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 0.001 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) 114301)

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 Lett. 491 (2010) 17, and unpublished work)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

ν1 C-H stretch – symmetric 3374 cm-1

ν2 C C stretch 1974 cm-1

ν3 C-H stretch – asymmetric 3289 cm-1

ν4 trans bend 612 cm-1

ν5 cis bend 730 cm-1

Vibrational basis state labels: v1 v2 v3 v4 v5, l4 l5 e/f g/u

Total vibrational angular momentum: k = l4 +l5

Polyad Structure of Acetylene

Based on the ratios of vibrational frequencies ν1 : ν2: ν3: ν4: ν5 = 5 : 3 : 5 : 1 : 1 Polyad number: Nr = 5v1+3v2+5v3+v4+v5 No. of stretch quanta: Ns = v1+v2+v3

Polyads without Coriolis coupling {Ns, Nr, ke/o, e/f, u/g, J}

Polyads with Coriolis coupling {Nr, 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

ν4 + ν5 at 1328 cm-1

Basic interactions: Anharmonic (+ Rotational l-resonance)

ν3 at 3288 cm-1

Basic interactions: More anharmonic couplings

Darling-Dennison bending resonance K4455

Crucial to bending dynamics and birth of new kinds of bending modes (Field, Kellman)

Darling-Dennison CH stretching resonance K1133

Responsible for normal to local mode transition (Lehmann)

Basic interactions: Coriolis (+ Rotational l-res + anharmonic)

ν3 at 3288 cm-1

Thermally rotationally averaged dynamics

A stretch-bend combination near 4500 cm-1

ν3 + 2ν4 at 4498 cm-1

A bending overtone near 4400 cm-1

7ν4 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.