D. Ahmasi Harris

Coherent mode-selective Raman excitation towards standoff detection

We report the detection of characteristic Raman lines for several chemicals using a single-beam coherent anti-Stokes Raman scattering (CARS) technique from a 12 meter standoff distance. Single laser shot spectra are obtained with sufficient signal to noise ratio to allow molecular identification. Background and spectroscopic discrimination are achieved through binary phase pulse shaping for optimal excitation of a single vibrational mode. These results provide a promising approach to standoff detection of chemicals, hazardous contaminants, and explosives.

Control of Molecular Fragmentation Using Shaped Femtosecond Pulses

The possibility that chemical reactions may be controlled by tailored femtosecond laser pulses has inspired recent studies that take advantage of their short pulse duration, comparable to intramolecular dynamics, and high peak intensity to fragment and ionize molecules. In this article, we present an experimental quest to control the chemical reactions that take place when isolated molecules interact with shaped near-infrared laser pulses with peak intensities ranging from 1013 to 1016 W/cm2. Through the exhaustive evaluation of hundreds of thousands of experiments, we methodically evaluated the molecular response of 16 compounds, including isomers, to the tailored light fields, as monitored by time-of-flight mass spectrometry. Analysis of the experimental data, taking into account its statistical significance, leads us to uncover important trends regarding the interaction of isolated molecules with an intense laser field. Despite the energetics involved in fragmentation and ionization, the integrated second-harmonic generation of a given laser pulse (ISHG), which was recorded as an independent diagnostic parameter, was found to be linearly proportional to the total ion yield (IMS) generated by that pulse in all of our pulse shaping measurements. Order of magnitude laser control over the relative yields of different fragment ions was observed for most of the molecules studied; the fragmentation yields were found to vary monotonically with IMS and/or ISHG. When the extensive changes in fragmentation yields as a function of IMS were compared for different phase functions, we found essentially identical results. This observation implies that fragmentation depends on a parameter that is responsible for IMS and independent from the particular time-frequency structure of the shaped laser pulse. With additional experiments, we found that individual ion yields depend only on the average pulse duration, implying that coherence does not play a role in the observed changes in yield as a function of pulse shaping. These findings were consistently observed for all molecules studied (p-, m-, o-nitrotoluene, 2,4-dinitrotoluene, benzene, toluene, naphthalene, azulene, acetone, acetyl chloride, acetophenone, p-chrolobenzonitrile, N,N-dimethylformamide, dimethyl phosphate, 2-chloroethyl ethyl sulfide, and tricarbonyl-[eta5-1-methyl-2,4-cyclopentadien-1-yl]-manganese). The exception to our conclusion is that the yield of small singly-charged fragments resulting from a multiple ionization process in a subset of molecules, were found to be highly sensitive to the phase structure of the intense pulses. This coherent process plays a minimal role in photofragmentation; therefore, we consider it an exception rather than a rule. Changes in the fragmentation process are dependent on molecular structure, as evidenced in a number of isomers, therefore femtosecond laser fragmentation could provide a practical dimension to analytical chemistry techniques.

Pulse shaping of octave spanning femtosecond laser pulses

Phase characterization, correction and shaping of an ultra-broad-bandwidth femtosecond laser were achieved using a grating-based pulse shaper. By using MIIPS, the compensated pulses generated a second harmonic spectrum spanning over 12,260 cm-1.

Standoff and arms-length detection of chemicals with single-beam coherent anti-Stokes Raman scattering

The detection of chemicals from safe distances is vital in environments with potentially hazardous or explosive threats, where high sensitivity and fast detection speed are needed. Here we demonstrate standoff detection of several solids, liquids, and gases with single-beam coherent anti-Stokes Raman scattering. This approach utilizes a phase coherent ultrabroad-bandwidth femtosecond laser to probe the fundamental vibrations that constitute a molecules fingerprint. Characteristic Raman lines for several chemicals are successfully obtained from arms-length and 12 m standoff distances. The sensitivity and speed of this approach are also demonstrated.

Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

Time-resolved transient absorption spectroscopy was used to investigate the primary geminate recombination and cage escape of alkyl radicals in solution over a temperature range from 0 to 80 degrees C. Radical pairs were produced by photoexcitation of methyl, ethyl, propyl, hexylnitrile, and adenosylcobalamin in water, ethylene glycol, mixtures of water and ethylene glycol, and sucrose solutions. In contrast to previous studies of cage escape and geminate recombination, these experiments demonstrate that cage escape for these radical pairs occurs on time scales ranging from a hundred picoseconds to over a nanosecond as a function of solvent fluidity and radical size. Ultrafast cage escape (<100 ps) is only observed for the methyl radical where the radical pair is produced through excitation to a directly dissociative electronic state. The data are interpreted using a unimolecular model with competition between geminate recombination and cage escape. The escape rate constant, k(e), is not a simple function of the solvent fluidity (T/eta) but depends on the nature of the solvent as well. The slope of k(e) as a function of T/eta for the adenosyl radical in water is in near quantitative agreement with the slope calculated using a hydrodynamic model and the Stokes-Einstein equation for the diffusion coefficients. The solvent dependence is reproduced when diffusion constants are calculated taking into account the relative volume and mass of both solvent and solute using the expression proposed by Akgerman (Akgerman, A.; Gainer, J. L. Ind. Eng. Chem. Fundam. 1972, 11, 373-379). Rate constants for cage escape of the other radicals investigated are consistently smaller than the calculated values suggesting a systematic correction for radical size or coupled radical pair motion.

Single-Beam Coherent Anti-Stokes Raman Scattering for Standoff Detection

Remote Sensing: We continue to develop more sensitive systems for the remote sensing of biological and chemical trace components.

Detection of chemicals at a standoff >10 m distance based on single-beam coherent anti-Stokes Raman scattering

The fundamental difficulty of achieving a coherently enhanced sensing method at standoff distances greater than 10 meters has been solved by single-beam coherent anti-Stokes Raman scattering and by actively measuring and eliminating chromatic dispersion experienced by the broad-bandwidth (100 nm) laser pulses. Characteristic Raman spectra for several chemicals in gas, liquid, and solid states, are successfully obtained from a 12 meter standoff distance. The results obtained indicate this is a promising approach to standoff detection of chemicals, hazardous contaminations, and explosives.

Standoff chemical detection using single-beam CARS

We introduce an improved method for standoff chemical detection of films and residues on solid targets which scatter or reflect the incident light using single-beam Coherent anti-Stokes Raman Scattering.

Automated phase characterization and adaptive pulse compression using multiphoton intrapulse interference phase scan in air.

We introduce a non-interferometric single beam method for automated spectral phase characterization and adaptive pulse compression of amplified ultrashort femtosecond pulses taking advantage of third order harmonic generation in air. This new method, air-MIIPS, compensates high-order phase distortions based on multiphoton intrapulse interference phase scan (MIIPS).

Controlling the excited state charge transfer in DMABN using shaped femtosecond pulses

The ability of multiphoton intrapulse interference (Mil) to regulate the formation of the twisted charge transfer state in DMABN upon three-photon excitation is explored and monitored via fluorescence.