INFRARED-ULTRAVILET DOUBLE RESONANCE SPECTROSCOPY OF ACETYLENE: UNRAVELLING THE MYSTERIES IN THE νCC+3νCH\nu_{CC} + 3\nu_{CH} REGION AT 11 600cm−1cm^{-1} USING THE CLUSTER/POLYAD MODEL

$^{h}$A. P. Milce, H.-D. Barth and B. J. Oty, {J Chem. Phys}. \textbf{100} 2398 (1994) A. P. Milce and B. J. Orr, {J. Chem. Phys}. 104, 6423 (1996), ibid 106 (1997) in peint. $^{i}$M. A. Temsamani and M. Herman, J. Chem Phys \textbf{102}. 6371 (1995); ibid. 105, 1355(1996); M. A. Temsamani, M. Herman, S. A. B. Solina, J.P.O'Brien and R. W. Field, J. Chem. Phys. \textbf{105}, 11357 (1996).Author Institution: School of Chemistry and Centre for Lasers and Applications, Macquarie UniversityTime-resolved fluorescence-detected infrared-ultraviolet optical double resonance (IR-UV DR) experiments have been. used to obtain detailed measurements of spectroscopic and dynamical processes in the ``$\nu_{CC} + 3\nu_{CH}$^{\prime\prime} region of acetylene ($C_{2}H_{2}$) at 11 $600 cm^{-1}$. The rotational energy transfer (RET) and vibrational energy transfer (RET) and vibrational energy transfer (V-V) kinetic data collected have already been well fit to 3 detailed rate-equation model that incorporates empirical exponential -gap fitting $laws.^{h}$ In order to explain some of the unusual effects observed in this region we now use the cluster/polyad model that has already successfully interpreted the vibrational energy pattern in $C_{2}H_{2}$ up to $12 000 cm^{-1}.^{i}$ Our work emphasises rotational perturbations, after including the rotational l-type resonance interaction. This enables us to identify the intramolecular couplings responsible for enhancement of rotationally-resolved V-V energy transfer between coupled rovibrational levels of the ``$\nu_{CC} + 3\nu_{CH}$^{\prime\prime} manifold. The model also provides qualitative evidence that Coriolis-type perturbations are responsible for other unusual symmetry-breaking phenomena in this region

The nCC + 3nCH rovibrational manifold of acetylene. II. Intramolecular perturbations and symmetry-breaking processes

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INFRARED-ULTRAVIOLET DOUBLE RESONANCE SPECTROSCOPY OF ACETYLENE: INTRAMOLECULAR COUPLINGS AT 11600CM−111 600 CM^{-1}

A.P. Milce, H.-D. Barth and B.J. Orr, J. Chem, Phys, 100, 2398 (1994).Author Institution: Macquarie University, Sydney, NSW, Australia 2109In the fluorescence-detected infrared-ultraviolet double resonance (IR-UV DR) experiments presented here, a Ramanshifted dye laser is used to prepare gas-phase $C_{2}H_{2}$ molecules in the $^{\prime\prime}\nu_{2+}3\nu_{3}^{\prime\prime}$ region followed by LIF probing in the $\tilde{A} - \tilde{X}$ vibronic band system. A complicated variety of intramolecular perturbations are found in the $\nu_{2} + 3\nu_{3}$ rovibrational manifold: anharmonic, Coriolis, l-type. These perturbations reveal themselves through the occurrence of only few rovibrational states with sufficiently large Frank-Condon factors to enable detection by LIF. A novel approach to the IR-UV DR experiment is adopted, in which the sum of the IR and UV frequencies is held constant, enabling the characterisation of IR-dark (UV-bright) rovibrational eigenstates associated with these few detectable IR-bright (UV-dark) rovibrational eigenstates found in the $\nu_{2} + 3\nu_{3}$ region. The appearance of unexpected $\Delta J = \pm 1$ features in collision-induced IR-UV DR spectra provide additional evidence of intramolecular perturbations

Rovibrational energy transfer in the 4v_CH 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 4νCH manifold of the electronic ground state Χ of acetylene (C₂H₂) is monitored by time-resolved infrared−ultraviolet double resonance (IR−UV DR) spectroscopy. Rotational J-states associated with the (ν₁ + 3ν₃) or (1 0 3 0 0)⁰ vibrational combination level, initially prepared by an IR pulse, are probed at ~299, ~296, or ~323 nm with UV laser-induced fluorescence via the Α 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-ΔJ features but also supposedly forbidden odd-ΔJ features. A preceding paper (J. Phys. Chem. A 2003, 107, 10759) focused on low-J-value rovibrational levels of the 4νCH 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 ΔJ.12 page(s

Rovibrational energy transfer in the 4ν_CH manifold of acetylene, viewed by IR-UV double-resonance spectroscopy. 4. Collision-induced quasi-continuous background effects

The 4vCH 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” (v₁ + 3v₃) or (1 0 3 0 0)⁰ vibrational combination level, and subsequent collision-induced state-to-state energy transfer is probed by UV laserinduced 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, UVscanned, and kinetic) on collision-induced rovibrational satellites, comprising both regular even-∆J features and unexpected odd-∆J features. This paper examines an unusual collision-induced quasi-continuous background (CIQCB) effect that is apparently ubiquitous, accompanying regular even-∆J rovibrational energy transfer and accounting for much of the observed collision-induced odd-∆J satellite structure; certain IRbright (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.13 page(s

Rovibrational energy transfer in the 4vCH manifold of acetylene, viewed by IR-UV double resonance spectroscopy. 3. State-to-state J-resolved kinetics

Time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy is used to study the kinetics of collision-induced state-to-state molecular energy transfer between rovibrational states in the 12700-cm⁻¹ 4νCH manifold of the electronic ground state of acetylene (C₂H₂). Particular initial and final rovibrational J-states are prepared and monitored by a pair of tunable laser pulses (IR PUMP and UV PROBE) and the kinetic results recorded by continuously varying the time delay between those pulses at a set sample pressure. After allowing for collision-induced quenching of fluorescence and mass transfer from the IR-UV optical excitation zone (by beam flyout and diffusion), an array of kinetic data for J-resolved energy-transfer channels can be interpreted by means of a mechanistically structured master-equation model. This paper focuses on kinetics derived by probing C₂H₂ in its 4νCH J = 12 state (which is affected by intramolecular perturbations and implicated in unusual collision-induced quasi-continuous background effects) and J-resolved collision-induced rovibrational energy transfer with both even ΔJ and (supposedly forbidden) odd ΔJ.33 page(s

'Symmetry-Breaking' Rovibrational Energy Transfer in Acetylene Molecules ... and Other Mysteries

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Rovibrational energy transfer in the 4vCH manifold of acetylene, viewed by IR-UV double resonance spectroscopy : kinetics of a collision-induced quasi-continuous background

The 4νCH rovibrational manifold at ~12 700 cm⁻¹ in the electronic ground state of acetylene (C₂H₂) has been studied by time-resolved IR-UV double resonance spectroscopy. An IR laser pulse prepares rotational J-states, associated with the "IR-bright" (ν₁ + 3ν₃) vibrational combination level, and subsequent collision-induced state-to-state energy transfer is probed by pulsed UV laser-induced fluorescence. Anharmonic, ℓ-resonance, and Coriolis couplings affect J-states of interest, resulting in a congested rovibrational manifold with complex intramolecular dynamics. These experiments reveal collision-induced rovibrational satellites, comprising regular even-ΔJ features and odd-ΔJ features that are unexpected in view of the ortho/para nuclear-spin symmetry of C₂H₂. An unusual collision-induced quasi-continuous background (CIQCB) effect is apparently ubiquitous, accompanying regular even-ΔJ rovibrational energy transfer and accounting for much of the observed collision-induced odd-ΔJ satellite structure. This CIQCB phenomenon is examined in terms of a congested IR-dark rovibrational manifold populated by collisional transfer from the nearby IR-bright (ν₁ + 3ν₃) submanifold. A kinetic master-equation model provides a satisfactory phenomenological fit to the processes of interest.4 page(s