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Molecular rotations and kinetics by light scattering
Molecular rotations and kinetics from spectral features of molecular Raman spectra, using light scattering spectroscopy
Theoretical study of the relativistic molecular rotational g-tensor
An original formulation of the relativistic molecular rotational g-tensor valid for heavy atom containing compounds is presented. In such formulation, the relevant terms of a molecular Hamiltonian for non-relativistic nuclei and relativistic electrons in the laboratory system are considered. Terms linear and bilinear in the nuclear rotation angular momentum and an external uniform magnetic field are considered within first and second order (relativistic) perturbation theory to obtain the rotational g-tensor. Relativistic effects are further analyzed by carrying out the linear response within the elimination of the small component expansion. Quantitative results for model systems HX (X=F, Cl, Br, I), XF (X=Cl, Br, I), and YH{sup +} (Y=Ne, Ar, Kr, Xe, Rn) are obtained both at the RPA and density functional theory levels of approximation. Relativistic effects are shown to be small for this molecular property. The relation between the rotational g-tensor and susceptibility tensor which is valid in the non-relativistic theory does not hold within the relativistic framework, and differences between both molecular parameters are analyzed for the model systems under study. It is found that the non-relativistic relation remains valid within 2% even for the heavy HI, IF, and XeH{sup +} systems. Only for the sixth-row Rn atom a significant deviation of this relation is found.
Molecular Rotational Dynamics in Mixed CH 4 –CO 2 Hydrates: Insights from Molecular Dynamics Simulations
The impact of guest molecule composition on the rotational dynamics in CH 4 , CO 2 , and mixed CH 4 –CO 2 gas hydrates is investigated with classical molecular dynamics simulations. Rotational autocorrelation functions are calculated for the guest and host molecules in each hydrate composition from simulation trajectories at 10, 40, 190, and 270 K. Analysis of these functions for each molecule is further decomposed into cage type for each CH 4 and CO 2 guest and cage face for the H 2 O host. CH 4 becomes more constrained, and CO 2 gains freedom in the mixed guest systems. Mixing guest species in gas hydrates alters the intermolecular interaction environment, impacting the rotational motion of the guest molecules. This effect is also seen in the host lattice H 2 O molecules, as evidence of molecular rotations is seen in the CO 2 hydrate at 270 K during longer simulations.
A 2000 Solar Mass Rotating Molecular Disk Around NGC 6334A
We present millimeter and centimeter wave spectroscopic observations of the H II region NGC 6334A. We have mapped the source in several transitions of CO, CS, and NH3. The molecular emission shows a distinct flattened structure in the east-west direction. This structure is probably a thick molecular disk or torus (2.2 x 0.9 pc) responsible for the bipolarity of the near-infrared (NIR) and radio continuum emission which extends in two "lobes" to the north and south of the shell-like H II region. The molecular disk is rotating from west to east (omega approximately equals 2.4 km/s.pc) about an axis approximately parallel to the radio and NIR emission lobes. By assuming virial equilibrium, we find that the molecular disk contains approximately 2000 solar mass. Single-component gas excitation model calculations show that the molecular gas in the disk is warmer and denser (T(sub k) approximately equals 60 K, n approximately equals 3000/cc) than the gas to the north and south (T(sub k) approximately equals 50 K, n approximately equals 400/cc). High resolution (approximately 5 sec) NH3 (3, 3) images of NGC 6334A reveal several small (approximately 0.1 pc) clumps, one of which lies southwest of the radio continuum shell, and is spatially coincident with a near-infrared source, IRS 20. A second NH3 clump is coincident with an H2O maser and the center of a molecular outflow. The dense gas tracers, CS J = 5 approaches 4 and 7 approaches 6, peak near IRS 20 and the H2O maser, not at NGC 6334A. IRS 20 has a substantial far-infrared (FIR) luminosity L(sub FIR) approximately 10(exp 5) solar luminosity, which indicates the presence of an O 7.5 star but has no detected radio continuum (F(sub 6 cm) < 0.02 Jy). The combination of dense gas, a large FIR luminosity and a lack of radio continuum can best be explained if IRS 20 is a protostar. A third clump of NH3 emission lies to the west of IRS 20 but is not associated with any other molecular or continuum features. The star formation activity in the region has moved west of NGC 6334A to IRS 20 and the H2O maser position. We suggest that NGC 6334A, IRS 20, and the H2O maser spot are part of a "protocluster" of stars which is condensing from the massive molecular disk. The similarity between the structure around NGC 6334A and other large (r approximately 1 pc), massive (M approximately 10(exp 3) solar mass), rotating disks (K3-50A and G10.6-0.4) suggests that this may be a common mechanism by which open clusters form.
Molecular rotation and reorientation - Microscopic and hydrodynamic contributions
The relative roles of microscopic and hydrodynamic contributions to molecular rotation and reorientation are examined within the framework of the microscopic boundary layer theory recently proposed by the authors. The theory is applied to rough spheres, for which computer simulation data are available and to experimental results on spherical top molecules. Attention is focused on rotational diffusion constants, the kappa parameter introduced by Kivelson et al., and orientational relaxation times. It is shown that, while collective effects are present and often nonnegligible, the motion of small molecules is dominated by its microscopic aspects. Experimental trends which can incorrectly suggest dominance by hydrodynamic contributions are discussed in some detail. Finally, the transition to the regime where collective effects are dominant is considered.
Observation of molecular rotational bands with a single Fabry-Perot interferometer
A method is described for using a single Fabry-Perot interferometer to study the emissions from an entire molecular rotation band, not just a single line to determine atmospheric temperature. It is shown that if the effective line spacing of the band to free spectral range is nearly a rational number, the resulting signal is highly temperature sensitive, particularly at low temperatures. It is shown that the finesse of the instrument is not extremely important but that the instrument function must be known very precisely. The method is illustrated with the O2 atmospheric band and an interferometer flown on the Dynamics Explorer 2 satellite.
Effects of molecular rotation after ionization and prior to fragmentation on observed recoil-frame photoelectron angular distributions in the dissociative photoionization of nonlinear molecules
Experimental angle-resolved photoelectron-photoion coincidence experiments measure photoelectron angular distributions (PADs) in dissociative photoionization (DPI) in the reference frame provided by the momenta of the emitted heavy fragments. By extension of the nomenclature used with DPI of diatomic molecules, we refer to such a PAD as a recoil-frame PAD (RFPAD). When the dissociation is fast compared to molecular rotational and bending motions, the emission directions of the heavy fragments can be used to determine the orientation of the bonds that are broken in the DPI at the time of the ionization, which is known as the axial-recoil approximation (ARA). When the ARA is valid, the RFPADs correspond to molecular-frame photoelectron angular distributions (MFPADs) when the momenta of a sufficient number of the heavy fragments are determined. When only two fragments are formed, the experiment cannot measure the orientation of the fragments about the recoil axes so that the resulting measured PAD is an azimuthally averaged RFPAD (AA-RFPAD). In this paper we consider how the breakdown of the ARA due to rotation will modify the observed RFPADs for DPI processes in nonlinear molecules for ionization by light of arbitrary polarization. This model is applied to the core C $1s$ DPI of ${\mathrm{CH}}_{4}$, with the results compared to experimental measurements and previous theoretical calculations done within the ARA. The published results indicate that there is a breakdown in the ARA for two-fragment events where the heavy-fragment kinetic energy release was less than 9 eV. Finally, including the breakdown of the ARA due to rotation in our calculations gives very good agreement with the experimental AA-RFPAD, leading to an estimate of upper bounds on the predissociative lifetimes as a function of the kinetic energy release of the intermediate ion states formed in the DPI process.
Molecular Rotations, Multiscale Order, Hyperuniformity, and Signatures of Metastability during the Compression/Decompression Cycles of Amorphous Ices
We model, via large-scale molecular dynamics simulations, the isothermal compression of low-density amorphous ice (LDA) to generate high-density amorphous ice (HDA) and the corresponding decompression extending to negative pressures to recover the low-density amorphous phase (LDA HDA ). Both LDA and HDA are nearly hyperuniform and are characterized by a dynamical HBN, showing that amorphous ices are nonstatic materials and implying that nearly hyperuniformity can be accommodated in dynamical networks. In correspondence with both the LDA-to-HDA and the HDA-to-LDA HDA phase transitions, the (partial) activation of rotational degrees of freedom activates a cascade effect that induces a drastic change in the connectivity and a pervasive reorganization of the HBN topology which, ultimately, break the samples’ hyperuniform character. Key to this effect is the rapid rate at which changes occur, and not their magnitude. The inspection of structural properties from the short- to the long-range shows that signatures of metastability are present at all length-scales, hence providing further solid evidence in support of the liquid–liquid critical point scenario. LDA and LDA HDA differ in terms of HBN and structural properties, implying that they are distinct low-density glasses. Our work unveils the role of molecular rotations in the phase transitions between amorphous ices and shows how the unfreezing of rotational degrees of freedom generates a cascade effect that propagates over multiple length-scales. Our findings greatly improve our basic understanding of water and amorphous ices and can potentially impact the field of molecular network-forming materials at large.
Optical Waveguiding Charge-Transfer Cocrystals: Examining the Impact of Molecular Rotations on Their Photoluminescence
Here, we present the first example of a binary optical waveguiding (OWG) cocrystal with large anisotropy featuring a fluorinated acceptor molecule (CPP-TFPN, 1) with on-plane rotational dynamics, confirmed by solid-state NMR ( 19 F T 1 ) and theoretical calculations. Spatially resolved microphotoluminescence and variable-temperature photoluminescence experiments allowed us to examine the OWG performance and photophysical properties of both single crystals and bulk microcrystalline samples. A comparison with an analogous cocrystal containing a regioisomeric acceptor (CPP-TFTN, 2) revealed that the photoluminescence characteristics of 1 are associated with the rotational motions of the acceptor, offering insights into how the molecular motion changes this property.
Saturated absorption in a rotational molecular transition at 2.5 THz using a quantum cascade laser
We report on the evidence of saturation effects in a rotational transition of CH{sub 3}OH around 2.5 THz, induced by a free-running continuous-wave quantum cascade laser (QCL). The QCL emission is used for direct-absorption spectroscopy experiments, allowing to study the dependence of the absorption coefficient on gas pressure and laser intensity. A saturation intensity of 25 μW/mm{sup 2}, for a gas pressure of 17 μbar, is measured. This result represents the initial step towards the implementation of a QCL-based high-resolution sub-Doppler THz spectroscopy, which is expected to improve by orders of magnitude the precision of THz spectrometers.
Propagating molecular rotational coherences through single-frequency pulses in the strong field regime
In the weak-field limit in which microwave spectroscopy is typically carried out, an application of a single-frequency pulse that is resonant with a molecular transition will create a coherence between the pair of states involved in the rotational transition, producing a free-induction decay (FID) that, after Fourier transform, produces a molecular signal at that same resonance frequency. With the advent of chirped-pulse Fourier transform microwave methods, the high-powered amplifiers needed to produce broadband microwave spectra also open up other experiments that probe the molecular response in the high-field regime. This paper describes a series of experiments involving resonant frequency pulses interrogating jet-cooled molecules under conditions of sufficient power to Rabi oscillate the two-state system through many Rabi cycles. The Fourier-transformed FID shows coherent signal not only at the applied resonant frequency but also at a series of transitions initially connected to the original one by sharing an upper or lower level with it. As the duration of the single-frequency excitation is increased from 250 to 1500 ns, the number of observed off-resonant, but dipole-allowed, molecular coherences grow. The phenomenon is quite general, having been demonstrated in Z-phenylvinylnitrile, E-phenylvinylnitrile (E-PVN), benzonitrile, guaiacol, and 4-pentynenitrile. In E-PVN, the highest power/longest pulse duration, coherent signal is also present at energetically nearby but not directly connected transitions. Even in molecular samples containing more than one independent species, only transitions due to the single species responsible for the original resonant transition are present. Here, we develop a time-dependent model of the molecular/photon system and use it in conjunction with the experiment to test possible sources of the phenomenon.
The Formation of Preplanetary Disks from the Collapse of Rotating Molecular Cloud Cores
Solutions that describe the collapse of a molecular cloud core that is initially in unstable equilibrium, embedded within an envelope of uniform density, and rotating at the same rate as the envelope are given. Hydrodynamic equations, including self gravity, are deduced to a set of ordinary differential equations, which are solved by the method of matched asymptotic expansions. Results of these calculations are: (1) the range of stellar masses derived seems to correspond to realistic ranges of observed stellar masses and interstellar cloud parameters, (2) the proper measure of dissipation rate is the ratio of accretion time to viscous diffusion time, and (3) the pressure distribution on the surface of an accreting protostar is nonuniform in a way that favors the channeling of a stellar wind into a bipolar flow directed along the rotation axis.
The rotating molecular core in G10.6 - 0.4: Synthesis maps in (12)C(18)O
Using the Owens Valley Radio Observatory millimeter-wave interferometer, the compact molecular cloud core surrounding the H II region G10.60-0.4 has been resolved in the C(18)O J = 1-0 transition. Since this line is most likely optically thin, it is an excellent tracer of the total column density. We find a centrally condensed and flattened (12.6 sec x 4.2 sec ; 0.3 pc x 0.1 pc) core, which is rotating rapidly with a substantial velocity gradient (17 +/- 2 km/s pc) along the major axis. The core is quite bright (greater than 10K), dense (greater than (10(exp 6)/cu cm), massive (10(exp 3) solar mass), and embedded in a more extended (greater than 60 seconds) envelope. There is no evidence for absorption, implying that the C(18)O emitting gas is both optically thin and substantially hotter than the equivalent continuum temperature at 3 mm. The systemic velocity of the rotating core is -2.5 +/- 0.5 km/s, the same as the systemic velocity of the extended cloud.
On the validity of collider-mass scaling for molecular rotational excitation
Rate coefficients for collisional processes such as rotational and vibrational excitation are essential inputs in many astrophysical models. When rate coefficients are unknown, they are often estimated using known values from other systems. The most common example is to use He-collider rate coefficients to estimate values for other colliders, typically H{sub 2}, using scaling arguments based on the reduced mass of the collision system. This procedure is often justified by the assumption that the inelastic cross section is independent of the collider. Here we explore the validity of this approach focusing on rotational inelastic transitions for collisions of H, para-H{sub 2}, {sup 3}He, and {sup 4}He with CO in its vibrational ground state. We compare rate coefficients obtained via explicit calculations to those deduced by standard reduced-mass scaling. Not surprisingly, inelastic cross sections and rate coefficients are found to depend sensitively on both the reduced mass and the interaction potential energy surface. We demonstrate that standard reduced-mass scaling is not valid on physical and mathematical grounds, and as a consequence, the common approach of multiplying a rate coefficient for a molecule-He collision system by the constant factor of ∼1.4 to estimate the rate coefficient for para-H{sub 2} collisions is deemed unreliable. Furthermore, we test an alternative analytic scaling approach based on the strength of the interaction potential and the reduced mass of the collision systems. Any scaling approach, however, may be problematic when low-energy resonances are present; explicit calculations or measurements of rate coefficients are to be preferred.
Spectroscopic measurement of increases in hydrogen molecular rotational temperature with plasma-facing surface temperature and due to collisional-radiative processes in tokamaks
Abstract Spatially resolved rotational temperature of ground state hydrogen molecules desorbed from plasma-facing surface was measured in QUEST, LTX-β, and DIII-D tokamaks, and the increases of the rotational temperature with the surface temperature and due to collisional-radiative processes in the plasmas were evaluated. The increase due to collisional-radiative processes was calculated by solving rate equations considering electron and proton collisional excitation and deexcitation and spontaneous emission. The calculation results suggest a high sensitivity for the rotational temperature to electron and proton densities, but a negligible sensitivity to the electron, proton, and surface temperatures. In the three tokamaks with different plasma parameters and plasma-facing surface materials, the spatial profile of the rotational temperature was estimated using Fulcher-α emission lines (600–608 nm). In QUEST, the spatial profile of the rotational temperature was estimated from spatially resolved spectra. In the other tokamaks, the rotational temperature was evaluated assuming a single point emission with a location determined from the Fulcher-α emission profile as measured with a filtered camera. In metal-walled devices QUEST and LTX-β, the rotational temperature increased with the surface temperature, and the calculated collisional-radiative increase is consistent with measured increase assuming that the rotational temperature at the surface is approximately 500–600 K higher than the surface temperature. In DIII-D with carbon walls, a larger collisional-radiative increase than the other tokamaks was observed because of the higher density leading to a large difference from the calculated increase compared to the other smaller tokamaks. Measurement of the Fulcher-α emission profile with higher spatial resolution in DIII-D may reduce the difference and reveal the effect of the surface temperature on the rotational temperature. These results show the increases in the rotational temperature with the surface temperature and due to the collisional-radiative processes.
The influence of molecular rotation on vibration-translation energy transfer
A three-dimensional vibration-rotation collision model is formulated within a semiclassical framework to study the effect of coupled rotational motion on the rate of vibrational energy transfer for collisions between anharmonic diatomic molecules and structureless atoms. Emphasis is on the net rates of vibrational energy transfer summed over all final rotational states rather than those for individual vibration-rotation transitions. The corresponding three-dimensional rate parameters are cast into forms that may be identified with equivalent parameters associated with the collinear approximation. Their comparisons allow the validity of a collinear model to be evaluated and some constraints on its application to be implied. The mechanisms of vibrational energy transfer including rotational transitions are shown to be separable into three classes which are identified and discussed
Methyl cyanide as a probe of the temperature and density in SgrB2 Quasi-equilibrium in molecular rotational levels
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