass spectrometry detection with offline classification.

The mass spectral detection of low vapor pressure, inorganic-based explosive signatures including ammonium nitrate, chlorate, perchlorate, sugar, and the constituents contained within black powder are reported using laser electrospray mass spectrometry. The ambient pressure mass spectrometry technique combining nonresonant, femtosecond laser vaporization with electrospray postionization revealed primary and secondary signatures for trace quantities of inorganic explosives. A mixture of complexation agents in the electrospray solvent enabled the simultaneous detection of vaporized cations, anions, and neutrals in a single measurement. An offline classifier discriminated the inorganic-based explosives based on the mass spectral signatures resulting in high fidelity identification.

Identification of explosives and explosive formulations using laser electrospray mass spectrometry

Mass analysis is demonstrated for the detection of sub-microgram quantities of explosive samples on a metallic surface at atmospheric pressure using laser electrospray mass spectrometry (LEMS). A non-resonant femtosecond duration laser pulse vaporizes native samples for subsequent electrospray ionization and transfer into a time-of-flight mass spectrometer. LEMS was used to detect 2,3-dimethyl-2,3-dinitrobutane (DMNB), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 3,4,8,9,12,13-hexaoxa-1,6-diazabicyclo[4.4.4]tetradecane (HMTD), and 3,3,6,6,9,9-hexamethyl-1,2,4,5,7,8-hexaoxacyclononane (TATP) deposited on a steel surface. LEMS was also used to directly analyze composite propellant materials containing an explosive to determine the molecular composition of the explosive pellets at atmospheric pressure.

Concerning the ionization of large polyatomic molecules with intense ultrafast lasers

The relative photoionization/dissociation probabilities are presented for the molecules benzene, naphthalene, and anthracene upon interaction with 780 nm laser radiation of duration 170 fs and intensity 3.8×1013 W cm−2. Both the ionization probability and the dissociation yield increase exponentially from benzene to anthracene as measured by time-of-flight mass spectra. A structure-based model is presented for the excitation of large polyatomic molecules by intense laser irradiation with pulse widths on the time scale of molecular vibration (100 fs) and with peak field strengths of 1–2 V A−1. The model accounts for molecular structure and is able to accurately predict the transition from multiphoton ionization (MPI) to tunnel ionization. It is also demonstrated that this structure-based model can quantitatively predict the experimentally measured ionization probabilities. In comparison, models employing the more conventional zero-range potential do not accurately predict either the transition or the relat...

Measuring the spatiotemporal electric field of ultrashort pulses with high spatial and spectral resolution

We demonstrate an experimentally simple and high-spectral-resolution version of spectral interferometry (SEA TADPOLE) that can measure complicated pulses (in time) at video rates. Additionally, SEA TADPOLE can measure spatial information about a pulse, and it is the first technique that can directly measure the spatiotemporal electric field [E(x,y,z,λ)] of a focusing ultrashort pulse. To illustrate and test SEA TADPOLE, we measured E(λ) of a shaped pulse that had a time-bandwidth product of approximately 100. To demonstrate that SEA TADPOLE can measure focusing pulses, we measured E(x,λ) at and around the focus produced by a plano-convex lens. We also measured the focus of a beam that had angular dispersion present before the lens. We have found that SEA TADPOLE can achieve better spectral resolution than an equivalent spectrometer, and here we discuss this in detail, giving both experimental and simulated examples. We also discuss the angular acceptance and spatial resolution of SEA TADPOLE when measuring the spatiotemporal field of a focusing pulse.

Calculating the Keldysh adiabaticity parameter for atomic, diatomic, and polyatomic molecules

A numerical model is presented to determine the Keldysh adiabaticity parameter for the interaction of an intense laser with a polyatomic molecule. The adiabaticity parameter is a guide to determining whether the ionization process is in the field or multiphoton ionization regime. The adiabaticity parameters are compared for potentials including the simple zero-range potential, the Coulomb potential, an atomic potential (Xe), a diatomic (N2) molecular potential, and a polyatomic (C6H6) molecular potential. It is demonstrated that the Coulomb potential is approximately equal to the atomic and diatomic potentials and differs from the zero-range potential employed in the Keldysh model in a way which is predominantly dependent upon the ionization potential. Both simple models substantially overestimate the adiabaticity parameter for C6H6 at all field strengths and at fields above 1.25 V/A both simple models become completely unphysical. This is because barrier suppression ionization is predicted to occur for b...