We used strong-field laser pulses that were tailored with closed-loop optimal control to govern specified chemical dissociation and reactivity channels in a series of organic molecules. Selective cleavage and rearrangement of chemical bonds having dissociation energies up to approximately 100 kilocalories per mole (about 4 electron volts) are reported for polyatomic molecules, including (CH3)2CO (acetone), CH3COCF3 (trifluoroacetone), and C6H5COCH3 (acetophenone). Control over the formation of CH(3)CO from (CH3)2CO, CF3 (or CH3) from CH3COCF3, and C6H5CH3 (toluene) from C6H5COCH3 was observed with high selectivity. Strong-field control appears to have generic applicability for manipulating molecular reactivity because the tailored intense laser fields (about 10(13) watts per square centimeter) can dynamically Stark shift many excited states into resonance, and consequently, the method is not confined by resonant spectral restrictions found in the perturbative (weak-field) regime.

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 Since a perturbative approach is no longer valid at these intensities, the full TDHF equations are used to propagate the electronic density.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

Strong field, closed-loop control of gas-phase photochemical reactivity is the focus of this article. The control of chemical reactivity is now possible using tailored laser pulses to circumvent previous laser bandwidth limitations. As an illustration of this capability, ketone rearrangements and dissociation reactions are considered. To introduce the experiments we discuss both optimal control theory (OCT) and optimal control experiments (OCE) with an emphasis on closed-loop control methods using near-infrared fs pulses. Because the experiments are in the strong field regime, we present the current state of the understanding of the electronic and nuclear photophysical processes that occur when polyatomic molecules are subjected to laser intensities ranging between 10 13 and 10 15 W cm -2 . Photoelectron spectroscopy measurements are presented that begin to elucidate the control mechanisms. These delineate the order of the multiphoton process, the presence of transient shifting of excited electronic state energies (on the order of 5 eV), and the phenomena of lifetime broadening of electronic states. Recent experiments probing the energy partitioning to nuclear modes are presented with an emphasis on detecting the final kinetic energy of fragment ions. The advances in laser pulse shaping technology slaved to pattern recognition learning algorithms have opened up the prospect of studying the dynamics and chemical manipulation of virtually any system that can be introduced into the closed-loop apparatus. Rather than operating under the limitation of finding the molecule to suit the laser capabilities, the closed-loop learning control procedure operating in the strong field regime now makes it possible to merely tailor the control laser to meet the molecule's dynamical capabilities in keeping with the chemical objectives. The prospects are very bright for exploring chemical reactivity with these tools.

The coupling mechanism between an intense (∼10 13 W cm -2 , 780 nm) near-infrared radiation field of duration 50-200 fs with molecules having 5-50 atoms is considered in this article. In general, the interaction of intense radiation fields with molecules can result in both electron emission and subsequent dissociation. For the laser excitation scheme employed here, intact ions are observed in addition to dissociative ionization channels for all classes of molecules investigated to date. An excitation mechanism is considered where the electric field of the laser mediates the coupling between the radiation and the molecule. This field-induced ionization is compared with the more common frequency-mediated coupling mechanism found in multiphoton processes. Measurements of intense-laser photoionization probability are presented for several series of molecules. An outline of our structure-based model is presented to enable calculation of relative tunneling rates and prediction of the laser-molecule coupling mechanism. The relative ion yields for various series of hydrocarbon molecules are found to be in good agreement with that predicted by the structure-based tunnel ionization model. Measurements of photoelectron kinetic energy distributions also suggest that the ionization phenomena proceed to a large degree through a field-mediated excitation process. The photoionization/ dissociation products are measured in an ion spectrometer and are interpreted in terms of a competition between electronic excitation and energy transfer to nuclear degrees of freedom. Evidence for field-induced dissociation is presented.

Recent advances in nonlinear optics and strong-field chemistry highlight the need for calculated properties of organic molecules and their molecular ions for which no experimental values exist. Both static and frequencydependent properties are required to understand the optical response of molecules and their ions interacting with laser fields. It is particularly important to understand the dynamics of the optical response of multielectron systems in the near-IR (λ ∼ 800 nm) region, where the majority of strong-field experiments are performed. To this end we used Hartree-Fock (HF) and PBE0 density functional theory to calculate ground-state firstorder polarizabilities (R) for two series of conjugated organic molecules and their molecular ions: (a) alltrans linear polyenes ranging in size from ethylene (C 2 H 4 ) to octadecanonene (C 18 H 20 ) and (b) polyacenes ranging in size from benzene (C 6 H 6 ) to tetracene (C 18 H 12 ). The major observed trends are: (i) the wellknown nonlinear increase of R with molecular size, (ii) a significant increase of R upon ionization for larger systems, and (iii) for larger ions, the dynamic polarizability at 800 nm is much larger than the static polarizability. We have also compared the HF and PBE0 polarizabilities of the linear polyenes up to octatetraene calculated with second-order Moller-Plesset perturbation theory (MP2) and coupled cluster theory with single and double excitations (CCSD). For neutral molecules the results at the PBE0 and HF levels are very similar and ca. 20% higher than the MP2 and CCSD results. For molecular ions, results at the HF, PBE0, MP2, and CCSD are all very close. We discuss the size scaling and frequency dependence of R, and provide simple models that capture the origin of the change in the static and dynamic polarization upon ionization.

Direct, multiphoton photolysis of aqueous metal complexes is found to play an important role in the formation of nanoparticles in solution by ultrafast laser irradiation. In situ absorption spectroscopy of aqueous [AuCl4](-) reveals two mechanisms of Au(0) nucleation: (1) direct multiphoton photolysis of [AuCl4](-) and (2) radical-mediated reduction of [AuCl4](-) upon multiphoton photolysis of water. Measurement of the reaction kinetics as a function of solution pH reveals zeroth-, first-, and second-order components. The radical-mediated process is found to be zeroth-order in [AuCl4](-) under acidic conditions, where the reaction rate is limited by the production of reactive radical species from water during each laser shot. Multiphoton photolysis is found to be first order in [AuCl4](-) at all pHs, whereas the autocatalytic reaction with H2O2, the photolytic reaction product of water, is second order.

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.

Simultaneous spatiotemporal focusing (SSTF) of femtosecond laser radiation is used to produce gold nanoparticles from aqueous [AuCl 4 ] − solutions. Multiphoton ionization and dissociation of water produces electrons and hydrogen atoms for the reduction of [AuCl 4 ] − to Au(0) during irradiation with the temporally chirped (36 ps) pulse and produces hydrogen peroxide (H 2 O 2 ) as a long-lived reducing agent which persists after irradiation is terminated. Aqueous H 2 O 2 is found to reduce [AuCl 4 ] − , remaining in solution after the laser irradiation is terminated, leading to growth and transformation of the existing Au(0) species. The highly efficient postirradiation reduction of [AuCl 4 ] − to Au(0) by H 2 O 2 is ascribed to reactions occurring on gold nanoparticle surfaces. In the absence of added surfactant, the negatively charged gold particles formed during irradiation are a complex mixture of irregularly shaped and spherical morphologies that are only metastable as aqueous dispersions. These particles become transformed into more perfectly shaped gold crystals, as the remaining [AuCl 4 ] − is reduced in the postirradiation period. The addition of polyethylene glycol (PEG 45 ) accelerates the rate of the [AuCl 4 ] − reduction during laser irradiation and directs the exclusive formation of spherical nanoparticles. Varying the concentration of PEG 45 tunes the diameter and size distribution of the Au nanoparticles formed by laser processing from 3.9 ± 0.7 to 11 ± 2.4 nm.