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

Abstract

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.

The 4νCH overtone of 12C2H2: Sub-MHz precision spectrum reveals perturbations

“…19,20 The local perturbation at J = 12 revealed here agrees well with such statement. The perturbations starting at the J = 8 level can be more easily identified from Fig.…”

“…The well-known splits of the J = 17 and J = 18 levels were first reported by Smith and Winn, 7 and later attracted more studies. 8,9,11,12,15,20 In our spectrum recorded under very low sample pressures, the doublets are well resolved and the R(16) and R (17) Table I. Smith and Winn 7 have proposed candidates for the perturber as (2ν 1 + 3ν 2 + ν 1 5 ) or (3ν 2 + 2ν 3 + ν 1 5 ).…”

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

“…8,32 Key issues such as these will be addressed in subsequent papers. 33 In the meantime, the present "foundation studies at low J" (to which the subtitle of this paper alludes) have demonstrated the attention to detail that is essential in IR-UV DR experiments that seek unambiguous characterization of the rovibrational dynamics of a small but not-so-simple polyatomic molecule such as C 2 H 2 .…”

“…Figure 3 is representative of an extensive body of IR-UV DR kinetic results that can be satisfactorily simulated by a phenomenological master-equation model. 32,33 The majority of IR-UV DR measurements presented in this paper are performed with the UV PROBE tuned to the 299 nm Α -˜1 Subsequent papers in this series 33 will explore the apparent energy-transfer gateway at J ) 12. They will also seek explanations of the unusual CIQCB effects 8,32 that underlie most of our 4ν CH IR-UV DR experiments, present additional IR-UV DR results for rovibrational levels with J > 12, and formulate spectroscopic and kinetic models of observed results.…”

“…They are virtually identical, confirming that the asterisked peaks in Figure 8b share a common rovibrational level. Figure 9c shows a template for (ν′ 3 + ν′ 5 ) Α -state rovibronic levels UV-excited from a J ) 10 rovibrational level; it comprises the 1 1 0 3 3 1 5 0 1 band rovibronic spectrum for J ) 10 from Polyad-model calculations 31,33 suggest that the vibrational eigenstate to which the IR-dark J ) 10 level belongs is most likely to have (0 3 1 6 0) 0 Σ u + as its dominant normal-mode basis state, with pseudo-quantum numbers n s ) 4, n res ) 20, and l ) 0 (as for IR-bright components of the 4ν CH polyad). The many CC stretching and trans-bending quanta are consistent with the UV brightness of this level, whereas its complexity should make it IR-dark.…”