Trans–cis isomerization of acetylene in the Ã1Au state as studied by dispersed fluorescence spectroscopy

Probing the predissociated levels of the S1 state of acetylene via H-atom fluorescence and photofragment fluorescence action spectroscopy

We report two new experimental schemes to obtain rotationally resolved high-resolution spectra of predissociated S₁ acetylene levels in the 47 000-47 300 cm^-1 energy region (∼1200 cm^-1 above the predissociation threshold). The two new detection schemes are compared to several other detection schemes (employed at similar laser power, molecular beam temperature, and number of signal averages) that have been used in our laboratory to study predissociated S₁ acetylene levels, both in terms of the signal-to-noise ratio (S/N) of the resultant spectra and experimental simplicity. In the first method, H-atoms from the predissociated S₁ acetylene levels are probed by two-photon laser-induced fluorescence (LIF). The H-atoms are pumped to the 3d level by the two-photon resonance transition at 205.14 nm. The resulting 3d-2p fluorescence (654.5 nm) is collected by a photomultiplier. The S/N of the H-atom fluorescence action spectrum is consistently better by ∼3× than that of the more widely used H-atom resonance-enhanced multiphoton ionization (REMPI) detection. Laser alignment is also considerably easier in H-atom fluorescence detection than H-atom REMPI detection due to the larger number-density of molecules that can be used in fluorescence vs. REMPI detection schemes. In the second method, fluorescence from electronically excited C₂ and C₂H photofragments of S₁ acetylene is detected. In contrast to the H-atom detection schemes, the detected C₂ and C₂H photofragments are produced by the same UV laser as is used for the à - X ̃ acetylene excitation. As a result, laser alignment is greatly simplified for the photofragment fluorescence detection scheme, compared to both H-atom detection schemes. Using the photofragment fluorescence detection method, we are able to obtain action spectra of predissociated S₁ acetylene levels with S/N ∼2× better than the HCCH REMPI detection and ∼10× better than H-atom and HCCH LIF detection schemes.

Simplified Cartesian basis model for intrapolyad emission intensities in the bent-to-linear electronic transition of acetylene

The acetylene emission spectrum from the trans-bent electronically excited à state to the linear ground electronic X̃ state has attracted considerable attention because it grants Franck–Condon access to local bending vibrational levels of the X̃ state with large-amplitude motion along the acetylene ⇌ vinylidene isomerization coordinate. For emission from the ground vibrational level of the à state, there is a simplifying set of Franck–Condon propensity rules that gives rise to only one zero-order bright state per conserved vibrational polyad of the X̃ state. Unfortunately, when the upper level involves excitation in the highly admixed ungerade bending modes, ν4′ and ν6′, the simplifying Franck–Condon propensity rule breaks down--as long as the usual polar basis (with v and l quantum numbers) is used to describe the degenerate bending vibrations of the X̃ state--and the intrapolyad intensities result from complicated interference patterns between many zero-order bright states. In this article, we show that, when the degenerate bending levels are instead treated in the Cartesian two-dimensional harmonic oscillator basis (with vx and vy quantum numbers), the propensity for only one zero-order bright state (in the Cartesian basis) is restored, and the intrapolyad intensities are simple to model, as long as corrections are made for anharmonic interactions. As a result of trans ⇌ cis isomerization in the à state, intrapolyad emission patterns from overtones of ν4′ and ν6′ evolve as quanta of trans bend (ν3′) are added, so the emission intensities are not only relevant to the ground-state acetylene ⇌ vinylidene isomerization, they are also a direct reporter of isomerization in the electronically excited state.

Ab Initio Calculation of the Low-Lying Vibrational States of C2H2(Ã) in Full Dimensionality

We report full-dimensional calculations of vibrational energies of trans-C2H2(A) using the code MULTIMODE and with a full-dimensional potential energy surface obtained by fitting singles and doubles coupled-cluster equations-of-motion (EOM-CCSD) energies using a [3s 2p 1d] atomic natural orbital basis. The EOM-CCSD calculations were done with the code "ACES II". We compare the properties of the potential surface to previous calculations at the trans minimum and also compare the vibrational energies to experimental ones.

A wave-packet simulation of the low-lying singlet electronic transitions of acetylene

The vibronic structure of the S0 --> S1 and the S0 --> S2 electronic transitions of acetylene is studied theoretically based on an ab initio quantum-dynamical approach. The underlying potential-energy surfaces and transition dipole moment functions are obtained from high-level multireference calculations, including the Davidson correction. Ensuing quantum-dynamical simulations rely on the wave-packet propagation method, using grid techniques, and including three nuclear degrees of freedom (C-C stretching and both HCC bending modes for J = 0). The importance of strong anharmonicity is assessed, especially for the S2 excited state with its unusual potential-energy surface. Good overall agreement with the experimental UV absorption spectrum of acetylene is achieved in the range of 6-8 eV.