A. B. Harvey

A Review of the Theory and Application of Coherent Anti-Stokes Raman Spectroscopy (CARS)

Coherent anti-Stokes Raman spectroscopy (CARS) is a relatively new kind of Raman spectroscopy which is based on a nonlinear conversion of two laser beams into a coherent, laser-like Raman beam of high intensity in the anti-Stokes region. The emission is often many orders of magnitude greater than normal Raman scattering and, because of the coherent and anti-Stokes character of radiation, the method is very useful for obtaining Raman spectra of fluorescing samples, gases in discharges, plasmas, combustion, atmospheric chemistry. In this paper we outline the basic theory behind CARS and describe its unusual effects and drawbacks. We review the research to date on various materials, and indicate the possible future direction, utility and applications of CARS such as surface studies, fluctuation phenomena, reaction dynamics, photochemistry, kinetics, relaxation, and energy transfer.

Direct determination of non‐Boltzmann vibrational level populations in electric discharges by CARS

The direct measurement of nonequilibrium vibrational level populations of N2 at the center of an electric discharge has been demonstrated by a new diagnostic technique: coherent anti‐Stokes Raman spectroscopy (CARS). On the assumption of a Boltzmann equilibrium among only the lowest vibrational levels, a method has been developed and utilized for the direct determination of vibrational populations of all other levels, even in the case of extreme deviations from a Boltzmann equilibrium. Limitations of the method are discussed and illustrated.

Vibrational Spectra and Structure of Dimethyl Diselenide and Dimethyl Diselenide‐d6

The vibrational behavior of the dimethyl diselenide and dimethyl diselenide‐d6 molecules has been studied in the infrared region (4000–70 cm−1) and by Raman shifts (4000–50 cm−1). The molecule has been found to belong to point group C2. The fundamental vibrations, with the exception of the methyl torsions, have been assigned and supported by a normal‐coordinate analysis.

Ring‐Puckering Vibration of 2,5‐Dihydrothiophene

The infrared spectrum of 2,5‐dihydrothiophene has been measured from 80 to 4000 cm−1. The far‐infrared region contained sharp Q branches at 87.0, 95.5, 102.0, 107.0, 111.2, 114.8, 118.1, 120.7, 123.2, 125.2, and 127.0 cm−1 which have been assigned to single quantum transitions between the first 12 levels of the ring‐puckering vibration. A mixed harmonic–quartic potential‐energy function has been fitted to the observed frequencies and is described by the following relation in reduced coordinates: V(cm−1) = 16.86 (x4 + 5.93x2). Such a potential function can be rationalized only in terms of a planar ring skeleton with no barrier to inversion. Two combination and one difference band series of the ring‐puckering transitions with fundamentals centered near 3064, 670, and 2865 cm−1, respectively, have also been studied. The ring‐puckering transitions obtained from the 670‐cm−1 band series agree favorably with the far‐infrared frequencies; however, those in reference to the two carbon–hydrogen stretching fundamen...

Vibrational Spectra and Structure of Four‐Membered Ring Molecules. XIV. Vibrational Analysis and Ring Puckering Vibration of Trimethylene Selenide and Trimethylene Selenide‐d4

The infrared and Raman spectra of trimethylene selenide (TMSe) and trimethylene selenide‐2,2,4,4‐d4 (TMSe‐d4) have been recorded from 4000 to 15 cm−1 in gaseous, liquid, and solid states. A vibrational assignment of the fundamentals is consistent with the expected Cs equilibrium configuration. A series of sharp bands associated with the ring puckering vibration have been observed in the mid‐ and far‐infrared regions. The frequencies of the transitions have been employed in determining the following double‐minimum potential, barrier‐to‐ring inversion, and the equilibrium separation of the ring diagonal, 2x in angstroms, for TMSe, respectively: V(cm−1 = 4.258 (± 0.02) × 105x4 − 2.5391 (± 0.007) × 104x2; 378.1 ± 4 cm−1; 0.35 A. For TMSe‐d4 these quantities are: V(cm−1) = 3.242(± 0.1) × 105x4 − 2.208 (± 0.033) × 104x2; 375 ± 10 cm−1; 0.37 A, respectively. The dihedral angle of both molecules is calculated to be 32.5 ± 2°.

Spectroscopic Determination of the Pseudorotation Barrier in Selenacyclopentane

The infrared and Raman spectra of selenacyclopentane have been studied. A series of far‐infrared absorption peaks is interpreted to indicate that the five‐membered ring molecule has a large barrier to pseudorotation. The derived potential function is V(φ) in cm−1 = − (1795.4 / 2)[1 − cos(2φ)] + (13.45 / 2) × [1 − cos(4φ)] − (86.19 / 2)[1 − cos(6φ)] with the pseudorotation constant B = 1.55 cm−1. Evidence that the molecule has a twisted (c2) configuration in its lowest few vibrational states has been obtained.

The vibrational spectra and structure of dimethyl ditelluride and dimethyl ditelluride-d6

Abstract The infrared spectra of dimethyl ditelluride and dimethyl ditelluride- d 6 have been recorded from 4000 to 33 cm −1 . The helium-neon excited Raman spectra of the liquids, neat, in benzene and in carbon tetrachloride solution, have also been recorded. Unfortunately, the dark red color of the materials was responsible for considerable absorption of the exciting light and no depolarization values were measured. The weak Raman spectra together with the inirared data suggest a hydrogen-peroxide type structure ( C 2 symmetry). All fundamental vibrational modes except the methyl torsional motions and a skeletal bending vibration have been accounted for and the assignments are substantiated by a normal coordinate analysis.

Power generation in coherent anti‐Stokes Raman spectroscopy with focused laser beams

Focusing effects for nonlinear power generation are explicitly discussed for coherent anti‐Stokes Raman spectroscopy (CARS). It is shown that very tight focusing only increases CARS power generation by about a factor of 4, while an extra λ−2 dependence introduced by focusing implies CARS signals scale with wavelength in the same manner as do the signals in normal spontaneous Raman scattering. Axial power generation in a gas phase medium is illustrated, and a modified plane wave approximation function is developed to analyze this behavior for loose focusing cases.

Modification of a Commercial Argon Ion Laser for Enhancement of Gas Phase Raman Scattering

Although the availability of high-powered argon ion lasers has significantly reduced the difficulty of obtaining Raman spectra of gases, nevertheless there are still many cases where enhancement of the scattered light from weakly scattering samples is required. One particular example is in the study of vibrationally excited molecules produced thermally and in an electrical discharge. Because the concentration of excited species is not very high, standard methods of laser excitation do not provide sufficiently intense Raman spectra. In this note, the application of intracavity excitation using a commercial laser (Coherent Radiation Laboratory, model 52A) is described. The resultant signal enhancement is much superior to that obtained with other techniques which excite the sample outside the cavity (e.g., a multiple pass device). Although spectra have been obtained by intracavity excitation with a laboratory-built laser, an ordinary commercial laser can also be modified for such use at a cost of less than

Vibrational Spectrum of Methanetellurol

1000. The modification described below outlines the minimum requirements of the apparatus and the techniques used for alignment. We feel the information in this report will help other Raman spectroscopists to take advantage of intracavity sampling.