Library

Notes: In centrosymmetric molecules such as acetylene, one expects one-photon and two-photon spectroscopy to be mutually exclusive by the g↔u electric dipole selection rule. An ultraviolet–ultraviolet double resonance (UVUVDR) spectrum of the 74 000–78 000 cm−1 region of acetylene, recorded using the A˜←X˜ transition for the first step in double resonance consists almost entirely of transitions terminating on levels of the E˜ state identified in one-photon vacuum ultraviolet (vuv) spectroscopy. Evidence for a nonplanar, noncentrosymmetric structure of the E˜ state obtained by rotationally resolved fluorescence dip detected UVUVDR is presented.

Notes: The dispersed fluorescence (DF) and stimulated emission pumping (SEP) spectra of acetylene originating from single rovibronic levels of the A˜ 1Au state were measured with resolutions of 30 and 0.5 cm−1, respectively, in order to examine the vibrational level structure of the electronic ground X˜ 1Σ+g state. The SEP spectra revealed that the number of vibrational levels under each peak in the DF spectra increases with vibrational energy from a single vibrational level below 8000 cm−1 to as many as ten vibrational levels above 16 500 cm−1. Taking account of the fact that a peak in the DF spectrum in the high energy region is composed of more than one level, a DF peak is called a feature state (or a feature). In the DF spectra from two trans-bending levels (v3=2 and 3) of the A˜ state a total of 140 DF features between 5 700 and 21 200 cm−1 were detected and long progressions in the trans bend (v‘4=6 –18) and CC stretch (v(large-closed-square)2=0 –6) were identified.Below 14 000 cm−1, 26 out of the 50 observed features were unambiguously assigned to these two modes and represented by a second order anharmonic expansion within the ∼20 cm−1 experimental error. At least three additional trans-bend progressions built on excitation in third vibrational mode were identified. Possible assignments of the third mode to the CH stretch (ν‘1) and the cis bend (ν(large-closed-square)5) are compared. The Darling–Dennison (DD) resonance between the two degenerate bending modes (trans and cis) was proposed as a mechanism to lend Franck–Condon (FC) intensity to the ν‘5 mode. The vibrational analysis of the DF features shows that the DF features correspond to the zero-order FC bright basis states. Each feature represents a group of levels which share the character of a zero-order FC bright level. Above 14 000 cm−1, characteristic groups of DF features with a width of around 300 cm−1 appear in the DF spectra originating from both v3=2 and v'3=3. The relative intensity patterns within each group of features in the two DF spectra are nearly identical. Three anharmonic resonances, including the DD resonance, are proposed as a plausible mechanism which splits a single FC bright state into several DF features. The SEP measurement revealed that a single DF feature splits further into several features with widths around 0.5 cm−1. The characteristic nested level structure identified in the DF and SEP spectra are explained in terms of a stepwise energy flow via a series of anharmonic resonances from the initially excited CC stretch/trans-bend vibrations to the remaining vibrational modes.

Notes: The signal-to-noise ratio in a time-resolved Fourier transform (FT) infrared emission experiment is improved by pulse-to-pulse normalization. The signal from the FT spectrometer is normalized by the total infrared fluorescence produced on each laser pulse. A factor of 20 enhancement in signal-to-noise ratio is demonstrated with normalization when the fluctuation of the laser pulse energy is the dominant noise source. Applications are discussed pertaining to cases where other noise sources such as detector and amplifier noise cannot be neglected and when information from the time evolution of the spectrum is required. © 1995 American Institute of Physics.

Notes: Six anomalous vibronic feature states [∼2 cm−1 full-width at half-maximum (FWHM), each consisting of ∼20 partially resolved eigenstates] have been observed in stimulated emission pumping (SEP) spectra of C2D2. Of the two plausible assignments for these features, the one most consistent with spectroscopic observations would imply that the lowest energy cis-bent triplet state of acetylene has T0≤25 820 cm−1, which is inconsistent with previous ab initio predictions. New higher level ab initio quantum mechanical methods have been used to predict the energy difference between X˜ 1Σg+ ground state and the cis-bent a˜ 3B2 lowest triplet state of acetylene. In conjunction with a triple zeta plus double polarization plus f function (TZ2Pf) basis set, the coupled cluster including single, double, and linearized triple excitations CCSD(T) method yields T0=ΔE(a˜ 3B2–X˜ 1Σg+)=30 500 cm−1. The true value of T0 for the a˜ 3B2 state is estimated to be ∼500 cm−1 higher. At the same level of theory the zero-point levels of the lowest triplet state of the trans-bent (a˜ 3Bu) and vinylidene (a˜ 3B2) isomers lie at still higher energies. This result conclusively rules out any triplet assignment for the anomalous feature states. The alternative assignment, as highly excited vibrational levels of the X˜ 1Σg+ state, is surprising in view of the Franck–Condon selectivity, dynamical stability, and nonselective relaxation of this special class of "bright states'' observed in the SEP spectra.Such an assignment would be implausible in the absence of the present ab initio calculations. Previous experimental observations [Lisy and Klemperer, J. Chem. Phys. 72, 3880 (1980) and Wendt, Hippler, and Hunziker, J. Chem. Phys. 70, 4044 (1979)] of acetylene triplet states are discussed and shown to be completely consistent with each other and with the present ab initio ordering of the cis and trans isomeric minima on the T1 potential energy surface: cis a˜ 3B2 below trans a˜ 3Bu .

Notes: The product state distributions for hot atom collisions of H(D) with HF(DF) with a broad range of collision energies between 1.2 eV and 2.3 eV are measured with time-resolved Fourier transform spectroscopy and rotational resolution under multiple collision conditions. In most cases the vibrational distributions of reactive and nonreactive channels can be distinguished. All rotational distributions have a similar appearance with a maximum at J=5 and an additional pronounced higher component, which cannot be described by a single Boltzmann distribution. The results are compared with recent three dimensional quasiclassical trajectory calculations (accompanying paper by Schatz) by applying a multiple collision model for both the H(D) atom slowdown and the rotational and vibrational relaxation. The rotational distributions can also be interpreted by a three dimensional asymmetric breathing ellipsoid model. © 1997 American Institute of Physics.