Theoretical and experimental studies of the infrared rovibrational spectrum of He2-N2O

Rovibrational spectra of the He(2)-N(2)O complex in the nu(1) fundamental band of N(2)O (2224 cm(-1)) have been observed using a tunable infrared laser to probe a pulsed supersonic jet expansion, and calculated using five coordinates that specify the positions of the He atoms with respect to the NNO molecule, a product basis, and a Lanczos eigensolver. Vibrational dynamics of the complex are dominated by the torsional motion of the two He atoms on a ring encircling the N(2)O molecule. The resulting torsional states could be readily identified, and they are relatively uncoupled to other He motions up to at least upsilon(t) = 7. Good agreement between experiment and theory was obtained with only one adjustable parameter, the band origin. The calculated results were crucial in assigning many weaker observed transitions because the effective rotational constants depend strongly on the torsional state. The observed spectra had effective temperatures around 0.7 K and involved transitions with J < or =3, with upsilon(t) = 0 and 1, and (with one possible exception) with Deltaupsilon(t)=0. Mixing of the torsion-rotation states is small but significant: some transitions with Deltaupsilon(t) not equal 0 were predicted to have appreciable intensity even assuming that the dipole transition moment coincides perfectly with the NNO axis. One such transition was tentatively assigned in the observed spectra, but confirmation will require further work.     

Rotational spectroscopy of the isotopic species of silicon monosulfide, SiS

Pure rotational transitions of silicon monosulfide ((28)Si(32)S) and its rare isotopic species have been observed in their ground as well as vibrationally excited states by employing Fourier transform microwave (FTMW) spectroscopy of a supersonic molecular beam at centimetre wavelengths (13-37 GHz) and by using long-path absorption spectroscopy at millimetre and submillimetre wavelengths (127-925 GHz). The latter measurements include 91 transition frequencies for (28)Si(32)S, (28)Si(33)S, (28)Si(34)S, (29)Si(32)S and (30)Si(32)S in upsilon = 0, as well as 5 lines for (28)Si(32)S in upsilon = 1, with rotational quantum numbers J'< or = 52. The centimetre-wave measurements include more than 300 newly recorded lines. Together with previous data they result in almost 600 transitions (J' = 0 and 1) from all twelve possible isotopic species, including (29)Si(36)S and (30)Si(36)S, which have fractional abundances of about 7 x 10(-6) and 4.5 x 10(-6), respectively. Rotational transitions were observed from upsilon = 0 for the least abundant isotopic species to as high as upsilon = 51 for the main species. Owing to the high spectral resolution of the FTMW spectrometer, hyperfine structure from the nuclear electric quadrupole moment of (33)S was resolved for species containing this isotope, as was much smaller nuclear spin-rotation splitting for isotopic species involving (29)Si. By combining the measurements here with previously published microwave and infrared data in one global fit, an improved set of spectroscopic parameters for SiS has been derived which include several terms describing the breakdown of the Born-Oppenheimer approximation. With this parameter set, highly accurate rotational frequencies for this important astronomical molecule can now be predicted well into the terahertz region.     

Collisionally assisted spectroscopy of water from 27,000 to 34,000 cm(-1)

We report here an experimental approach that enables measurement of weak transitions to a wide range of rovibrational levels of water in the energy region 27,000-34,200 cm(-1). We have previously demonstrated the use of laser double-resonance overtone excitation to access highly excited vibrational levels from single rovibrational states. Although this approach simplifies the assignment of the spectra, it strongly reduces the number of observed transitions and hence our ability to test theoretical predictions. Here, we increase significantly the number of observed transitions by allowing rotational relaxation of H2O at intermediate levels of the double-resonance excitation scheme to the levels of the same nuclear spin (ortho or para). Our recently developed semiempirical potential energy surface PES12 enables assignment of the resulting complex spectra and reproduction of the measured transitions with accuracy better than 1 cm(-1).     

High-resolution FTIR, microwave, and ab initio investigations of CH2 79BrF: ground, v(5) = 1, and v(6) = 1, 2 state constants

A spectroscopic study of CH279BrF in the infrared and microwave regions has been carried out. The rovibrational spectrum of the nu5 fundamental interacting with 2nu6 has been investigated by high-resolution FTIR spectroscopy. Owing to the weakness of the 2nu6 band, the v6 = 2 state constants have been derived from v6 = 1. For this reason, the rotational spectra of the ground and v6 = 1 states have been observed by means of microwave spectroscopy. Highly accurate ab initio computations have also been performed at the CCSD(T) level of theory in order to support the experimental investigation. As far as the nu5 band is concerned, the analysis of the rovibrational structure led to the identification of more than 3000 transitions, allowing the determination of a set of spectroscopic parameters up to sextic distortion terms and pointing out first-order c-type Coriolis interaction with the v6 = 2 state. With regard to the pure rotational spectra measurements, the assignment of several DeltaJ = 0, +1 transitions allowed the determination of the rotational, all the quartic, and most of the sextic centrifugal distortion constants, as well as the full bromine quadrupole coupling tensor for both the ground and v6 = 1 states.     

Infrared spectra of seeded hydrogen clusters: (para-H2)N-N2O and (ortho-H2)N-N2O, N = 2-13

High-resolution infrared spectra of clusters containing para-H2 and/or ortho-H2 and a single nitrous oxide molecule are studied in the 2225-cm(-1) region of the upsilon1 fundamental band of N2O. The clusters are formed in pulsed supersonic jet expansions from a cooled nozzle and probed using a tunable infrared diode laser spectrometer. The simple symmetric rotor-type spectra generally show no resolved K structure, with prominent Q-branch features for ortho-H2 but not para-H2 clusters. The observed vibrational shifts and rotational constants are reported. There is no obvious indication of superfluid effects for para-H2 clusters up to N=13. Sharp transitions due to even larger clusters are observed, but no definite assignments are possible. Mixed (para-H2)N-(ortho-H2)M-N2O cluster line positions can be well predicted by linear interpolation between the corresponding transitions of the pure clusters.     

The role of axis embedding on rigid rotor decomposition analysis of variational rovibrational wave functions

Approximate rotational characterization of variational rovibrational wave functions via the rigid rotor decomposition (RRD) protocol is developed for Hamiltonians based on arbitrary sets of internal coordinates and axis embeddings. An efficient and general procedure is given that allows employing the Eckart embedding with arbitrary polyatomic Hamiltonians through a fully numerical approach. RRD tables formed by projecting rotational-vibrational wave functions into products of rigid-rotor basis functions and previously determined vibrational eigenstates yield rigid-rotor labels for rovibrational eigenstates by selecting the largest overlap. Embedding-dependent RRD analyses are performed, up to high energies and rotational excitations, for the H(2) (16)O isotopologue of the water molecule. Irrespective of the embedding chosen, the RRD procedure proves effective in providing unambiguous rotational assignments at low energies and J values. Rotational labeling of rovibrational states of H(2) (16)O proves to be increasingly difficult beyond about 10,000 cm(-1), close to the barrier to linearity of the water molecule. For medium energies and excitations the Eckart embedding yields the largest RRD coefficients, thus providing the largest number of unambiguous rotational labels.     

Planar study of H2O+Cl<-->HO+HCl reactions

The first four dimensional (4D) quantum scattering calculations on the tetra-atomic H2O+Cl<-->HO+HCl reactions are reported. With respect to a full (6D) treatment, only the planar constraint and a fixed length for the HO spectator bond are imposed. This work explicitly accounts for the bending and local HO stretching vibrations in H2O, for the vibration of HCl and for the in-plane rotation of the H2O, HO and HCl molecules. The calculations are performed with the potential energy surface of Clary et al. and use a Born-Oppenheimer type separation between the motions of the light and the heavy nuclei. State-to-state cross sections are reported for a collision energy range 0-1.8 eV measured with respect to H2O+Cl. For the H2O+Cl reaction, present results agree with previous (3D) non planar calculations and confirm that excitation of the H2O stretching promotes more reactivity than excitation of the bending. New results are related to the rotation of the H2O molecule: the cross sections are maximal for planar rotational states corresponding to 10相似文献   

Non-adiabatic transitions from I2(E0g+ and D0u+) states induced by collisions with M = I2(X0g+) and H2O

The stepwise two-step two-color and three-step three-color laser excitation schemes are used for selective population of rovibronic levels of the first-tier ion-pair E0(g)(+) and D0(u)(+) states of molecular iodine and studies of non-adiabatic transitions to the D and E states induced by collisions with M = I(2)(X) and H(2)O. Collection and analysis of the luminescence after excitation of the v(E) = 8, 13 and v(D) = 13, 18 vibronic levels of the E and D states in the pure iodine vapor and the gas-phase mixtures with H(2)O provide rate constants for the non-adiabatic transitions to the D and E state induced by collisions with these molecules. Vibrational distributions for the [formula: see text] collision-induced non-adiabatic transitions (CINATs) are obtained. Rather strong λ(lum)(max) ≈ 3400 ? luminescence band is observed in the I(2) + H(2)O mixtures, whereas its intensity is ~100 times less in pure iodine vapor. Radiative lifetimes and quenching rate constants of the I(2)(E,v(E) = 8, 13 and D,v(D) = 13, 18) vibronic state are also determined. Rate constants of the [formula: see text], v(E) = 8-54, CINATs are measured again and compared with those obtained earlier. New data confirm resonance characters of the CINATs found in our laboratory about 10 years ago. Possible reasons of differences between rate constant values obtained in this and earlier works are discussed. It is shown, in particular, that differences in rate constants of non-resonant CINATs are due to admixture of water vapor in iodine.     

Calculation of rovibrational energy states for water vapor using the symmetric top approximation in the Pade form

Considering the water vapor molecule for ka > or = J/2 > 1 as a symmetric top, the simple analytical Pade form for vibrational rotational energy levels was proposed. The rotational spectroscopic parameters for ground and 010 vibrational states of H2O molecule in the framework of this model have been obtained with good prediction. The absorption coefficient of water vapor in the spectral range 725-925 cm(-1) has been calculated for 1000-6000 K temperatures taking into account the transitions to high-excited states up to J < or = 35 and J < or = 30 for the 000 and 010 vibrational states, respectively. It is shown that the prominent role of transitions to high-excited states is in 8-12 microm atmospheric transparency window.     

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.     

Variational quantum mechanical and active database approaches to the rotational-vibrational spectroscopy of ketene, H2CCO

A variational quantum mechanical protocol is presented for the computation of rovibrational energy levels of semirigid molecules using discrete variable representation of the Eckart-Watson Hamiltonian, a complete, "exact" inclusion of the potential energy surface, and selection of a vibrational subspace. Molecular symmetry is exploited via a symmetry-adapted Lanczos algorithm. Besides symmetry labels, zeroth-order rigid-rotor and harmonic-oscillator quantum numbers are employed to characterize the computed rovibrational states. Using the computational molecular spectroscopy algorithm presented, a large number of rovibrational states, up to J = 50, of the ground electronic state of the parent isotopologue of ketene, H(2) (12)C=(12)C=(16)O, were computed and characterized. Based on 12 references, altogether 3982 measured and assigned rovibrational transitions of H(2) (12)C=(12)C=(16)O have been collected, from which 3194 were validated. These transitions form two spectroscopic networks (SN). The ortho and the para SNs contain 2489 and 705 validated transitions and 1251 and 471 validated energy levels, respectively. The computed energy levels are compared with energy levels obtained, up to J = 41, via an inversion protocol based on this collection of validated measured rovibrational transitions. The accurate inverted energy levels allow new assignments to be proposed. Some regularities and irregularities in the rovibrational spectrum of ketene are elucidated.     

A study of the radical-radical reaction dynamics of O(3P)+t-C4H9 --> OH+iso-C4H8

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with t-butyl radicals (t-C4H9) in the gas phase were investigated using high-resolution laser spectroscopy in a crossed-beam configuration, together with ab initio theoretical calculations. The radical reactants, O(3P) and t-C4H9, were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of the precursor, azo-t-butane, respectively. A new exothermic channel, O(3P)+t-C4H9 --> OH+iso-C4H8, was identified and the nascent rovibrational distributions of the OH (X 2Pi: upsilon" = 0,1,2) products were examined. The population analyses for the two spin-orbit states of F1(2Pi32) and F2(2Pi12) showed that the upsilon" = 0 level is described by a bimodal feature composed of low- and high-N" rotational components, whereas the upsilon" = 1 and 2 levels exhibit unimodal distributions. No noticeable spin-orbit or Lambda-doublet propensities were observed in any vibrational state. The partitioning ratio of the vibrational populations (Pupsilon") with respect to the low-N" components of the upsilon" = 0 level was estimated to be P0:P1:P2 = 1:1.17+/-0.24:1.40+/-0.11, indicating that the nascent internal distributions are highly excited. On the basis of the comparison of the experimental results with the statistical theory, the reaction mechanism at the molecular level can be described in terms of two competing dynamic pathways: the major, direct abstraction process leading to the inversion of the vibrational populations, and the minor, short-lived addition-complex process responsible for the hot rotational distributions. After considering the reaction exothermicity, the barrier height, and the number of intermediates along the addition reaction pathways on the lowest doublet potential energy surface, the formation of CH3COCH3(acetone)+CH3 was predicted to be dominant in the addition mechanism.     

Close-coupling study of rotational energy transfer of CO (upsilon=2) by collisions with He atoms

Quantum close-coupling scattering calculations of rotational energy transfer in the vibrationally excited CO due to collisions with He atom are presented for collision energies between 10(-5) and approximately 1000 cm-1 with CO being initially in the vibrational level upsilon=2 and rotational levels j=0,1,4, and 6. The He-CO interaction potential of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)] was adopted for the calculations. Cross sections for rovibrational transitions and state-to-state rotational energy transfer from selected initial rotational levels were computed and compared with recent measurements of Carty et al. [J. Chem. Phys. 121, 4671 (2004)] and available theoretical results. Comparison in all cases is found to be excellent, providing a stringent test for the scattering calculations as well as the reliability of the He-CO interaction potential by Heijmen et al.     

Spectroscopic characterization of the C2-Ne van der Waals complex

Binary complexes of C2 with rare-gas atoms (C2-Rg) have attracted theoretical interest as their potential-energy surfaces are predicted to support linear equilibrium geometries, without the local minimum for the T-shaped geometry that would be expected using a standard pair-potential model. In the present work we have explored the properties of C2-Ne using laser-induced fluorescence detection of the D 1Sigmau +-X 1Sigmag + transition. Bands of the complex were observed in association with the monomer 0-0 and 1-1 transitions. Rotationally resolved data yielded rotational constants of B'=0.099(3) cm(-1) and B"=0.100(3) cm(-1) for the excited and ground states, respectively. Analysis of the rovibrational energy-level structure for C2(D)-Ne indicates that the complex has a linear equilibrium structure with a barrier to internal rotation of approximately 15 cm(-1). Data for the ground state validate a recent high-level ab initio calculation of the potential-energy surface for C2(X)-Ne.     

The bending vibrational levels of the acetylene cation: a case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Forty three vibronic levels of C2H2+, X 2Pi u, with upsilon4 = 0-6, upsilon5 = 0-3, and K = 0-4, lying at energies of 0-3520 cm(-1) above the zero-point level, have been recorded at rotational resolution. These levels were observed by double resonance, using 1+1' two-color pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. The intermediate states were single rovibrational levels chosen from the A1Au, 4nu3 (K = 1-2), 5nu3 (K = 1), nu2+4nu3 (K = 0), and 47,206 cm(-1) (K = 1) levels of C2H2. Seven of the trans-bending levels of C2H2+ (upsilon4 = 0-3, K = 0-2) had been reported previously by Pratt et al. [J. Chem. Phys. 99, 6233 (1993)]; our results for these levels agree well with theirs. A full analysis has been carried out, including the Renner-Teller effect and the vibrational anharmonicity for both the trans- and cis-bending vibrations. The rotational structure of the lowest 16 vibronic levels (consisting of the complete set of levels with upsilon4 + upsilon5 < or = 2, except for the unobserved upper (2Pi u component of the 2nu4 overtone) could be fitted by least squares using 16 parameters to give an rms deviation of 0.21 cm(-1). The vibronic coupling parameter epsilon5 (about whose magnitude there has been controversy) was determined to be -0.0273(7). For the higher vibronic levels, an additional parameter, r45, was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the upsilon4 + upsilon5 = 3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with upsilon4 + upsilon5 < or = 5, an rms deviation of 0.34 cm(-1) was obtained using 20 parameters. An interesting finding is that Hund's spin-coupling cases (a) and (b) both occur in the Sigmau components of the nu4 + 2nu5 combination level. The ionization potential of C2H2 (from the lowest rotational level of the ground state to the lowest rotational level of the cation) is found to be 91,953.77 +/- 0.09 cm(-1) (3sigma).     

The role of rotational excitation in the activated dissociative chemisorption of vibrationally excited methane on Ni(100)

We have measured the sticking probability of methane excited to v = 1 of the v3 antisymmetric C-H stretching vibration on a clean Ni(100) surface as a function of rotational state (J = 0, 1, 2 and 3) and have investigated the effect of Coriolis-mixing on reactivity. The data span a wide range of kinetic energies (9-49 kJ mol-1) and indicate that rotational excitation does not alter reactivity by more than a factor of two, even at low molecular speeds that allow for considerable rotation of the molecule during the interaction with the surface. In addition, rotation-induced Coriolis-splitting of the v3 mode into F+, F0 and F- states does not significantly affect the reactivity for J = 1 at 49 kJ mol-1 translational energy, even though the nuclear motions of these states differ. The lack of a pronounced rotational energy effect in methane dissociation on Ni(100) suggests that our previous results for (v = 1, v3, J = 2) are representative of all rovibrational sublevels of this vibrational mode. These experiments shed light on the relative importance of rotational hindering and dynamical steering mechanisms in the dissociative chemisorption on Ni(100) and guide future attempts to accurately model methane dissociation on nickel surfaces.     

A five-dimensional potential energy surface and predicted infrared spectra for the N2O-hydrogen complexes

We present a five-dimensional potential energy surface for the N(2)O-hydrogen complex using supermolecular approach with the full counterpoise correction at the coupled-cluster singles and doubles with noniterative inclusion of connected triple level. The normal mode Q(3) for the nu(3) antisymmetric stretching vibration of the N(2)O molecule was included in the calculations of the potential energies. The radial discrete variable representation/angular finite basis representation method and Lanczos algorithm were employed to calculate the rovibrational energy levels for four species of N(2)O-hydrogen complexes (N(2)O-para-H(2), -ortho-H(2), -ortho-D(2), and -para-D(2)) without separating the inter- and intramolecular vibrations. The calculated band origins are all blueshifted relative to the isolated N(2)O molecule and in good agreement with the experimental values. The calculated rotational spectroscopic constants and molecular structures agree well with the available experimental results. The frequencies and line intensities of the rovibrational transitions in the nu(3) region of N(2)O for the van der Waals ground vibrational state were calculated and compared with the observed spectra. The predicted infrared spectra are consistent with the observed spectra and show that the N(2)O-H(2) complexes are mostly a-type transitions while both a-type and b-type transitions are significant for the N(2)O-D(2) complexes.     

High resolution spectral analysis of 13CH3OH in the excited torsional states

In this work, we have extended our previous analysis of the Hamiltonian of 13C substituted methanol to include a large number of spectral lines involving the second excited torsional state using an improved model. The data set consisted of 2529 Fourier transform and microwave transitions with the rotational angular momentum J < or = 10, K < or = 6 and n < or = 2 (with 336 MW lines). The data set was fitted with the new Hamiltonian model to derive the molecular parameters. The results indicate that the model developed for the other methanol species (CH3OH, CH3(18)OH and CH3OD) is also valid for the C-13 substituted species. The results will allow the energy levels of the molecule to be calculated for higher torsional levels above the internal rotational barrier with improved precision and allow the analysis to be carried out for more excited torsional states.