Low Rotational Quantum Numbers Research Articles

Rate constants and molecular recombination pathways of oxygen from quasi-classical trajectory simulations of theO3system

• Investigation of recombination mechanisms using quasi-classical trajectory method. • Direct collisions are more effective for recombination. • Indirect recombinations proceed through orbiting pairs. • Recombination products result in strong non-Boltzmann internal energy distributions. • Recombination products exhibit large vibrational and low rotational quantum numbers. A lack of understanding of the dynamics of molecular recombination has led to the development of a large number of recombination models and methods to quantify recombination rate constants, each with their own set of assumptions and ranges of applicability. This work aims to unravel the complex dynamics behind the recombination of atomic oxygen to molecular oxygen using the quasi-classical trajectory method for the O 3 system. Recombination pathways considered include a single-step “direct” reaction, and a two-step mechanism that involves the formation of a long-lived intermediate which is stabilized through collision with a third body. The latter is further classified as Lindemann or Chaperon recombination pathways depending on the identity of the intermediate and the final product. The classical versions of all three mechanisms are explored in detail for the O 3 system. A time-lag parameter is introduced to quantify the time interval between the intermediate formation and third-body stabilization steps, and the direct collisions are treated as a limiting case when the time lag is zero. It is found that the recombination probabilities reduce as the time-lag is increased. Direct recombination events are dominated by head-on collisions, while the two step recombination events are characterized by a combination of glancing and head-on collisions. The resulting recombination products follow a strongly non-Boltzmann distribution. Recombination rate coefficients are computed through a novel rate coefficient formula that takes into consideration the Poisson distribution in time of third body stabilizing collisions with the orbiting pair characterized by finite lifetimes (on the order of picoseconds) that lead to recombination. Good agreement is obtained with recombination rates computed from detailed balance based on known dissociation rate coefficients. This approach thus enables the direct calculation of recombination rate coefficients at low temperatures, without the need for dissociation rate coefficients and detailed balance.

Microwave-microwave double-resonance spectroscopy of acetone in the torsional excited state ν17

Since rotational transitions between energy levels with low rotational quantum numbers J have not been identified in previous investigations of the rotational spectrum in the ν17 state of acetone, such transitions have now been searched for and assigned by microwave-microwave double-resonance spectroscopy. Eventually, 15, 23, 4 and 0 transitions have been found in the (00) = AA, (01) = EE, (11) = AE and (12) = EA torsional substates, respectively. Even though the dimensionless standard deviation of the least-squares fit of the parameters of the Effective Rotational Hamiltonian (ERHAM) to their frequencies combined with those used in earlier work increased by ≈ 23%, the standard errors of most spectroscopic parameters decreased by more than 20%, but increased for others, probably due the changes in fitting blended transitions. A comparison of the results with those of other recent investigations of acetone confirms that the ERHAM code is unable to fit the ν17 state of acetone without accounting for the interactions with other vibrational states.

Radiative decay of HeLi+(b3[formula omitted

Radiative lifetimes of the ro-vibrational bound states of HeLi+(b3∑+) are calculated quantum-mechanically when both bound-bound and bound-free processes are taken into account. The calculations are restricted to the initial states with low rotational quantum numbers. For the rotationless molecular ion, the shortest lifetime is 1.30×10-6s of the ground vibrational state. With increasing vibration excitations the radiative lifetimes slightly increase, but keep below 10-5s for the next 17 vibrational states, then they start to increase more rapidly up to 2.41×10-2s for the highest vibrational state. The radiative lifetimes also tend to slightly prolong with increasing rotational excitations but stay on the same magnitude for the low rotational quantum numbers.

Effective potential energy surface of HD16O for calculation of highly excited states of nν3 and ν1 + nν3 types

A new spectroscopically determined potential energy surface (PES) for HD16O is presented, and rotational-vibrational transitions are calculated using it for low rotational quantum numbers J ≤ 4. This surface is constructed by adjusting a high-accuracy PES by fitting to experimental energy levels of nν3 and ν1 + nν3 types. Seven hundred and forty rotational levels with energies up to 25600 cm−1 and J ≤ 8 were used for the refinement. To improve the extrapolation properties of the empirical PES, the fitting was applied to experimental and ab initio energy levels.

Spectroscopy of jet-cooled methane in the lower icosad region: Empirical assignments of low-J″ spectral lines from two-temperature analysis

Complete 12CH4 spectrum in the lower icosad spectral range (7050–7350cm−1) has been measured in supersonic jet at 28K for the first time using direct absorption tunable diode laser spectrometer. Due to efficient rotational cooling only spectral lines with lower rotational quantum number J<4 are observed in the spectrum. Spectral lines in the supersonic jet spectra have been matched with those in liquid nitrogen cooled cell spectra (81K) and two-temperature method for empirical lower state rotational quantum number assignments has been applied to obtain accurate experimental values of the lower state rotational quantum numbers J″. Especially positions of lines with of J″=0 are relevant for vibrational band origin determination, as they are unambiguously the R(0) transitions of the respective bands.

Direct MP2 molecular dynamics studies of H atom reaction with CD4 and CH4

The mechanism and dynamics of the H + CD(4) -> CD(3) + HD (I) and H + CH(4) -> CH(3) + H(2) (II) reactions have been investigated by electronic structure methods. The minimum-energy path and vibrational frequencies along the intrinsic reaction coordinate are calculated at MP2/cc-pVDZ level. Energy distributions of the products are also obtained by the direct classical trajectory calculations at the MP2/cc-pVDZ level. It is found that most of the available energy appears as product translational energy, and very little of the available energy is partitioned into internal excitation of the HD (H(2)) product for reaction I (II), which is in agreement with the experimental evidence. The results indicate that the experimental results could be reproduced by the direct MP2 molecular dynamics calculations. The rotational state distributions of the products show the HD (H(2)) products are formed with lower rotational quantum numbers than the CD(3) (CH(3)) products. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 4433-4442, 2011

Accurate determination of low state rotational quantum numbers (J &lt; 4) from planar-jet and liquid nitrogen cell absorption spectra of methane near 1.4 micron

An improved procedure for accurate determination of empirical lower state rotational quantum numbers from molecular absorption spectra is demonstrated for methane. We combine the high resolution absorption spectra in the 7070-7300 cm(-1) frequency range obtained in liquid nitrogen cooled cryogenic cell (T = 81 K) and in supersonic planar jet expansion (T(R) = 25 K). Empirical lower state energies of 59 transitions are determined from the ratio of the absolute absorption line strengths at 25 and 81 K. The procedure relies on the realistic description of rotational state populations in the supersonic jet expansion where non-equilibrium nuclear spin isomer distributions are generated due to the rapid cooling. The accuracy of the experimental determination of the lower state energies with J < or = 3 is found to considerably improve the results of the same approach applied to spectra at 296 and 81 K. The 59 transitions with determined lower J values provide a good starting point for the theoretical interpretation of the highly congested icosad region of methane. In particular, the centres of nine vibrational bands are estimated from the transitions with J = 0 lower state rotational quantum number.

Shifting temperature dependence of nitrogen-broadened lines in the ν 2 band of H2O

We report the nitrogen-induced shifting coefficients of 37 water vapour rovibrational transitions in the 1845–2140 cm−1 spectral region. The measurements were made with a Fourier transform spectrometer at Paris University in the 258–330 K temperature range at a spectral resolution of 0.005 cm−1 for lines with low state rotational quantum number up to 14. The measured exponents of the temperature dependence of the width exhibit a large range of values from 0 to 2.5. Theoretical calculations are based on impact theory modified by introducing additional parameters to extend the use of empirical data. The model parameters are determined by fitting the broadening and shifting coefficients to experimental data. The calculated line shift coefficients as well as the temperature exponents for the line shift agree satisfactorily with measured values.

Manipulation of ro-vibronic wave packet composition using chirped ultrafast laser pulses

Linearly chirped femtosecond laser pulses are used to excite ro-vibrational wave packets in lithium dimers from a single well-prepared launch state. Two-photon resonant Raman transitions excite a manifold of rotational states in a coherent superposition. Excitation with positively chirped laser pulses is shown to enhance a quantum beat involving states with lower rotational quantum numbers, whereas negatively chirped pulses enhance a quantum beat between states with higher rotational quantum numbers, thus steering the rotational coherences to higher or lower rotational states. Time-dependent perturbation theory calculations verify the magnitudes and directions of the enhancement as a function of the applied chirp. The quantitative calculations and experimental results demonstrate that excitation with chirped pulses is an effective method for enhancing specific rotational coherences involving higher or lower rotational states.

Cross sections and low temperature rate coefficients for the H + CH+reaction: a quasiclassical trajectory study

The H + CH(+) reaction is studied by quasiclassical trajectory (QCT) calculations, along with phase space theory (PST) and quantum rigid rotor calculations, employing a global single-valued potential energy surface recently derived by our group. We report QCT total cross sections for each of the three channels, for low collision energies and different reactant rotational quantum numbers. At the lowest collision energies, all cross sections exhibit a capture-like behaviour, as expected from a barrierless reaction. At higher energies, there are important dynamical effects coming from the opening of new channels in the inelastic and reactive exchange collisions. The inelastic cross sections turn out to largely increase, while the reactive abstraction cross sections are declining faster than predicted by the capture theory. A large value of the reactant rotational quantum number tends to suppress these dynamical effects. The QCT rate coefficients are reported for a temperature range from 1-700 K. Below 20 K, the abstraction and exchange QCT rate coefficients are almost constant, as predicted by the capture theory. Above this temperature, the abstraction rate coefficient declines, while the exchange and inelastic rate coefficients are increasing, due to the opening of new channels. A good agreement is observed between the experimental abstraction rate coefficient and the QCT and PST ones. The QCT inelastic results are also compared with those obtained from rigid rotor close coupling (CCRR) calculations in order to check the ability of this approach to provide a reliable estimate of the inelastic rate coefficients for a reactive system without a barrier. The laws of variation as a function of temperature are found to be very similar and the curves are parallel above 20 K. However, reaction is not allowed in the rigid rotor approximation, therefore the CCRR results are about twice as large as their QCT counterparts.

The Millimeter‐ and Submillimeter‐Wave Spectrum of Iso ‐Propanol [(CH 3 ) 2 CHOH

Iso-propanol [(CH3)2CHOH], an isomer of n-propanol, has been studied in the millimeter- and submillimeter-wave region of the electromagnetic spectrum with our FASSST spectrometer through 360 GHz. Spectra arising from the ground vibrational state of all three hydroxyl torsional substates, given the labels symmetric gauche, antisymmetric gauche, and trans in order of increasing energy, have been observed. We have successfully assigned ~7600 pure rotational transitions within the torsional substates as well as ~4700 torsional-rotational transitions between the symmetric and antisymmetric gauche substates through the lower rotational quantum number J'' = 68. Spectral lines involving one or both of the two gauche forms have been simultaneously analyzed with a 2 × 2 effective torsional-rotational Hamiltonian, which includes terms through fifth order in the torsional-rotational interaction. Excluding perturbed transitions, the assigned transitions were fitted to a root mean square deviation of 76 kHz. The trans substate was analyzed as a semirigid rotor, and its unperturbed transitions fitted to a root mean square deviation of 63 kHz. A perturbation was seen at transitions with J'' > 50 in the trans substate. The torsional excitation energy for the trans substate above ground was estimated from intensity ratios to be about 120 K.

Seven-dimensional microcanonical treatment of hydrogen dissociation dynamics on Cu(111): Clarifying the essential role of surface phonons

A simple picture of the hydrogen dissociation/associative desorption dynamics on Cu(111) emerges from a two-parameter, full dimensionality microcanonical unimolecular rate theory (MURT) model of the gas-surface reactivity. Vibrational frequencies for the reactive transition state were taken from density functional theory calculations of a six-dimensional potential energy surface [Hammer et al., Phys. Rev. Lett. 73, 1400 (1994)]. The two remaining parameters required by the MURT were fixed by simulation of experiments. These parameters are the dissociation threshold energy, E(0)=79 kJmol, and the number of surface oscillators involved in the localized H(2)Cu(111) collision complex, s=1. The two-parameter MURT quantitatively predicts much of the varied behavior observed for the H(2) and D(2)Cu(111) reactive systems, including the temperature-dependent associative desorption angular distributions, mean translational energies of the associatively desorbing hydrogen as a function of rovibrational eigenstate, etc. The divergence of the statistical theory's predictions from experimental results at low rotational quantum numbers, J < or approximately 5, suggests that either (i) rotational steering is important to the dissociation dynamics at low J, an effect that washes out at high J, or (ii) molecular rotation is approximately a spectator degree of freedom to the dissociation dynamics for these low J states, the states that dominate the thermal reactivity. Surface vibrations are predicted to provide approximately 30% of the energy required to surmount the activation barrier to H(2) dissociation under thermal equilibrium conditions. The MURT with s=1 is used to analytically confirm the experimental finding that partial differential "E(a)(T(s))" partial differential E(t)= -1 for eigenstate-resolved dissociative sticking at translational energies E(t)<E(0)-E(v)-E(r). Explicit treatment of the surface motion (i.e., surface not frozen at T(s)=0 K) is a relatively novel aspect of the MURT theoretical approach.

Are HBO− and BOH− electronically stable?

The binding of an excess electron to HBO and BOH was studied at the coupled cluster level of theory with single, double and non-iterative triple excitations and with extended basis sets to accommodate the loosely bound excess electron. The bent BOH molecule, with a dipole moment of 2.803 D, binds an electron by 39cm−1, whereas the linear HBO tautomer possesses a similar dipole moment (2.796 D) yet binds the electron by less than 1 cm−1. It is therefore likely that HBO− is not stable when rotational energies are included whereas BOH− is for low rotational quantum numbers.

A close-coupling study of collision-induced dissociation: three-dimensional calculations for the He+H 2→He+H+H reaction

Three-dimensional quantum mechanical close-coupling calculations using discretized continuum states have been carried out for the He+H 2→He+H+H collision-induced dissociation reaction in order to study the contribution of sequential two-body and direct three-body collisions just above the dissociation threshold. Although the calculations were done only for zero total angular momentum, we have found that the nonresonant, direct three-body mechanism is generally dominant for collisions of H 2 having a low rotational quantum number and that the two-body mechanism via quasibound resonance states is important only for H 2 having a higher rotational quantum number. We have also found that the magnitude of the total collision-induced dissociation probabilities is exclusively determined by the energy difference between the rovibrational energy level of H 2 and the dissociation threshold. This indicates that collision-induced dissociation can be described with a simple collisional energy transfer process. The semiclassical calculations, in which the relative radial motion is treated in classical mechanics while the other motions are treated quantum mechanically, have also been performed for comparison. We found that the semiclassical method does not work well for dissociation processes with small probabilities, as expected.

Rotational alignment in the photodesorption of CO from Cr2O3(0001): A systematic three-dimensional ab initio study

In recent experiments, the rotational alignment of the laser induced nonthermal desorption of CO adsorbed on an epitaxially grown film of Cr2O3(0001) has been studied [Beauport, Al-Shamery, and Freund, Chem. Phys. Lett. 256, 641 (1996)]. At low-rotational quantum numbers J, the molecules desorb like helicopters (J-vector perpendicular to the surface) while at high J-values cartwheel motion is preferred (J-vector parallel to the surface). These stereodynamic effects and the experimental state resolved velocity distributions of the desorbing species are simulated by means of an exact time-dependent wave packet method in three dimensions. As a basis for this quantum-mechanical treatment of the CO desorption ab initio potential energy surfaces (PES) were used. The PES for the electronic ground state of the CO–Cr2O3(0001) system has been calculated previously by Pykavy et al. [Surf. Sci. 479, 11 (2001)] in an embedded cluster approach. As the intermediate state, generated by the laser irradiation, an internal CO excited state (5σ→2π*) was considered. The PES of this a 3Π-like state of CO adsorbed on Cr2O3(0001) was calculated at the ab initio CASSCF-level. Our key findings in the subsequent wave packet calculations are (1) a high-dimensional treatment of the photodesorption process is very important in this system, essentially the angular coordinates, very often neglected in similar studies, are responsible for a “successful” desorption event; (2) the change from the strongly tilted equilibrium geometry in the electronic ground state to the preferred upright position in the electronically excited state after laser irradiation is essential for the mechanistic picture of the desorption process; (3) the experimental phenomemon of rotational alignment can only be explained if the topologies of the PES of both the electronically excited and the ground state are accounted for; (4) the lifetime of the CO*-intermediate is in the order of 10 fs; (5) the molecule–surface vibrations in the electronic ground state do not much influence the asymptotic results at the experimental temperature of T=100 K. However, the inclusion of excited levels of the hindered rotation helps to gain insight into the desorption mechanism on a microscopic level.

Pressure Broadening and Temperature Dependence of Microwave and Far Infrared Rotational Lines in OH Perturbed by N2, O2, and Ar

Absorption linewidths associated with microwave transitions across Λ-doublets and pure rotational transitions of the free hydroxyl radical OH perturbed by N2, O2, and Ar are computed for temperatures ranging from 194 to 300 K within the framework of the Robert–Bonamy semiclassical formalism using exact trajectories. The calculation is done for the Hund's coupling case (a) which is expected to be a good approximation for the low rotational quantum numbers so far experimentally studied. As intermolecular potential, the usual sum of electrostatic and atom–atom interactions is taken. Unlike previous theoretical predictions, our computed values are found to be in good agreement with measurements.

The Millimeter‐ and Submillimeter‐Wave Spectrum of Methyl Mercaptan (CH3SH)

Methyl mercaptan (CH3SH), a threefold internal rotor and the sulfur analog of methanol, has been detected in the interstellar medium, although little is known about its chemistry or about the physical conditions most favorable for its synthesis. Until the present, the assigned and fitted rotational-torsional spectral lines of methyl mercaptan have been mainly confined to transitions involving low rotational quantum numbers. We have measured many new lines of the rotational spectrum of this species through the rotational quantum number J=20 using two spectrometers: our standard klystron-based system and a new fast scan submillimeter spectroscopic technique (FASSST). The newly measured lines have been combined with previous data at mainly lower frequencies to form a data set consisting of approximately 1000 lines that has been fitted to nearly the accuracy of our experiments using the internal axis method we have previously applied to methanol. The spectral constants obtained from the fit have allowed us to predict the frequencies of many additional lines up to 600 GHz in frequency and up to J=25 in rotational quantum number.

Rotational alignment in the UV-laser induced desorption of CO from Cr 2O 3(0001)

The rotational alignment of the laser-induced non-thermal desorption of CO adsorbed on an epitaxially grown film of Cr 2O 3(0001) has been studied using 193 nm (6.4 eV) laser light and a (1 + 1′)-REMPI technique. At low rotational quantum numbers J the molecules desorb like a ‘helicopter’ ( J- vector perpendicular to the surface) while at high J-values a ‘cartwheel’ like motion is preferred ( J- vector parallel to the surface). CO is adsorbed in a flat geometry as a single species. The resulting angular momentum alignment in the laser desorption is interpreted qualitatively as a sum of effects due to corrugation of the surface in the initial state and a change of the adsorption geometry in the intermediate state before desorption.

近赤外領域におけるメタン，二酸化炭素の吸収線強度の測定

High resolution measurements of the CH4 and CO2 absorption bands in the near infrared region were carried out with a Fourier Transform Spectrometer at room temperature. Line strengths were determined from the measured equivalent widths with other line parameters fixed at the values given in the HITRAN database. It has been found in the 2ν3 band of CH4, the line strengths obtained in this study are very close to those in HITRAN92 but of large difference with those in HITRAN86. The measured line strengths of the (30012←00001) and (30013←00001) bands of CO2 agree well with the HITRAN92's values for the lines of the low rotational quantum number J, but the difference between the measured and HITRAN92's value gradually expands in the band wing regions. The line strengths of the HITRAN92 are 20-25% larger than the measured values for the (30011←00001) and (30014←00001) bands.

A more accurate potential energy surface and quantum mechanical cross section calculations for the F+ H 2 reaction

A new potential energy surface (called 6SEC) obtained by iterative refinement of the previously published 5SEC surface is presented. The new surface was obtained using accurate three-dimensional quantum mechanical scattering calculations to test the effects of various modifications of the 5SEC surface. We also calculated well converged quantum mechanical vibrational branching ratios and differential cross sections for the 6SEC surface for four sets of initial conditions; these results show good agreement with experiment for low initial rotational quantum number; increasing the rotational quantum number diminishes the forward scattering, but not as much as has been inferred from experiment.