The infrared-ultraviolet dispersed fluorescence spectrum of acetylene: New classes of bright states

Communication: Observation of local-bender eigenstates in acetylene

We report the observation of eigenstates that embody large-amplitude, local-bending vibrational motion in acetylene by stimulated emission pumping spectroscopy via vibrational levels of the S1 state involving excitation in the non-totally symmetric bending modes. The N(b) = 14 level, lying at 8971.69 cm(-1) (J = 0), is assigned on the basis of degeneracy due to dynamical symmetry breaking in the local-mode limit. The level pattern for the N(b) = 16 level, lying at 10 218.9 cm(-1), is consistent with expectations for increased separation of ℓ = 0 and 2 vibrational angular momentum components. Increasingly poor agreement between our observations and the predicted positions of these levels highlights the failure of currently available normal mode effective Hamiltonian models to extrapolate to regions of the potential energy surface involving large-amplitude displacement along the acetylene ⇌ vinylidene isomerization coordinate.

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.

Full dimensional Franck-Condon factors for the acetylene à (1)A(u)-X̃ (1)Σ(g)(+) transition. I. Method for calculating polyatomic linear-bent vibrational intensity factors and evaluation of calculated intensities for the gerade vibrational modes in acetylene

Franck-Condon vibrational overlap integrals for the à Au1-X̃ 1Σg+ transition in acetylene have been calculated in full dimension in the harmonic normal mode basis. The calculation uses the method of generating functions first developed for polyatomic Franck-Condon factors by Sharp and Rosenstock [J. Chem. Phys. 41(11), 3453-3463 (1964)], and previously applied to acetylene by Watson [J. Mol. Spectrosc. 207(2), 276-284 (2001)] in a reduced-dimension calculation. Because the transition involves a large change in the equilibrium geometry of the electronic states, two different types of corrections to the coordinate transformation are considered to first order: corrections for axis-switching between the Cartesian molecular frames and corrections for the curvilinear nature of the normal modes at large amplitude. The angular factor in the wavefunction for the out-of-plane component of the trans bending mode, ν4(″), is treated as a rotation, which results in an Eckart constraint on the polar coordinates of the bending modes. To simplify the calculation, the other degenerate bending mode, ν5(″), is integrated in the Cartesian basis and later transformed to the constrained polar coordinate basis, restoring the conventional v and l quantum numbers. An updated Ã-state harmonic force field obtained recently in the R. W. Field research group is evaluated. The results for transitions involving the gerade vibrational modes are in qualitative agreement with experiment. Calculated results for transitions involving ungerade modes are presented in Paper II of this series [G. B. Park, J. H. Baraban, and R. W. Field, "Full dimensional Franck-Condon factors for the acetylene à Au1-X̃ 1Σg+ transition. II. Vibrational overlap factors for levels involving excitation in ungerade modes," J. Chem. Phys. 141, 134305 (2014)].

Electronic Signatures of Large Amplitude Motions: Dipole Moments of Vibrationally Excited Local-Bend and Local-Stretch States of S0 Acetylene

A one-dimensional local bend model is used to describe the variation of electronic properties of acetylene in vibrational levels that embody large amplitude local motions on the S0 potential energy surface. Calculations performed at the CCSD(T) and MR-AQCC levels of theory predict an approximately linear dependence of the dipole moment on the number of quanta in either the local bending or local stretching excitation. In the local mode limit, one quantum of stretching excitation in one CH bond leads to an increase of 0.025 D in the dipole moment, and one quantum of bending vibration in the CCH angle leads to an increase of 0.068 D. The use of a one-dimensional model for the local bend is justified by comparison to the well-established polyad model which reveals a decoupling of the large amplitude bending from other degrees of freedom in the range of Nbend = 14-22. We find that the same one-dimensional large amplitude bending motion emerges from two profoundly different representations, a one-dimensional cut through an ab initio, seven-dimensional Hamiltonian and the three-dimensional (l = 0) pure-bending experimentally parametrized spectroscopic Hamiltonian.

Bending Dynamics of Acetylene: New Modes Born in Bifurcations of Normal Modes

Semiclassical techniques are used to analyze highly excited pure bending vibrational dynamics from spectra of C2H2. An analytic bifurcation approach is developed, based on critical points of a classical version of the quantum fitting Hamiltonian. At high energy four new types of anharmonic modes are born in bifurcations of the normal modes: local, orthogonal, precessional, and counter-rotator. Visual insight into their nature is obtained with the help of computer-generated three-dimensional animations. The connection between the local mode and the acetylene-vinylidene isomerization "reaction mode" is considered.

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene, Viewed by IR−UV Double-Resonance Spectroscopy. 4. Collision-Induced Quasi-Continuous Background Effects

The 4nu(CH) rovibrational manifold around 12 700 cm(-1) in the electronic ground state, X, of acetylene (C2H2) is monitored by time-resolved infrared-ultraviolet double-resonance (IR-UV DR) spectroscopy. An IR laser pulse initially prepares rotational J states, associated with the "IR-bright" (nu1 + 3nu3) or (1 0 3 0 0)0 vibrational combination level, and subsequent collision-induced state-to-state energy transfer is probed by UV laser-induced fluorescence. Anharmonic, l-resonance, and Coriolis couplings affect the J states of interest, resulting in a congested rovibrational manifold that exhibits complex intramolecular dynamics. In preceding papers in this series, we have described three complementary forms of the IR-UV DR experiment (IR-scanned, UV-scanned, and kinetic) on collision-induced rovibrational satellites, comprising both regular even-DeltaJ features and unexpected odd-DeltaJ features. This paper examines an unusual collision-induced quasi-continuous background (CIQCB) effect that is apparently ubiquitous, accompanying regular even-DeltaJ rovibrational energy transfer and accounting for much of the observed collision-induced odd-DeltaJ satellite structure; certain IR-bright (1 0 3 0 0)0 rovibrational states (e.g., J = 12) are particularly prominent in this regard. We examine the mechanism of this CIQCB phenomenon in terms of a congested IR-dark rovibrational manifold that is populated by collisional transfer from the nearly isoenergetic IR-bright (1 0 3 0 0)0 submanifold.

The vibrational energy pattern in acetylene VII: C12C13H2

In (12)C(13)CH(2) 129 vibrational term values up to 10,000 cm(-1) are merged, about 60% of which are newly reported. They are fitted using an effective Hamiltonian with a standard deviation of 0.22 cm(-1). The vibrational assignments and vibrational constants are listed and discussed. The energy pattern is found to be very similar to the one in (12)C2H2 with additional anharmonic resonances arising from the lack of u/g character in the asymmetric isotopolog.

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene Viewed by IR−UV Double Resonance Spectroscopy. 2. Perturbed States with J = 17 and 18

Collision-induced state-to-state molecular energy transfer between rovibrational states in the 12,700 cm(-1) 4nu(CH) manifold of the electronic ground state X of acetylene (C(2)H(2)) is monitored by time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy. Rotational J-states associated with the (nu(1) + 3nu(3)) or (1 0 3 0 0)(0) vibrational combination level, initially prepared by an IR pulse, are probed at approximately 299, approximately 296, or approximately 323 nm with UV laser-induced fluorescence via the Alpha electronic state. The rovibrational J-states of interest belong to a congested manifold that is affected by anharmonic, l-resonance, and Coriolis couplings, yielding complex intramolecular dynamics. Consequently, collision-induced rovibrational satellites observed by IR-UV DR comprise not only regular even-DeltaJ features but also supposedly forbidden odd-DeltaJ features. A preceding paper (J. Phys. Chem. A 2003, 107, 10759) focused on low-J-value rovibrational levels of the 4nu(CH) manifold (particularly those with J = 0 and J = 1) whereas this paper examines locally perturbed states at higher values of J (particularly J = 17 and 18, which display anomalous doublet structure in IR-absorption spectra). Three complementary forms of IR-UV DR experiments (IR-scanned, UV-scanned, and kinetic) are used to address the extent to which intramolecular perturbations influence the efficiency of J-resolved collision-induced energy transfer with both even and odd DeltaJ.