Coherent Control of Multiphoton Transitions in the Gas and Condensed Phases with Shaped Ultrashort Pulses

Controlling laser-molecule interactions has become an integral part of developing devices and applications in spectroscopy, microscopy, optical switching, micromachining and photochemistry. Coherent control of multiphoton transitions could bring a significant improvement of these methods. In microscopy, multi-photon transitions are used to activate different contrast agents and suppress background fluorescence; coherent control could generate selective probe excitation. In photochemistry, different dissociative states are accessed through two, three, or more photon transitions; coherent control could be used to select the reaction pathway and therefore the yield-specific products. For micromachining and processing a wide variety of materials, femtosecond lasers are now used routinely. Understanding the interactions between the intense femtosecond pulse and the material could lead to technologically important advances. Pulse shaping could then be used to optimize the desired outcome. The scope of our research program is to develop robust and efficient strategies to control nonlinear laser-matter interactions using ultrashort shaped pulses in gas and condensed phases. Our systematic research has led to significant developments in a number of areas relevant to the AMO Physics group at DOE, among them: generation of ultrashort phase shaped pulses, coherent control and manipulation of quantum mechanical states in gas and condensed phases, behavior of isolated molecules under intense laser fields, behavior of condensed phase matter under intense laser field and implications on micromachining with ultrashort pulses, coherent control of nanoparticles their surface plasmon waves and their nonlinear optical behavior, and observation of coherent Coulomb explosion processes at 10^16 W/cm^2. In all, the research has resulted in 36 publications (five journal covers) and nine invention disclosures, five of which have continued on to patentin

Femtosecond real-time probing of reactions. I. The technique

When a chemical bond is broken in a direct dissociationreaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10^(−13) s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as "transition states." Femtosecond transition‐state spectroscopy (FTS) is a real‐time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real‐time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s)

Femtosecond real-time probing of reactions. II. The dissociation reaction of ICN

Experimental results obtained for the dissociation reaction ICN^*→[I⋅⋅⋅CN]^(‡*)→I+CN using femtosecond transition‐state spectroscopy (FTS) are presented. The process of the I–CN bond breaking is clocked, and the transition states of the reaction are observed in real time. From the clocking experiments, a "dissociation" time of 205±30 fs was measured and was related to the length scale of the potential. The transition states live for only ∼50 fs or less, and from the observed transients we deduce some characteristics of the relevant potential energy surfaces (PES). These FTS experiments are discussed in relation to both classical and quantum mechanical models of the dynamical motion, including features of the femtosecondcoherence and alignment of fragments during recoil. The observations are related to the radial and angular properties of the PES

Waste Minimization and Process Integration Applied to the Retrofit Design of Chemical Processes

The chemical processing industry is faced with a need to manufacture quality products while minimizing production costs and complying with a variety of safety and environmental regulations. These regulations include the Clean Air Act (CAA), the Clean Water Act (CWA), the Resource Conservation and Recovery Act (RCRA), and the most recent the Hazardous Organic National Emission Standards for Hazardous Air Pollutants (HON) (Zanetti 1994). In 1990, the Environmental Protection Agency (EPA) promulgated the Pollution Prevention Act (Freeman, et al 1992). This act declared that the national policy of the United States is to prevent or reduce pollution at the source, that pollution which cannot be prevented should be recycled in an environmentally safe manner, and that waste disposal should be employed only as a last resort. Hydrocarbon Processing (1993) reports on an increased rate in pollution control costs and estimates that by 1995 the hydrocarbon processing industry will spend 152.6 billion dollars in pollution control. As a result of the Pollution Prevention Act, the constant change in regulations, and the increasing pollution control costs, and because waste treatment is not the solution to the pollution problem, end-of-the-pipe treatment is no longer feasible or recommended. Therefore, a recent approach that has been taken is to apply sotlrce reduction instead of end-of-the-pipe treatment. In this way, industry complies with all regulations and reduces waste treatment costs, thus increasing the overall profit of operation

Milliradian precision ultrafast pulse control for spectral phase metrology

A pulse-shaper-based method for spectral phase measurement and compression with milliradian precision is proposed and tested experimentally. Measurements of chirp and third-order dispersion are performed and compared to theoretical predictions. The single-digit milliradian accuracy is benchmarked by a group velocity dispersion measurement of fused silica

Broadband 2.12 GHz Ti : sapphire laser compressed to 5.9 femtoseconds using MIIPS

CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOWe report a self-starting prismless femtosecond Ti:sapphire ring laser whose repetition rate has been gradually increased from 1 to 2.12 GHz. A broadband spectrum extending from 650 to 1040 nm, in which 17% of the intracavity power is generated in a single-pass through the crystal, is preserved in spite of the reduction in peak power. An average power of 0.95 W was obtained for 7.5 W of pump power, with very stable operation verified over 22 hours. Pulses from this laser have been fully characterized in spectral phase, and then compressed to 5.9 femtoseconds using multiphoton intrapulse interference phase scan (MIIPS).We report a self-starting prismless femtosecond Ti:sapphire ring laser whose repetition rate has been gradually increased from 1 to 2.12 GHz. A broadband spectrum extending from 650 to 1040 nm, in which 17% of the intracavity power is generated in a single-pass through the crystal, is preserved in spite of the reduction in peak power. An average power of 0.95 W was obtained for 7.5 W of pump power, with very stable operation verified over 22 hours. Pulses from this laser have been fully characterized in spectral phase, and then compressed to 5.9 femtoseconds using multiphoton intrapulse interference phase scan (MIIPS).16141003310038CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSem informaçãoSem informaçãoSem informaçãoG. T. Nogueira acknowledges a scholarship from CAPES. F. C. Cruz ([email protected]) acknowledges financial support from FAPESP, CEPOF, and CNPq. The MSU team acknowledges partial funding for the project from the National Science Foundation

Femtosecond transient-grating techniques: population and coherence dynamics involving ground and excited

Time-resolved transient grating techniques ͑TG͒ arising from four-wave mixing ͑FWM͒ processes are explored for the study of molecular dynamics in gas-phase systems ranging from single atoms to large polyatomic molecules. For atomic species such as Ar and Xe, each TG signal shows only a peak at zero time delay when all three incident pulses are overlapped temporally. For diatomic O 2 and N 2 and linear triatomic CS 2 molecules, the TG signals exhibit ground state rotational wave packet recurrences that can be analyzed to obtain accurate rotational constants for these molecules. With heavier systems such as HgI 2 , ground state vibrational and rotational wave packet dynamics are observed. Resonant excitation allows us to select between measurements that monitor wave packet dynamics, i.e., populations in the ground or excited states or coherences between the two electronic states. To illustrate these two cases we chose the X→B transition in I 2 . TG measurements yield dynamic information characteristic of vibrational and rotational wave packets from the ground and excited states. Reverse transient grating ͑RTG͒ experiments monitor the time evolution of an electronic coherence between the ground and excited states which includes vibrational and rotational information as well. Early time TG signal for the polyatomic samples CH 2 Cl 2 , CH 2 Br 2 , benzene, and toluene exhibit a coherence coupling feature at time zero followed by rotational dephasing. Differences in the amplitude of these two components are related to the contributions from the isotropic and anisotropic components of the molecular polarizability. A theoretical formalism is developed and used successfully to interpret and simulate the experimental transients. The measurements in this study provide gas-phase rotational and vibrational dephasing information that is contrasted, in the case of CS 2 , with liquid-phase measurements. This comparison provides a time scale for intramolecular dynamics, intermolecular collisions, and solvation dynamics

Femtosecond real-time probing of reactions. I. The technique

When a chemical bond is broken in a direct dissociationreaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10^(−13) s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as "transition states." Femtosecond transition‐state spectroscopy (FTS) is a real‐time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real‐time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s)