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  • 1
    Electronic Resource
    Electronic Resource
    Very low temperature helium pressure broadening of DCl in a collisionally cooled cell (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 898-902 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Very low temperature pressure broadening parameters and cross sections for the J=0–1 rotational transition of DCl broadened by helium are presented. Data was taken using a collisionally cooled cell and measured cross sections range from 10.76 A(ring)2 at 4.21 K to 8.10 A(ring)2 at 1.85 K. This compares with a 300 K cross section of 9.36 A(ring)2. Predictions from two currently available anisotropic He–DCl potentials do not reproduce accurately the experimental results. Modifications were made to one of these potentials to reproduce more accurately both the observed low temperature and 300 K pressure broadening cross sections. The resulting modified potential has a well depth De=32.7 cm−1 at a separation Re=3.46 A(ring) in the linear He–Cl–D configuration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462110
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  • 2
    Electronic Resource
    Electronic Resource
    Collisionally cooled spectroscopy: Pressure broadening below 5 K (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 122-125 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: New results from pressure broadening measurements between 4.3 and 1.7 K for the J=2←1 and 3←2 transitions of CO broadened by helium are reported. Unlike the results of helium pressure broadening experiments at higher temperature, significant variations in cross section occur with changes in temperature. These measurements are compared with the results of close coupling calculations and show both the effects of resonances in the collision channels and energy defects at the very low collision energies. Room-temperature measurements of helium pressure broadening have also been made on CO, NO, CH3F, and H2S. These provide a normalization for our previous studies of these species at very low temperatures and a basis for the qualitative comparison of the diverse results we have obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457498
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  • 3
    Electronic Resource
    Electronic Resource
    A time-resolved study of rotational energy transfer into A and E symmetry species of 13CH3F (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 3520-3527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotational energy transfer processes into the A and E symmetry species of the symmetric top molecule 13CH3F have been studied. In this time-resolved double resonance experiment a tunable millimeter/submillimeter wave spectrometer was used to monitor the change in strength of rotational transitions in the ν3 vibrational state after a Q-switched CO2 laser pumped the K=3, J=5 level in ν3. A simple numerical simulation of rotational energy transfer allowed the 13CH3F system to be modeled and collisional energy transfer rates to be obtained from the data. Two important processes were studied. The first, a process that obeys the spin statistic selection rule ΔK=3n has a rate of 29±6 ms−1 mTorr−1. The second, a vibrational quantum number swapping collision that effectively transfers population between the A and E symmetry species and thereby transcends the spin statistic selection rule, has a rate of 6.6±0.7 ms−1 mTorr−1, about 1.4 gas kinetic collisions. The numerical simulations and these rates, along with previous measurements of the ΔJ=±1 rate and vibrational decay rates, provide an accurate characterization for a large body of varied experimental data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456665
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  • 4
    Electronic Resource
    Electronic Resource
    Very low temperature spectroscopy: The pressure broadening coefficients for CH3F between 4.2 and 1.9 K (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6147-6149 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Pressure broadening parameters and cross sections for the CH3F–He system have been measured between 4.2 and 1.9 K by means of a recently developed technique based on collisional cooling in a cryogenic cell. The cross sections for the K=0 and K=1 components of the J=1–2 transition were found to be 75.2 and 66.6 A(ring)2, respectively, at 4.2 K and 55.5 and 54.5 A(ring)2, respectively, at 1.9 K. The cross sections at 4.2 K are about 50% larger than the 300 K values. These results are compared with earlier experimental and theoretical results obtained for the CO–He system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455430
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  • 5
    Electronic Resource
    Electronic Resource
    Very low temperature spectroscopy: The pressure broadening coefficients for CO–He between 4.3 and 1.7 K (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1923-1928 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The attributes of very low temperature spectroscopy in the context of a simple collisionally cooled cell are described. Among them are narrow linewidths, large absorption coefficients, and the simplification of complex spectra. Most importantly, the technique makes possible the study of gas phase collisional processes under equilibrium conditions in a variable temperature environment below 4 K. In this regime the rotational energy level spacing is typically larger than kT, thereby both significantly simplifying the most exact theoretical calculations and giving a more direct link between experimental observables and fundamental molecular parameters. The results of a pressure broadening experiment in the CO–He system in the region between 4.24–1.78 K are described. It was found that the pressure broadening parameters varied from 21.3 to 40.0 MHz/Torr over this temperature region. Comparison with earlier theoretical calculations showed excellent agreement at the higher temperatures; however, at the lowest observed temperature the theoretical value was about 20% greater than the measurement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455089
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  • 6
    Electronic Resource
    Electronic Resource
    A fast scan submillimeter spectroscopic technique (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 1675-1683 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new fast scan submillimeter spectroscopic technique (FASSST) has been developed which uses a voltage tunable backward wave oscillator (BWO) as a primary source of radiation, but which uses fast scan (∼105 Doppler limited resolution elements/s) and optical calibration methods rather than the more traditional phase or frequency lock techniques. Among its attributes are (1) absolute frequency calibration to ∼1/10 of a Doppler limited gaseous absorption linewidth (〈0.1 MHz, 0.000 003 cm−1), (2) high sensitivity, and (3) the ability to measure many thousands of lines/s. Key elements which make this system possible include the excellent short term spectral purity of the broadly (∼100 GHz) tunable BWO; a very low noise, rapidly scannable high voltage power supply; fast data acquisition; and software capable of automated calibration and spectral line measurement. In addition to the unique spectroscopic power of the FASSST system, its implementation is simple enough that it has the prospect of impacting a wide range of scientific problems. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147970
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  • 7
    Electronic Resource
    Electronic Resource
    Frequency stability and reproducibility of optically pumped far-infrared lasers (1990)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 2882-2884 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct measurements of the gain profile of optically pumped far-infrared lasers show that large shifts in the laser frequency can be caused by the absorption from thermal molecules on the laser transition. The absorption shifting greatly exacerbates pump frequency deviations, resulting in an extreme sensitivity to pump offsets and drifts. This pressure-dependent shifting mechanism is not present in transversely pumped lasers, which explains their superior frequency reproducibility compared to longitudinally pumped lasers, and reconciles two apparently conflicting results regarding laser stability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.103765
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  • 8
    Electronic Resource
    Electronic Resource
    Femtosecond demodulation source for high-resolution submillimeter spectroscopy (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 3810-3812 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new continuously tunable submillimeter source for spectroscopy and other high-resolution applications has been developed. In this source the optical spectrum of a mode-locked femtosecond laser is downconverted into the submillimeter region by the demodulation process of a photoconductive switch. The power generated is subsequently radiated into free space by an antenna which is integrated along with the switch on low-temperature grown GaAs. The very high resolution is ultimately traceable to the cavity length of the laser and the stable mode-lock frequency which results. Among the most important attributes of the sources are straightforward absolute frequency calibration, very high spectral purity, and the potential for spectral multiplexing. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.115391
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  • 9
    Electronic Resource
    Electronic Resource
    Time-resolved double resonance study of J- and K-changing rotational collisional processes in CH3Cl (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 5666-5683 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time-resolved double resonance spectroscopy using infrared pump radiation and millimeter-wave and submillimeter-wave probe radiation (IRMMDR) has been used to study rotational energy transfer (RET) in CH3Cl. A collisional energy transfer model using only five parameters for RET plus those needed for vibrational processes is shown to accurately model 350 IRMMDR time responses for two different pump states and 43 probe transitions covering a wide range of rotational states. Previous studies in this laboratory have revealed that J- and K-changing RET have vastly different characters in CH3F [J. Chem. Phys. 92, 6480 (1990)]. Both J- and K-changing RET were accurately modeled with four parameters—one for dipole–dipole collisions, two for the ΔJ scaling law, and one for the cumulative rate of K-changing collisions. As was found for CH3F, J-changing rotational collision rates in CH3Cl are modeled accurately by both the statistical power gap (SPG) law and the infinite order sudden approximation using a power law expression for the basis rates (IOS-P). However, in contrast to CH3F, where all IRMMDR time responses for K-changing collisions have the same shape, many time responses of CH3Cl states populated by K-changing collisions contain an additional early time feature (ETF) that varies with pump and probe states. Nonetheless, a simple generalization of the previously reported model for K-changing collisions is shown to account for all of the additional features observed in CH3Cl. Rather than observing a fixed temperature for K-changing collisions as was the case for CH3F, the temperature is found to be a function of time for CH3Cl. Moreover, the two new parameters this adds to the RET model are related to known physical quantities. A qualitative argument of K-changing collisions based on a classical picture is offered to explain the difference between the measured J- and K-changing state-to-state rates in CH3Cl.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467133
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  • 10
    Electronic Resource
    Electronic Resource
    Erratum: Very low temperature pressure broadening of DCl in a collisionally cooled cell [J. Chem. Phys. 96, 898 (1992)] (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2402-2402 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467279
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