Long wavelength near-infrared transmission spectroscopy of barley seeds using a supercontinuum laser: Prediction of mixed-linkage beta-glucan content.

A supercontinuum laser was used to perform the first transmission measurements on intact seeds with long wavelength near-infrared spectroscopy. A total of 105 barley seeds from five different barley genotypes (Bomi, lys5.f, lys5.g, lys16 and lys95) were measured from 2275 to 2375 nm. The mixed-linkage (1→3,1→4)-ß-D-glucan (BG) and protein content was measured with wet chemical analysis for each single seed. A partial least squares model correlated the BG % (w/w) with the spectral measurements with a R2CV and R2PRED of 0.83 and 0.90, respectively. The predictive model for BG could be improved by averaging spectra from the same seed and by replacing the individual seed BG content with the average BG of each barley genotype.

Transient IR Spectroscopic Observation of Singlet and Triplet States of 2-Nitrofluorene: Revisiting the Photophysics of Nitroaromatics.

The dynamics of 2-nitrofluorene (2-NF) in deuterated acetonitrile is studied using UV pump, IR probe femtosecond transient absorption spectroscopy. Upon excitation to the vibrationally excited S1 state, the excited-state population of 2-NF branches into two different relaxation pathways. One route leads to intersystem crossing (ISC) to the triplet manifold within a few hundred femtoseconds and the other to internal conversion (IC) to the ground state. The experiments indicate that after relaxation to the energetic minimum on S1, 2-NF undergoes internal conversion to the ground state in about 15 ps. IC within the triplet manifold is also observed as the initially populated triplet state relaxes to T1 in about 6 ps. Rotational anisotropy measurements corroborate the assignment of the transient IR frequencies and indicate a rotational diffusion time of 2-NF in the solvent of about 14 ps. The combined set of results provides a unified picture of the dynamics in photoexcited 2-NF. This to our knowledge is the first example using femtosecond vibrational spectroscopy for the study of the fundamental photoinduced processes in nitroaromatic compounds.

Hydration dynamics of aqueous nitrate.

Aqueous nitrate, NO3(-)(aq), was studied by 2D-IR, UV-IR, and UV-UV time-resolved spectroscopies in combination with molecular dynamics (MD) simulations with the purpose of determining the hydration dynamics around the anion. In water, the D3h symmetry of NO3(-) is broken, and the degeneracy of the asymmetric-stretch modes is lifted. This provides a very sensitive probe of the ion-water interactions. The 2D-IR measurements reveal excitation exchange between the two nondegenerate asymmetric-stretch vibrations on a 300-fs time scale concomitant with fast anisotropy decay of the diagonal-peak signals. The MD simulations show that this is caused by jumps of the transition dipole orientations related to fluctuations of the hydrogen bonds connecting the nitrate ion to the nearest water molecules. Reorientation of the ion, which is associated with the hydrogen-bond breaking, was monitored by time-resolved UV-IR and UV-UV spectroscopy, revealing a 2-ps time constant. These time scales are very similar to those reported for isotope-labeled water, suggesting that NO3(-)(aq) has a labile hydration shell.

Pulsed laser manipulation of an optically trapped bead: averaging thermal noise and measuring the pulsed force amplitude.

An experimental strategy for post-eliminating thermal noise on position measurements of optically trapped particles is presented. Using a nanosecond pulsed laser, synchronized to the detection system, to exert a periodic driving force on an optically trapped 10 µm polystyrene bead, the laser pulse-bead interaction is repeated hundreds of times. Traces with the bead position following the prompt displacement from equilibrium, induced by each laser pulse, are averaged and reveal the underlying deterministic motion of the bead, which is not visible in a single trace due to thermal noise. The motion of the bead is analyzed from the direct time-dependent position measurements and from the power spectrum. The results show that the bead is on average displaced 208 nm from the trap center and exposed to a force amplitude of 71 nanoNewton, more than five orders of magnitude larger than the trapping forces. Our experimental method may have implications for microrheology.

Nonlinear soliton matching between optical fibers.

In this Letter, we propose a generic nonlinear coupling coefficient, η(NL)²=η|γ/ß2|(fiber2)/|γ/ß2|(fiber1), which gives a quantitative measure for the efficiency of nonlinear matching of optical fibers by describing how a fundamental soliton couples from one fiber into another. Specifically, we use η(NL) to demonstrate a significant soliton self-frequency shift of a fundamental soliton, and we show that nonlinear matching can take precedence over linear mode matching. The nonlinear coupling coefficient depends on both the dispersion (ß2) and nonlinearity (γ), as well as on the power coupling efficiency η. Being generic, η(NL) enables engineering of general waveguide systems, e.g., for optimized Raman redshift or supercontinuum generation.

A higher-order-mode fiber delivery for Ti:Sapphire femtosecond lasers.

We report the first higher-order-mode fiber with anomalous dispersion at 800nm and demonstrate its potential in femtosecond pulse delivery for Ti:Sapphire femtosecond lasers. We obtain 125fs pulses after propagating a distance of 3.6 meters in solid-silica fiber. The pulses could be further compressed in a quartz rod to nearly chirp-free 110fs pulses. Femtosecond pulse delivery is achieved by launching the laser output directly into the delivery fiber without any pre-chirping of the input pulse. The demonstrated pulse delivery scheme suggests scaling to >20meters for pulse delivery in harsh environments not suited for oscillator operation or in applications that require long distance flexibility.

Primary formation dynamics of peroxynitrite following photolysis of nitrate.

The photolysis of nitrate, NO3-, in D2O solution has been investigated by femtosecond infrared spectroscopy. In accordance with previous investigations, we observe that the peroxynitrite ion, ONOO-, is the dominant photochemical product following the excitation of nitrate at 200 nm. Moreover, we are able to identify the cis/trans isomers of peroxynitrite and the dynamics of their formation in solution. We observe that the trans- ONOO- isomer is formed directly and solely from the excited NO3- ion within the first two picoseconds after excitation. Subsequently, about half of the trans-ONOO- isomerizes to cis-ONOO- in 25 ps; thereafter, the ratio between the two isomers remains constant for the 300 ps duration of the experiment. The observed vibrational frequencies of the terminal O=N bonds are at 1515 and 1580 cm(-1) for trans- and cis-peroxynitrite, respectively. The detailed analysis of the infrared bands of cis- and trans-peroxynitrite is facilitated by electronic structure calculations on the conformers in a cluster of 11 D2O molecules and by steady-state infrared spectroscopy of ONOO- in D2O. In addition to the formation of ONOO-, the experiments also reveal a slow approximately 50 ps formation of NO2 following the photolysis of nitrate.

Solvent response to solute photo-dissociation.

Ultraviolet-visible and infrared transient-absorption spectroscopy are used to investigate the transfer of energy from nitrite to water during the photo-dissociation of NO2-(aq). Nitrite is dissociated by photo-excitation at 200 nm. About 40% of the photo-fragments recombine and relax on a 3 ps timescale, while diffusive recombination accounts for another 10% of the fragments during the subsequent 50 ps. The infrared transient-absorption spectra of the photo-dissociation of nitrite solvated in H2O and D2O show no evidence of excited vibrations after 0.5 ps. Instead they reveal a sub-0.5 ps change in the infrared absorption similar to what is observed when the temperature of water is increased. Since this spectral change is associated with the weakening of the hydrogen-bond network, we infer that excess energy from the dissociation of nitrite is dissipated to the local hydrogen-bonded water network in less than 0.5 ps. The rapid change in the infrared absorption is followed by a slower (50 ps) component associated with the energy dissipation to the solvent as the photo-fragments diffusively recombine and relax.

Electron detachment and relaxation of OH-(aq).

Femtosecond transient absorption spectroscopy is used to study the primary reaction dynamics of photoinduced electron detachment of the hydroxide ion in water, OH- (aq). The electron is detached by excitation of OH- (aq) to the charge-transfer-to-solvent (CTTS) state at 200 nm. The subsequent relaxation processes are probed in the spectral range from 193 to 800 nm with femtosecond time resolution. We determine both the time-dependent quantum yields of OH- (aq), OH(aq), and e-(aq), and we observe a transient spectral signature which is assigned to relaxation of hot (OH-)* ions formed via solvent-assisted conversion of the excited CTTS state to OH-. The primary quantum yield of OH(aq) is 65 +/- 5%, while recombination with e-(aq) reduces the yield to 34% after 5 ps and 12% after 200 ps. The yield of hot (OH-)* ions is 35 +/- 5%. Rotational anisotropy measurements of OH- (aq) and OH(aq) indicate a reorientation time for OH- (aq) of 1.9 ps, while no rotational anisotropy is resolved for the OH(aq) radical within our time resolution of 0.3 ps. This is consistent with the notion that OH(aq) radicals formed after electron detachment are only weakly bound to the hydrogen bond network of water. The assignment of the experimental data is supported by a series of electronic structure calculations of simple complexes of OH- (H(2)O)(n).

Reproductive death of cancer cells induced by femtosecond laser pulses.

PURPOSE: High intensity femtosecond (1 fs = 10(-15) s) laser pulses may, via multi-photon processes, cause reproductive cell death at wavelengths that otherwise are harmless. We study the efficacy of inducing reproductive death of cancer cells by ultraviolet (UV), visible (VIS) and near infrared (IR) femtosecond laser pulses. MATERIALS AND METHODS: Human squamous carcinoma cervical cancer cells are irradiated by femtosecond laser pulses at 800 nanometers (nm), 400 nm, 266 nm and 200 nm. The reproductive death is assessed by colony forming assay. The contribution from multi-photon processes is evaluated by comparing the cell reproduction subsequent to irradiation by collimated (low intensity) and focused (high intensity), pulsed laser beams with identical fluences. RESULTS: Suitable femtosecond pulses are capable of arresting cell reproduction at all the tested wavelengths. Irradiation at 266 nm is far more efficient than the other wavelengths, both in terms of the fluence and the absorbed dose needed to induce reproductive cell death. The collimated 800 nm beam is unable to induce reproductive cell death even at a fluence of 230 Joule/square centimeters (J/cm2). However, focused 800 nm pulses with much higher intensities, but lower fluences efficiently arrest cell reproduction, thus highlighting the dramatic effect of multi-photon processes. At the intensities used in the present work focusing the 400 nm beam improves its efficacy by an order of magnitude, whereas focusing the 266 nm beam does not improve its efficacy. CONCLUSION: Femtosecond pulses at 200, 266, 400 and 800 nm induce reproductive cell death if the intensity is sufficiently high. Multi-photon processes can improve the efficacy substantially and even result in reproductive cell death at wavelengths, where single-photon processes are harmless.

Fiber laser-based light source for coherent anti-Stokes Raman scattering microspectroscopy.

We demonstrate an alternative light source for CARS microspectroscopy based on a fiber laser and a photonic-crystal fiber. The light source simultaneously delivers a near-transform-limited picosecond pump pulse at 1033.5 nm and a frequency-shifted, near-transform-limited femtosecond Stokes pulse, tunable from 1033.5 nm to 1400 nm. This corresponds to a range 0 - 2500 cm(-1), so that Raman-active vibrations in this frequency range can be probed. The spectral resolution is 5 cm(-1), given by the spectral width of the pump pulse. The frequency range that can be probed simultaneously is 200 cm(-1)-wide, given by the spectral width of the Stokes pulse. The achievable pulse powers are 50 mW for the pump and 2 mW for the Stokes pulse. The repetition rate is 35 MHz. We demonstrate the capability of this light source by performing CARS microspectroscopy and comparing CARS spectra with Raman spectra.

The primary photodynamics of aqueous nitrate: formation of peroxynitrite.

We have examined the photochemical reactions occurring after irradiation at 200 nm of the aqueous nitrate ion, NO3(-)(aq). Using femtosecond transient absorption spectroscopy over the range 194-388 nm, we have characterized the formation and subsequent relaxation of the primary photoproducts of nitrate photolysis. The dominant photoproduct is the cis-isomer of peroxynitrite, which accounts for 48% of the excited state molecules initially produced. A slightly smaller fraction, 44%, of the excited molecules return to the electronic ground state of NO3(-) and relax to the vibrational ground state in 2 ps. The remaining 8% of the molecules initially excited react via the *NO + *O2(-) or the NO- + O2 dissociation channels. Formation of NO2(-) and *NO2 is not observed, suggesting that the previous observations of these species in steady-state photolysis are caused by reactions occurring on a longer time scale.