Tissa C. Gunaratne

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.

Femtosecond Laser-Induced Ionization/Dissociation of Protonated Peptides

Although tandem mass spectrometry has revolutionized the identification and structural characterization of peptides and proteins, future advances in comprehensive proteome analysis will depend on the development of improved methods for ion activation that yield greater sequence information, and with selective control over the fragmentation chemistry. This report presents initial findings that demonstrate the utility of a novel ion activation method using ultrashort (approximately 30 fs) laser pulses as a means to overcome the limitations of current technologies, while opening the door to solving significant challenges in protein and peptide analysis.

Single-beam coherent anti-Stokes Raman scattering spectroscopy of N2 using a shaped 7 fs laser pulse

The feasibility is explored by single-beam coherent anti-Stokes Raman scattering CARS spectroscopy of gas-phase diatomic molecules related to combusting flows, with implications for gas-phase thermometry. We demonstrate CARS of gas-phase N2 using a shaped 7 fs laser pulse, investigate the dependence of the CARS signal on the total pressure of the probed environment, both in pure N2 and in mixtures with Ar, discuss the observed signal-to-noise ratio, and suggest improvements to be considered for reliable single-shot measurements at flame temperatures.

Atmospheric pressure femtosecond laser imaging mass spectrometry.

We present a novel imaging mass spectrometry technique using femtosecond laser pulses to ionize the sample at atmospheric pressure and without the need of a laser-absorbing matrix. A 10µm-resolution image of biological tissue is demonstrated.

Atmospheric pressure femtosecond laser imaging mass spectrometry

We present a novel imaging mass spectrometry technique using femtosecond laser pulses to ablate and ionize the sample at ambient conditions with improved lateral resolution (1μm), as demonstrated here with an image of vegetable cells.

In-situ femtosecond laser pulse characterization and compression during micromachining

We report on phase measurements and adaptive phase distortion compensation of femtosecond pulses using multiphoton intrapulse interference phase scan (MIIPS) based on second harmonic generation in the plasma generated on the surface of silicon and metals.

Comment on "closing the loop on bond selective chemistry using tailored strong field laser pulses".

The work by Levis andRabitz in the field of laser control achieved great importancedue to their result of having controlled a complex bondrearrangement reaction (Scheme 2, in ref 2; Scheme 3c in ref3), specifically the production of toluene starting from acetophe-none (see Figure 1 insert). Their original experiment wasreported to

Influence of the temporal shape of femtosecond pulses on silicon micromachining

The influence of femtosecond laser pulse shaping on silicon wafer micromachining is explored. Surface second harmonic generation provides in situ pulse characterization of the laser pulses, and plasma and atomic emissions were identified as valuable indicators of the micromachining process. The ablation threshold was found to decrease as the bandwidth of the pulses increases, as well as for shorter pulses. Dependence of atomic and plasma emissions on temporal shape of the pulses confirmed that emission preceded ablation and has a threshold as well. The morphology of micromachined holes was observed to be dependent upon pulse duration.

Multidimensional molecular identification by laser control mass spectrometry

Controlled molecular photofragmentation and ionization achieved with shaped femtosecond laser pulses is coupled with mass spectrometry to achieve a powerful multidimensional tool for fast, accurate, reproducible and quantitative molecular identification. Specific pulse shaping functions are introduced to enhance structure-dependent differences in fragmentation fingerprints. Identification of geometric and structural isomer mixtures is demonstrated. Receiver operational (ROC) curves from our experimental data demonstrate the enhanced reliability that can be achieved by femtosecond laser control mass spectrometry. The potential use of this method for identification of chemicals and explosives with no false alarms is discussed.

Ultrafast dynamics of Cu(I)-phenanthrolines in dichloromethane

Transient absorption spectrometry of Cu(I)-phenanthrolines in CH2Cl2 reveals ligand-independent dynamic processes lasting 15 ps, which are associated with the peculiar structural rearrangements occurring for this class of compounds upon photoexcitation.