LASER PHOTOFRAGMENTATION INFRARED EMISSION SPECTROSCOPY OF $CH_{2} I$ RADICALS
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Date
1979
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Ohio State University
Abstract
Time and wavelength resolved infrared fluorescence spectra of vibrationally excited $CH_{2}^{I}$ radicals have been obtained. The radicals are produced by single photon photolysis of $CH_{2}I_{2}$ with a rare gas-halide excimer laser. The technique provides information on the highly excited radical immediately after dissociation, on its vibrational frequencies, and on subsequent energy transfer and relaxation processes. Pulsed irradiation of $CH_{2}I_{2}$ at 249 nm using a defocused KrF excimer laser provides single photon dissociation to an excited spin-orbit state of I ($^{2}P_{1/2}, 7603 cm^{-1}$ above the ground state) and highly vibrationally excited $CH_{2}I$ radical. Using circular variable interference filters, low resolution ($\sim 50 cm^{-1}$) spectra of the radical emission are obtained as a function of time after the laser pulse. Immediately following the dissociating pulse, broad-banded fluorescence is observed throughout the entire $1300-3900 cm^{-1}$ region with peaks at 3850, 2850 and $1350 cm^{-1}$, as well as other smaller features at other frequencies. These peaks correspond to emission from both combination and fundamental bands. In addition, the peak at $2850 cm^{-1}$ is extremely broad, indicative of both high rotational excitation and possibly overtone bands. Upon collisions with argon, the emission from the combination states quickly decays away. A few microsecond after the laser pulse the most intense peak shifts from $2850 cm^{-1}$ to 3050$cm^{-1}$ and the band sharpens significantly as higher vibrational states of the C-H stretching mode are collisionally relaxed to lower states. The underlying broad-banded fluorescence is undoubtedly due to the summation of combination bands, hot bands, overtones, and high rotational excitation arising from the highly vibrationally excited radicals. Under conditions of high laser power, visible and infrared fluorescence from multiphoton processes have been observed, as well as the production of ions. These consults will also be discussed briefly.
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$^{*}$NRC-NBS Postdoctoral Fellow, 1978-1979. $^{\ast\ast}$Staff member, Quantum Physics Division, National Bureau of Standards and Alfred P. Sloan Fellow.
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