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  • 1
    Electronic Resource
    Electronic Resource
    Fourier-transform spectrophotometer for time-resolved emission measurements using a 100-point transient digitizer (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 95-102 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144409
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  • 2
    Electronic Resource
    Electronic Resource
    Repetitively sampled time-of-flight mass spectrometry for gas-phase kinetics studies (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 3259-3264 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An apparatus has been constructed to study radical–radical reactions in the gas phase. It consists of a tubular quartz reactor in which radicals are produced by flash photolysis using an excimer laser as light source. The composition of the gas mixture is analyzed in situ by photoionizing sampled gases using the vacuum ultraviolet emission of a hollow cathode lamp and subsequent time-of-flight mass spectrometry. A simple arrangement of grids at the entrance to the flight tube is used to interrupt the constant flux of ions by application of a combination of constant and pulsed voltages. Individual mass spectra can be taken at a repetition rate of around 20 kHz following each photolysis event. Signal counts from a specified number of consecutive mass spectra are fed into a 2 GHz multiscaler and accumulated as a sampling-time-indexed series of mass spectra. This allows simultaneous observation of the concentrations of multiple transient or stable species on a millisecond time scale. To achieve a suitable signal-to-noise ratio, signals were typically accumulated over several tens of thousands of laser shots at a pulse rate of 10–15 Hz. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149944
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  • 3
    Electronic Resource
    Electronic Resource
    Time-resolved Fourier transform infrared study of the photodissociation of 1,1-difluoroethene at 193 nm (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3679-3687 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured vibrational and rotational population distributions for the excited HF product of the 193 nm photolysis of CH2CF2. Approximately 15% of the available energy is released as vibrational energy of HF, and the estimated nascent HF rotational energy corresponds to about 10% of the total available energy. The relative vibrational populations can be fit to a linear surprisal. We have compared the vibrational distribution to that observed from this reaction with different amounts of available energy, and to the HF vibrational distributions produced in the dissociation of other fluoroethenes. While results differ among the various experiments considered, overall more than the amount of vibrational energy expected statistically is deposited in HF. The rotational population distributions we observe fit both a Boltzmann distribution and an information theoretic model corresponding to a nearly statistical energy distribution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467552
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  • 4
    Electronic Resource
    Electronic Resource
    Studies of the 193 nm photolysis of diethyl ketone and acetone using time-resolved Fourier transform emission spectroscopy (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6660-6668 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have observed the infrared emission from the products of the 193 nm photolysis of diethyl ketone (3-pentanone) in comparison with acetone (2-propanone) using time-resolved Fourier transform spectroscopy. In the photolysis of diethyl ketone, two bands are apparent: The first, spanning the region 1950 to 2250 cm−1, is assigned to CO rovibrational transitions; the other band, spanning the region 2800 to 3400 cm−1 and not exhibiting resolved line structure, is assigned to the ethyl radical. Spectral simulations of the CO bands under conditions of minimal, but not negligible, relaxation produce a lower bound for the nascent CO rotational temperature of ∼2100 K. The CO vibrational population distribution varies slowly over the ∼80 μs time spanned by our experiment. Both the rotational and vibrational energies of CO exceed statistical partitioning in the dissociation of acetone. In comparison to the case of acetone, absolute energies in CO vibration and rotation decrease only modestly for diethyl ketone, corresponding to a dramatic increase in the excess above the statistically partitioned energies. Several simple dissociation models are compared to these results. None is fully satisfactory. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469139
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  • 5
    Electronic Resource
    Electronic Resource
    Time-resolved frequency modulation spectroscopy of photochemical transients (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1717-1720 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467793
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  • 6
    Electronic Resource
    Electronic Resource
    Near-infrared vibronic spectrum of the CH2 b˜ 1B1←a˜ 1A1 transition (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 9236-9245 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468015
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  • 7
    Electronic Resource
    Electronic Resource
    Photofragment vector correlations measured by transient absorption spectroscopy: cyanogen fragments from ethyl thiocyanate photodissociation (1993)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 97 (1993), S. 10911-10919 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100144a003
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  • 8
    Electronic Resource
    Electronic Resource
    Vector and scalar correlations in statistical dissociation: The photodissociation of NCCN at 193 nm (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 60-76 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Nascent Doppler profiles of CN (X 2Σ+) fragments from the 193 nm photodissociation of NCCN have been measured using high-resolution transient frequency modulated (FM) absorption spectroscopy. This new method is highly suited for Doppler spectroscopy of nascent photoproducts. The experimental line shapes suggest an asymptotic available energy of 5300±100 cm−1 and are well described by a model in which the available energy is partitioned between a statistical reservoir (4700 cm−1) and a modest exit barrier (600 cm−1). We have determined state dependent v⋅j correlations. A trend of j becoming increasingly perpendicular to v for the higher rotational states is in accord with phase space theory, although the observed correlations are more than twice as strong. The v⋅j correlations can be quantitatively modeled by further restricting the phase space model with an approximate conservation of the K-quantum number, the projection of total angular momentum about the linear axis of NCCN. Global rotational and vibrational product distributions have also been measured. The highest accessible rotational states are underpopulated, compared to a phase space calculation. The global vibrational distribution is substantially colder than the phase space theory predictions. Vibrational branching ratios for coincident fragments have been measured as a function of the detected CN state from a close analysis of high signal-to-noise Doppler profiles. The correlated vibrational distribution, P(v1,v2), shows an excess of vibrationless coincident fragments, at the expense of dissociation to give one ground state and one vibrationally excited CN fragment. The correlated formation of two vibrationally excited CN fragments is as likely as the phase space prediction, yet the formation of v=2 is strongly suppressed. The fragment vector and scalar correlations provide a highly detailed view of the loose transition state typical for reactions well described by statistical reaction theories. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473023
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  • 9
    Electronic Resource
    Electronic Resource
    Line shape analysis of Doppler broadened frequency-modulated line spectra (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 2129-2135 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a method for recovering Doppler broadened absorption line shapes from frequency modulated (FM) line spectra. The method of analysis is calibrated and demonstrated with thermalized CN radicals produced by photodissociation of cyanogen (NCCN), probed on the A–X system near 800 nm with a frequency modulated Ti: sapphire ring laser. Nonthermal, Doppler broadened lines from translationally nascent photofragments can also be recovered by direct transformations of experimental FM line profiles acquired with a time resolution exceeding 100 ns. The superior signal-to-noise afforded by FM spectroscopy, relative to other direct absorption methods, should encourage the application of transient FM spectroscopy to problems in photoinitiated reaction dynamics. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470969
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  • 10
    Electronic Resource
    Electronic Resource
    Quantum phase space theory for the calculation of v⋅j vector correlations (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 1864-1874 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The quantum state-counting phase space theory commonly used to describe "barrierless'' dissociation is recast in a helicity basis to calculate photofragment v⋅j correlations. Counting pairs of fragment states with specific angular momentum projection numbers on the relative velocity provides a simple connection between angular momentum conservation and the v⋅j correlation, which is not so evident in the conventional basis for phase space state counts. The upper bound on the orbital angular momentum, l, imposed by the centrifugal barrier cannot be included simply in the helicity basis, where l is not a good quantum number. Two approaches for an exact calculation of the v⋅j correlation including the centrifugal barrier are described to address this point, although the simpler helicity state count with no centrifugal barrier correction is remarkably good in many cases. An application to the photodissociation of NCCN is consistent with recent classical phase space calculations of Klippenstein and Cline. The experimentally observed vector correlation exceeds the phase space theory prediction. We take this as evidence of incomplete mixing of the K states of the linear parent molecule at the transition state, corresponding to an evolution of the body-fixed projection number K into the total helicity of the fragment pair state. The average over a thermal distribution of parent angular momentum in the special case of a linear molecule does not significantly reduce the v⋅j correlation below that computed for total J=0. Predictions of the v⋅j correlations for the unimolecular dissociation of NCNO and CH2CO are also provided. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471533
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