Walter J. Lafferty

The HITRAN molecular spectroscopic database: edition of 2000 including updates through 2001

This paper describes the status circa 2001, of the HITRAN compilation that comprises the public edition available through 2001. The HITRAN compilation consists of several components useful for radiative transfer calculation codes: high-resolution spectroscopic parameters of molecules in the gas phase, absorption cross-sections for molecules with very dense spectral features, aerosol refractive indices, ultraviolet line-by-line parameters and absorption cross-sections, and associated database management software. The line-by-line portion of the database contains spectroscopic parameters for 38 molecules and their isotopologues and isotopomers suitable for calculating atmospheric transmission and radiance properties. Many more molecular species are presented in the infrared cross-section data than in the previous edition, especially the chlorofluorocarbons and their replacement gases. There is now sufficient representation so that quasi-quantitative simulations can be obtained with the standard radiance codes. In addition to the description and justification of new or modified data that have been incorporated since the last edition of HITRAN (1996), future modifications are indicated for cases considered to have a significant impact on remote-sensing experiments

Molecular structures of gas‐phase polyatomic molecules determined by spectroscopic methods

Spectroscopic data related to the structures of polyatomic molecules in the gas phase have been reviewed, critically evaluated, and compiled. All reported bond distances and angles have been classified as equilibrium (re), average (rz), substitution (rs), or effective (ro) parameters, and have been given a quality rating which is a measure of the parameter uncertainty. The surveyed literature includes work from all of the areas of gas‐phase spectroscopy from which precise quantitative structural information can be derived. Introductory material includes definitions of the various types of parameters and a description of the evaluation procedure.

Vibrational predissociation, tunneling, and rotational saturation in the HF and DF dimers

The high‐resolution spectra of the intramolecular stretching bands of the HF and DF dimers have been recorded with a tunable difference‐frequency laser. These measurements yield considerable information about the dynamics of hydrogen bonding in these complexes. Vibrational predissociation is observed as a non‐pressure‐dependent excess linewidth for the ‘‘bound‐H’’stretching band of the HF dimer, but no excess linewidth is observed for the ‘‘free‐H’’ stretching band of the HF dimer or for either band of the DF dimer. An unusually large vibrational dependence to the interconversion tunneling frequency is observed for both species, with about a factor of three reduction from the ground state splitting upon excitation of any of the intramolecular stretches. The K subband origins obtained from the A/B hybrid free‐H stretching band of the HF dimer exhibit an irregular pattern indicating anomalous centrifugal distortion effects suggestive of rotational saturation of the angular orientation of the hydrogen bond.

Rotational structure and vibrational predissociation in the HF stretching bands of the HF dimer

The rotational structure in the HF stretching bands of the HF dimer has been recorded with nearly Doppler‐limited resolution using a tunable difference‐frequency laser spectrometer and a long‐path cell held at low temperatures and pressures. Two bands are observed; the higher frequency band, corresponding principally to the ‘‘free’’ hydrogen stretch, has the appearance of a B‐type or perpendicular band with two prominent Q subbranches RQ0 and PQ1; the lower band is an A‐type or parallel band arising primarily from the ‘‘bonded’’ hydrogen vibration. The K=0 subbands of both vibrations have been fully assigned and fit with polynomial expansions in J(J+1) to yield ground state constants in excellent agreement with a previous microwave resonance molecular beam study of the HF dimer. The subbands exhibit doubling and 10:6 intensity ratios for alternate J indicative of the internal rotation tunneling motion proposed in the microwave study. A pressure‐independent broadening of the lines to about twice the Dopple...

High resolution infrared spectra of C212H2, C12C13H2, and C213H2☆

Abstract Two parallel bands in Fermi resonance, ν3, and ν2 + ν41 + ν51, have been studied in the 3200–3400 cm−1 region of C212H2. The calculated unperturbed frequencies for ν3 and ν2 + ν41 + ν51 are 3288.66 cm−1 and 3288.11 cm−1, respectively. The assignment of ν3 to the higher frequency transition is confirmed by the study of this diad in C13C12H2 and C213H2. In addition the absorption spectra of C13C12H2 and C213H2 have been obtained in the 2600–2800 cm−1, 3300–3400, and 6400–6600 cm−1 spectral regions. The unperturbed vibrational frequencies have been obtained for C13C12H2, and a few of these frequencies have been calculated for C213H2. Equilibrium B values have been obtained for the isotopic carbon molecules, and these values have been used to calculate accurate rc distances for acetylene. The “substitution structure” has also been calculated.

Structure and vibrational dynamics of the CO2 dimer from the sub‐Doppler infrared spectrum of the 2.7 μm Fermi diad

Sub‐Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm−1 using an optothermal molecular beam color‐center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm−1 from the corresponding ν1+ν3/2ν02+ν3 CO2 bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C2h symmetry. The C‐‐C separation and C‐‐C–O angle are determined to be 3.599(7) A and 58.2(8)°, respectively. The nearest neighbor O‐‐C distance is 3.14 A which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm−1. No interconversion tunneling is observed.

The structure of the carbon dioxide dimer from near infrared spectroscopy

An F‐center laser–molecular beam spectrometer has been used to obtain a sub‐Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν1+ν3 combination mode of the monomer located at 3716 cm−1. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T‐shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.

SUB-DOPPLER INFRARED SPECTRUM OF THE CARBON DIOXIDE TRIMER

A spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm−1 at sub‐Doppler resolution using an optothermal (bolometer‐detected) molecular‐beam color‐center laser spectrometer. A planar, cyclic structure with C3h symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) A. The observed perpendicular band, corresponding to an in‐plane E′‐symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν02 +ν3 combination mode of a CO2 subunit by virtue of its small blue shift (∼0.98 cm−1) from that of the isolated monomer.

Infrared and microwave investigations of interconversion tunneling in the acetylene dimer

A sub‐Doppler infrared spectrum of (HCCH)2 has been obtained in the region of the acetylene C–H stretching fundamental using an optothermal molecular‐beam color‐center laser spectrometer. Microwave spectra were obtained for the ground vibrational state using a pulsed‐nozzle Fourier transform microwave spectrometer. In the infrared spectrum, both a parallel and perpendicular band are observed with the parallel band being previously assigned to a T‐shaped C2v complex by Prichard, Nandi, and Muenter and the perpendicular band to a C2h complex by Bryant, Eggers, and Watts. The parallel band exhibits three Ka=0 and three asymmetry‐doubled Ka=1 series. The transitions show a clear intensity alternation with Kc with two of the Ka=0 series missing every other line. In addition, the perpendicular band has the same ground‐state combination differences as the parallel band. To explain these apparent anomalies in the spectrum, we invoke a model consisting of a T‐shaped complex with interconversion tunneling between f...

Microwave spectra of the (HF)2, (DF)2, HFDF, and DFHF hydrogen-bonded complexes

Abstract The microwave spectra of the tunneling-rotation bands of the hydrogen-bonded complexes (HF)2 and (DF)2 have been measured in the 50- to 126-GHz region. In addition, the pure rotation spectra of both the HFDF and DFHF molecules have been obtained. Transitions with K = 0 through K = 2 have been observed for all isotopic species. The hydrogen fluoride dimer is a very nonrigid molecular species. In order to fit the observed transitions adequately, empirical expressions for the energy levels were used, and each K subband was separately fitted. Constants obtained from a Pade approximant fitting of the microwave data of (HF)2 together with infrared ground state combination differences are given.