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  • 1
    Digitale Medien
    Digitale Medien
    High-resolution infrared spectroscopy of jet-cooled allyl radical (CH2–CH–CH2): In-phase (ν1) and out-of-phase (ν13) antisymmetric CH2 stretching vibrations (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 7793-7802 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: High-resolution infrared spectra of jet-cooled allyl (CH2–CH–CH2) radical in the CH2 stretching region are obtained in a slit jet discharge spectrometer. Over 200 transitions are observed in the 3100–3140 cm−1 region, which have been rigorously assigned to "in-phase" (ν1) and "out-of-phase" (ν13) coupling of antisymmetric CH stretch excitations on each of the equivalent CH2 subunits. Origins for the two bands are determined to be 3113.98 488(89) cm−1 (ν1) and 3110.59 857(36) cm−1 (ν13), respectively. The data are reasonably well fit to semirigid asymmetric rotor behavior for the Ka≤2 levels populated in the slit jet at Trot(approximate)20 K, though not to within the experimental precision, which provides some indication of perturbations in the upper states. The sign and magnitude of the +3.4 cm−1 splitting between"in-phase" and"out-of-phase" excitations are in good qualitative agreement with a simple electrostatic model of dipole–dipole coupling between the two "local mode" CH2 groups. Due to sub-Doppler resolution (Δν(approximate)70 MHz) in the slit jet expansion, quantum-state-dependent excess broadening of the rovibrational transitions is observed, which can be ascribed to spin–rotation interactions. Based on a least squares analysis of the high-resolution line shapes, the data are consistent with a spin rotation constant of cursive-epsilonaa(approximate)−67(25) MHz. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.477425
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  • 2
    Digitale Medien
    Digitale Medien
    Jet-cooled molecular radicals in slit supersonic discharges: Sub-Doppler infrared studies of methyl radical (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 5661-5675 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A novel high-intensity source of jet-cooled molecular radicals is described based on the combination of (i) slit supersonic expansions with (ii) pulsed electric discharges. The electrode bias configuration effectively confines the discharge to a region upstream of the supersonic expansion, which results both in efficient rotational cooling (Trot(approximate)25 K) and high radical densities (〉1014/cm3). In conjunction with direct absorption laser probe methods, this discharge source provides a general technique for high-resolution IR studies of jet-cooled radicals. Performance of the slit discharge system is demonstrated on v=1←0 rovibrational transitions in jet-cooled OH radicals, which indicate sub-Doppler linewidths (Δν(approximate)100 MHz) when probed along the slit expansion axis. The enhanced spectral resolution of the slit discharge geometry is utilized to probe the v3=1←0 asymmetric CH stretch vibration–rotation spectra of CH3 radical. Under sub-Doppler conditions, spin–rotation splittings are fully resolved and nuclear hyperfine splittings partially resolved in all of the transitions, permitting the first measure of Fermi contact interactions [af″=−65.5(9) MHz, εbb″=−354(5) MHz, af′=−65(2) MHz, εbb′=−353(2) MHz] and therefore both the sign and magnitude of spin-polarization effects for CH3 under isolated gas-phase conditions. The results permit direct comparison with high level ab initio calculations, and highlight a clear trend in spin-polarization effects between condensed and gas-phase behavior. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.474259
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  • 3
    Digitale Medien
    Digitale Medien
    Laser spectroscopy of jet-cooled ethyl radical: Infrared studies in the CH2 stretch manifold (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1823-1834 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A glow discharge, slit supersonic expansion in conjunction with direct infrared laser absorption methods has been utilized to record high resolution vibration–rotation spectra of the CH3–CH2 ethyl radical. The slit supersonic expansion results in efficient rotational cooling from discharge temperatures down to Trot(approximate)14 K, permitting unambiguous rotational assignment and spectral analysis for the first time. Furthermore, a discharge on/discharge off data collection scheme permits clean discrimination between spectral contributions from radical vs precursor absorption. Spectra for both symmetric and asymmetric CH2 stretch manifolds are observed. Least-squares fits of transition frequencies out of the K=0 ground state manifold to a near prolate top model Hamiltonian reproduce the data to within the 7 MHz experimental uncertainty and provides rotational constants for both ground and vibrationally excited symmetric/asymmetric CH2 stretch states. The band origins for the CH2 stretch vibrations [3037.018 96(12) cm−1 and 3128.693 69(13) cm−1] are in reasonable agreement with ab initio theory; though predictions for relative intensities of the two bands are off by nearly an order of magnitude and indicate that the transition moment vector is tilted 33° away from each C–H bond toward the C–C bond axis. Structural analysis based on the measured B and C rotational constants imply a C–C bond distance of 1.49 Å. This is consistent with partial ((approximate)15%) double bond character for the ethyl radical carbon frame and in excellent agreement with theoretical predictions. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.480746
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  • 4
    Digitale Medien
    Digitale Medien
    Sequential solvation of HCl in argon: High resolution infrared spectroscopy of ArnHCl (n=1,2,3) (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 1115-1127 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: High-resolution near-infrared spectra of the vHCl=1←0 fundamental stretch in Ar2HCl and Ar3HCl have been characterized using a slit-jet infrared spectrometer. Analysis of the jet-cooled, rotationally resolved spectra (i) permits unambiguous identification of the cluster size, (ii) provides vibrationally averaged geometries in the vHCl=1 excited state, and (iii) allows the vibrational shift of the HCl chromophore to be measured as a function of the number of Ar atoms in the complex. The equilibrium structures of ArnHCl (n=1–3) clusters calculated using accurate Ar–Ar and Ar–HCl pair potentials are consistent with the vibrationally averaged structures inferred spectroscopically. The vibrational red-shifts for ArnHCl (n=1–3) reflect a near-linear dependence on the number of Ar atoms, which is qualitatively reproduced by simple classical calculations on vHCl=0 and 1 pairwise additive potential surfaces. Theoretical predictions of the ArnHCl red-shifts in a fcc lattice indicate good agreement with experimental matrix results. However, to achieve this asymptotic limit requires up to n(approximate)54 Ar atoms; this underscores a clear sensitivity to non-nearest neighbor Ar–HCl interactions significantly outside the first solvation shell. Finally, for smaller ArnHCl clusters with only one solvation shell (n=12), the potentials predict an energetic preference for HCl in surface vs interior sites. Analysis indicates that this effect is predominantly due to Ar/HCl size mismatch, which destabilizes the nearest neighbor Ar shell for HCl solvated in the center of the cluster. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.474458
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  • 5
    Digitale Medien
    Digitale Medien
    Probing hydrogen bond potentials via combination band spectroscopy: A near infrared study of the geared bend/van der Waals stretch intermolecular modes in (HF)2 (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6225-6243 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: High resolution near infrared spectra of the two lowest frequency intermolecular modes in HF-stretch excited states of (HF)2 have been characterized using a slit-jet infrared spectrometer. In the spectral region surveyed, ten vibration–rotation–tunneling (VRT) bands are observed and assigned to the low frequency "van der Waals stretch'' (ν4) and "geared bend'' (ν5) intermolecular modes, in combination with either the hydrogen bond acceptor (ν1) or donor (ν2) high-frequency intramolecular HF stretches. Analysis of the rotationally resolved spectra provide intermolecular frequencies, rotational constants, tunneling splittings, and predissociation rates for the ν4/ν5 intermolecular excited states. The intermolecular vibrational frequencies in the combination states display a systematic dependence on intramolecular redshift that allows far-IR intermolecular frequencies to be reliably extrapolated from the near-IR data. Approximately tenfold increases in the hydrogen bond interconversion tunneling splittings with either ν4 or ν5 excitation indicate that both intermolecular modes correlate strongly to the tunneling coordinate. The high resolution VRT line shapes reveal mode specific predissociation broadening sensitive predominantly to intramolecular excitation, with weaker but significant additional effects due to low frequency intermolecular excitation. Analysis of the high resolution spectroscopic data for these ν4 and ν5 combination bands suggests strong state mixing between what has previously been considered van der Waals stretch and geared bend degrees of freedom. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.471285
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  • 6
    Digitale Medien
    Digitale Medien
    Plucking a hydrogen bond: A near infrared study of all four intermolecular modes in (DF)2 (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6645-6664 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The near ir combination band spectra of supersonically cooled (DF)2 in the 2900 to 3300 cm−1 region have been recorded with a high resolution slit jet spectrometer. Twelve vibration–rotation–tunneling (VRT) bands are observed, representing each of the four intermolecular modes (van der Waals stretch ν4, geared bend ν5, out-of-plane torsion ν6, and antigeared bend ν3) built as combination bands on either the ν1 (free) or ν2 (bound) DF stretches. Analysis of the rotationally resolved spectra provide spectroscopic constants, intermolecular frequencies, tunneling splittings, and predissociation rates as a function of both intra- and intermolecular excitation. The intermolecular frequencies demonstrate a small but systematic dependence on intramolecular mode, which is exploited to yield frequency predictions relevant to far-ir studies, as well as facilitate direct comparison with full 6-D quantum calculations on trial potential surfaces. The tunneling splittings demonstrate a much stronger dependence upon intermolecular mode, increasing by as much as an order of magnitude for geared bend excitation. Conversely, high resolution line shape analysis reveals that vibrational predissociation broadening is only modestly affected by intermolecular excitation, and instead exhibits mode specific behavior controlled predominantly by intramolecular excitation. Detailed H/D isotopic vibrational shifts are obtained by comparison with previous combination band studies of all four intermolecular modes in (HF)2. In contrast to the strong state mixing previously observed for (HF)2, the van der Waals stretch and geared bend degrees of freedom are largely decoupled in (DF)2, due to isotopically "detuning'' of resonances between bend–stretch intermolecular vibrations. Four-dimensional quantum calculations of the (HF)2 and (DF)2 eigenfunctions indicate that the isotopic dependence of this bend–stretch resonance behavior is incorrectly predicted by current hydrogen bond potential surfaces. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.471978
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  • 7
    Digitale Medien
    Digitale Medien
    Dependence of level-resolved energy transfer on initial vibrational level in Li2A1Σu+−Ne collisions (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 1415-1426 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have investigated collision induced rotational and vibrational energy transfer in the Li2 A1Σu+(vi,ji=30)−Ne system experimentally under single-collision conditions at an effective temperature of 691 K. Over 800 inelastic rate constants have been measured, with the initial vibrational level vi ranging from 2 to 24 and −2≤Δv≤+2. Increasing vi results in a linear increase in the vibrational transition rate constants, which is accompanied by a decrease in the rotationally inelastic transition rate constant. The total inelastic rate constant increases with vi only at the highest values of vi. Net vibrational energy transfer 〈ΔE〉 calculated using rotationally summed rate constants is qualitatively consistent with a simple model. However, explicit inclusion of rotation gives quite different values of 〈ΔE〉. The experimental results are compared with our three-dimensional trajectory calculations on an ab initio potential surface and on a simple repulsive potential surface. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.470908
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  • 8
    Digitale Medien
    Digitale Medien
    Probing nuclear quadrupole interactions in the rotationally resolved S1←S0 electronic spectrum of 2-chloronaphthalene (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 225-235 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Rotationally resolved S1←S0 fluorescence excitation spectra of 2-chloronaphthalene (2ClN) are measured using a newly constructed ultraviolet (UV) laser/molecular beam spectrometer. More than 1000 well-resolved rotational lines are recorded at a resolution of ∼3 MHz for the two overlapping band origins of the 35Cl and 37Cl isotopomers (Δνorigin∼3 GHz) over a 3.5 cm−1 spectral region at a rotational temperature of 10 K. Both spectra are hybrid bands, having 18% a-/82% b-type character in accordance with 1La←1A1 excitations observed for other 2-substituted naphthalenes. Additionally, Cl nuclear quadrupole splittings are observed as broadened asymmetric line shapes with linewidths from 5 to 20 MHz. Least-squares fits of these line shapes coupled with high-precision Fourier-transform microwave measurements of the ground state have provided accurate S1 quadrupole coupling constants. Specified relative to the principal quadrupole tensor orientation of S0, the S0 and S1 values of eQqzz are −71.198(5) MHz and −67.8(30) MHz, and the asymmetry parameters, η=(eQqyy−eQqxx)/eQqzz, are 0.076 38(5) and 0.13(5), respectively. The experimental line shapes are fit to Voigt functions, yielding 4.9(3) MHz homogeneous (lifetime) and 3.0(2) MHz inhomogeneous (Doppler) contributions. All of the observed structure are well reproduced by more than 20 000 transitions predicted for the nuclear-quadrupole-split rotational spectra of the two hybrid-band origins. Furthermore, the homogeneous linewidths show no dependence on either the upper state energy or angular momentum quantum numbers. The absence of additional level structure at the resolution of 1 MHz or more, from heavy-atom-enhanced intersystem crossing mechanisms, is discussed within the framework of radiationless transition theory. The quadrupole tensor components are used in simple theoretical models to investigate the orbital character of the S1 state and its radiationless decay mechanisms. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1376629
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  • 9
    Digitale Medien
    Digitale Medien
    Probing potential surfaces for hydrogen bonding: Near-infrared combination band spectroscopy of van der Waals stretch (ν4) and geared bend (ν5) vibrations in (HCl)2 (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6132-6145 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: High resolution infrared spectra of the two "low" frequency intermolecular modes—van der Waals stretch (ν4) and geared bend (ν5) of (HCl)2—have been characterized in HCl-stretch excited states using a slit jet spectrometer. In a high resolution high sensitivity search covering the range between 2880 and 3070 cm−1 four (HCl)2 combination bands associated with in-plane vibrations ν4 and ν5 have been observed. The vibrational assignment of these bands is based on comparison between observed intermolecular mode energies and predictions from recent six-dimensional (6D) quantum mechanical (QM) calculations [Y. Qiu, J. Z. H. Zhang, and Z. Bacic, J. Chem. Phys. 108, 4804 (1998)], though additional confirmation is provided by ancillary spectroscopic information such as rotational constants, predissociation linewidths, and 35Cl/37Cl isotopic band shifts. The experimentally observed intermolecular energies agree with theoretical predictions to (approximate)2–4 cm−1 out of 60–90 cm−1, suggesting that the 6D potential energy surface can describe combination band excitation in these lower frequency intermolecular coordinates fairly well. Three of the four observed combination bands arise from the upper tunneling level (B+), and all four bands are built exclusively on bound HCl stretch (ν2). To account for these striking intensity anomalies, a simple model for three-dimensional QM calculation of transition moments is introduced, which correctly reproduces the experimental trends. In this model, the propensity for ν2 based combination bands arising from upper tunneling levels can be successfully ascribed to the unusually "floppy" nature of the intermolecular vibrations, which results in a "harmonic oscillator" Δv=+1 propensity for excitations between tunneling levels along the geared bend coordinate. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1436105
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  • 10
    Digitale Medien
    Digitale Medien
    Beyond the Born–Oppenheimer approximation: High-resolution overtone spectroscopy of H2D+ and D2H+ (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6146-6158 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Transitions to overtone 2ν2 and 2ν3, and combination ν2+ν3 vibrations in jet-cooled H2D+ and D2H+ molecular ions have been measured for the first time by high-resolution IR spectroscopy. The source of these ions is a pulsed slit jet supersonic discharge, which allows for efficient generation, rotational cooling, and high frequency (100 KHz) concentration modulation for detection via sensitive lock-in detection methods. Isotopic substitution and high-resolution overtone spectroscopy in this fundamental molecular ion permit a systematic, first principles investigation of Born–Oppenheimer "breakdown" effects due to large amplitude vibrational motion as well as provide rigorous tests of approximate theoretical methods beyond the Born–Oppenheimer level. The observed overtone transitions are in remarkably good agreement (〈0.1 cm−1) with non-Born–Oppenheimer ab initio theoretical predictions, with small but systematic deviations for 2ν2, ν2+ν3, and 2ν3 excited states indicating directions for further improvement in such treatments. Spectroscopic assignment and analysis of the isotopomeric transitions reveals strong Coriolis mixing between near resonant 2ν3 and ν2+ν3 vibrations in D2H+. Population-independent line intensity ratios for transitions from common lower states indicate excellent overall agreement with theoretical predictions for D2H+, but with statistically significant discrepancies noted for H2D+. Finally, H2D+ versus D2H+ isotopomer populations are analyzed as a function of D2/H2 mixing ratio and can be well described by steady state kinetics in the slit discharge expansion. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1458244
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