Decoding Dynamical Information from Vibrational Spectra

Vibrational excitation sensitive probe of molecular structure, conformation and intramolecular dynamics reaction control or alteration of product branching role of vibrational energy in promoting chemical processes

Vibrational spectra absorption absorption narrow bandwidth laser and double resonance methods in supersonically cooled free-jets single eigenstates of molecules excited to fundamental or low overtones narrow bandwidth laser and double resonance methods in supersonically cooled free-jets single eigenstates of molecules excited to fundamental or low overtones

Vibrationally mediated photodissociation (VMP) monitoring of high overtones via jet- cooled action spectroscopy following monitoring of high overtones via jet- cooled action spectroscopy following VMP action spectra benefit from improved Franck- Condon factors action spectra benefit from improved Franck- Condon factors

H + VMP 1s 2 S v” = 0, J” (2 + 1) REMPI H R-H* R + H v’,J’ X R-H 2s 2 S

Experimental Setup Nd:YAG/OPO or Nd:YAG/Dye/DFM Pump Nd:YAG/Dye Laser+SHG PA Cell Preamp Pump DG BC DO HV TOFMS → PC HV Gas sample MSP PV

two C-H oscillator types two C-H oscillator types acetylenic C-H higher in frequency than methyl C-H - somewhat decoupled acetylenic C-H higher in frequency than methyl C-H - somewhat decoupled frequencies of stretch and bend acetylenic C-H occur with a 5 : 1 ratio - weaker coupling frequencies of stretch and bend acetylenic C-H occur with a 5 : 1 ratio - weaker coupling Propyne (CH 3 C  CH) - Why?

PA and action spectra of the 4 1 region of propyne (CH 3 C  CH) (d (c (b (a 3ν 1 + ν 3 + ν 5 3ν 1 + ν 3 + ν 5 + ν 10 - ν 10 4ν14ν1 3ν 1 + ν 3 + ν 5 + 2ν ν 10 4ν 1 + ν 10 - ν 10 4ν 1 + 2ν ν 10 N (b (a 4ν14ν ps 0.3 ps

Time evolution of propyne states in the 4 1 region the Hamiltonian matrix the Hamiltonian allows following the temporal evolution of the density matrix the Hamiltonian allows following the temporal evolution of the density matrix ; P(t) = U(t)P(t = 0)U † (t) ;

Temporal evolution of propyne states in the 4 1 region

Acetylenic C-H stretches in propyne new bands, N, observed in the regions of the new bands, N, observed in the regions of the second and third C-H acetylenic overtones second and third C-H acetylenic overtones multi time scales for intramolecular vibrational energy redistribution (IVR) multi time scales for intramolecular vibrational energy redistribution (IVR)

of the V = 2 manifold of the C-H methyl stretch of propyne PA and action spectra of the V = 2 manifold of the C-H methyl stretch of propyne wavenumber (cm -1 )

The Model Hamiltonian

the Hamiltonian is block-diagonal in V = m + n + p + ½( ) matrix diagonalization resulted in eigenvalues and eigenvectors best fit parameters were obtained by comparing the band origins to the eigenvalues and the correspondence between calculated intensities and simulated band areas V=1 V=2 V=3 The Local mode/Normal mode model

of the V = manifolds of the C-H methyl stretches of propyne PA and action spectra of the V = manifolds of the C-H methyl stretches of propyne Wavenumber (cm -1 )

Temporal evolution of methyl states of propyne

x6 Variation of the Fermi resonance strength as a function of the vibrational quantum number - resonance between the pure C-H stretches |V 0 0, 0 0> and combinations including, |V , 0 2> - resonance between the pure C-H stretches |V 0 0, 0 0> and combinations including, |V , 0 2> - between C-H stretches and, |V , 2 0> - between C-H stretches and, |V , 2 0>

Methyl C-H stretches in propyne simulation of the action spectra allowed retrieving the homogeneous broadening - increases with manifold number of the C-H stretch action spectra were analyzed by a simplified joint LM/NM model including Fermi resonances and by calculating the intensities evolution of the C-H stretching states occurs on picosecond timescale the coupling for low and higher V-states is doorway and bath states dominated, respectively

SUMMARY measurements and analysis of room temperature PA and jet-cooled action spectra of the acetylenic and methyl C-H stretching overtones of propyne action spectra significantly narrower than PA spectra enabling observation of new features and IVR time scales the lifetime of a state is related to the coupling strength to close resonating states and bath states action spectra together with their simulation and modeling - key players in obtaining a global accounting of the studied regions these findings speak to the utility of the VMP method as a new “window” providing a different view of the vibrational dynamics

S. Rosenwaks S. Rosenwaks A. Portnov, E. Bespachiansky, Y. Ganot, L. Blockstein A. Portnov, E. Bespachiansky, Y. Ganot, L. Blockstein R. Marom, C. Levi, A. Golan R. Marom, C. Levi, A. Golan $$$ ISF, JF $$$ ISF, JF

Intensity calculations The oscillator strength is given by: The dipole moment function is expressed as a series of expansion around the equilibrium configuration: