Coherent anti-Stokes Raman Spectroscopy Data Research Articles

Development of ultrafast spectroscopic techniques to study rapid chemical and physical changes in materials under extreme pressure and temperature conditions

ABSTRACTIn the study of materials at extreme pressures and temperatures, there is an enduring need to extend the range of experiments to previously inaccessible regimes. To accomplish this, improvements in diagnostics for in situ material characterization at extremes must proceed in parallel with techniques used to generate extreme states. Simultaneously, there is a need to study material phenomena – e.g. phase transformations and chemical reactions triggered by the application of extreme conditions – on their natural timescales. Here we report on recent developments in the application of ultrafast laser spectroscopic techniques to high-pressure hightemperature experiments on materials confined in a diamond-anvil cell. Using a bright broadband source coupled to ultrafast detection to discriminate signal from high thermal and fluorescent backgrounds, we conducted broadband optical spectroscopy up to 60 GPa and 1560 K. By coupling the broadband source to a monochromatic pulse, nonlinear Coherent Anti- Stokes Raman Spectroscopy (CARS) with high signal brightness was achieved. Optical absorption data in hot compressed O2 and CARS data in N2 at extreme pressures are reported.

Simultaneous time and frequency detection in femtosecond coherent Raman spectroscopy. II. Application to acetonitrile

The preceding paper showed that, in principle, a high-resolution coherent Raman spectrum can be recovered using femtosecond probe pulses by combined detection in both time and frequency. This measurement is possible even when the pulses are too broad in frequency for conventional frequency-domain spectroscopy and too broad in time for conventional time-domain spectroscopy. In this paper, the method is tested on experimental coherent anti-stokes Raman spectroscopy data from acetonitrile. Compared to theoretical models, experimental data are complicated by noise and incomplete knowledge of the pulse structure. Despite these complications, most of the information in the Raman spectrum is recovered from the data: weak transitions are detected and natural-linewidth resolution is achieved across an 800 cm(-1) spectral range. However, circumstances in which experimental limitations result in missed features or ambiguities in the recovered spectrum are also identified. These results suggest where improvements in measurement and data analysis can be made.

On the accuracy of temperature measurements in turbulent jet diffusion flames by coherent anti-stokes-raman spectroscopy

Coherent anti-Stokes-Raman spectroscopy (CARS) has been applied to determine profiles of mean and standard deviation of temperature and of probability density functions for the Delft turbulent jet diffusion flames fed with Dutch natural gas. Several improvements to the CARS data reduction are introduced and special attention is paid to the effect of spatial averaging. It is shown that the error due to the limited resolution is at most 40 K for mean temperatures around 750 K. The CARS results are compared to the Raman-Rayleigh results obtained earlier for the same flames. Within experimental uncertainty, the results from the two independent nonintrusive laser diagnostic techniques are the same, except for the low central part of the flames. The difference can be a consequence of the occurrence of large methane concentrations and of the sensitive dependence on upstream boundary conditions.

Comparison of CARS Temperature Profile Measurements with Flow-Field Model Calculations in an MHD Diffuser

This paper reports on recent gas temperature profile measurements made on a large-scale coal-fired flow facility (CFFF) with a coherent anti-Stokes Raman spectroscopy (CARS) system. These measurements further demonstrate the application of advanced optical diagnostic methods to characterize large-scale processes. Data from the CARS gas temperature measurements taken in the CFFF diffuser during a recent long-duration experiment are discussed and analyzed. Details are given on the CARS system. Comparisons are drawn between the CARS data and flow-field modeling results. These results define the flow field and heat transfer in the diffuser and provide a basis for interpreting the CARS temperature measurements and further evaluating the flow-field model.

Comparative study of temperature measurement with folded BOXCARS and collinear CARS

A theoretical model has been developed to simulate temperature profile measurements performed with Coherent Anti-Stokes Raman Spectroscopy (CARS) using collinear and BOXCARS phase matching geometries. Spatial averaging is a more serious problem for collinear geometry in which the observed spectra have CARS signal contributions generated outside the focal volume. The shape of the extracted temperature profile close to the boundary of a gas stream is quite sensitive to the temperature gradient at the boundary for both collinear and BOXCARS measurements. Based on the modeling results, collinear CARS phase-matching optics have been modified to reduce the contribution of the CARS signal from the cold region. The error in the temperature extracted from a spectrum generated in a bimodal temperature distribution medium has been studied. Temperature and temperature profile measurements have been performed with both collinear and folded BOXCARS geometries in a CH 4/air, flat flame burner and the Diagnostic Instrumentation and Analysis Laboratory (DIAL)/MSU test stand to compare the results of these two phasematch geometries. Collinear CARS data taken with the usual and modified collinear CARS in the DIAL test stand have been compared to study the application of collinear CARS in practical combustion. A temperature profile obtained from the measurements with modified collinear CARS and folded BOXCARS phase matching technique is in agreement with the theoretically predicted results. The studies also show that BOXCARS is the best choice for the temperature profile measurement. However, collinear CARS can be used with a modified optics arrangement and an expanded laser beam to improve the spatial resolution whenever folded BOXCARS is not able to provide a sufficient signal.

Electronic coherent anti-Stokes Raman spectroscopy in CeF_3

The coherent anti-Stokes Raman spectroscopy (CARS) of electronic states of the Ce3+ ion in a CeF3 crystal at 3.8 K is reported. CARS resonances involving the ground state of the 2F5/2 manifold and the Stark components of the 2F7/2 manifold at 2161 and 2239 cm−1 have been detected. Measurements were made with both visible (λ1 ≈ 476 nm, λ2 = 532 nm) and near-ultraviolet (λ1 = 355 nm, λ2 ≈ 385 nm) lasers. The enhancement of the third-order susceptibility owing to the electronic transitions, |χ(3)R/χ(3)NR|max, is as great as 4.8. This represents an order-of-magnitude increase over that in singly resonant electronic CARS experiments in other rare-earth crystals. The CARS data also yielded accurate measurements of the Ce3+ electronic Raman cross sections. In the visible region, σzz(2161 cm−1) = (5.0 ± 1.1) × 10−30 cm2 and σzz(2239 cm−1) = (1.9 ± 0.7) × 10−30 cm2. One could calculate both the absolute magnitude and the dispersion of |χ(3)R|max by modeling the virtual intermediate states as a single degenerate 5d state at 45 000 cm−1.

Raman linewidths of hydrogen fluoride determined by using low-resolution coherent anti-Stokes Raman spectroscopy

The Q-branch linewidths of hydrogen fluoride were measured by using coherent anti-Stokes Raman spectroscopy (CARS). The first seven transitions were excited by a broadband CARS system, and low-resolution spectra were recorded on an optical multichannel analyzer at pressures from 50 to 700 Torr in 50-Torr increments. A nonlinear least-squares program was employed to fit calculated spectra to experimental spectra by adjusting the linewidths. All lines displayed pressure broadening over the pressure region investigated, with relative magnitudes being consistent with theory and available infrared data. Although the Q-branch lines of the observed spectra were partially blended, the CARS data and calculations permitted consistent linewidth measurements over the pressure range of the experiment.