James R. Nestor

Bio: James R. Nestor is an academic researcher from Princeton University. The author has contributed to research in topics: Raman spectroscopy & Raman scattering. The author has an hindex of 5, co-authored 6 publications receiving 214 citations.

Coherent anti-Stokes Raman scattering (CARS) spectra, with resonance enhancement, of cytochrome c and vitamin B12 in dilute aqueous solution.

Abstract: Coherent anti-Stokes Raman scattering (CARS) spectra have been obtained for ferrocytochrome c and cyano cobalamin in aqueous solution at millimolar concentrations, using a pair of tunable dye lasers pumped by a pulsed nitrogen laser. Resonance enhancement was obtained by tuning the omega1 laser to the visible absorption bands of the samples. The spectral features correspond to those observed in the conventional resonance Raman spectra. It appears that CARS spectroscopy, with its advantageous fluorescence rejection, can be usefully applied to biological samples by exploiting resonance enhancement. While the background scattering from water is 10 times higher than that of benzene and other aromatic solvents, it is actually at the low end of the scale for most liquids. The anomalously low background of aromatic liquids is thought to result from competition by the unusually efficient stimulated Raman scattering which they display. Off-resonance spectra for both cobalamin and cytochrome c contain negative peaks, i.e., absorption bands in the background. These are interpreted as inverse Raman processes induced by the omega1 photons in the presence of the continuum provided by the background scattering. While both CARS and the inverse Raman effect are subject to resonance enhancement, the wavelength dependence of CARS is evidently steeper.

Resonance coherent anti-Stokes Raman scattering spectra of fluorescent biological chromophores: Vibrational evidence for hydrogen bonding of flavin to glucose oxidase and for rapid solvent exchange

Abstract: Coherent anti-Stokes Raman scattering (CARS) spectra are reported for flavin adenine dinucleotide (FAD) and glucose oxidase (β-D-glucose:oxygen 1-oxidoreductase; EC 1.1.3.4), in resonance with the 450 nm flavin absorption band. Several isoalloxazine ring modes were observed (1635, 1584, 1507, 1416, and 1359 cm-1). A 12 cm-1 increase in one component of the 1359 cm-1 band upon binding to glucose oxidase was attributed to hydrogen bonding of the N3 proton to a protein acceptor. This interpretation is consistent with deuteration results. Smaller decreases (5-7 cm-1) on binding were observed for the 1635 and 1416 cm-1 modes (and also a 1297 cm-1 mode of deuterated FAD) and were attributed to environmental effects. Deuteration of bound FAD was observed within 5 min of mixing glucose oxidase with D2O, demonstrating ready access to solvent. Lorentzian CARS peaks were observed with ωas at the peak of the 450 nm absorption band, a condition which corresponds to maximum resonance enhancement if the peak corresponds to the envelope of 0-1 vibronic transitions. If ω1 was tuned to the peak, then dispersion lineshapes were observed, reflecting a loss of Raman enhancement relative to the electronic background.

Optogalvanic spectra of neon and argon in glow discharge lamps

Abstract: Optogalvanic spectra of atomic neon and argon have been obtained by irradiating miniature glow discharge lamps with a pulsed, tunable dye laser. The method is shown to be useful for wavelength calibration of tunable lasers, and the resonant lines are cataloged. Two-photon absorption lines in neon are observed when the laser is focused into a small region of the discharge.

Stimulated Raman Gain Spectroscopy Utilizing Counter-Propagating Pulsed Dye Lasers

Abstract: A simplified experimental arrangement, in which the laser beams are counter-propagating, has allowed the acquisition of Raman gain spectra without the aid of optically dispersive devices. The method allows high spectral resolution with the spatial rejection of fluorescence typical of coherent Raman techniques. Access to the low-frequency spectral region has revealed stimulated gain involving mechanical waves of the bulk medium. Quasi-continuous scans of the entire vibrational region have been made using only two laser dyes.

Local modes of benzene and benzene dimer, studied by infrared–ultraviolet double resonance in a supersonic beam

Abstract: We used rotational cooling of molecules to ∼5 K by supersonic expansion and state‐selective, multilevel saturation spectroscopy to obtain high‐resolution spectra of the fundamental and first and second overtone transitions of C–H stretching modes in ground‐electronic‐state benzene and its dimer. Greatly reduced linewidths (<3 cm−1 FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C–H stretches are at least a few ps, even at the energy of the second overtone (8800 cm−1). The ‘‘local mode’’ picture appears to apply when at least three quanta of C–H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm−1, the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer’s predissociation lifetime at ∼3000 cm−1 vibrational energy exceeds ∼3 ps.

Infrared–ultraviolet double resonance studies of benzene molecules in a supersonic beam

Abstract: We have used IR excitation to selectively create populations in admixtures of the zeroth‐order states comprising the ∼3000 cm−1 ‘‘C–H stretching Fermi triad’’ of benzene. UV spectra of the 260 nm A(1B2u)←X(1A1g) transition in the IR‐excited molecules show several new bands, which we have assigned. Final states in the UV transitions are some vibrational levels which have not been detected before, allowing us to find several excited‐state vibrational frequencies. We have determined ν’3 =1327±3 cm−1, ν19 =1405±3 cm−1, and ν’20 =3084±5 cm−1. Also, vibrational structure which was unresolved in IR spectra of the ‘‘Fermi triad’’ was resolved in the UV double resonance spectra, confirming that the C–H stretching admixture is really a tetrad. The 3048, 3079, and 3101 cm−1 states had formerly been given the labels ν‘20, ν‘8+ν‘19, and ν‘1+ν‘6+ν‘19, respectively. Actually, the middle level most nearly resembles ν‘1+ν‘6+ν‘19, and the 3101 cm−1 level is strongly mixed with ν‘3+ν‘6+ν‘15. As predicted by molecular orbi...

Coherent Anti-Stokes Raman Spectroscopy

Abstract: The development and application of Raman spectroscopy has followed closely the development of excitation sources of increased brightness and power. The laser provided a quantum jump in improvement over mercury arc sources, and CW visible lasers with average powers of 1–10 W are now in common use. Despite the fact that Raman scattering cross sections are quite small (Chap.4), the technique has proven an extremely valuable tool in molecular spectroscopy. Many of the more important applications are reviewed in the preceding chapters and, in Chap.6, ways to increase Raman cross sections via resonance enhancement have been discussed.

Airborne remote sensing of tropospheric water vapor with a near-infrared differential absorption lidar system.

Abstract: A near-infrared airborne differential absorption lidar (DIAL) system has become operational. Horizontal and vertical water vapor profiles of the troposphere during summer (nighttime) conditions extending from the top of the planetary boundary layer (PBL) up to near the tropopause are investigated. These measurements have been performed in Southern Bavaria, Germany. The system design, the frequency control units, and an estimation of the laser line profile of the narrow-band dye laser are discussed. Effective absorption cross sections in terms of altitude are calculated. Statistical and systematic errors of the water vapor measurements are evaluated as a function of altitude. The effect of a systematic range-dependent error caused by molecular absorption is investigated by comparing the DIAL data with in situ measurements. Typical horizontal resolutions range from 4 km in the lower troposphere to 11 km in the upper troposphere, with vertical resolutions varying from 0.3 to 1 km, respectively. The lower limit of the sensitivity of the water vapor mixing ratio is calculated to be 0.01 g/kg. The total errors of these measurements range between 8% and 25%. A sine-shaped wave structure with a wavelength of 14 km and an amplitude of 20% of its mean value, detected in the lower troposphere, indicates an atmospheric gravity wave field.