Biomedical subjects

Stephen R Leone

Publications and source records attributed to Stephen R Leone.

At least 19 recordsLinked to original sources

PubMed2006

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states.

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.

Journal Article↗

PubMed2006

Energy-resolved photoionization of alkylperoxy radicals and the stability of their cations.

The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ã (1)A' state of C(2)H(5)OO(+).

Journal Article↗

PubMed2006

Heterodyne mixing of laser fields for temporal gating of high-order harmonic generation.

The concept of heterodyne mixing of laser fields is theoretically applied to the process of high-harmonic generation to enhance and modulate the kinetic energy of the active electron on subcycle time scales. A very small amount of intensity in the heterodyne field creates a significant modification of the electron kinetic energy, due to its amplification by the strong fundamental field in the kinetic-energy term, in which the heterodyne mixing occurs. Quantum calculations are carried out to verify the predictions of the classical results, demonstrating very good qualitative and quantitative agreement. Applications of the heterodyne-mixing concept are the extension of the harmonic cutoff to higher photon energies and the temporal gating of attosecond pulse production.

Journal Article↗

PubMed2006

High-resolution apertureless near-field optical imaging using gold nanosphere probes.

An apertureless near-field scanning optical microscope (ANSOM) that utilizes the enhanced field around a gold nanosphere, which is attached to the end of an atomic force microscope (AFM) tip, is used to image the local dielectric constant of the patterned metallic surfaces and local electric field around plasmonic nanosphere samples. A colloidal gold nanosphere (approximately 50 nm diameter) is linked to the extremity of the conventional etched-silicon probe. The scattering of laser radiation (633 or 532 nm) is modulated by the oscillating nanosphere-functionalized silicon tip, and the scattered radiation is detected. The approach curve (scattering intensity as a function of the tip-sample distance), the polarization dependence (scattering intensity as a function of the excitation polarization direction), and ANSOM image contrast confirm that the spherical nanosphere attached to the silicon tip acts as a point dipole that interacts with the sample surface via a dipole-dipole coupling, in which the dipole created by the field at the tip interacts with its own image dipole in the sample. The image obtained with the nanoparticle functionalized tip provides a dielectric map of the sample surface with a spatial resolution better than 80 nm. In addition, we show that the functionalized tip is capable of imaging the local electric field distribution above the plasmonic nanosphere samples. Overall, the result shows that high-resolution ANSOM is possible without the aid of the lightning-rod effect. With an improved tip-fabrication method, we believe that the method can provide a versatile high-resolution chemical imaging that is not available from usual forms of ANSOM.

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PubMed2006

Two-color visible/vacuum ultraviolet photoelectron imaging dynamics of Br2.

An experimental two-color photoionization dynamics study of laser-excited Br2 molecules is presented, combining pulsed visible laser excitation and tunable vacuum ultraviolet (VUV) synchrotron radiation with photoelectron imaging. The X 1Sigmag + -B 3Pi0+u transition in Br2 is excited at 527 nm corresponding predominantly to excitation of the v' = 28 vibrational level in the B 3Pi0+u state. Tunable VUV undulator radiation in the energy range of 8.40-10.15 eV is subsequently used to ionize the excited molecules to the X 2Pi32,12 state of the ion, and the ionic ground state is probed by photoelectron imaging. Similar experiments are performed using single-photon synchrotron ionization in the photon energy range of 10.75-12.50 eV without any laser excitation. Photoelectron kinetic energy distributions are extracted from the photoelectron images. In the case of two-color photoionization using resonant excitation of the intermediate B 3Pi0+u state, a broad distribution of photoelectron kinetic energies is observed, and in some cases even a bimodal distribution, which depends on the VUV photon energy. In contrast, for single-photon ionization, a single nearly Gaussian-shaped distribution is observed, which shifts to higher energy with photon energy. Simulated spectra based on Franck-Condon factors for the transitions Br2(X 1Sigmag+, v" = 0)-Br2 +(X 2Pi12,32, v+) and Br2(B 3Pi0+u, v' = 28)-Br2 +(X 2Pi12,32, v+) are generated. Comparison of these calculated spectra with the measured images suggests that the differences in the kinetic energy distributions for the two ionization processes reflect the different extensions of the vibrational wave functions in the v" = 0 electronic ground state (X 1Sigmag+) versus the electronically and vibrationally excited state (B 3Pi0+u, v' = 28).

Journal Article↗

PubMed2006

Circular phase mask for control and stabilization of single optical filaments.

We experimentally demonstrate an efficient way to control and stabilize single optical filaments initiated by ultrashort laser pulses in a rare gas medium. This is done by the application of a stationary two-dimensional phase mask to the laser beam prior to focusing. Simple circular phase-step patterns of a given radius and relative phase are sufficient to stabilize the pointing of the filament output and to optimize the spectral bandwidth of the light without any resulting loss of input laser power.

Journal Article↗

PubMed2006

Dynamics of the CH(A2Delta) product from the reaction of C2H with O2 studied by Fourier transform visible spectroscopy.

The reaction C(2)H + O(2) --> CH(A(2)Delta) + CO(2) is investigated using Fourier transform visible emission spectroscopy. C(2)H radicals, produced by 193 nm photolysis of C(2)H(2), react with O(2) molecules at low total pressures to produce electronically excited CH(A(2)Delta). Observation of the CH(A(2)Delta-X(2)Pi) electronic emission to infer nascent rotational and vibrational CH(A(2)Delta) distributions provides information about energy partitioning in the CH(A(2)Delta) fragment during the reaction. The rotational and vibrational populations of the CH(A(2)Delta) product are determined by fitting the rotationally resolved experimental spectra with simulated spectra. The CH(A(2)Delta) product is found to be rotationally and vibrationally excited with T(rot) congruent with 1150 K and T(vib) congruent with 1900 K. The mechanism for this reaction proceeds through one of two five-atom intermediates and requires a crossing between electronic potential surfaces. The rotational excitation suggests a bent geometry for the final intermediate of this reaction before dissociation to products, and the vibrational excitation involves an elongation of the C-H bond from the compressed transition state to the final CH(A) state.

Journal Article↗

PubMed2006

Coherent ultrafast photoelectron spectroscopy in Br2: an alternative method for measuring Delta v not equal 0 transitions in Rydberg-to-Rydberg excitations.

A two-state vibrational wave packet is prepared in a low-lying 4d[12](1 or 2) Rydberg state of jet cooled Br(2) (4d, v(')=3 and v(')=4) by two-photon excitation with 266.5 nm pulses from an ultrafast laser. The wave packet is detected by autoionization following excitation with time-delayed 800 nm pulses to the n=8 (v(+)=4) and n=9 (v(+)=3) Rydberg states in the (2)Pi(12) angular momentum core state. Autoionization of each state occurs to the (2)Pi(32) state of the ion through spin-orbit ionization. Photoelectron spectroscopy is used to differentiate between the n=8 and n=9 ejected photoelectrons. Detection of the wave packet recurrences via the n=8 and n=9 Rydberg states reveals a pi phase-shift difference of the recurrences between the two final states. In each case, Delta v not equal 0 transitions are observed since wave packet recurrences are detected. By fitting the observed phase change of the recurrences with a simple model for the overlap amplitudes and assumptions about the potentials, we estimate, within the context of the model, that approximately 0.6% of the transitions may be attributed to Delta v= +/- 1 transitions between the initial Rydberg superposition state and the final Rydberg detection state.

Journal Article↗

PubMed2006

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field.

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.

Journal Article↗

PubMed2006

Chemical imaging by single pulse interferometric coherent anti-stokes Raman scattering microscopy.

A single pulse interferometric coherent anti-Stokes Raman (CARS) spectroscopy method is used to obtain broadband CARS spectra and microscopy images of liquid and polymer samples. The pump, Stokes, and probe pulses are all selected inside a single broadband ultrafast pulse by a phase- and polarization-controlled pulse shaping technique and used to generate two spectral interference CARS signals simultaneously. The normalized difference of these two signals provides an amplified background-free broadband resonant CARS spectrum over the 400-1500 cm(-1) range with 35 cm(-1) spectral resolution. Chemically selective microscopy images of multicomponent polymer and liquid samples are investigated with this new CARS method. Multiplex CARS spectra at 10,000 spatial points are measured within a few minutes, and used to construct chemically selective microscopy images with a spatial resolution of 400 nm. The spectral bandwidth limits, sensitivity, homodyne amplification advantages, spatial resolution, depolarization, chromatic aberration, and chemical imaging aspects of this new technique are discussed in detail.

Journal Article↗

PubMed2006

State-resolved dynamics of the CH(A2Delta) channels from single and multiple photon dissociation of bromoform in the 10-20 eV energy range.

Single photon dissociation of bromoform using synchrotron radiation has been investigated by Fourier transform visible fluorescence spectroscopy (FTVIS). The photodissociation of bromoform in the 12-18 eV energy range results in several products, among which are the CH(A2Delta) and CH(B2Sigma) radicals. Vibrational and rotational state distributions of the CH(A2Delta) are determined from their fluorescence spectra. From the threshold photon energy above which emission from the CH(A2Delta) radicals is observed, the most likely process leading to CH(A) formation is CHBr3 --> CH + 3Br rather than CHBr3 --> CH + Br + Br2. The rotational Boltzmann temperatures in the CH(A --> X) emission spectra for v' = 0 and v' = 1 range between 1570 and 3650 K, depending on the excitation photon energy. From the high rotational excitation, the results suggest that the mechanism for the loss of three bromine atoms is most likely sequential. A small negative emission anisotropy of the CH(A) radicals [(Ipar - Iper)/(Ipar + 2Iper) = -0.024 +/- 0.005] is constant across the action spectrum; a small net absorption dipole of CHBr3 in the vacuum ultraviolet is parallel to the 3-fold symmetry axis of the CHBr3 molecule. The state distributions of the CH(A2Delta) radicals from multiphoton dissociation of bromoform using the 266 nm output (three photons) of a femtosecond laser (Boltzmann temperatures: T(v'=0)(rot)= 4250 +/- 300 K; T(v'=1)(rot)= 3100 +/- 550 K) are compared to those from the single photon dissociation results (Boltzmann temperatures: T(v'=0)(rot)= 3650 +/- 150 K; T(v'=1)(rot)= 2400 +/- 200 K) at the same total excitation energy under collision free conditions. The analysis of the CH(A) rotational populations shows hotter rotational populations for the femtosecond experiment, also suggesting sequential dissociation of the bromoform in the femtosecond experiment. The duration of the femtosecond laser pulse is approximately 180 fs, setting a limit on the time scales for the multiple dissociations.

Journal Article↗

PubMed2006

VUV photoelectron imaging of biological nanoparticles: ionization energy determination of nanophase glycine and phenylalanine-glycine-glycine.

The ionization energies of biological nanoparticles are determined using the velocity map photoelectron imaging technique. A beam of nanoparticles produced by aerosol methods is photoionized with tunable vacuum ultraviolet (VUV) synchrotron radiation. The resulting photoelectrons are detected and their angular and energy distributions are measured, yielding an angle-resolved photoelectron spectrum. The ionization energies of the nanoparticles are derived from plots of the photoelectron spectrum versus incident photon energy. The ionization energies of nanophase glycine and phenylalanine-glycine-glycine are 7.6 +/- 0.2 eV, and 7.5 +/- 0.2 eV, respectively. X-Ray powder diffraction studies on the glycine nanoparticles indicate that they are crystalline in nature. The reduced ionization energy when compared to gas phase results suggests that the polarization energy in the solid is significant. The difference in the ionization energy between the nano and gas phase reflects this polarization energy and is derived to be 1.7 +/- 0.2 eV and 1.6 +/- 0.2 eV for glycine and phenylalanine-glycine-glycine, respectively. Using these results the molecular polarizability of glycine is estimated to be 4.7 +/- 0.3 A3 (31.9 +/- 1.9 au).

Glycine↗

PubMed2006

Thermal vaporization of biological nanoparticles: fragment-free vacuum ultraviolet photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and beta-carotene.

A simple, new way to introduce fragile biomolecules into the gas phase via thermal vaporization of nanoparticles is described. The general utility of this technique for the study of biomolecules is demonstrated by coupling this source to tunable synchrotron vacuum ultraviolet radiation. Fragment-free photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and beta-carotene are detected with signal-to-noise ratios exceeding 100. The 8.0 eV photoionization mass spectrum of tryptophan nanoparticles vaporized at 373 K is dominated by a single parent ion peak that exhibits a 20-fold enhancement over the methylene indole fragment ion. The degree of dissociative photoionization of tryptophan can be precisely controlled either by the thermal energy imparted into the neutral tryptophan molecule or by the energy of the ionizing photon. The results reveal how approximately 0.5 eV changes in internal energy affect both the photoionization mass spectrum of tryptophan and the appearance energy of the daughter ion fragments. This method allows the ionization energies of glycine (9.3 +/- 0.1 eV), tryptophan (7.3 +/- 0.2 eV), phenylalanine (8.6 +/- 0.1 eV), phenylalanine-glycine-glycine (9.1 +/- 0.1 eV), and beta-carotene (<7.0 eV) molecules to be determined directly from the photoionization efficiency spectra.

Energy Transfer↗

PubMed2006

Low-temperature rate coefficients for the reaction of ethynyl radical (C2H) with benzene.

The reaction of the C2H radical with benzene is studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193-nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Delta) chemiluminescence from the C2H + O2 reaction. Measurements at very low photolysis energy are performed using CF3C2H as the C2H precursor to study the influence of benzene photodissociation on the rate coefficient. Rate coefficients are obtained over a temperature range between 105 and 298 K. The average rate coefficient is found to be five times greater than the estimated value presently used in the photochemical modeling of Titan's atmosphere. The reaction exhibits a slight negative temperature dependence which can be fitted to the expression k(cm3 molecule(-1) s(-1)) = 3.28(+/-1.0) x 10(-10) (T/298)(-0.18(+/-0.18)). The results show that this reaction has no barrier and may play an important role in the formation of large molecules and aerosols at low temperature. Our results are consistent with the formation of a short lifetime intermediate that decomposes to give the final products.

Journal Article↗

PubMed2006

Control of Li2 wave packet dynamics by modification of the quantum mechanical amplitude of a single state.

Sequences of pulses with different spectra are used to control rotational wave packet dynamics in Li(2) by exploiting quantum interference phenomena. Wave packet superpositions are excited in a two-step resonant Raman process by two different pulses. Interferences between individual states shared by both wave packets can be used to enhance or destroy specific components of a superposition by varying the time delay between the pulses and/or the relative phase within the pulses. Elimination of selected quantum beats is achieved by greater than 94% for each case. A simple, yet effective, method for generating different color phase-locked pairs of laser pulses in a liquid-crystal pulse shaper setup without the need for interferometric stabilization schemes is described. The ability to manipulate single states of a superposition is an important advancement for intuitive control schemes and provides a potential new approach for initialization schemes in the field of quantum information.

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PubMed2006

Elastic light scattering from nanoparticles by monochromatic vacuum-ultraviolet radiation.

Elastic light scattering is reported using monochromatic vacuum-ultraviolet radiation to study free, spherical silica nanoparticles prepared by approaches from colloidal chemistry, with diameters between 100 and 240 nm. The colloidal nanoparticles of defined size are transferred from an aqueous solution into the gas phase using a particle beam experiment. After focusing of the particle beam by an aerodynamic lens, the scattered light from monochromatic synchrotron radiation is measured. Angle-resolved elastically scattered light is detected, showing a strong forward-scattering component. Additional evidence for the detection of elastically scattered light comes from plotting the scattered light intensity as a function of the dimensionless parameter qR, where q is the magnitude of the scattering wave vector and R is the particle radius. This yields different power-law regimes that are assigned to scattering from the surface and the bulk of the nanoparticles. Furthermore, there is evidence for modulations in the scattered light intensity as a function of scattering angle, which is clearly distinguished from the forward-scattering component. The experimental results are compared to Mie scattering simulations for isolated particles, yielding general agreement with the experimental results. Deviations from Mie simulations are observed for samples consisting of significant amounts of aggregates. The present results indicate that the optical properties of free nanoparticles are sensitively probed by vacuum-ultraviolet radiation.

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PubMed2006

Vacuum ultraviolet photoionization of C3.

Photoionization efficiency (PIE) curves for C(3) molecules produced by laser ablation are measured from 11.0 to 13.5 eV with tunable vacuum ultraviolet undulator radiation. A step in the PIE curve versus photon energy, obtained with N(2) as the carrier gas, supports the conclusion of very effective cooling of C(3) to its linear (1)Sigma(g)(+) ground state. The second step observed in the PIE curve versus photon energy could be the first experimental evidence of the C(3)(+)((2)Sigma(g)(+)) excited state. The experimental results, complemented by ab initio calculations, suggest a state-to-state vertical ionization energy of 11.70 +/- 0.05 eV between the C(3)(X(1)Sigma(g)(+)) and the C(3)(+)(X(2)Sigma(u)(+)) states. An ionization energy of 11.61 +/- 0.07 eV between the neutral and ionic ground states of C(3) is deduced using the data together with our calculations. Accurate ab initio calculations are performed for both linear and bent geometries on the lowest doublet electronic states of C(3)(+) using Configuration Interaction (CI) approaches and large basis sets. These calculations confirm that C(3)(+) is bent in its electronic ground state, which is separated by a small potential barrier from the (2)Sigma(u)(+) minimum. The gradual increase at the onset of the PIE curve suggests a geometry change between the ground neutral and cationic states. The energies between several doublet states of the ion are theoretically determined to be 0.81, 1.49, and 1.98 eV between the (2)Sigma(u)(+) and the (2)Sigma(g)(+),( 2)Pi(u), (2)Pi(g) excited states of C(3)(+), respectively.

Journal Article↗

PubMed2005

Vibrational and rotational distributions of the CH(A2Delta) product of the C(2)H + O(3P) reaction studied by fourier transform visible (FTVIS) emission spectroscopy.

The C(2)H + O((3)P) --> CH(A) + CO reaction is investigated using Fourier transform visible emission spectroscopy. The O((3)P) and C(2)H radicals are produced by simultaneous 193 nm photolysis of SO(2) and C(2)H(2) precursors, respectively. The nascent vibrational and rotational distributions of the CH(A) product are obtained under time-resolved, but quasi-steady-state, conditions facilitated by the short lifetime of the CH(A) emission. The vibrational temperature of the CH(A) product is found to be appreciably hotter (2800 +/- 100 K) than the rotational distributions in the v' = 0 (1400 +/- 100 K) and v' = 1 (1250 +/- 250 K) levels. The results suggest that the reaction may proceed through an electronically excited HCCO() intermediate; moreover, the vibrational excitation compared to rotational excitation is higher than expected based on a statistical distribution of energy and may be the result of geometrical changes in the transition state. The CH(A) emission is also observed in a C(2)H(2)/O/H reaction mixture using a microwave discharge apparatus to form O atoms, with subsequent H atom production. The nascent rotational and vibrational distributions of the CH(A) determined by the microwave discharge apparatus are very similar to the CH(A) distributions obtained in the photodissociation experiment. The results support the idea that the C(2)H + O((3)P) reaction may play a role in low-pressure C(2)H(2)/O/H flames, as previously concluded.

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