Rotational line structure in three-photon scattering by symmetric top molecules

Molecular and nuclear magnetic moments in microwave Zeeman spectra

Using an asymptotic expansion of the angular momentum coupling coefficients which appear in the infinite-order-sudden formulation of collisions between symmetric top molecules and structureless atoms [S. Green, J. Chem. Phys. 70, 816 (1979)], we demonstrate that in certain cases, transitions which conserve the ’’parity index’’ of the rotational wave function will be strongly favored over those in which the parity index changes. This propensity rule will apply both to transitions which are inelastic in J but elastic in K, the projection of J along the molecular axis, as well as to transitions which are elastic in J but inelastic in K. Identical propensity rules will emerge within a Born treatment of the collision dynamics. These rules are completely general and independent of the particular form of the atom–molecule potential. The propensity toward conservation of the parity index will also be apparent in collisions of slightly asymmetric tops and l-doubled linear polyatomic molecules. A simple criteria is developed to determine the minimum value of J for which these propensity rules will be valid. Examination of accurate close-coupling and coupled-states cross sections for the NH3–He and H2CO–He systems confirms the accuracy of the new propensity rules. These rules can be used to predict intensity changes in four-level double resonance experiments; excellent agreement is obtained in a comparison with available experimental data for collisions of both HCN and NH3 with He, Ar, Xe, and H2.

Ultrafast electron diffraction (UED) from aligned molecules in the gas phase has successfully retrieved structures of both linear and symmetric top molecules. Alignment of asymmetric tops has been recorded with UED but no structural information was retrieved. We present here the extraction of two-dimensional structural information from simple transformations of experimental diffraction patterns of aligned molecules as a proof-of-principle for the recovery of the full structure. We align 4-fluorobenzotrifluoride with a linearly polarized laser and show that we can distinguish between atomic pairs with equal distances that are parallel and perpendicular to the aligned axis. We additionally show with numerical simulations that by cooling the molecules to a rotational temperature of 1 K, more distances and angles can be resolved through direct transformations.

Context. Water is an important molecule in interstellar and circumstellar environments. Previous observations of mid-infrared (IR) rotational lines of OH toward star-forming regions suggest that OH emission may be used to probe the photodissociation of water. Aims. Our goal is to propose a method to quantify H2O photodissociation and measure the local ultraviolet (UV) flux from observations of mid-IR OH lines. Methods. Cross sections for the photodissociation of H2O resolving individual electronic, vibrational, and rotational states of the OH fragment are collected. The state distribution of nascent OH following H2O photodissociation is computed for various astrophysically relevant UV radiation fields (e.g., a single Lyα line or a broadband spectrum). These distributions are incorporated in a new molecular excitation code called GROSBETA, which includes radiative pumping, collisional (de)excitation, and prompt emission (i.e., following the production of OH in excited states). The influence of the photodissociation rate of H2O, the spectral shape of the UV radiation field, the density, the temperature of the gas, and the strength of the IR background radiation field on the integrated line intensities are studied in detail. As a test case, our model is compared to Spitzer-IRS observations at the tip of the HH 211 bow-shock. Results. The OH rotational line intensities in the range 9–16 μm, covering rotational transitions with Nup = 18–45, are proportional to the column density of H2O photodissociated per second by photons in the range 114–143 nm (denoted as ΦB̃) and do not depend on other local properties such as the IR radiation field, the density, or the kinetic temperature. Provided an independent measurement of the column density of water is available, the strength of the local UV radiation field can be deduced with good accuracy, regardless of the exact shape of the UV field. In contrast, OH lines at longer far-IR wavelengths are primarily produced by IR radiative pumping and collisions, depending on the chemical pumping rate defined as 𝒟B̃ = ΦB̃/N(OH) and on the local physical conditions (nH, TK, IR radiation field). Our model successfully reproduces the OH mid-IR lines in the 10–16 μm range observed toward the tip of the HH 211 bow-shock and shows that the jet shock irradiates its surroundings, exposing H2O to a UV photon flux that is about 5 × 103 times larger than the standard interstellar radiation field. We also find that chemical pumping by the reaction H2 + O may supplement the excitation of lines in the range 16–30 μm, suggesting that these lines could also be used to measure the two-body formation rates of OH. Conclusions. The mid-IR lines of OH constitute a powerful diagnostic for inferring the photodissociation rate of water and thus the UV field that water is exposed to. Future JWST-MIRI observations will be able to map the photodestruction rate of H2O in various dense (nH ≳ 106 cm−3) and irradiated environments and provide robust estimates of the local UV radiation field.

Analytic expressions of the distribution function of figure axes of symmetric and asymmetric top molecules under a polarized one‐photon excitation are de rived by the density matrix theory. To prepare maximally aligned reactants, those formulas are useful in stereodynamic experiments when the optical alignment techniques are utilized.

A method that combines basic calculations for the moments of inertia and a computer iteration technique over experimental data was employed to evaluate the constantAv in an excited vibrational state for a symmetric top molecule. This method has been used to give a good estimate for the constantAv and provides a reasonable interval of fluctuation, less than 1%, from its average value for computer fitting of experimental microwave data for vibrations up tovt=4. The angle of bending and the orientation of the molecular system with respect to an axis passing through the center of mass of the system was explored and the results are discussed in this contribution. The method is then extended to thev10=1 vibration of CH3CCH with13C isotopic species and to thev10=1,2,3,4 vibrations of CH3CCH. The results were promising so the method was applied to the13C isotopes of CH3C15N forv8=1 vibrations. The results for each of the molecules are quite good and the calculated values ofAv were found to be in very good agreement with those deduced from experimental data.

The effect of Coriolis perturbations on the perpendicular infra-red bands of symmetric top molecules is studied in the first approximation. The sum of the Coriolis coefficients for the vibrations in any symmetry class is shown to assume an extremely simple form, and its value is given for the general case. A straightforward method is described for the calculation of first-order Coriolis coefficients, and rules are derived whereby the rotational spacing of an overtone or combination band may be calculated from the Coriolis coefficients of the component vibrations.

K-dependence of broadening coefficients for CH3F–N2 and for other systems involving a symmetric top molecule

For symmetric top molecules the role of nuclear spin symmetry is included in the derivation of the nuclear magnetic relaxation rate. It is assumed that this is dominated by a spin-rotation mechanism. Of the three nuclear spin species which exist for ZX3Y' symmetric tops, it is found that the "zero frequency" K = −1 to +1 term results in two of the species relaxing with a different rate from the third. The difference, which allows a direct NMR evaluation of the relative values of [Formula: see text] and [Formula: see text], is enhanced at low temperatures.

The viscosity dependence of reorientation correlation times τϑ for a number of neat liquid monosubstituted benzenes and several symmetric top molecules has been analyzed in terms of Hu and Zwanzig’s hydrodynamic slip limit and also the parameter κ introduced by Kivelson. The slip limit predictions are improved by introducing a weighting factor which emphasizes molecular shape. The analysis of our high pressure data shows that the parameter κ is independent of pressure and temperature, in agreement with the results of other studies. The coupling between translational and rotational motions as characterized by κ is dominated by the molecular shape.

Detailed studies of vibrational relaxation in pure liquid CH3I, t-butyl chloride (CH3)3CCl, t-butyl bromide (CH3)3CBr and in solutions in carbon tetrachloride and in n-hexane, over the −90 to 50°C temperature range, have been made using Raman scattering. Assuming that only phase relaxation contributes to the isotropic band shapes, the mechanism of vibrational relaxation in these liquids was first studied by investigating the characteristics of the fluctuations of the vibrational frequencies, which are caused by the intermolecular interactions. A first study allowed us to determine the correlation time of these fluctuations. The obtained values are between 0.1 and 0.5 ps and are consistent with a binary collision model. This model has been used to develop an efficient computational model to describe the experimental data relative to vibrational relaxation and orientational motions as well.

The structure of the $\ensuremath{\lambda}4050$ group in comets appears to be incompatible with the assumption of a diatomic emitter. Rather, the structure is in conformity with that expected for a \ensuremath{\perp} band of a nearly symmetric top molecule if the moment of inertia about the top axis is approximately 0.35\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}40}$ g ${\mathrm{cm}}^{2}$. Such a small value is possible only for a slightly bent X${\mathrm{H}}_{2}$ molecule with X = C, N, or O. For C${\mathrm{H}}_{2}$ and N${\mathrm{H}}_{2}^{+}$ a \ensuremath{\perp} band is to be expected in the region 4500-4000A. Of these two possibilities C${\mathrm{H}}_{2}$ is the most likely. Since the CH radicals observed in the comets must necessarily be formed from saturated hydrocarbons by successive photodecompositions one should indeed expect to find the spectra of intermediate molecules that lie in the accessible region.

A technique which was employed earlier to calculate the rotational constants of CH3CCH has been extended to the ground and two vibrational levels in thev 8 vibration of CH3CN for several isotopic species. The moments of inertia and a computer iteration technique over experimental data for each isotopic species were employed to evaluate the constantA v in an excited vibrational state for a symmetric top molecule. This method gave good estimates forA v for each isotope. The angle of bending and the orientation of each molecular system in reference frames, one fixed on the carbon atom at the -C≡N site and the other at the center of mass, were explored. These results are discussed in this paper. The method, which was applied by Tam and Roberts to the nv10,n=1, 2, 3, 4, vibrations of CH3CCH earlier and which was extended to thev 10=1 vibration of CH3CCH with13C isotopic species, has been applied to13C isotopic species of CH3CN and seems to be a useful tool to extract the value ofA 0. Each of these molecules shows reasonable dependency ofA v over vibrational levels. Values ofA v calculated from the geometrical model are in good agreement with those obtained by fitting the terms in the frequency equations, which containedA v , to the experimental data through an iteration technique in which the value ofA was allowed to vary.

Detection of Short-Lived PH 2I by High-Resolution Infrared and Millimeter-Wave Spectroscopy: The Ground, ν 3, ν 4, and ν 6 Excited States

The Rayleigh depolarized profiles of three symmetric top molecules, cyclopropane, allene, and benzene, are analysed. It is shown that these profiles cannot be interpreted assuming only orientational fluctuations but can be well reproduced, together with the [Formula: see text] profiles, assuming that they result from a weighted sum of an orientational profile and a damped oscillator profile. The former does not exhibit orientational cooperative effects at room temperature for the three molecules. The shape and intensity of the latter present some characteristics expected for a collision-induced profile.