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  • 1990-1994  (24)
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  • 1
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1717-1720 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We report the use of time-resolved frequency modulation (FM) spectroscopy for the measurement of photochemically generated radicals. CN radicals from the 193 nm photodissociation of cyanogen (NCCN) have been detected using a phase-modulated cw Ti:sapphire ring laser, probing single rotational lines of the A 2Π←X 2Σ system. The combination of sensitivity with time and frequency resolution is more than adequate to record Doppler-broadened line shapes of collisionless photofragments. Significant signal-to-noise enhancement is demonstrated compared to a dual-beam transient absorption technique.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 9236-9245 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A Doppler-limited high resolution vibronic spectrum of the methylene (CH2) b˜ 1B1←a˜ 1A1 transition in the near-infrared wavelength region has been obtained using transient absorption techniques. The radical was produced by 308 nm excimer laser photolysis of ketene (CH2CO) in a flow system. The analysis of this spectrum confirms the reassignments of some previously observed bands as well as the presence of new vibronic bands predicted by a recent ab initio calculation [Green et al., J. Chem. Phys. 94, 118 (1991)]. We also measured the Doppler broadened profiles of low-J rotational lines of CH2 under collisionless conditions. From the line profile analysis, we find that the 308 nm photolysis of ketene produces fragments with isotropic and uncorrelated velocity and angular momentum distributions. The Doppler profiles also provide a view of the coincident product state distributions. The measured Doppler profiles are consistent with a rotational distribution of CO produced in coincidence with low energy CH2 states given by statistical phase space theory. The vibrationally excited coincident CO appears, however, to be produced about 2–3 times more efficiently than is predicted by various statistical theories. A frequency modulation (FM) technique has been adopted to improve the sensitivity of the transient absorption experiment. There is a significant improvement in the observed signal to noise ratio of the CH2 spectrum over the dual beam method previously used. © 1994 American Institute of Physics.
    Materialart: Digitale Medien
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  • 3
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: An infrared time-resolved Fourier-transform emission spectrophotometer has been constructed and its use has been demonstrated. The instrument is based on a commercial interferometer combined with a CAMAC-based data acquisition system. The use of a transient digitizer permits acquisition of a single interferogram point at 100 time intervals following a single photoexcitation. The instrument operates in the "smooth scan'' mode. The combination of this mode of operation with the use of a transient digitizer provides great time efficiency for data acquisition because there is no mirror settling time at each optical retardation. Complete interferograms free from the artifacts associated with assembly of interferograms arising from multiple scans are available after each mirror scan. The maximum resolution of the present instrument is 0.1 cm−1, limited only by data storage considerations; the maximum resolution of the commercial interferometer is 0.02 cm−1 (with apodization) and could be utilized with long scans. The shortest time that can be resolved by the instrument, currently ∼1–2 μs, is limited only by the infrared detector/preamplifier combination. The longest time interval which is permitted between successive photochemical or photophysical events (nominally 3.16 ms) is limited by the slowest scan velocity of the moving mirror, (0.01 cm/s). Usable data can be acquired from 10 to 50 mirror scans, where acquisition, storage, and coaddition of a single 1 cm−1 resolution scan takes ∼5 min.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 6624-6631 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Far infrared laser magnetic resonance spectroscopy is used to measure components of 12 rotational transitions in the ground state of the HOCO radical. The transitions are all b-dipole in character in contrast to the a-dipole rotational spectrum previously reported [Radford, Wei, and Sears, J. Chem. Phys. 97, 3989 (1992)]. The new data determine the A rotational constant to high precision and allow the determination of several centrifugal distortion constants for the first time. The hyperfine coupling in the radical leads to observable splittings in several of the observed transitions and these are used to estimate two of the four expected nonzero hyperfine parameters in the radical.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3989-3995 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Part of the rotational spectrum of the hydroxyformyl radical, HOCO, and its deuterated analog has been detected at frequencies between 230 and 300 GHz. The radical was formed in a flow system by the reaction between chlorine atoms and formic acid. Analysis of the spectra yields reliable estimates of the rotational, centrifugal distortion and spin–rotational parameters describing the ground state of this species. The rotational constants derived for HOCO and DOCO are consistent with a planar equilibrium structure and are used to confirm that the carrier of the spectrum is the trans-geometrical isomer.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2337-2345 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The 248 and 193 nm photodissociation of the series of molecules RNCS and RSCN (R=H, CH3, C2H5) has been investigated. Laser-induced fluorescence of the radical NCS was used to probe the energy deposition in the R+NCS channel of this dissociation, however excitation to many vibrational states of NCS caused spectral congestion which prohibited direct measurement of populations of individual levels. A single-photon dissociation channel leading to RS and CN was observed for both isomers of all of the precursor molecules. While this was expected for the thiocyanate species, no direct channel to these products is available for the isothiocyanates. This observation is discussed in terms of an excited state isomerization of these molecules. Measured ratios of CN to NCS production for all compounds provide additional support for this mechanism and suggest a significant barrier to the process. Comparison of nascent laser-induced fluorescence (LIF) spectra of NCS with spectra obtained following different amounts of collisional relaxation suggests that all three vibrational modes were equally excited with a vibrational temperature of roughly 4500 K. The rotational temperature was estimated to be less than 1000 K. A spin–orbit population inversion was observed for all vibrational levels with approximately 75% of the population in the upper spin–orbit component of the vibrationless and low lying bending excited levels. No simple model is found to explain the photodissociation dynamics or the isomerization.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 7021-7026 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The infrared absorption spectrum of the ν3 fundamental band of the CD3 radical has been detected by diode laser absorption spectroscopy. The CD3 radical was produced by excimer laser photolysis of CD3I at 248 nm or (CD3)2CO at 193 nm. Molecular parameters of the v3=1 vibrational state were determined from a least-squares fit to 62 rotation–vibration transitions. In this fit, molecular parameters describing the ground state were constrained to those obtained from previous spectroscopic studies of the ν2 parallel IR band [J. M. Frye, T. J. Sears, and D. Leitner, J. Chem. Phys. 88, 5300 (1988)]. The molecular parameters determined in the present work are the band origin ν0=2381.088 60(84), B'=4.758 737(40), C'=2.373 297(34), (ζC)3=0.476 278(72), q3=0.003 76(59), D'N =0.000 187 9(5), DNK =−0.000 341 0(12), D'K =0.000 143 7(8), ηN =−0.000 005 5(36), η'K =0.000 060(35), and qN =0.000 063(17), all in cm−1 with one standard deviation in parentheses. The derived molecular parameters were compared with those for the CH3 radical v3=1 level determined previously [T. Amano, P. Bernath, C. Yamada, Y. Endo, and E. Hirota, J. Chem. Phys. 77, 5284 (1982)]. The molecular parameters of the v3=1 state of the CD3 and CH3 radicals follow the expected isotopic relationships. We have also found that the determined molecular parameters reasonably satisfy the approximate planarity relationships [J. K. G. Watson, J. Mol. Spectrsoc. 65, 123 (1977)] and the sign of the l-type doubling constant is consistent with a planar equilibrium structure.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7218-7228 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Laser induced fluorescence measurements of NCO formed in the reaction CN+O2→NCO+O in a free jet expansion are reported. The rotational temperature achieved in the experiments was typically 10–15 K and absorption from vibrationally excited levels was very much reduced in intensity. Two bands in the A˜–X˜ system, the v1, v2, v3=200–000 and 120–000, of the radical were rotationally resolved for the first time, and wavelength resolved laser excited fluorescence measurements have been used to characterize many ground state vibronic energy levels below 6000 cm−1 in energy. Stimulated emission pumping experiments were carried out on vibronic levels associated with the ν3 vibration with 1≤v3≤3 in the electronic ground state. Harmonic and anharmonic constants were determined, together with the spin–orbit coupling parameters and rotational constants for these levels.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4182-4188 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Diode laser transient absorption/gain spectroscopy is used to monitor time-dependent populations of CD3 fragments formed in the photodissociation of acetone-d6 at 193 nm. Selected rotational lines have been measured in the ν2 "umbrella'' fundamental and first two hot bands, and in the ν3 asymmetric stretching fundamental band. Substantial growth is observed in the vibrationless state on the time scale of vibrational relaxation. We estimate that only about 15% of the nascent CD3 population is formed in the vibrational states we detect: ν2=1 and 2, ν3=1, and the vibrationless state. Most of the nascent methyl population is evidently spread among many undetected vibrational states. These results complement previous measurements of acetone photofragments by infrared emission, multiphoton ionization, and laser-induced fluorescence. Our inferred global vibrational distribution is consistent with a two-step fragmentation.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2346-2356 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Photodissociation of the molecules RNCS and RSCN (R=H, CH3, and C2H5) was investigated at 248 and 193 nm and the internal energy distributions in the CN resulting from the RS+CN fragmentation channel were probed by laser induced fluorescence. These CN distributions were identical for formation from the isomer pairs in agreement with an excited state isomerization postulated earlier. At 248 nm, all precursors lead to nearly thermal CN rotational distributions with rotational temperatures of approximately 1100 K in v=0 and 800 K in v=1. The vibrational distributions could not be characterized by the same temperatures. At 193 nm, contributions from two apparently different dissociation channels were observed. The CN rotational populations showed a narrow, strongly peaked distribution lying on a broad, approximately statistical distribution. For HNCS, this peak occurs near N=10 suggesting dissociation from an excited state with a linear NCS skeleton. For the alkyl precursors, the peak occurs at very high rotational quantum number (N=70) indicating an excited state with a bent NCS framework. Vibrational distributions with population out to v=6 were observed. In all cases, the apparent statistical part of the rotational distribution and the vibrational distributions can be described by a prior function derived assuming that the internal modes of the alkyl substituent are not involved in any energy redistribution during the dissociation.
    Materialart: Digitale Medien
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