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  • 1990-1994  (7)
  • 1
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 873-882 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Radiative lifetimes are reported for CH2(b 1B1) in a wide range of rotational states in the (0,14,0), (0,15,0), (0,16,0), and (0,17,0) vibrational levels. Laser photolysis of ketene in a supersonic-jet expansion produced CH2(a 1A1) which was excited to single rovibrational levels of the (b 1B1) state by a second laser. Analysis of the temporal evolution of the fluorescence (b 1B1)–(a 1A1) yielded collision-free radiative lifetimes for the (b 1B1) state. The measured lifetimes range from about 4 to 10 μs and decrease with increasing vibrational energy. For the (0,14,0) overtone, the lifetimes increase slightly as a function of J (Ka=0). However, the lifetimes of rotational levels with Ka=0 in the (0,16,0) vibrational state are found to be independent of the rotational state. Calculations of the vibronic lifetimes show that the considerable lifetime lengthening when Ka≥1 is due to Renner–Teller coupling to the a˜ 1A1 state. The random lifetime fluctuation observed in some vibronic bands is probably due to spin–orbit coupling to the X˜ 3B1 state, e.g., 413 and 414 of (0,15,0). Here the radiative lifetime of the singlet component is shorter than that of the corresponding triplet component.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1744-313X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: HLA-DPB1 allele frequencies in 181 unrelated control individuals and 70 rheumatoid factor-positive RA patients from Seville (Spain) were determined using oligonucleotide typing methods. All frequencies shown concern the percentage of individuals positive for a certain allele. HLA-DPB1*0401 was the most common DPB1 allele in the healthy individuals, possessed by 65.7% of them. In addition to HLA-DPB1 *0401, only the following alleles were found in normal subjects at frequencies greater than 10%: DPB1*0101 (15.5%), DPB1*0201 (12.2%), DPB1*0301 (16.6), and DPB1*0402 (29.3%). When HLA-DPB1 allelic frequencies were compared between seropositive RA patients and controls, a negative association for DPB1*0301 and DPB1*0401 was found in RA patients, although it failed to reach statistical significance after correction for the number of comparisons made. The other DPB1 alleles exhibited almost identical frequencies in both groups. However, when only DR4+ patients and controls were considered, the decrease in the frequency of the DPB1*0301 and DPB1*0401 alleles lacked statistical significance. On the other hand, when DR4- RA patients and controls were compared, the frequency of DPB1*0301 was found decreased significantly again, even more than in the whole group of patients.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 8890-8901 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from the threshold for production of CH2(a˜ 1A1)(0,0,0)+CO(X˜ 1Σ+) to 2900 cm−1 above. By scanning the 1CH2 probe laser wavelength, the CH2 laser-induced fluorescence spectrum is recorded and the 1CH2 product state distributions determined. The appearance thresholds and yield curves of individual 1CH2 rotational states are obtained by scanning the photolysis laser frequency with a fixed 1CH2 probe laser frequency. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. PHOFEX yield curves combined with the distribution data demonstrate that the ketene absorption cross section is constant within 10% over the 0–1000 cm−1 range of the PHOFEX spectra. PHOFEX curves up to 200 cm−1 and rotational state distributions for singlet methylene at 50, 100, and 170 cm−1 above its threshold are in good agreement with phase space theory (PST) calculations. The lowest energy methylene rotational states of + and − symmetry are formed in the ratio predicted by phase space theory at all energies. States of mixed singlet/triplet parentage are populated with equal probability for triplet characters ranging from 30% to 70%. For energies greater than 300 cm−1 above the threshold, the 1CH2 rotational distribution is substantially colder than the statistical distribution given by PST. These differences increase with excess energy above the singlet threshold. In this energy range, the dynamics of motion between the transition state and separated product fragments is in neither the adiabatic nor the statistical limit though the data do suggest that motion may well be adiabatic for molecules fragmenting to yield the lowest energy rotational states of ortho methylene.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6429-6435 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The b˜ 1B1→a˜ 1A1 emission spectrum of the CH2 radical has been studied in the 13 330–18 180 cm−1 region. Methylene (a˜ 1A1) is produced by photolysis of ketene at 308 nm and excited to single rovibrational levels of the b˜ 1B1 state by a tunable pulsed dye laser. The emission from the b˜ 1B1 state is spectrally dispersed with a resolution of 0.25 A(ring). Term values and transitions are reported for new levels of the b˜ 1B1(0,v2,0) (v2=14 and 15) and a˜ 1A1(0,0,0) states with J≥6 and 0≤Ka≤6. Unusually intense ΔKa=3 transitions and several ΔKa=5 transitions are observed. Analysis of the emission spectra gives the relative intensities of transitions from a selected rovibrational level in the upper b˜ 1B1 state to rovibrational levels in the a˜ 1A1 state. These relative intensities together with the fluorescence lifetimes of the b˜ 1B1 levels determine the absolute Einstein emission coefficients for rovibronic transitions in the visible spectrum of methylene. These data permit accurate measurement of rovibrational state populations for CH2(a˜ 1A1) by laser-induced fluorescence spectroscopy.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 9208-9209 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Raman spectrum of ketene has been recorded for the first time since 1936. The rotational part of this spectrum shows Δkp=2, ΔJ=0 peaks with an intensity nearly four orders of magnitude smaller than that of the usual R/S structure. Computer simulation is used to reproduce the whole rotational spectrum. The observation of these peaks allows a fairly accurate determination of the R20 intensity ratio.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 8902-8906 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from 1460 to 2900 cm−1 above the threshold for singlet methylene CH2(a˜ 1A1) production. By scanning the 1CH2 probe laser wavelength, the CH2 laser-induced fluorescence (LIF) excitation spectrum is recorded and the 1CH2(0,1,0), (0,2,0), and (1,0,0) product state rotational distributions determined. As observed previously for the (0,0,0) state, the rotational state distributions of vibrationally excited methylene are in good agreement with phase space theory (PST) for excess energies less than 200 cm−1 above the appearance threshold of each vibrational state probed. For higher excess energies, 1CH2(0,1,0) and (0,2,0) rotational distributions like those for (0,0,0) are substantially colder than the statistical distribution given by PST. The quantum yields of vibrationally excited states are determined at several excess energies above the singlet threshold. These values are larger than predicted by PST and match values predicted by variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method.
    Type of Medium: Electronic Resource
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