Pure rotational CARS

Temperature information from CARS spectra derives from spectral shapes either of the 2-branches or of the pure rotational CARS spectra of the molecular constituents. In combustion research it is most common to perform thermometry from nitrogen since it is the dominant constituent and present everywhere in large concentration despite the extent of chemical reaction. The 2-branch of nitrogen changes its shape due to the increased contribution of higher rotational levels which become more populated when the temperature increases. Figure 6.1-21 displays a calculated temperature dependence of the N2 CARS spectrum for experimental parameters typically used in CARS thermometry (Hall and Eckbreth, 1984). Note that the wavenumber scale corresponds to the absolute wavenumber value for the 2320 cm 2-branch of N2 when excited with the frequency doubled Nd.YAG laser at 532 nm ( 18796 cm ), i. e. = 18796 -1- 2320 = 21116 cm. The bands lower than about 21100 cm are due to the rotational structure of the first vibrational hot band. 

For the case that there are not too many constituents in the gas under investigation the use of the pure rotational CARS technique (Zheng et al., 1984 Alden et al., 1986) may be superior to vibrational CARS thermometry since the spectra are easily resovable (for N2 the adjacent rotational peaks have a spacing of appr. 8 cm ) compared with the... 

Over the last two decades many CARS measurements have been reported in literature with other configurations such as dual Stokes, dual pump [52-54] laser beams. A more recent example of pure rotational CARS spectroscopy at high pressures can be found in Vestin et al. [55]. 

This approach should also lead to better single pulse quality of pure rotational CARS spectra since many frequency combinations drive each rotational transition as discussed earlier. If the quality were sufficiently high, it might eliminate the necessity of resonant referencing currently necessary in pure rotational CARS (or CARS in any constituent... 

Fig. 8. New approach to pure rotational CARS. The broadband Stokes laser can be spectrally positioned arbitrarily.

Temperature information from CARS spectra derives from spectral shapes either of the Q-branches or of the pure rotational CARS spectra of the molecular constituents. In combustion research it is most common to perform thermometry from nitrogen since it is the dominant constituent and present everywhere in large concentration despite the extent... 

For the case that there are not too many constituents in the gas under investigation, the use of the pure rotational CARS technique may be superior to vibrational CARS thermometry since the spectra are easily resolvable (for N2 the adjacent rotational peaks have a spacing of approximately 8 cm i) compared with the congestion of the rotational lines in the vibrational bands of the Q-branch spectra (see Figure 11). An experimental comparison of rotational and vibrational CARS techniques, under similar conditions has been made that demonstrates that rotational CARS may be viable for flame-temperature measurements up to 2000 K. Of course, the pure rotational approach cannot be applied for spherical molecules which have no pure rotational CARS spectrum. An elegant method, using Fourier analysis based on the periodicity of pure rotational CARS spectra has been introduced recently. 

Fig. 3.15, The CARS spectrum rotational width versus methane density for various values of parameter y (1) y = 0, (2) y = 0.3, (3) y = 0.5, (4) y = 0.7, (5) y = 0.75, (6) y = 0.9, (7) y = 0.95, (8) y = 1. Curves (4) and (6) are obtained by subtraction of the dephasing contribution from the line width calculated taking account of vibrational broadening. The other dependences are found assuming purely rotational broadening (vibrational relaxation neglected).

Figure 5 Normalised enthalpy and optical rotation vs temperature curves of pure i-car-rageenan (solid line enthalpy, circles optical rotation) and i-carrageenan containing 12.3% v-units (dotted line enthalpy triangles optical rotation)...

The essential features of CARS can be summarized as follows. A typical CARS set-up involves the use of a pump laser beam at frequency cOi and a red-shifted laser beam at frequency cOg (Stokes beam). These beams are crossed within the Raman medium and, when their frequency difference equals a vibration-rotation or a purely rotational transition in the molecule, a third external laser beam at frequency (O2 can probe (in a fashion... 

It is concluded from Fig. 4 that vibrational dephasing contributes little to the measured relaxation rates. Comparison of the transient IR data (AJ=1 transitions) with picosecond CARS results (AJ=0,2 transitions) gives evidence that apart from rotational population decay also pure rotational dephasing is an important relaxation mechanism. ... 

Fig. 3.12. The room temperature CARS spectra of CH4 obtained in [162] at the following densities (1) 0.1 amagat of pure CH4 (2) 5 amagat CH4 (3) 5 amagat CH4 + 35 amagat Ar (4) 5 amagat CH4 + 85 amagat Ar. The position of the vibration frequency wv is indicated as well as the centre of gravity of the Q0i branch rotational structure wv + coq.

This is also suggested by the data of syn and anti-2-carbomethoxy-l-(carbomethoxymethylene)cyclopropane (86, 87). Changing the position of the car bo-methoxy group at the double bond (syn (86) - anti (87)) makes the rotation more negative by ca. 300°. Though only minimum rotations for the compounds are known, one can assume that the samples of 86 and 87, obtained from thermal isomerization of 84, have the same optical purities. Hence, the comparison between the rotation values below is justified. (Optically pure 86 should have [0]d +229.7°" .)... 

More complex molecules require also that their internal motions— bending, internal rotations, etc.— be represented. The molecules are then usually treated as purely classical objects, although modem treatments may also incorporate quantum degrees of freedom in a consistent manner. The Car-Parrinello method, described as ab initio MD (Car and Parrinello, 1985), is an example. 

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