Unexpected simplicity in the S1–S dispersed fluorescence spectra of C213H2

Abstract

We have recorded dispersed fluorescence (DF) spectra (18 cm−1 resolution) from the JKaKc=110 rotational level of six vibrational levels in the S1 (Ã 1Au) state of C213H2. Improvements in our methods of recording and calibrating DF spectra, that have enhanced the quality of our data sets, are briefly discussed. More than 50 fractionated bright state patterns associated with approximately conserved polyad quantum numbers have been extracted from our DF data sets using a spectroscopic pattern recognition technique, extended cross correlation. These polyads extend to internal energies as high as 20 000 cm−1 above the zero-point vibrational level. The polyad fractionation patterns observed at high energy are surprisingly simple relative to the corresponding patterns of C212H2. Comparison between the DF spectra of C213H2 and C212H2 reveals slower intramolecular vibration redistribution in C213H2, particularly with nonzero quanta of CC stretch excitation. More than 15 patterns were extracted above the energy at which acetylene↔vinylidene isomerization is predicted to be energetically feasible (∼15 200 cm−1) and the dynamical information encoded in these patterns is addressed. In particular, we have analyzed a subset of the C213H2 polyads, the pure bending polyads, those with zero quanta of excitation in each of the stretch modes, (Ns=v1+v2+v3=0). The observed pure bending levels are reproduced to a root-mean-square error of <1.5 cm−1 by two different effective Hamiltonian models: an 11 parameter normal-mode and a 13 parameter local-mode model.

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

“…As part of this family, directly observed infrared absorption spectra have yielded an extensive vibrational-spectroscopic database − ,,− and have enabled a global fit of the vibrational energy pattern to available spectroscopic data up to ∼19 000 cm -1 in terms of the polyad/cluster model. Likewise, measurement of dispersed rovibronic LIF from the S 1 (trans-bent à 1 A u ) electronic state, entailing laser excitation of a single low- J Ã−X̃ transition (either under gas-phase conditions that allow collisional relaxation or collision-free in a molecular beam) combined with elegant numerical pattern-recognition methods, also provide an alternative direct spectroscopic view of vibrational states of C 2 H 2 at high energies (typically 6 000−20 000 cm -1 ). ,, A complementary spectroscopic technique, also relatively direct, is stimulated emission pumping (SEP), which has been applied extensively to high-energy vibrational states of C 2 H 2 . 60d,, Spectroscopic databases derived as above include many important homogeneous vibrational perturbations (e.g., anharmonic mixing and vibrational l -doubling) that can be characterized in the low- J limit. However, local J -dependent perturbations (e.g., rotational l -resonance and Coriolis coupling) tend to be treated on a case-by-case basis. ,,,, …”

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene, Viewed by IR−UV Double Resonance Spectroscopy. 1. Foundation Studies at Low J

“…Numerous other acetylene high-overtone experiments entail techniques such as IR absorption, ,− stimulated emission pumping (SEP), ,− dispersed LIF (both rovibronic ,− ,, and rovibrational − ), and molecular-beam laser-Stark spectroscopy. − Pulsed two-step IR−UV excitation of C 2 H 2 has also been used in spectroscopic , and molecular-action − experiments.…”

“…[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] Much of our own work 7,8,[28][29][30][31]40,41,[43][44][45][47][48][49] has pursued the role of intramolecular perturbations in promoting (or, in some cases, inhibiting) the efficiency of rotationally selective vibrational energy transfer. [55][56][57] Numerous other acetylene high-overtone experiments entail techniques such as IR absorption, 6,9-15 stimulated emission pumping (SEP), 4,[58][59][60] dispersed LIF (both rovibronic 20,[23][24][25][26][27]61,62 and rovibrational [63][64][65] ), and molecular-beam laser-Stark spectroscopy. [66][67][68] Pulsed two...…”

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene, Viewed by IR−UV Double Resonance Spectroscopy. 5. Detailed Kinetic Model

“…The IR-bright 6ν 3 J = 2 and J = 4 levels exhibit splittings that are attributed to local perturbations via couplings to the full density of vibrational states expected in in the 6ν CH manifold of C 2 H 2 . However, there is no evidence of tunneling splittings that would indicate the mechanistically significant isomerization of C 2 H 2 to vinylidene (H 2 CC:) that might be expected at such a high vibrational energy. ,,86g,,, …”

“…This is because IR−UV DR spectroscopy alone employs Franck−Condon factors associated with UV PROBE excitation to project out IR-dark/UV-bright rovibrational levels ( V , J , K ) final that are populated by collision-induced transfer from IR-bright rovibrational levels ( V , J , K ) init selectively prepared by the IR PUMP. Several other versatile Franck−Condon-assisted detection techniques, including stimulated emission pumping 85 and dispersed rovibronic LIF, , are also useful in elucidating the high-energy rovibrational manifolds of C 2 H 2 . However, none of these approaches have the distinctive capability of IR−UV DR spectroscopy to reveal the combined IR and UV darkness/brightness of rovibrational levels involved in collision-induced energy transfer …”