Algorithms for calculating coherent anti-stokes Raman spectra: Application to several small molecules

The cross section of Raman processes involving light molecules takes into account the unavoidable coupling between rotational and vibrational modes. This coupling is quantified by the Herman-Wallis (HW) factor and its use was recently criticized in view of the known thermometric application of vibrational spectra of coherent antiStokes Raman scattering (CARS) of hydrogen. Following this line of research, this paper addresses the physical limitations of the HW factors employed ordinarily in Q-branch CARS measurements. Furthermore, new determinations of the vibration-rotation coupling are considered. These are based on a more realistic scenario consisting of anharmonic oscillations and polarizability variations during vibration. Numerical examples for fundamental diatomics, namely H 2 and N 2 , are given throughout the text. It is found that, as expected, the Raman cross section of H 2 is strongly dependent on the HW factor and the consequent correction to CARS line intensities is of the order of 10% or greater. Surprisingly, the analysis suggests that N 2 spectral data are also affected, to a certain extent, by the new HW factors. Usually, this correction is completely disregarded, but under the new understanding of the vibration-rotation coupling, Q-branch CARS lines of N 2 are predicted to undergo variations between 1 and 5% for rotational levels that are sufficiently populated at temperatures of interest for CARS studies. Oxygen is also prone to vibration-rotation corrections and is finally shown as an example between the two limits of hydrogen and nitrogen.

Section: Analysis Of Old Q-branch Hw Factorsmentioning

confidence: 99%

Section: Analysis Of New Q-branch Hw Factorsmentioning

confidence: 99%

Comparative analysis of Herman‐Wallis factors for uses in coherent anti‐Stokes Raman spectra of light molecules

Marrocco

2009

“…However, the point is that population of rotational levels is not the only factor affecting the amplitudes of rotational lines. Two other factors are the J-dependence of Raman cross-sections arising from the interaction between the vibrational and rotational modes of a molecular motion [18,19] and the J-dependence of Raman linewidths.…”

Section: Temperature Measurementsmentioning

confidence: 99%

CARS temperature measurement in a LOX/CH₄ spray flame

Fabelinsky

Смирнов

Stelmakh

et al. 2010

Coherent anti-Stokes Raman scattering (CARS) spectroscopy has been used to investigate cryogenic liquid oxygen/gaseous methane (LOX/CH 4 ) flames on a medium-size test facility at a pressure of 0.24 MPa and mass flow of 0.025 kg/s. Single-shot, broadband CARS spectra with simultaneous detection of the Q-branches of hydrogen and water molecules were recorded with good signal-to-noise ratio. Temperature was deduced from the H 2 and H 2 O CARS profiles. The spatial temperature distribution in a comparatively harsh environment has been measured successfully. The measurements took place in the windowed combustion chamber of the DLR M3 test facility, aiming to provide data for validation of rocket combustor modeling. Copyright

“…On the contrary, the resonant contribution originates from the movement of nuclei in the electrical field of the light waves and has a pronounced dependence on frequency, gas density N and temperature T. In the simple case of ro-vibrational branch without a spectral exchange, χ (3) R can be expressed [8] as a sum of a set of terms, where each term represents the contribution from an individual transition. Resonant nonlinear susceptibility is a complex quantity, and its real and imaginary parts differently depend on the detuning from a resonance ( J , γ J ).…”

Section: Line Positions' Measurementsmentioning

confidence: 99%

CARS investigation of collisional shift of the hydrogen Q‐branch transitions by water at high temperatures

Vereschagin

Смирнов

et al. 2010

A high-resolution (∼0.1 cm −1 ) spectroscopic method based on the application of a Fabry-Pérot interferometer to the spectral analysis of the coherent anti-Stokes Raman scattering (CARS) signal from an individual Raman transition was used to obtain single-shot spectra of hydrogen Q-branch transitions directly in the flame of a pulsed, high-pressure H 2 /O 2 combustion chamber. Simultaneously with the Fabry-Pérot pattern, a broadband CARS spectrum of the complete H 2 Q -branch structure was recorded in order to measure the temperature of the probe volume. During every cycle of the combustion chamber, a pressure pulse together with single-shot CARS spectra, providing information on individual line shapes and medium temperature, was recorded. On the basis of the experimental data, the temperature dependences of lineshift coefficients for several Q-branch lines of hydrogen molecules under collisions with water molecules were determined in the temperature range 2100 < T < 3500 K, and an empirical 'fitting law' for H 2 -H 2 O lineshift coefficients is proposed.