ALBERT

Notes: A direct absorption, difference frequency, infrared laser spectrometer with 10−4 cm−1 resolution combined with slit supersonic jet optical pathlengths is presented as a tool for the study of mode–mode vibrational coupling in laser-excited hydrocarbons. These weak mode–mode couplings are evidenced in our frequency domain studies by virtue of transitions to isolated upper J states that are split into multiplets under sub-Doppler resolution. Instrument performance is demonstrated by investigating vibrational coupling in the 3000–3300 cm−1 C–H stretch fundamental region of 12C3 propyne, as well as the 12C213C propynes observed in natural isotopic abundance. No appreciable state mixing is observed in ν1=1←0 and ν6=1←0 spectra at T=4 K. However, near-resonant two-state mixing of ν2 and ν5+ν8+3ν10 in the ν2=1←0 transition of 12C3 propyne is detected and deperturbed to provide an anharmonic coupling matrix element of 0.096 41(38) cm−1. This matrix element is independent of J' and thus arises from purely anharmonic, non-Coriolis-mediated couplings. The implications of anharmonic coupling matrix elements of this magnitude in overtone vibrational dynamics are discussed.

Notes: Continuous wave difference frequency mixing of a single mode Nd:YAG laser at 1.06 μm and a scanning, single mode ring dye laser (R6G) in a LiNbO3 crystal generates a novel source of widely tunable near infrared radiation in the 1.2–2.2 μm region. In conjunction with the high sensitivity of a pulsed slit nozzle expansion with multipass optics (0.48 m path length), this narrow band source of tunable ir light allows the high resolution study of overtone (v=2←0) spectra for a wide variety of molecular complexes with H stretching vibrations. In this paper, we report the first rotationally resolved spectra of (HF)2 in the first HF stretching overtone region. In particular, we observe Ka=1←0 and 0←0 subbands for a vibrational state from one member of the v=2 overtone triad in (HF)2 with a band center of 7682.8228(5) cm−1. We tentatively assign this state as the hydrogen bond acceptor (i.e., free) HF stretching overtone 2νacc based on predissociation line widths and excellent agreement with predictions based on an anharmonic local mode description of (HF)2. Splittings of 0.2119(5) cm−1 (K'a = 0) and 0.0942(3) cm−1 (K'a = 1) due to interconversion tunneling are found.From the observed intensity alternation due to nuclear spin statistical weights, the overall vibrational symmetry for K'a = 0 and 1 is unambiguously determined to be Γvib=A+ and B+ for the lower and upper tunneling levels, respectively. These A+ and B+ symmetry designations correspond to irreducible representations of the MS4 molecular symmetry group, which allows for large amplitude motion and exchange of the identical HF subunits. Predissociation line broadening is observed in each of the four upper vibrational levels which varies between 56(20) and 175(25) MHz and depends sensitively on both K'a and the tunneling symmetry. This tunneling symmetry dependence, together with the unusual K'a dependence of the tunneling splitting and the anomalously large intensity ratio between the parallel and perpendicular transitions, indicates the presence of vibrational resonances in the overtone region not clearly evidenced in the analysis of the corresponding fundamental HF stretch region. Our results are discussed in the context of earlier static cell FTIR spectra and recent ab initio predictions for this overtone state. The data suggest that the overtone dynamics in (HF)2 can not be satisfactorily described as an oscillator pair connected by a 1D interconversion pathway, and may instead involve substantial coupling to other intermolecular vibrational degrees of freedom.

Notes: Mode–mode vibrational coupling in the acetylinic CH stretch at 3330 cm−1 of 1-butyne and 1-pentyne is studied via high-resolution, direct absorption infrared spectroscopy. As in our previous study of propyne, mixing of the CH stretch vibration carrying oscillator strength (the bright state) with the bath of multiquantum combination states (the dark, or background, states) manifests itself in the spectrum via fragmentation of the isolated bright state transitions into clusters of closely spaced spectral lines in a ∼0.01 cm−1 window about the zeroth order acetylinic CH stretch position. In the 1-butyne spectrum, we find an experimental density of mixed states of 114±30 states/cm−1 compared to a direct state count prediction of 90 total states/cm−1, and thus quantitatively determine that all possible states appear in the spectrum. The 1-butyne line spacing distribution suggests the Wigner distribution expected for a quantum mechanically ergodic system. Analysis of the mode mixing as a function of J' shows that anharmonic terms in the potential, rather than Coriolis effects, contribute most strongly to the coupling. The acetylinic CH stretch spectrum of 1-pentyne (2400 states/cm−1) reveals only broad rovibrational transitions with ∼0.01 cm−1 Lorentzian width, even at our 10−4 cm−1 resolution. J' independent, anharmonic coupling with a minimum of 1/3 of all states must be invoked to reproduce the observed broadening. In contrast, the 1-pentype methyl CH stretch spectrum shows broadening greater than five times larger than that observed at the acetylinic end. Via Fourier transform methods, the spectra for both 1-butyne and 1-pentyne indicate vibrational energy localization in the CH stretch for ∼500 ps. However, for the methyl CH stretch, energy redistribution takes place in 〈40 ps, clearly indicating the presence of mode specific, nonRRKM vibrational relaxation pathways.

Notes: The first high-resolution spectra of ArHF excited to the vHF=2←0 manifold near 7800 cm−1 are recorded via direct infrared absorption in a slit supersonic expansion. The tunable difference frequency light is generated via nonlinear subtraction of a cw Nd:YAG laser from a tunable cw ring dye laser in temperature phase matched LiNbO3, and permits continuous single-mode access to the 1–2 μm near-IR region. Rotationally resolved spectra are presented for the pure HF stretching overtone (2000)←(0000), as well as for combination band excitation into the Σ bend (2100)←(0000) and Π bend (2110)←(0000) internal rotor levels built on the vHF=2 overtone stretch. Local perturbations in the Π bend spectrum are observed which arise from a resonant crossing of rotational levels with the (2002) van der Waals stretch and allow spectroscopic analysis of this state. Nonresonant coupling between the Σ and Π bend vibrational levels is evidenced by anomalous P branch/R branch transition intensities and is analyzed as Coriolis interactions in a tumbling, hindered rotor. The spectra reveal Doppler limited line shapes [Δν=79(11) MHz] characteristic of the temperature and geometry of the slit expansion. An upper limit of Δνprediss≤2 MHz Lorentzian broadening is established, indicating an 80 ns lower limit to the predissociation lifetime. Comparison of intermolecular vibrational levels in ArHF vHF=0, 1, and 2 indicates a systematic increase in both angular anisotropy and radial well depth upon excitation of the high-frequency HF stretch. In conjunction with previous results from the vHF=1 and vHF=0 vibrational levels, these studies provide the necessary data for fitting an atom+diatom potential energy surface as a function of all intermolecular and intramolecular internal degrees of freedom.

Notes: We investigate the reaction of CH(X 2Π) with H2 as a function of temperature in the range 240–470 K at 8.2 and 750 Torr of helium pressure and as a function of helium pressure in the range 8–750 Torr at 294 K. Methylidyne forms upon excimer-laser photolysis of CHBr3 or CHClBr2 in a slow-flow reactor and we time-resolve its concentration profile using cw laser-induced fluorescence. The title reaction proceeds through the formation of an excited methyl radical with multiple open decay channels. We observe dramatically different temperature dependencies at high and low helium bath-gas pressures. At high pressure, collisional stabilization of CH*3 to CH3 dominates the reaction mechanism and the CH-loss rate constant exhibits a negative temperature dependence over the range studied. At low pressure, the predominant product channel switches from CH3 to CH2+H as the temperature increases. We employ a Rice–Ramsperger–Kassel–Marcus-master-equation calculation to model the experimental results.

Notes: The eigenstate-resolved 2ν1 (acetylenic CH stretch) absorption spectrum of propane has been observed for J'=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full-width at half-maximum resolution of 6×10−4 cm−1 (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm−1 wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν1 to the near resonant bath states with an average matrix element of 〈V2〉1/2=0.002 cm−1 that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum.The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z-axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z-axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z-axis Coriolis coupled to the dark states. Z-axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν1 normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν1 do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.

Notes: The high resolution, slit jet cooled infrared v=1←0 methyl asymmetric stretch spectra of trans-2-butene and 1-butene are reported. Both of these molecules are singly unsaturated butene chains, have 30 vibrational degrees of freedom, and yield nearly equivalent vibrational state densities (ρvib≈200 states/cm−1) at CH stretch levels of excitation. The key difference between these two molecules is the presence of a large amplitude C–C–C skeletal torsional coordinate in 1-butene corresponding to a low barrier, internal isomerization pathway which is completely absent in trans-2-butene. The trans-2-butene asymmetric CH stretch (ν16) spectrum is fully discrete at 0.002 cm−1 resolution, and the coarse structure readily assigned to zero order rovibrational transitions (J'K'aK'c ← J‘K‘aK‘c) in an asymmetric top. Fragmentation of these zero order transitions into spectral "clumps'' of fine structure provides direct evidence for coupling of the CH stretch to vibrational bath states, but no evidence for loss of Ka' and Kc' as good quantum labels in the spectrum.The average density of coupled states is found directly from the spectrum to be 114 states/cm−1, i.e., on the order of 0.5 ρvib. In contrast to the behavior in trans-2-butene, the 1-butene v=1←0 methyl asymmetric stretch spectrum exhibits an essentially continuous absorption contour even at Trot=6 K and 0.002 cm−1 resolution. On closer inspection, the 1-butene spectral envelope exhibits reproducible, intramolecular vibrational relaxation (IVR) induced fine structure limited by apparatus resolution and characteristic of highly congested IVR coupling. Analysis of this fine structure indicates a density of coupled states on the order of 1 000–10 000 states/cm−1, i.e., 20–30-fold in excess of ρvib, and 1–2 orders of magnitude larger than observed in trans-2-butene. In order to model the degree of fine structure observed in the spectrum, this level of spectral congestion essentially requires complete mixing of all ρvib⋅(2J'+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively dramatic differences between 1-butene and trans-2-butene behavior support a simple model for strong vibration-rotation (V-R) coupling in the bath states due to large amplitude skeletal motion in the C–C–C torsional mode which greatly enhances the available state density for IVR. Hence, the presence of a low barrier, skeletal isomerization coordinate may prove to be a general, moiety specific promoter for IVR processes in CH stretch excited hydrocarbons.

Notes: The availability of pairwise additive "two-body'' potentials for van der Waals systems from near-IR, far-IR and microwave data permits detailed prediction of librational behavior for isolated HF chromophores solvated by successive numbers of rare gas Ar atoms. This paper describes theoretical calculations of ArnHF equilibrium structures and intermolecular HF vibrational frequencies based on an ArnHF "two-body'' potential energy surface developed from previously determined Ar–Ar and Ar–HF potentials. Isomeric structures are predicted from local minima on these multidimensional surfaces, and are found to be in excellent qualitative agreement with near-IR observations of ArnHF clusters with n=1,2,3, and 4 Ar atoms. Quantum mechanical calculations are performed for the HF librational and van der Waals stretching modes against a rigid Arn frame. These pairwise additive potentials predict a strongly increasing angular anisotropy for the HF bending coordinate with number of Ar atoms (for small n), and provide predictions of HF intermolecular van der Waals bend and stretch vibrational frequencies. Fourier transform (FT)-microwave and near-IR data, on the other hand, demonstrate only a minor dependence of the anisotropy on n; this suggests the pairwise additive potentials may systematically overestimate the angular anisotropy for HF bending. Selected cuts through these potential surfaces indicate significant coupling between the Arn–HF stretch, Ar–Ar stretch, and Ar–Ar bending vibrations. This strong vibrational coupling indicates that a full quantum treatment of all intermolecular coordinates may be required in order to make quantitative comparison with van der Waals vibrational data. In the limit of sufficient Ar atoms to fill the first coordination sphere around the HF, the calculations indicate a nearly perfect cancellation of angular anisotropy for HF librational motion, consistent with the nearly free internal rotation of the HF observed in cryogenic Ar matrices.