Diode-laser measurements of O2, N2 and air-pressure broadening and shifting of NH3 in the 10 μm spectral region

Using a tunable diode-laser spectrometer, we have measured the O₂, N₂, air-shift and broadening coefficients for 5 lines of ammonia in the R branch of the ν₂ band. These lines are located in the spectral range 1030-1070 cm⁻¹. The pressure shift and broadening are obtained by fitting the measured shapes of these lines by a Voigt profile. The broadening parameters and shift coefficients are compared to the results of theoretical calculations based on the semiclassical Robert-Bonamy formalism (RB) in which the intermolecular potential includes electrostatic, induction, and dispersion energy contributions. The variation of these coefficients with rotational and vibrational quantum numbers is examined. The results are generally in satisfactory agreement with experimental data.     

The adsorption of ammonia on a Fe(110) single crystal surface studied by high resolution electron energy loss spectroscopy (EELS)

EELS spectra of ammonia adsorbed on a Fe(110) single crystal surface at 120 K reveal four different molecular adsorption states:1. At very low exposures (0.05 L) three vibrational losses at 345 cm^?1, 1170 and 3310 cm^?1 are observed which are attributed to the symmetric Fe-N stretching-, N-H₃ deformation and N-H₃ stretching modes of chemisorbed molecular ammonia, respectively. The observation of only three vibrational losses indicates an adsorption complex of high symmetry (C_3v).2. Further exposures up to 0.5 L cause the appearance of additional losses at 1450 cm^?1, 1640 cm^?1 and 3370 cm^?1. The latter two are interpreted as the degenerate NH₃ deformation and - stretching modes of molecularly adsorbed NH₃. The 1450 cm^?1 loss is a combination of the losses at 345 cm^?1 and 1105 cm^?1. The observation of 5 vibrational losses is consistent with an adsorption complex of C_s symmetry.3. In the exposure range from 0.5 to 2 L adsorption of molecular ammonia in a second layer is observed. This phase is characterized by a symmetric deformation mode at 1190 cm^?1 and by two additional very intense modes at 160 cm^?1 and 350 cm^?1 which are due to rotational and translational modes.4. Exposures above 2 L cause multilayer condensation of ammonia characterized by translational and rotational bands at 190 cm^?1, 415 cm^?1 and 520 cm^?1, and a symmetric deformation mode at 1090 cm^?1. A broad loss feature around 3300 cm^?1 is attributed to hydrogen bonding in the condensed layer.Thermal processing of a Fe(110) surface ammonia covered at 120 K leads to decomposition of the ammonia into hydrogen and nitrogen above 260 K. No vibrational modes due to adsorbed NH or HN₂ species were detected.     

Water dimer vibration-rotation tunnelling levels from vibrationally averaged monomer wavefunctions

The vibration-rotation tunnelling (VRT) spectra for the water dimer obtained by vibrationally averaging the dimer potential over accurate water monomer wavefunctions is reported. The vibrational averaging requires evaluation of the 12D dimer potential energy surface at more than 10¹² distinct geometries. The resulting vibrational spectra of the low-lying dimer states are presented and compared with both less computationally expensive methods based on fixed nuclei approximations and the recent (6+6)d adiabatic calculations of Leforestier et al. [38] (2009). The procedure gives some modest improvement in the agreement with experimental values for the vibration-rotation tunnelling (VRT) states of (H₂O)₂ and (D₂O)₂. This approach can be extended to treat dimer states involving monomer overtone excitations, which is important in obtaining water dimer spectra at infrared and visible wavelengths at atmospheric temperatures, and in characterizing the dimer contribution to the so-called water continuum absorption at these wavelengths.     

The ν5 band of CH3CH3: High resolution stimulated Raman spectrum and global three band analysis

Rotationally resolved spectrum of ¹²CH₃¹³CH₃ in the region of ν₅ vibrational fundamental (CC stretch) was observed using stimulated Raman spectroscopy. This spectrum was analyzed with data from the ν₁₂ fundamental and transitions from the ν₆, 2ν₆-ν₆, and 3ν₆ torsional bands using a 3-state fit. One torsional component of the ν₅ fundamental is perturbed, interacting with its partner in the ν₆=4 of the torsional stack of the ground vibrational state. As for normal ethane, the coupling was successfully modeled using a Fermi-type interaction. The results mirror that of ¹²CH₃¹²CH₃ in that the inclusion of the Fermi-type interaction reduces the required number of terms in the Fourier expansion of the torsional potential for the ground vibrational state from three (in the 2-state fit) to one, only the term in the barrier height is required.     

Electronic transition of C3H− in the vicinity of the lowest photodetachment threshold

The electronic spectrum of the C³H(D)^? anion has been studied near the lowest electron detachment threshold associated with the chain isomer. On the basis of the vibrational and rotational analyses, the photodetachment spectrum is assigned to the ³A″ ← [Xtilde]³A″ electronic transition of the cumulene-like C³H(D)^? anion. The spectrum is characterized by a rich vibrational structure leading to the determination of almost all fundamental frequencies. The vibrational progression involving the CCH(D) in plane bending mode (V¹ ₆) is fitted with a 1-dimensional potential energy curve along the CCH(D) angle. This fit and the rotational analysis enable the geometry of the upper state to be determined. Although this excited state is expected to have dipole bound character, its neutral counterpart has a significantly different structure. The observation of an excited triplet state near the electron detachment threshold and close to the the calculated transition state for the electron catalysed isomerization reaction leads to new insight into this process.     

A refined potential energy function for the electronic ground state of H2Se

The potential energy surface for the electronic ground state of the hydrogen selenide molecule has been determined previously by Jensen and Kozin [J. Mol. Spectrosc. 160 (1993) 39] in a fitting to experimental data by means of the MORBID computer program. We report here a further refinement of this surface, also made with the MORBID program. With the refined potential surface, we can make predictions of rotation-vibration transition wavenumbers for H₂Se, D₂Se, and HDSe, and with these predictions we can assign weak spectra of these molecules. We assign here two very weak bands of HD⁸⁰Se, ν₁+ν₂+ν₃ and 2ν₁+ν₃. The refinement of the potential energy surface was made possible because (1) the number of vibrational states characterized experimentally for various isotopomers of H₂Se has approximately doubled since 1993, and (2) we now have access to larger computers with which we can fit energy spacings of states with J?8, whereas Jensen and Kozin could only use J?5. In the present work, we fitted rotation-vibration energy spacings associated with 24 vibrational states of H₂⁸⁰Se with v₁?6, v₂?3, and v₃?2; 11 vibrational states of D₂⁸⁰Se with v₁?2, v₂?3, and v₃?2, and 17 vibrational states of HD⁸⁰Se with v₁?3, v₂?3, and v₃?3. The input data set comprised 3611 energy spacings. In the fitting, we could usefully vary 29 potential energy parameters. The standard deviation of the fitting was 0.12 cm⁻¹ and the root-mean-square deviation for 49 vibrational term values was 0.59 cm⁻¹.     

Ion-electron coincidence study of the thermal He(23S) + H2 Penning reaction

A new Penning-electron-Penning-ion coincidence method is described. It is applied to the study of the thermal reaction of He(2³S) with H₂. The main results reported are separate electron energy spectra that are coincident with the three different ions formed: HeH₂⁺, HeH⁺ and H₂⁺. Based on these results it is shown that the Penning reaction of the He(2³S)/H ₂ system proceeds in two well-separated steps: (i) ionization at distances R (HeH₂) ? 6a₀ in which H₂⁺ (v) is formed in different vibrational states; and (ii) reactive collision of H₂⁺ (v) with He. For the second step the variation of the branching ratios with vibrational quantum numbers v = 0 to v = 10 is derived, and it is shown that these branching ratios may be regarded as relative vibrational-energy-dependent cross-sections for the collision of H₂⁺ (v) with He at an average relative kinetic energy of ～20 meV.     

Electromagnetically induced transparency in a molecular gas

The dynamic Stark effect on the as Q(5,4) transition of the 0 → 1 vibrational band of the ν₂ mode of ammonia ¹⁵NH₃ was studied by the method of copropagating waves of radiation of two ¹³CO₂ lasers operating at the R(18)_I line. For waves with mutually orthogonal polarizations, which propagated in a waveguide cell, the intensity of the saturating radiation reached 225 W/cm², and the absorption line of the probing radiation had the form of a two-hump curve with a splitting that was consistent with the Rabi frequency for the ammonia transition being investigated. At the maximum intensity of the saturating radiation, complete transparency was observed at the center of the absorption line of ¹⁵NH₃. When identically polarized saturating and probing waves propagating in a cell at a small angle with respect to each other were used, the splitting was more weakly manifested. The effect of a spatial inhomogeneity of the optical fields on the shape of the line of saturated absorption is discussed.     

The rotational spectra of the υ8 = υ9 = 1 and υ6 = υ7 = 1 interacting vibrational states of nitric acid (HNO3)

The analysis of the rotational spectrum of HNO₃ has been extended to include the υ₈ = υ₉ = 1 state at 1205.7 cm⁻¹ and the υ₆ = υ₇ = 1 state at 1223.4 cm⁻¹. Based on 78-519 GHz data, the assignments in the 8¹9¹ vibrational state have been significantly expanded from the previously reported microwave measurements [T.M. Goyette, F.C. De Lucia, J. Mol. Spectrosc. 139 (1990) 241-243]. A new microwave analysis is also reported for the 6¹7¹ vibrational state. A simultaneous analysis takes into account the localized ΔK_a = ±2 Fermi resonances between the vibrational states, describes the torsional splitting of 3.3 and 1.4 MHz for the 8¹9¹ and 6¹7¹ states respectively, and fits to experimental accuracy over 1500 rotational transition frequencies that extend up to J = 59. Infrared energy levels [A. Perrin, J.-M. Flaud, F. Keller, A. Goldman, R. D. Blatherwick, F. J. Murcray, C. P. Rinsland, J. Mol. Spectrosc. 194 (1999) 113-123] were also included in the analysis and fit to experimental accuracy. Measurement of strongly perturbed transitions in each vibrational state provide a determination of the band origin difference of 17.733184(17) cm⁻¹. The rotational constants agree well with those predicted by vibrational-rotational constants of the fundamental modes. Furthermore, the analysis will provide a very accurate simulation of the infrared spectrum of HNO₃ in the 8.3 μm region.     

Mechanism of reactions induced by 7 MeV deuterons on9Be [(d,p), (d,d), (d,t), (d,4He)]

Angular distributions of protons, deuterons, tritons and alpha-particles emitted from the reactions in thed+⁹Be-system atE _d =7 MeV as well as excitation functions at selected angles in the energy rangeE _d =6.5–7.5 MeV (LAB) were measured. The potential part of the elastic scattering is described by the phenomenological optical model. The compound nucleus contribution to all exit channels is determined using the Hauser-Feshbach model. The collective excitation of the 2.43 MeV excited state of⁹Be and transfer processes are analysed within the DWBA formalism. The analyses suggest a significant contribution of five-nucleon transfer to the (d,⁴He) channel.     

High resolution Fourier transform infrared spectra and analysis of the ν14, ν15 and ν16 bands of azetidine

Rotationally resolved vibrational spectra of the three lowest frequency bands of the four-membered heterocycle azetidine (c-C₃H₆NH) have been collected with a resolution of 0.00096 cm⁻¹ using the far infrared beamline at the Canadian Light Source synchrotron. The modes observed correspond principally to motions best described as: β-CH₂ rock (ν₁₄) at 736.701310(7) cm⁻¹, ring deformation (ν₁₅) at 648.116041(8) cm⁻¹, and the ring puckering mode (ν₁₆) at 207.727053(9) cm⁻¹. A global fit of 14 276 rovibrational transitions from the three bands provided an accurate set of ground state spectroscopic constants as well as excited state parameters for each of the three vibrational modes. The ground state structure was determined to be that of the puckered conformer having the NH bond in an equatorial arrangement.     

C3O2 as a semirigid bender: The degenerate ν5 state

The semirigid bender Hamiltonian for carbon su?ide C₃O₂ [P. R. Bunker, J. Mol. Spectrosc.80, 422–437 (1980)] is extended in a manner similar to the extension previously described for HCNO [P. Jensen, J. Mol. Spectrosc.101, 422–439 (1983)]. The extended Hamiltonian describes the manifold of large-amplitude vibrational states (due to the ν₇ CCC bending mode) superimposed on a high-frequency vibrational state involving excited quanta of the CCO bending modes ν₅ and ν₆. The extended model is used to fit CCC bending and rotation energy level separations for¹²C₃¹⁶O₂ superimposed on the ν₅ fundamental level. Due to the severely limited experimental data it is not possible to unambiguously determine the effective CCC bending potential energy function in the ν₅ state, but estimates of the potential energy parameters are obtained by determining them in two limiting cases.     

Laser Stark and laser microwave double resonance spectroscopy of fluoroacetylene with the CO2 laser

The CO₂ laser Stark spectrum of fluoroacetylene was identified for the ν₃, ν₃ + ν₄ ? ν₄, and ν₃ + ν₅ ? ν₅ vibrational bands. The origins of these bands were precisely determined (±0.0003 cm^?1) to be 1061.4452, 1059.0639, and 1064.6960 cm^?1. A CO₂ laser microwave double resonance experiment in the presence of the Stark field is described. This technique was applied to assignment of the Stark spectrum, calibration of the Stark electrode spacing, and precise determination (±0.0003 D) of dipole moment. The dipole moments of the HCCF molecule in the ground, ν₃, ν₄, ν₅, ν₃ + ν₄, and ν₃ + ν₅ vibrational states are 0.7207, 0.7447, 0.6557, 0.7441, 0.6769, and 0.7689 D.     

The study of ungerade electronic states of the Xe2 molecules in the region of Xe*(5p 56p, 5d, 7s, 6d) by the resonance multiphoton ionization method

Electronic spectra of the Xe2 molecules in the energy range of 77700?C89300 cm^?1 are recorded. The method of resonance enhanced multiphoton ionization of molecules in a supersonic molecular beam was used, in which excitation of the molecules by three photons was followed by ionization caused by a fourth photon (the (3+1) REMPI method). Analysis of the vibrational structure of observed systems of bands yielded information about the dissociation energy and the molecular constants for ungerade states of molecules. On the basis of the Franck-Condon principle, the equilibrium distances for potential curves were estimated from the relative intensities in vibrational progressions. Data on 16 new electronic states of diatomic xenon molecules with the dissociation limits Xe ₂ ^* ?? XE(5p ^{6 1} S ₀) + Xe*(5p ⁵6p,5d, 7s, 7p) were obtained.     

On the study of the vibrational energy levels of Arsine molecule

We compare two formalisms applied to the vibrational modes of the molecule of AsH₃ of C_3v molecular symmetry group. Indeed, the close stretching modes of this molecule may be considered as those of a three-dimensional oscillator whereas the bending modes may be considered either as a one-dimensional oscillator of symmetry A₁ and a two-dimensional oscillator of symmetry E or as an approximate three-dimensional oscillator. So, we have applied the U(p + 1) formalism to the both stretching and bending modes and introduced coupling terms acting on an appropriate coupled vibrational basis through a local mode formalism. We have then compared the result of our fitting with those obtained with the coupling of a local mode formalism adapted to the stretching vibrations with a normal mode formalism for the bending ones. Finally we compare our results with other methods recently proposed in the literature.     

A new Morse-oscillator based Hamiltonian for H3+: Calculation of line strengths

In two recent publications [V. Špirko, P. Jensen, P. R. Bunker, and A. Čejchan, J. Mol. Spectrosc. 112, 183–202 (1985); P. Jensen, V. Špirko, and P. R. Bunker, J. Mol. Spectrosc. 115, 269–293 (1986)], we have described the development of Morse oscillator adapted rotation-vibration Hamiltonians for equilateral triangular X₃ and Y₂X molecules, and we have used these Hamiltonians to calculate the rotation-vibration energies for H₃⁺ and its X₃⁺ and Y₂X⁺ isotopes from ab initio potential energy functions. The present paper presents a method for calculating rotation-vibration line strengths of H₃⁺ and its isotopes using an ab initio dipole moment function [G. D. Carney and R. N. Porter, J. Chem. Phys. 60, 4251–4264 (1974)] together with the energies and wave-functions obtained by diagonalization of the Morse oscillator adapted Hamiltonians. We use this method for calculating the vibrational transition moments involving the lowest vibrational states of H₃⁺, D₃⁺, H₂D⁺, and D₂H⁺. Further, we calculate the line strengths of the low-J transitions in the rotational spectra of H₃⁺ in the vibrational ground state and in the ν₁ and ν₂ states. We hope that the calculations presented will facilitate the search for further rotation-vibration transitions of H₃⁺ and its isotopes.     

Electronic absorption spectra of the 5d3 hexafluororhenate(IV) ion

The Γ₈(⁴A_2g) →Γ₇(²T_2g), Γ₈(²T_2g) electronic transitions for the 5d³ hexafluororhenate(IV) ion have been observed at liquid hydrogen temperature in a single Cs₂GeF₆ crystal and in a mixed crystal where the ReF^?2₆ ion is doped in the cubic Cs₂GeF₆ lattice. The electronic transitions have been assigned with a crystal field model to give information about the parameters B, C, Dq, and spin-orbit coupling. The vibrational structure in the mixed crystal system may be assigned to the ungerade modes of the ReF^?2₆ moiety. Comparison of the mixed and pure crystal vibrational structure shows that the pure crystal vibrational structure can be interpreted on the basis of K ≠ 0 lattice effects and a small distortion in the pure crystal.     

The two-dimensional anharmonic oscillator: The CCC bending mode of C3O2

Newly observed data on the rotational constants of carbon su?ide in excited vibrational states of the low-wavenumber bending vibration ν₇ have been successfully interpreted in terms of the two-dimensional anharmonic oscillator wavefunctions associated with this vibration. By combining these results with published infrared and Raman spectra the vibrational assignment has been extended and a refined bending potential for ν₇ has been derived: this has a minimum at a bending angle of about 24° at the central C atom, with an energy maximum at the linear configuration some 23 cm^?1 above the minimum. From similar data on the combination and hot bands of ν₇ with ν₄ (1587 cm^?1) and ν₂ (786 cm^?1) the effective ν₇ bending potential has also been determined in the one-quantum excited states of ν₄ and ν₂. The effective ν₇ potential shows significant changes from the ground vibrational state; the central hump in the ν₇ potential surface is increased to about 50 cm^?1 in the v₄ = 1 state, and decreased to about 1 cm^?1 in the v₂ = 1 state. In the light of these results vibrational assignments are suggested for most of the observed bands in the infrared and Raman spectra of C₃O₂.     

Shifting and line mixing parameters in the ν4 band of NH3 perturbed by CO2 and He: Experimental results and theoretical calculations

The pressure-induced shifting coefficients and line mixing parameters have been studied in the ν₄ band of NH₃ perturbed by CO₂ and He at room temperature. Measurements have been made using a high-resolution Fourier transform spectrometer. The measurements cover the ^PP and ^RP branches of the ν₄ band and are located in the spectral range 1470-1600 cm⁻¹. The line shift and line mixing parameters have been derived from a non-linear least-squares multi-pressure fitting technique. The shift coefficients are compared to a semiclassical calculation based on the Robert-Bonamy formalism employing two types of intermolecular interactions. It is shown that the line shifts mainly originate from the vibrational dephasing effects. The observed interference parameters are compared with calculations based on state-to-state collisional cross sections calculated from the intermolecular potential with a semiclassical approach. The results of computation are in reasonable agreement with the experimental data. It is demonstrated also that the line mixing process mainly originates from the energy transfer between symmetric and antisymmetric components of the inversion doublets.     

Diode-laser spectroscopy of supersonic free jets

A supersonic-free-jet infrared spectrometer has been constructed for investigation of molecular vibrational spectra at low rotational and vibrational temperatures. The sensitivity of measurement in a pulsed jet is increased by employing a phase-sensitive detection method synchronized with the pulse frequency. The performance of the spectrometer is examined for the absorption lines of the NH₃ v ₂ band. A rotational temperature as low as 16K is attained when seeded in He. Cold-jet spectra are demonstrated for thev ₃ bands of PF₅,³⁴SF₆, and¹⁸²WF₆.