Atmospheric pressure femtosecond laser imaging mass spectrometry.

A novel atmospheric pressure imaging mass spectrometry approach that offers improved lateral resolution (10 microm) using near-infrared femtosecond laser pulses for nonresonant desorption and ionization of sample constituents without the need of a laser-absorbing matrix is demonstrated. As a proof of concept the method was used to image a two-chemical pattern in paper. To demonstrate the ability of the approach to analyze biological tissue, a monolayer of onion epidermis was imaged allowing the chemical visualization of individual cells using mass spectrometry at ambient conditions for the first time. As the spatial resolution is currently limited by the limit of detection of the setup (approximately 500 fmol limit of detection for citric acid), improvements in sensitivity will increase the achievable spatial resolution.

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.

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.

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.

State preparation and excited electronic and vibrational behavior in hemes.

The temporally overlapping, ultrafast electronic and vibrational dynamics of a model five-coordinate, high-spin heme in a nominally isotropic solvent environment has been studied for the first time with three complementary ultrafast techniques: transient absorption, time-resolved resonance Raman Stokes, and time-resolved resonance Raman anti-Stokes spectroscopies. Vibrational dynamics associated with an evolving ground-state species dominate the observations. Excitation into the blue side of the Soret band led to very rapid S2 --> S1 decay (sub-100 fs), followed by somewhat slower (800 fs) S1 --> S0 nonradiative decay. The initial vibrationally excited, non-Boltzmann S0 state was modeled as shifted to lower energy by 300 cm(-1) and broadened by 20%. On a approximately 10 ps time scale, the S0 state evolved into its room-temperature, thermal distribution S0 profile largely through VER. Anti-Stokes signals disappear very rapidly, indicating that the vibrational energy redistributes internally in about 1-3 ps from the initial accepting modes associated with S1 --> S0 internal conversion to the rest of the macrocycle. Comparisons of anti-Stokes mode intensities and lifetimes from TRARRS studies in which the initial excited state was prepared by ligand photolysis [Mizutani, T.; Kitagawa, T. Science 1997, 278, 443, and Chem. Rec. 2001, 1, 258] suggest that, while transient absorption studies appear to be relatively insensitive to initial preparation of the electronic excited state, the subsequent vibrational dynamics are not. Direct, time-resolved evaluation of vibrational lifetimes provides insight into fast internal conversion in hemes and the pathways of subsequent vibrational energy flow in the ground state. The overall similarity of the model heme electronic dynamics to those of biological systems may be a sign that the protein's influence upon the dynamics of the heme active site is rather subtle.

Tunable ultrafast infrared/visible laser to probe vibrational dynamics.

A tunable, ultrafast (approximately 100 fs-approximately 1 ps) laser system generating mid-IR (3-10 microm) and UV/visible (392-417 nm, 785-835 nm) radiation is described and its output characterized. The system is designed to explore vibrational dynamics in the condensed phase in a direct, two-pulse, time-resolved manner, using Raman spectroscopy as the probe. To produce vibrational resolution, probe pulses are spectrally narrowed by use of a long doubling crystal. Frequency-resolved optical gating is used to evaluate beam characteristics. An effective method for determining the temporal overlap of the pump and probe pulses for a one-color, 400 nm configuration is illustrated. Representative results from studies of heme and paranitroaniline vibrational dynamics illustrate the effectiveness of the visible pump-visible probe portion of the system in illuminating fast structure and energy dynamics.

Energy flow in push-pull chromophores: vibrational dynamics in para-nitroaniline.

para-Nitroaniline (PNA) plays an essential role as the prototype model of push-pull chromophores. The nature and degree of participation of vibrational degrees of freedom in the charge-transfer and internal-conversion processes are current issues of great theoretical and practical importance. Ultrafast time-resolved anti-Stokes resonance Raman spectroscopy (TRARRS) experiments on PNA in dimethyl sulfoxide with three different excitation wavelengths were performed to probe these dynamical influences. The vibrational dynamics associated with S0 were independent of incident wavelength, and this supports the picture that the S1 dynamics are fast relative to the rate of intersystem crossing. The phenyl breathing mode nu(19) (860 cm(-1)) and the symmetric NO2 stretch nu(29) (1310 cm(-1)) exhibited vibrational lifetimes in S0 of 8.1 and 5.2 ps, respectively. No evidence for inhomogeneous broadening of the charge-transfer band in the UV/Vis absorption spectrum was found.

Photophysics of octabutoxy phthalocyaninato-Ni(II) in toluene: ultrafast experiments and DFT/TDDFT studies.

Reported herein is a combination of experimental and DFT/TDDFT theoretical investigations of the ground and excited states of 1,4,8,11,15,18,22,25-Octabutoxyphthalocyaninato-nickel(II), NiPc(BuO)(8), and the dynamics of its deactivation after excitation into the S(1)(pi,pi) state in toluene solution. According to X-ray crystallographic analysis NiPc(BuO)(8) has a highly saddled structure in the solid state. However, DFT studies suggest that in solution the complex is likely to flap from one D(2)(d)-saddled conformation to the opposite one through a D(4)(h)-planar structure. The spectral and kinetic changes for the complex in toluene are understood in terms of the 730 nm excitation light generating a primarily excited S(1) (pi,pi) state that transforms initially into a vibrationally hot (3)(d(z)2,d(x)2(-)(y)2) state. Cooling to the zeroth state is complete after ca. 8 ps. The cold (d,d) state converted to its daughter state, the (3)LMCT (pi,d(x)2(-)(y)2), which itself decays to the ground state with a lifetime of 640 ps. The proposed deactivation mechanism applies to the D(2)(d)-saddled and the D(4)(h)-planar structure as well. The results presented here for NiPc(BuO)(8) suggest that in nickel phthalocyanines the (1,3)LMCT (pi,d(x)2(-)(y)2) states may provide effective routes for radiationless deactivation of the (1,3)(pi,pi) states.

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.