ALBERT

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: An extensive series of near-infrared absorption spectra are recorded for jet-cooled (6–14 K) hydrogen chloride dimer (HCl)2. Both ΔKa=0 and ΔKa=±1 bands are observed for both the free (ν1) and bonded (ν2) HCl stretches; all three chlorine isotopomers (H 35Cl–H 35Cl, H 35Cl–H 37Cl, and H 37Cl–H 37Cl) are observed and analyzed for K‘a ≤ 2. The slit jet spectrum extends significantly the previous cooled cell infrared study of this complex and provides a measure of tunneling splittings for Ka=0 and 1 for each of the HCl ground (v=0) and excited (v=1) states. Mode specific vibrational predissociation is observed via analysis of the absorption line shapes, with Lorentzian contributions to the line profiles of Δν1(approximately-less-than)1.6 MHz and Δν2=5.1±1.2 (2σ) MHz full width at half-maximum for ν1 and ν2 excitation, respectively. Stronger coupling in (HCl)2 of the bonded (ν2) vs free (ν1) HCl vibration to the dissociation coordinate is consistent with the comparable trends observed in other hydrogen bonded dimers. Quantum mechanical variational calculations on an electrostatic angular potential energy surface are used to model the internal HCl rotor dynamics using a coupled rotor formalism; analysis of the internal rotor eigenfunctions provides direct evidence for large amplitude "geared'' internal rotation of the HCl subunits.

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: Both D- and H-bonded isomers of the mixed dimers formed between HCl and DCl are investigated via high resolution infrared difference frequency and diode laser spectroscopy in the 2885 and 2064 cm−1 regions. From an analysis of the relative integrated absorption intensities, the D-bonded complex (i.e., HCl–DCl) is determined to be more stable by 16±4 cm−1 than the H-bonded (i.e., DCl–HCl) species. All four chlorine isotopic combinations of the lower energy (HCl–DCl) complex are probed via excitation of both HCl (vHClacc=1←0) and DCl (vDCldon=1←0) stretches. Additionally, two chlorine isotopomers of the higher energy (DCl–HCl) complex are investigated through HCl excitation. Compared to the facile tunneling observed in both (HCl)2 or (DCl)2 complexes, these mixed dimers exhibit more rigid behavior characteristic of two distinct isomeric species. However, the relatively small energy difference (16±4 cm−1) between the two isomers still allows the wave functions for both species to sample both the HCl–DCl and DCl–HCl local minima on the potential surface. This intermediate level of angular localization of the wave function is modeled via 3D quantum mechanical calculations including all three internal rotor angular degrees of freedom. Additionally, a 1D treatment along the minimum energy tunneling path is investigated, which quantifies the asymmetry in the tunneling coordinate due to isotopic dependence of the H- and D-bonded zero point bending and torsion energies.Vibrational predissociation lifetimes in excess of the slit jet instrument line shape are determined from homogeneous broadening of the spectral line widths. The HCl stretch excited lifetime of H-bonded DCl–HCl [ΔνHCldon=44(6) MHz, τHCldon=3.6(5) ns] is threefold shorter than the corresponding lifetime of D-bonded HCl–DCl [ΔνHClacc=16(3) MHz, τHClacc=9.6(16) ns]. This ratio is quite comparable to the results obtained in investigations of (HCl)2 and consistent with a stronger, mode specific coupling to the dissociation coordinate for excitation of the bonded-HX vs free-HX moiety. However, the absolute lifetimes of both vHClacc=1 HCl–DCl and vHCldon=1 DCl–HCl complexes are tenfold shorter than the corresponding excited vibrational state lifetimes in (HCl)2. This suggests a near resonant channel for predissociation into HCl(v=0)+DCl(v=1) which minimizes the energy deposited into rotation and relative translation of the diatomic fragments.

Notes: IR spectra of jet cooled ArHF are obtained via direct absorption of a high resolution tunable difference frequency laser in a 2.54 cm path length, slit supersonic pulsed expansion at 〈10 K. Detection limits of 2×109 molecules/cm3/quantum state permit observation of the high frequency ν1 fundamental stretch (1000) ← (0000), the ν1+ν2 van der Waals bend plus stretch combination band (1110) ← (0000), as well as transitions to the (1002) triply vibrationally excited state that are weakly allowed via Coriolis interactions with the Π+ component of the (1110) manifold. The ground state (0000) molecular constants are in excellent agreement with previous microwave data. From the changes in rotational and centrifugal distortion constants, the vibrationally averaged van der Waals well depth is estimated to increase (+15%) with ν1 excitation, but decrease dramatically (−42%) upon subsequent excitation of the l=1 ν2 bend. L-doubling in the ν1+ν2 (1110) perpendicular bending state is large and negative [−69.8(18) MHz] and indicates the presence of a near resonant Coriolis coupledvibration of Σ+ symmetry at lower energy. A second, localized Coriolis perturbation is observed in the (1110) state and assigned to the near resonant (1002) Σ+ fundamental plus van der Waals stretch overtone at higher energy. Analysis of this Coriolis interaction indicates that coupling can be significant even for a three quantum change in vibration. However, a perturbative, small amplitude oscillator model predicts Coriolis matrix elements only 18% of the observed values, suggesting that large amplitude, bend–stretch interactions can strongly enhance Coriolis coupling. The decrease in the B rotational constant and the vibrationally averaged well depth upon ν2 excitation confirms the strong coupling between van der Waals stretch and bend coordinates. The slit expansion geometry quenches perpendicular velocity distributions and therefore offers intrinsically sub-Doppler resolution in an unskimmed molecular beam. Residual linewidths in the ArHF spectra are all below the apparatus resolution limit of ±25 MHz, which translates into a lower limit for the predissociation lifetime of 3 ns, i.e., in excess of 2×106 ν1 vibrational periods.

Notes: High sensitivity, tunable laser direct absorption methods are exploited to obtain high resolution IR spectra (Δν(approximately-less-than)0.001 cm−1) of weakly bound CO2HF complexes in a pulsed supersonic slit jet expansion. Transitions from the ground vibrational state corresponding to a single quantum excitation of the ν1 HF stretch are observed and analyzed with a semirigid linear molecule Hamiltonian. The observed increase in both B (+1.75%) and D (+55%) upon ν1 excitation is inconsistent with the commonly used diatomic approximation, and is not possible to rationalize for a nearly linear upper state geometry with small amplitude zero point motion of the intermolecular CO2 bend coordinate. We consider an alternative centrifugal straightening mechanism which predicts large centrifugal distortion effects due to end over end rotation of a complex with a nonlinear vibrationally averaged geometry in a weak bending potential. In support of this interpretation, hot band spectra are observed arising from bend excited complexes significantly populated in the 16 K expansion; intensity based estimates of the internal excitation indicate a surprisingly low bend frequency of 10±5 cm−1. A preliminary analysis of the spectra as l doubling in a Π←Π vibrational hot band for a linear equilibrium geometry is presented. An alternative interpretation of the spectrum as asymmetry doubling of a K=1←1 rotational hot band for a bent geometry is also considered. This latter interpretation is more consistent with the data and predicts a CO2 bend angle in the complex for K=1 between 25° and 30°. Linewidths for the upper vibrational states in CO2HF exhibit homogeneous broadening 3–4 times in excess of the apparatus resolution. Voigt analysis of the absorption line shapes indicates linewidths (FWHM) of 136±16 MHz and independent of J state; this corresponds to a relaxation lifetime of 1.1±0.2 ns for HF in the complex.

Notes: High resolution infrared spectra of the two "low" frequency intermolecular modes—van der Waals stretch (ν4) and geared bend (ν5) of (HCl)2—have been characterized in HCl-stretch excited states using a slit jet spectrometer. In a high resolution high sensitivity search covering the range between 2880 and 3070 cm−1 four (HCl)2 combination bands associated with in-plane vibrations ν4 and ν5 have been observed. The vibrational assignment of these bands is based on comparison between observed intermolecular mode energies and predictions from recent six-dimensional (6D) quantum mechanical (QM) calculations [Y. Qiu, J. Z. H. Zhang, and Z. Bacic, J. Chem. Phys. 108, 4804 (1998)], though additional confirmation is provided by ancillary spectroscopic information such as rotational constants, predissociation linewidths, and 35Cl/37Cl isotopic band shifts. The experimentally observed intermolecular energies agree with theoretical predictions to (approximate)2–4 cm−1 out of 60–90 cm−1, suggesting that the 6D potential energy surface can describe combination band excitation in these lower frequency intermolecular coordinates fairly well. Three of the four observed combination bands arise from the upper tunneling level (B+), and all four bands are built exclusively on bound HCl stretch (ν2). To account for these striking intensity anomalies, a simple model for three-dimensional QM calculation of transition moments is introduced, which correctly reproduces the experimental trends. In this model, the propensity for ν2 based combination bands arising from upper tunneling levels can be successfully ascribed to the unusually "floppy" nature of the intermolecular vibrations, which results in a "harmonic oscillator" Δv=+1 propensity for excitations between tunneling levels along the geared bend coordinate. © 2002 American Institute of Physics.