ALBERT

Notes: Ultrasensitive infrared laser absorption spectroscopy in a slit supersonic expansion is used to obtain the spectrum of the HF stretching fundamental of D2HF. Both a Π←Π band due to para-D2HF and a ∑←∑ band due to ortho-D2HF are observed, in contrast to the H2HF spectrum which consists of the Π←Π band alone. Analysis of the spectrum indicates that the D2HF Π states are more strongly bound than the ∑ states. Doublet splittings in the Π←Π band are analyzed to determine barriers to internal rotation of D2 within the complex. The vibrationa1 predissociation rate of D2HF is approximately 25 times faster than that of H2HF, suggesting the opening of a channel which results in vibrational excitation of the D2 fragment.

Notes: The relative intensities of nine pairs of rovibrational transitions of OH in the v=1←0 fundamental have been measured by flash kinetic infrared absorption spectroscopy. Each pair of transitions originates from a common rotational and spin–orbit state, so that relative intensities are independent of the OH number density and quantum state distribution. The relative intensities are strongly J dependent and this dependence provides detailed information about the shape of the OH dipole moment function, μ(r), and hence the absolute infrared transition strengths. In an accompanying paper we present the theoretical basis for extracting μ(r), for an open shell diatomic like OH, from relative infrared intensities and permanent dipole moment measurements (Peterson et al.). In this work we implement those ideas and determine the OH dipole moment function to be: μ(r)=1.6498(6) D+0.561(32) D/A(ring) (r−re )−0.75(17) D/A(ring)2 (r−re )−1.5(11) D/A(ring)3(r−re )3. The accuracy of μ(r) is excellent near re (re =0.970 A(ring)), since the data used to derive it are from low vibrational states. The useful range of this function extends from approximately 0.75 to 1.35 A(ring). The rotationless Einstein A coefficient for the OH fundamental is determined from μ(r) to be 16.7(19) Hz. This is in considerable disagreement with most other experimental and theoretical results, but is in good agreement with theoretical calculations by Mies (18.3 Hz) and by Langhoff et al. (13.8 Hz).

Notes: The relative intensities of 88 pairs of rovibrational transitions of OH (X 2Π) distributed over 16 vibrational bands (v'≤9, Δv=−1,−2) have been measured using Fourier transform infrared (FTIR) emission/absorption spectroscopy. Each pair of transitions originates from a common vibrational, rotational, and spin–orbit state, so that the measured relative intensities are independent of the OH number density and quantum state distribution. These data are combined with previous v=1←0 relative intensity absorption measurements and v=0, 1, and 2 permanent dipole moments to determine the OH dipole moment function as a cubic polynomial expanded about re, the equilibrium bond length. The relative intensities provide detailed information about the shape of the OH dipole moment function μ(r) and hence the absolute Einstein A coefficients.The intensity information is inverted through a procedure which takes full account of the strong rotation–vibration interaction and spin uncoupling effects in OH to obtain the dipole moment function (with 95% confidence limits): μ(r)=1.6502(2) D+0.538(29) D/A(ring) (r−re)−0.796(51) D/A(ring)2 (r−re)2−0.739(50) D/A(ring)3 (r−re), 3 with a range of quantitative validity up to the classical turning points of the v=9 vibrational level (i.e., from 0.70 to 1.76 A(ring)). The μ(r) determined in this study differs significantly from previous empirical analyses which neglect the strong effects of rotation–vibration interaction and spin uncoupling. The present work also permits distinguishing between the various ab initio efforts. Best agreement is with the dipole moment function of Langhoff, Werner, and Rosmus [J. Mol. Spectrosc. 118, 507 (1986)], but their theoretical predictions for higher overtone transitions are still outside of the 2σ experimental error bars. Absolute Einstein A coefficients from the present μ(r) are therefore presented for P, Q, R branch transitions for Δv=1, 2, 3, v'≤9, J'≤14.5, in order to provide the most reliable experimental numbers for modeling of near IR atmosphere OH emission phenomena.

Notes: The van der Waals complexes, NeDCl and ArDCl, are produced in a slit jet supersonic expansion and observed via direct absorption of tunable mid-infrared Pb-salt diode laser radiation. For the NeDCl complex, the DCl stretch fundamental [ν0=2091.3717(4) cm−1 ] and the DCl Σ and Πe, f bend combination bands [ν0=2099.5760(4) and 2104.9465(4) cm−1, respectively] are observed. The DCl stretch fundamental and Πe, f combination band are observed for ArDCl at 2089.4180(2) and 2117.4443(3) cm−1, respectively. The relative fundamental vs bend combination band intensity distributions are very different for the two complexes. The ArDCl fundamental to Π bend combination band intensity ratio is 4:1, whereas for NeDCl the corresponding ratio is 1:8. This anomalous intensity pattern for NeDCl and the proximity of the bend combination bands to the DCl R(0) line indicate that the DCl diatomic is exhibiting nearly free rotation within this complex, compared to more restricted librational motion of DCl in ArDCl. Strong Coriolis interactions between Πe and Σ bend states are observed for both complexes and analyzed quantitatively for NeDCl. The observed NeDCl and ArDCl absorption linewidths are only slightly larger than the instrumental limit determined from nearby OCS monomer absorptions in the slit jet, but the differences are not of high statistical significance. This FWHM of the observed transitions dictates a rigorous lower limit to the vibrational predissociation lifetime of 3 ns. Experimentally determined rotational constants, vibrational frequencies, and relative intensities are compared to predictions based on existing empirical potential surfaces.

Notes: A general approach to the determination of the dipole moment function and of the absolute vibrational transition moments for diatomic molecules is presented. This method utilizes the variation of intensity with J within a vibrational transition, together with permanent dipole moment information, to extract the absolute transition moments. An essential feature of the model is its use of algebraic expressions for calculating vibration–rotation line intensities. These expressions can be rapidly evaluated in a least squares fit which determines the dipole moment function. This approach is general in that it is not limited to 1Σ state molecules, nor to the simplest of Hund's case couplings of spin, orbital and mechanical angular momentum. It is also not limited to molecules with essentially linear dipole moment functions. The model is successfully applied to the OH molecule which violates each of these restrictions. In the accompanying work we report experimental measurements of relative infrared absorption intensity measurements for OH v=1←0 transitions and the extraction of an experimental μ(r) using the approach presented here.

Notes: The first high resolution spectra of (DCl)2 are reported using direct IR laser absorption spectroscopy in a slit supersonic expansion. The spectral data are analyzed to obtain vibrational frequencies, rotational constants, and tunneling (interconversion) level splittings for isotopically symmetric (D35Cl)2 and (D37Cl)2, and mixed D35Cl–D37Cl dimers. Six dimer absorption bands are observed and analyzed for both (D35Cl)2 and D35Cl–D37Cl. These include two perpendicular Ka=1←0, v1=1←0 (i.e., "free'' DCl stretch) bands, one each originating from the lower (+) and the upper (−) tunneling sublevels in the ground vibrational state. Four parallel v2=1←0 (i.e., "bound'' DCl stretch) bands are also observed, one for each of the Ka=0←0 and Ka=1←1 subbands originating from both the lower (+) and upper (−) tunneling components.In addition, two bands are observed only for the isotopically mixed dimer (i.e., complexes from D35Cl and D37Cl), which acquire oscillator strength by virtue of the breaking of inversion symmetry. This complete set of bands provides the necessary data to determine interconversion splittings for the mixed dimer in the ground [5.9595(6) cm−1] and the two DCl vibrationally excited states [3.2286(6) cm−1 for v1=1 and 2.9935(6) cm−1 for v2=1], as well as to make accurate predictions for the symmetric (D35Cl)2 dimer. These experimental splittings for the ground state are compared to results from (i) a 1D quantum calculation for adiabatic motion over a minimum energy tunneling path; and (ii) a 3D variational calculation in a basis set of free DCl rotors which treats all three internal bend and torsion angles (aitch-theta1, aitch-theta2, and φ1–φ2). These calculations, performed on an approximate dipole and quadrupole model of the electrostatic potential surface, reproduce the ground state tunneling splittings to within 15%. The corresponding eigenfunctions provide direct evidence for highly correlated, "geared'' internal rotation of the two DCl subunits over a low barrier. The fivefold decrease in tunneling splitting for the symmetric (DCl)2 upon v1=1 or v2=1 excitation is qualitatively consistent with previous models of vibrationally diminished tunneling rates due to intramolecular V→V energy transfer at the C2h transition state. However, this decrease is nearly identical to the 4.8-fold decrease observed in (HCl)2, which is quantitatively inconsistent with a simple dipole–dipole vibrational energy transfer mechanism. Measured linewidths in these dimer spectra are all at the resolution limit of the diode laser apparatus, which translates into vibrational predissociation lifetimes in excess of 3 ns.

Notes: We have developed an electrostatically driven tuning fork chopper which is compatible with an ultrahigh vacuum (UHV) environment. Constructed with a commercially available tuning fork using stainless steel and alumina parts, the chopper is capable of operating while exposed to high temperature samples and corrosive gases. Operation in an UHV environment while modulating both optical and molecular beams is demonstrated.