325 research outputs found
Coherent Raman spectro-imaging with laser frequency combs
Optical spectroscopy and imaging of microscopic samples have opened up a wide
range of applications throughout the physical, chemical, and biological
sciences. High chemical specificity may be achieved by directly interrogating
the fundamental or low-lying vibrational energy levels of the compound
molecules. Amongst the available prevailing label-free techniques, coherent
Raman scattering has the distinguishing features of high spatial resolution
down to 200 nm and three-dimensional sectioning. However, combining fast
imaging speed and identification of multiple - and possibly unexpected-
compounds remains challenging: existing high spectral resolution schemes
require long measurement times to achieve broad spectral spans. Here we
overcome this difficulty and introduce a novel concept of coherent anti-Stokes
Raman scattering (CARS) spectro-imaging with two laser frequency combs. We
illustrate the power of our technique with high resolution (4 cm-1) Raman
spectra spanning more than 1200 cm-1 recorded within less than 15 microseconds.
Furthermore, hyperspectral images combining high spectral (10 cm-1) and spatial
(2 micrometers) resolutions are acquired at a rate of 50 pixels per second.
Real-time multiplex accessing of hyperspectral images may dramatically expand
the range of applications of nonlinear microscopy.Comment: 8 pages, 3 figure
Experimental Determination of the 1 Sigma(+) State Electric-Dipole-Moment Function of Carbon Monoxide up to a Large Internuclear Separation
Experimental intensity information is combined with numerically obtained vibrational wave functions in a nonlinear least squares fitting procedure to obtain the ground electronic state electric-dipole-moment function of carbon monoxide valid in the range of nuclear oscillation (0.87 to 1.01 A) of about the V = 38th vibrational level. Mechanical anharmonicity intensity factors, H, are computed from this function for delta V + = 1, 2, 3, with or = to 38
Adaptive dual-comb spectroscopy in the green region
Dual-comb spectroscopy is extended to the visible spectral range with a
set-up based on two frequency-doubled femtosecond ytterbium-doped fiber lasers.
The dense rovibronic spectrum of iodine around 19240 cm-1 is recorded within 12
ms at Doppler-limited resolution with a simple scheme that only uses
free-running femtosecond lasers
Raman-induced Kerr-effect dual-comb spectroscopy
We report on the first demonstration of nonlinear dual-frequency-comb
spectroscopy. In multi-heterodyne femtosecond Raman-induced Kerr-effect
spectroscopy, the Raman gain resulting from the coherent excitation of
molecular vibrations by a spectrally-narrow pump is imprinted onto the
femtosecond laser frequency comb probe spectrum. The birefringence signal
induced by the nonlinear interaction of these beams and the sample is
heterodyned against a frequency comb local oscillator with a repetition
frequency slightly different from that of the comb probe. Such time-domain
interference provides multiplex access to the phase and amplitude Raman spectra
over a broad spectral bandwidth within a short measurement time. Experimental
demonstration, at a spectral resolution of 200 GHz, a measurement time of 293
{\mu}s and a sensitivity of 10^-6, is given on liquid samples exhibiting a C-H
stretch Raman shift.Comment: 7 pages, 4 figure
The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks
Aims. Photodissociation by UV light is an important destruction mechanism for
CO in many astrophysical environments, ranging from interstellar clouds to
protoplanetary disks. The aim of this work is to gain a better understanding of
the depth dependence and isotope-selective nature of this process.
Methods. We present a photodissociation model based on recent spectroscopic
data from the literature, which allows us to compute depth-dependent and
isotope-selective photodissociation rates at higher accuracy than in previous
work. The model includes self-shielding, mutual shielding and shielding by
atomic and molecular hydrogen, and it is the first such model to include the
rare isotopologues C17O and 13C17O. We couple it to a simple chemical network
to analyse CO abundances in diffuse and translucent clouds, photon-dominated
regions, and circumstellar disks.
Results. The photodissociation rate in the unattenuated interstellar
radiation field is 2.6e-10 s^-1, 30% higher than currently adopted values.
Increasing the excitation temperature or the Doppler width can reduce the
photodissociation rates and the isotopic selectivity by as much as a factor of
three for temperatures above 100 K. The model reproduces column densities
observed towards diffuse clouds and PDRs, and it offers an explanation for both
the enhanced and the reduced N(12CO)/N(13CO) ratios seen in diffuse clouds. The
photodissociation of C17O and 13C17O shows almost exactly the same depth
dependence as that of C18O and 13C18O, respectively, so 17O and 18O are equally
fractionated with respect to 16O. This supports the recent hypothesis that CO
photodissociation in the solar nebula is responsible for the anomalous 17O and
18O abundances in meteorites.Comment: Accepted by A&
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