Graduate Thesis Or Dissertation
 

Coherent anti-Stokes Raman spectroscopy of gases

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  • The development and applications of a 0.2 cm⁻¹ resolution Nd-YAG laser powered coherent anti-Stokes Raman spectroscopy, CARS, spectrometer for gas phase studies is chronicled in this thesis. Applications including CARS lineshape analysis, resonant CARS, and CARS of transient species and excited state molecules is reported. The intensity of the signal generated at the CARS frequency, w₃, is governed by the behavior of the square of the nonlinear third order electronic susceptibility, Ix(³)I² which contains resonant, XRes' and nonresonant, XNR' terms. The various nonlinear optical three wave mixing, 3WM, processes which contribute to the intensity at w₃ are discussed in terms of a semiclassical derivation of x(³)Res. From Maxwell's equations, a wave equation is obtained from expressing the induced nonlinear polarization, pNL, as a function of the applied electric fields. A damped harmonic oscillator model is assumed for the response of the electrons to the applied fields. Two similar expressions for x(³)Res result from introducing the nonlinearity into either the oscillator response (anharmonic term) or the driving force. x(³) is a function of various molecular parameters such as molecular number densities, Raman cross sections and Raman active vibration-rotation transition frequencies. A computer program is reported for calculating Ix(³)I² for homonuclear diatomic molecules. The program features a convolution over an analytical line-shape function to account for probe laser linewidths. Results are presented for calculated and observed spectra of 0₂ gas at room temperature and in the free jet region of a supersonic molecular beam. Rotational cooling to 10K is indicated in the supersonic jet by CARS lineshape analysis. The design and performance of the CARS spectrometer is discussed in terms of the various components. The line-width of the primary w₁ beam was reduced to about 0.03 cm⁻¹ by employing two intracavity etalons and an electronic line narrowing device. Two dye laser designs and two optical pumping schemes are evaluated in terms of stability, linewidth, and ease of operation. The spectrometer resolution is limited by the dye laser linewidth of about 0.2-0.3 cm⁻¹. Wavelength tuning ranges and optimal concentrations are reported for 16 commercially available laser dyes pumped with the third harmonic of the Nd-YAG laser at 355 nm. The laser dye outputs cover the visible range from 410 to 715 nm. High and low resolution broadband CARS spectra were obtained using an intensified optical multichannel analyzer as a detector. Resonant CARS spectra are reported for nitrogen dioxide gas for frequency shifts of 1200-3400 cm⁻¹ from a 532 nm w₁ pump source. The spectra change dramatically with slight changes in w₁ frequency. Much vibrational-rotational structure is observed but the analysis is complicated by the contribution of more than one resonant process. Various possible resonances are considered and absorption spectra and intensity measurements are used to assess the importance of some of these. Intense 3WM spectra are reported for transient fragments produced by 266 nm laser photolysis of benzene, several substituted benzenes, and acetylene. Single pulse broadband 3WM spectra taken with an optical multichannel analyzer establish that the fragments are primary photoproducts obtained under collision-free conditions. The spectra consist of many features at anti-Stokes frequency shifts of 900-3100 cm⁻¹ from a 532 nm w₁ pump. Ninety degree fluorescence studies of the photolysis zone show that C₂ is produced in various electronic states and energetic consideration require that dissociation of C₆H₆ must involve two or more photons at 266 nm. Three wave mixing spectra of C₆D₆ are identical to those of C₆H₆ in the anti-Stokes shift region near 3000 cm⁻¹ and hence the transients do not contain CH bonds. Three wave mixing spectra of C₂H₂ fragments are also identical to those of benzene in the 3000 cm⁻¹ region so that C₂ is believed to be responsible for both 3WM and fluorescence spectra. The 3WM spectra cannot be interpreted in terms of simple CARS vibrational resonances of C₂. Intensity considerations suggest that enhancement due to multiple resonance is likely, and various electronic-electronic and vibrationalelectronic 3WM processes are discussed. Calculations of possible resonances in the Swan system involving overtone Raman transitions indicate that many of the spectral features could arise from such processes.
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