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  • 1
    Electronic Resource
    Electronic Resource
    Molecular-beam infrared–infrared double-resonance spectroscopy study of the vibrational dynamics of the acetylenic C–H stretch of propargyl amine (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4290-4301 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The acetylenic C–H stretch spectrum of propargyl amine near 3330 cm−1 has been measured at 0.0002 cm−1 (6 MHz) resolution with a tunable color-center laser in an electric-resonance optothermal spectrometer. The spectrum has been fully assigned through IR–IR double resonance measurements employing a tunable, microwave sideband-CO2 laser. The 10 μm spectrum of propargyl amine displays splittings in the two nuclear spin symmetry states arising from amino-proton interchange, allowing double-resonance assignment of the –NH2 group resultant proton nuclear spin quantum number in the highly fragmented 3 μm spectrum. The experimental state density is consistent with a (2J+1) increase that is expected if all near-resonant states are coupled. From this J-dependent growth in the state density we determine the density of states at J=0 to be 22 states/cm−1. This value is in reasonable agreement with the direct state count result of 16 states/cm−1. The unperturbed transition frequencies for the two different nuclear spin species at a given rotational level do not coincide, differing on average by about 50 MHz. The nonresonant coupling effects which produce effective splittings in the 10 μm spectrum appear to survive into the high state density regime. The measured IVR lifetimes are on the order of 500 ps for the low Ka values studied here (Ka〈4) and show a Ka-dependence with the IVR rate increasing as Ka increases. The statistical properties of the spectrum have been compared to predictions from random matrix theory. The level spacings are not well represented by Wigner statistics as would be expected for underlying chaotic classical dynamics. However, the intensity fluctuations are consistent with a χ2-distribution, expected for classically chaotic systems, as measured by Heller's F-statistic. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477033
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  • 2
    Electronic Resource
    Electronic Resource
    The rotational spectrum of highly vibrationally mixed quantum states of propynol near 3330 cm−1 (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4407-4414 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational spectra of molecular eigenstates of propynol in the region of the acetylenic C–H stretch (3330 cm−1) have been measured using infrared-microwave saturation spectroscopy. These spectra illustrate the basic properties of the rotational spectra of highly vibrationally mixed quantum states. From the measurements we are able to measure the average value of the rotational constant and the width of the rotational constant distribution. We determine that the average value of the quantity 〈fraction SHAPE="CASE"〉12 (B+C) is 17 MHz smaller than the ground state value (a decrease of 0.4%). The width of the distribution (FHWM) is 90 MHz (1% of the ground state value). The distribution is approximately Gaussian. Narrowing of the rotational spectrum of single eigenstates by intramolecular vibrational energy redistribution (IVR) exchange processes is observed for the Ka=2 eigenstates. From the spectral narrowing we determine that the average IVR lifetime for vibrational states with Ka=2 near 3330 cm−1 is approximately 75 ps, about five times faster than the IVR lifetime of the Ka=2 states following coherent vibrational excitation of the acetylenic C–H stretch bright state (400 ps). Weak narrowing of the Ka=0 and Ka=1 eigenstates is observed below J=2. We estimate the IVR lifetime for Ka=0 and Ka=1 states as approximately 600 ps. The strong Ka dependence of the IVR rates of the bath states indicates that strong parallel Coriolis interactions play a primary role in the energy redistribution process. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477044
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  • 3
    Electronic Resource
    Electronic Resource
    Dressed states of molecules and microwave–infrared double-resonance spectroscopic techniques employing an electric quadrupole focusing field (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 10430-10439 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-sensitivity, microwave–infrared double-resonance measurements can be made in molecular-beam spectrometers employing a single state-focusing device. The key feature of the double-resonance technique is the achievement of large signal modulations of infrared signals using microwave transitions, even in cases where the infrared transition cannot be saturated. A series of measurements is presented that shows that the technique is based on the transition moment and state-focusing properties of dressed molecular states in the presence of a strong microwave field. Using a state-focusing device, the spectroscopic measurements are doubly sensitive to the composition of the dressed states. The technique can be extended to other types of spectroscopy, such as electronic spectroscopy and the spectroscopy of weakly bound complexes. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474207
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  • 4
    Electronic Resource
    Electronic Resource
    Structurally mixed molecular eigenstates of 2-fluoroethanol resulting from conformational isomerization (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 8189-8192 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a newly developed method of molecular-beam, infrared-microwave double-resonance spectroscopy, we are able to measure the rotational spectrum of a single molecular eigenstate of the molecular Hamiltonian near 3000 cm−1 of energy above the ground rovibrational state. This energy lies above the barrier to conformation isomerization in many molecules. In the –CH2F asymmetric C–H stretch of the Gg conformer of 2-fluoroethanol, near 2983 cm−1, we demonstrate the contribution of vibrational states localized around the Tt conformer structural minimum to the individual molecular eigenstates. The measurement demonstrates the ability of isolated molecules to use vibrational excitation to achieve geometrical rearrangement. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475119
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  • 5
    Electronic Resource
    Electronic Resource
    Doorway state enhanced intramolecular vibrational energy redistribution in the asymmetric (Double Bond)CH2 hydride stretch of methyl vinyl ether (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4302-4315 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The microwave-infrared double-resonance capabilities of an electric-resonance optothermal spectrometer have been used to assign the high resolution (5 MHz) infrared spectrum of the asymmetric (Double Bond)CH2 stretch of the cis conformer of methyl vinyl ether near 3130 cm−1. This vibrational state is anharmonically coupled to a near-resonant bath state by a 0.69 cm−1 matrix element resulting in two vibrational bands separated by about 1.44 cm−1. The two mixed states resulting from this interaction are further coupled to other near-resonant bath states with an average matrix element of about 0.01 cm−1. The coupled state density increases weakly with the total angular momentum, J, however, the intramolecular vibrational energy distribution (IVR) rate is approximately independent of the total angular momentum quantum number. Therefore, the rotationally mediated coupling mechanisms are weaker than the anharmonic terms in the redistribution process. A two-state analysis of the strong coupling, which includes a phenomenological IVR rate constant, suggests that the IVR rate in the two mixed states is dominated by the contribution from the coupled dark state. From the deconvolution of the IVR rates to remove the contribution from the dark state, the IVR lifetime of the asymmetric (Double Bond)CH2 stretch is determined to be 660 ps. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477034
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  • 6
    Electronic Resource
    Electronic Resource
    The rotational spectrum of a highly vibrationally mixed quantum state. II. The eigenstate-resolved spectroscopy analog to dynamic nuclear magnetic resonance spectroscopy (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 1990-1999 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The description of the rotational spectrum of a single molecular eigenstate in an energy region where conformational isomerization can occur is presented. The conformational isomerization rate can be determined from an analysis of the overall line shape of the spectrum. The isomerization dynamics are investigated through a time-domain analysis. It is shown that the nonstationary state produced by coherent microwave excitation has a well-defined conformational structure. However, the individual molecular eigenstates are a mixture of rovibrational states associated with the two conformations. The structural localization decays to the eigenstate equilibrium value at a rate defined as the isomerization rate. It is also shown that the line shape of the spectrum coalesces as the isomerization rate exceeds the difference between the characteristic rotational frequencies of the two conformers. The line shape profile of the eigenstate-resolved spectrum is well described by the Bloch equations modified for chemical exchange. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477864
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  • 7
    Electronic Resource
    Electronic Resource
    Intramolecular vibrational energy redistribution and conformational isomerization in vibrationally excited 2-fluoroethanol: High-resolution, microwave-infrared double-resonance spectroscopy investigation of the asymmetric –CH2(F) stretch near 2980 cm−1 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 1979-1989 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The asymmetric –CH2(F) stretch spectrum of 2-fluoroethanol near 2980 cm−1 has been rotationally assigned using microwave-infrared double-resonance spectroscopy methods in an electric-resonance optothermal molecular-beam spectrometer. The eigenstate-resolved infrared spectrum shows the effects of intramolecular vibrational energy redistribution (IVR) through the fragmentation of each rotational level of the vibrationally excited state into a set of transitions. From the spectrum we determine the IVR lifetime of the asymmetric –CH2(F) stretch to be 275 ps. The measured vibrational state density at 2980 cm−1 is 44 states/cm−1, and matches the value for the total state density obtained from a direct count. This agreement suggests that vibrational states of both the Gg′ and Tt conformers are coupled by the intramolecular dynamics. From measurements of the c-type pure rotational transitions of the Gg′ conformer we determine that the tunneling splitting for the Gg′ ground state is less than 35 kHz. The infrared spectrum is characterized by a large number of closely spaced infrared transitions. The clustering of vibrational energy levels is attributed to the weak interaction between the degenerate Gg′ and nondegenerate Tt vibrational states. This lifting of the effective Gg′ parity degeneracy is quantitatively investigated through the shape of the nearest-neighbor level spacing distribution. From this analysis we estimate that the isomerization lifetime for the Gg′ conformer of 2-fluoroethanol is 2 ns. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477975
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  • 8
    Electronic Resource
    Electronic Resource
    The rotational spectra of single molecular eigenstates of 2-fluoroethanol: Measurement of the conformational isomerization rate at 2980 cm−1 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 2000-2009 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational spectroscopy of single molecular eigenstates has been used to measure the conformational isomerization rate in 2-fluoroethanol. Eigenstates in the asymmetric –CH2(F) stretch spectrum of the Gg′ conformer near 2980 cm−1 are prepared with an infrared laser. These eigenstates are approximately 2000 cm−1 above the barrier to Gg′−Tt conformational isomerization. The rotational spectrum is measured using an infrared-microwave double-resonance technique based on the Autler–Townes splitting of states in a strong microwave field. This technique does not require saturation of the infrared preparation step. Two types of rotational transitions are observed. These are assigned to rotational transitions from vibrational states with Tt conformation (near 15.8 GHz) and to "isomerization states" (near 17.1 GHz) where the torsional wave functions are above the isomerization barrier. The isomerization kinetics are obtained from the linewidth of the ensemble eigenstate rotational spectrum. The lifetime for the Tt conformer is 2.7 ns. The isomerization states relax at approximately twice the rate of the Tt states (1.5 ns lifetime). This result is consistent with a kinetics model where the isomerization proceeds by "over-the-barrier" pathways. Both lifetimes are longer than the bright-state IVR lifetime (275 ps) indicating that the intramolecular dynamics occur on two distinct time scales. The isomerization rate for the Tt states is three orders-of-magnitude slower than predicted by a simple RRKM rate expression. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477865
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  • 9
    Electronic Resource
    Electronic Resource
    The rotational spectrum of a highly vibrationally mixed quantum state. I. Intramolecular vibrational energy redistribution (IVR) exchange narrowing of the rotational spectrum (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4396-4406 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The description of the rotational spectrum of a single, highly vibrationally mixed molecular eigenstate is given using two formulations. The model studied is a set of anharmonically coupled normal-mode rovibrational states. In the first approach, the rotational problem is cast into the form of the single bright-state model of IVR. This eigenstate-level formulation reveals the fragmentation of the rotational spectrum as the magnitude of the anharmonic coupling is increased. It is also seen that the center frequencies of all of the molecular eigenstate rotational spectra approach the same value, determined by the ensemble average rotational constant, as the IVR rate is increased. Furthermore, this formulation provides a generalization to rotational spectroscopy. When there is extensive state mixing, the center frequency of the pure rotational spectrum of a single molecular eigenstate is determined by the expectation value of the rotational constant, and the width of the spectrum is determined by the quantum mechanical fluctuation of the rotational constant in the molecular eigenstate. The lineshape properties of the spectrum are addressed using the motional (exchange) narrowing formalism for the ensemble spectrum. This formulation provides a quantitative description of the narrowing of the rotational spectrum by an IVR exchange mechanism. Finally, the convergence of the line shape profile of the eigenstate rotational spectrum to the line shape of the ensemble spectrum is investigated using a statistical model Hamiltonian. Convergence is observed when the number of overlapping states, defined as Wrms*ρ, reaches 1. These results show that an experimental measurement of the rotational spectrum of single molecular eigenstates can provide important average properties of the rotational constant distribution, dipole moment distribution, and the IVR rate at a well-defined total energy. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477043
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  • 10
    Electronic Resource
    Electronic Resource
    Intramolecular vibrational energy redistribution in the acetylenic C–H and hydroxyl stretches of propynol (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4316-4326 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The high-resolution infrared spectra of the acetylenic C–H and O–H stretches of propynol have been measured using an electric-resonance optothermal molecular beam spectrometer (EROS). Both spectra display extensive fragmentation of the hydride-stretch oscillator strength characteristic of the intramolecular vibrational energy redistribution (IVR) process. The IVR lifetime is strongly mode-specific. The IVR lifetime of the acetylenic C–H stretch is approximately 400 ps, with a slight increase in the lifetime with increasing values of the Ka quantum number. The lifetime of the O–H stretch is 60 ps and is independent of the rotational quantum numbers. The experimental upper limit for the anharmonic state densities are 30 and 40 states/cm−1 for the acetylenic C–H and O–H stretches, respectively. These values are in good agreement with the values obtained by a direct state count (19 and 32 states/cm−1, respectively). The measured density of states increases with an approximate (2J+1)-dependence. These results indicate that all energetically accessible states are involved in the IVR dynamics. However, neither the acetylenic C–H nor the O–H stretch shows a decrease in lifetime as the total angular momentum (J) increases. This result shows that Coriolis coupling of these two hydride stretches to the near-resonant bath states is much weaker than the anharmonic coupling. For the O–H stretch, we are able to obtain the root-mean-squared (rms) matrix element for the Coriolis coupling prefactor, 0.0015(5) cm−1. The rms anharmonic coupling matrix element is 0.03 cm−1. For the low J values measured in the O–H spectrum, the Coriolis-induced IVR rate is much slower than the initial redistribution rate resulting from the stronger anharmonic interactions leading to an IVR process with two distinct time scales. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477075
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