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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
    Sampling of semiclassically quantized polyatomic molecule vibrations by an adiabatic switching method: Application to quasiclassical trajectory calculations (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 5695-5707 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We apply the adiabatic switching (AS) method to determine the polyatomic classical motions that correspond to selected vibrational quantum states on multidimensional, anharmonic potential energy surfaces, and use these semiclassically quantized motions as initial conditions for quasiclassical trajectory (QCT) calculations of state-to-state reaction dynamics. Specifically, we calculate the classical motion corresponding to the quantum mechanical zero-point vibration of deuterated methane, CD4, and run QCT calculations on the H+CD4→HD(v',j')+CD3 reaction. The distribution of CD4 vibrational zero-point energy (ZPE) associated with the AS-sampled motions is compared with that from normal-mode-sampled motions. The spread of total zero-point energy in the AS calculations is much narrower than with normal-mode sampling, and the ZPE's are appropriately shifted to lower energy due to anharmonic effects. Reverse adiabatic switching is used as an indirect check of the quantum numbers of the adiabatically sampled motion, but numerical limitations made this test inconclusive. The AS method thus appears to be superior to normal-mode sampling, but this superiority cannot be demonstrated conclusively for the fully anharmonic CD4 potential. However, the AS method is shown to perform very well for transformation from one CD4 harmonic potential to another and for transformation from an harmonic to an anharmonic, but decoupled potential in which CD4 is described by Morse oscillators. Evidence is presented that suggests the AS calculations are limited by numerical inaccuracies or intrinsic features of the potential energy surface, both of which are unavoidable. H+CD4→HD(v',j')+CD3 QCT calculations of state-to-state dynamics using CD4 with no ZPE, the ZPE from AS sampling, and the ZPE from normal-mode sampling are reported and compared. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469300
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  • 4
    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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  • 5
    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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  • 6
    Electronic Resource
    Electronic Resource
    Mode-selective infrared excitation of linear acetylene (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 3897-3910 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum-mechanical simulations of excitation of linear acetylene (HCCH) with a few synchronized, infrared, linearly polarized, transform-limited, subpicosecond laser pulses reveal optimal pathways for the selective laser-controlled excitation of the stretching modes in the molecule. Examples presented include a double-resonance excitation of a CH-stretching local mode state, a single-pulse excitation of a predominantly symmetrical CH-stretching state, an optimal two-pulse dissociation of the molecule into C2H + H, and a two-pulse sequence which induces stimulated emission and dumps the energy from a highly excited CH-stretching state into a CC-stretching state. The resulting optimal laser pulses fall within the capabilities of current powerful, subpicosecond, tunable light sources. The spectroscopy of the model that is relevant for finding selective excitation pathways is discussed. The wave function of the molecule is represented in a harmonic normal-mode basis, a discrete variable representation, and in an eigenstate basis. A real-time Lanczos propagator and an energy-shifted, imaginary-time Lanczos propagator are employed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468567
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  • 7
    Electronic Resource
    Electronic Resource
    Optimally controlled five-laser infrared multiphoton dissociation of HF (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 4211-4228 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Simulations of the quantum dynamics of the HF molecule immersed in a field of five overlapping, intense, linearly polarized, infrared laser pulses of subpicosecond duration are performed. The HF molecule, initially in its ground state, is modeled as a rotating oscillator interacting with a classical laser field via electric dipole interaction. Realistic potential and dipole functions are used. Optimal overlaps of the five laser pulses, as well as the optimal carrier frequencies of the laser pulses, are found which maximize the HF dissociation yield. A maximal yield of 45% in a single combined pulse is achieved using the best available potential and dipole moment functions. The optimal infrared multiphoton dissociation pathway for the HF molecule includes a series of the Δv=1 vibrational-rotational transitions followed by a series of Δv≥2 vibrational-rotational transitions. The latter is necessary as a consequence of the vanishing Δv=1 transition moment around v=12. In the Δv=1 regime, both P and R branch transitions are found to be important. The angular distribution of the dissociative flux is computed. Robustness of the results with respect to changes in the interatomic potentials, dipole functions and reduced mass, as well as to changes in laser pulse parameters (carrier frequencies, timings, phases, field amplitudes, and pulse durations) is investigated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466305
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  • 8
    Electronic Resource
    Electronic Resource
    Bond selective infrared multiphoton excitation and dissociation of linear monodeuterated acetylene (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 535-550 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum mechanical simulations of vibrational excitation of monodeuterated linear acetylene (HCCD) with linearly polarized, frequency-swept, intense but nonionizing infrared laser pulses are performed. The aim is selective dissociation of either H or D atoms by optimal shaping of the laser pulses. We use a discrete variable representation and a compact (〈400 states) bright-state expansion to represent the wave function during and after the pulse. Wave packet propagations in the bright-state expansion are at least an order of magnitude faster than discrete variable representation wave packet propagations. This enables optimal-control calculations to find the best parameters for the laser pulses. The dynamics of CH-bond breaking with infrared pulses are very different from the dynamics of CD-bond breaking. This is a direct consequence of CH being the highest-frequency mode in the molecule. Selective CH-bond breaking is possible with two synchronized pulses, the first being quasi-resonant with the Δv=1 transitions in the CH stretch between v=0 and v=8, and the second being quasiresonant with Δv=2 transitions at higher v. H-atom yields as high as 7.7%, with H to D yield ratio as high as 2.1, are demonstrated. Selective CD-bond breaking is possible using a single, subpicosecond, frequency-swept pulse. D-atom yields as high as 3%, or D to H atom yield ratios as high as 3.9, are calculated. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471907
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  • 9
    Electronic Resource
    Electronic Resource
    Strong-field optical control of vibrational dynamics: Vibrational Stark effect in planar acetylene (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 2446-2451 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We perform quantum mechanical simulations of vibrational excitation of planar (5D) acetylene (HCCH) with linearly polarized, intense but nonionizing, infrared laser pulses, exploring one particular pathway for exciting the HCCH molecule into a CC-stretching state via the fundamental excitation in a two-pulse scheme. We optimize the pulse widths, time centers, and carrier frequencies of the two pulses to achieve the maximal projection onto the target CC-stretching state, (0,3,0,0,0) A1, subject to penalties related to peak electric field and pulse duration. The influences of Fermi resonance, the vibrational Stark effect, and avoided crossings on the selective excitation are discussed. Different sizes of "essential-states" representation are used and checked against the underlying 299 475-point discrete variable representation (DVR) basis. We find that an essential-states basis consisting of 362 A1 and B2 eigenstates represents the results of a full-grid calculation for the excitation process under study. Moreover, we demonstrate that despite the complications associated with the higher dimensionality of the 5D model, we can nevertheless exert infrared laser control over the vibrational dynamics of selective excitation. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477950
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  • 10
    Electronic Resource
    Electronic Resource
    Vibrational eigenvalues and eigenfunctions for planar acetylene by wave-packet propagation, and its mode-selective infrared excitation (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3402-3416 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational eigenvalues with estimated errors 〈5×10−2 cm−1 and their corresponding eigenfunctions for J=0 5D (planar) acetylene modeled by the Halonen–Child–Carter potential-energy surface are obtained using an energy-shifted, imaginary-time Lanczos propagation of symmetry-adapted wave packets. A lower resolution (∼4 cm−1) vibrational eigenspectrum of the system is also calculated by the Fourier transform of the autocorrelation of an appropriate wave packet. The eigenvalues from both approaches are in excellent agreement. The wave function of the molecule is represented in a direct-product discrete variable representation (DVR) with nearly 300 000 grid points. Our results are compared with the previously reported theoretical and experimental values. We use our 69 computed eigenstates as a basis to perform an optimal control simulation of selective two-photon excitation of the symmetric CH-stretch mode with an infrared, linearly polarized, transform-limited, and subpicosecond–picosecond laser pulse. The resulting optimal laser pulses, which are then tested on the full DVR grid, fall within the capabilities of current powerful, subpicosecond, and tunable light sources. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474714
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