Rotational dependence of intramolecular dynamics in acetylene as deduced from high resolution spectroscopy David Perry, Anthony Miller B. Amyay, A. Fayt, and M. Herman Fonds National de la Recherche Scientifique (Belgium) Action de Recherches Concertées de la Communauté française de Belgique U. S. Department of Energy

Outline Acetylene rotation-vibration spectroscopy Polyad structure Time-dependent dynamics Basic interactions Low-energy dynamics Intermediate energy dynamics

Rotation-Vibration Spectroscopy of Acetylene 15,562 lines up to 8600 cm -1 fit to cm -1 B. Amyay, S. Robert, M. Herman, A. Fayt, B. Raghavendra, A. Moudens, J. Thiévin, B. Rowe, and R. Georges, J. Chem. Phys. 131 (2009) ) 18,507 lines up to 13, 227 cm -1 fit to lower precision. IR, NIR, and THz data (B. Amyay, M. Herman, A. Fayt, L. Fusina, and A. Predoi- Cross, Chem. Phys. Lett. 491 (2010) 17, and unpublished work) 369 constants 4 kinds of coupling terms: anharmonic, vibrational l-type, rotational l-type, and Coriolis Michel Herman Badr Amyay

Acetylene vibrational modes ν 1 C-H stretch – symmetric 3374 cm -1 ν 2 C C stretch 1974 cm -1 ν 3 C-H stretch – asymmetric 3289 cm -1 ν 4 trans bend 612 cm -1 ν 5 cis bend 730 cm -1 Vibrational basis state labels: v 1 v 2 v 3 v 4 v 5, l 4 l 5 e / f g / u Total vibrational angular momentum: k = l 4 +l 5

Polyad Structure of Acetylene Based on the ratios of vibrational frequencies ν 1 : ν 2 : ν 3 : ν 4 : ν 5 = 5 : 3 : 5 : 1 : 1 Polyad number: N r = 5 v 1 +3 v 2 +5 v 3 + v 4 + v 5 No. of stretch quanta: N s = v 1 + v 2 + v 3 Polyads without Coriolis coupling { N s, N r, k e/o, e / f, u / g, J } Polyads with Coriolis coupling { N r, e / f, u / g, J } 45 coupled states at 4,500 cm -1 ; 897 at 10,500 cm -1

Time-dependent dynamics n coupled levels: If is the bright state, the spectral intensities are The time-dependent wave function following a coherent excitation: Probability of basis state j as a function of time: Tony Miller

Basic interactions: Rotational l-resonance ν 4 + ν 5 at 1328 cm -1

Basic interactions: Anharmonic (+ Rotational l-resonance) ν 3 at 3288 cm -1

Basic interactions: More anharmonic couplings Darling-Dennison bending resonance K 4455 Crucial to bending dynamics and birth of new kinds of bending modes (Field, Kellman) Darling-Dennison CH stretching resonance K 1133 Responsible for normal to local mode transition (Lehmann)

Basic interactions: Coriolis (+ Rotational l-res + anharmonic) ν 3 at 3288 cm -1

Thermally rotationally averaged dynamics

A stretch-bend combination near 4500 cm -1 ν 3 + 2ν 4 at 4498 cm -1

A bending overtone near 4400 cm -1 7ν 4 at 4419 cm -1

Conclusions: Lower energy dynamics Sparse coupling with quantum beats. An inhomogeneous thermal rotational average produces the appearance of irreversible decay. The nature of the bright state is crucial. similar behavior across polyads. Rotational l-resonance dominates at high J. Coriolis coupling localized and contributes little to overall dynamics

Intermediate energies: A bending overtone near 10,500 cm -1 Polyad connects 897 vibrations Jonathan Martens

Strong rotational effects Intermediate energies: A bending overtone near 10,500 cm -1

Exploration of phase space

Conclusions: Intermediate energy region Dynamics span 3 orders of magnitude in time. A hierarchy of coupling stages. The volume of phase space explored increases dramatically with rotational excitation. The nature of the bright state remains crucial.