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Mode-Specific Vibrational Excitation and Control of Internal Conversion in Polyatomic Molecules

This research study investigates the possibility of controlling internal conversion in polyatomic molecules through mode-specific vibrational excitation. The study focuses on methylamine isotopologues and examines the branching ratios and resonant enhancements in two-photon spectroscopy. Experimental techniques such as resonance-enhanced multiphoton ionization and velocity map imaging are employed to study the dynamics of photofragments. The findings reveal the presence of dynamic resonances that strongly depend on the vibrational modes and initial rovibronic states.

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Mode-Specific Vibrational Excitation and Control of Internal Conversion in Polyatomic Molecules

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  1. Can Internal Conversion be Controlled by Mode-specific Vibrational Excitation in Polyatomic Molecules? Michael Epshtein, Alexander Portnov and Ilana Bar Department of Physics Ben-Gurion University of the Negev Beer-Sheva 84105 ISRAEL

  2. Introduction • Conical intersections - points of degeneracy between electronic states, which act as dynamic funnels for radiationless deactivation of the excited state – playing crucial roles in driving internal conversion in different processes Energy RN-H

  3. Methylamine (CH3NH2) - Why? *appears quite often in organic and biologic building blocks * the smallest member of primary amines possessing, in addition to stretching and bending, inversion motion and internal-rotation * interesting dynamics v7 Energy v9 RN-H

  4. Resonant enhanced two-photon spectroscopy of methylamine isotopologues M. H. Park, K.-W. Choi, Y.S. Choi, and S. K. Kim, J. Chem. Phys. 125, 084311 (2006).

  5. Primary dissociation channels D0 (cm-1) • CH3NH2 H + CH3NH 35,900 • H + CH2NH2 30,600 • CH3 + NH2 29,300 • CH4 + NH 25,570 • CH2NH + H2 10,600 M. N. R. Ashfold, R. N. Dixon, M. Kono, D. H. Mordaunt, and C. L. Reed, Phil. Trans. R. Soc. Lond. A 355, 1659 (1997) . J. O. Thomas, K. E. Lower, and C. Murray, J. Phys. Chem. A 118,9844 ( 2014).

  6. Most studies lacked characterization of the starting quantum states on the S1 state and direct examination of state-selective predissociation dynamics • Would specific vibrational modes on S1 of CH3NH2isotopologues affect the branching ratios?

  7. Velocity map imaging (2 + 1) resonance-enhanced multiphotonionization (REMPI) H+ H+ 2s 2S 2s 2S 1s 2S 1s 2S H+ H+ I *Cylindrically symmetric ion cloud * s(x,y) - a slice through the 3D distribution perpendicular to the symmetry axis * f(x) - the integrated distribution s(x,y) along the y axis - raw ion image * Conversion to a central slice in a 3D velocity map by inverse Abel transform

  8. Ion imaging High velocity photofragments lead to strong Doppler broadening (DB) For an atom traveling in the laboratory frame, the frequency it observes is v - velocity w = wl – kvwl- laser frequency k – wavevector Photofragment detection is accomplished by scanning the probe over the DB profile H atoms can be probed in each laser pulse by two-color reduced Doppler (TCRD) or Doppler-free (DF) (2 + 1) REMPI w = wR+wL – (kR + kL)v selection rules: Dl= 0 and Dm= 0 M. Epshtein, A. Portnov, R. Kupfer, S. Rosenwaks, and I. Bar, J. Chem. Phys. 139, 184201 (2013) M. Epshtein, Y. Monsa, A. Portnov, and I. Bar, Chem. Phys. Lett. 677, 1 (2017)

  9. H photofragment action spectra obtained in CH3NH2 and CD3NH2predissociation R. Marom, C. Levi, T. Weiss, S. Rosenwaks, Y. Zeiri, R. Kosloff, and I. Bar, J. Phys. Chem. A 114, 9623 (2010); R. Marom, T. Weiss, S. Rosenwaks, and I. Bar, J. Chem. Phys. 132, 244310 (2010).

  10. TCRD velocity map images of H atoms, Abel inverted images and TKEDs CH3NH2 CD3NH2 M. Epshtein, A. Portnov, and I. Bar, Phys. Chem. Chem. Phys. 17, 19607 (2015).

  11. Branching ratios and banisotropy parameters for CH3NH2 and CD3NH2

  12. TCRD images, Abel inverted images,TKEDs and action spectrum of CH3ND2 M. Epshtein, Y. Yifrach, A. Portnov, and I. Bar, J. Phys. Chem. Lett. 7, 1717 (2016).

  13. TKEDs of D photofragments released in CH3ND2 predissociation

  14. Summary • The TCRD and DF techniques allowed to measure entire velocity distributions of the photofragments in each laser pulse • The N-H(D) bond fission leads to fast and slow H(D) photofragments • Anomalous distributions in the branching ratios of the H(D) photofragments, Eint(R) and in b parameters for the isotopologues releasing H atoms  dynamic resonances •These resonances strongly depend on the energy of the initially excited rovibronic states, the evolving vibrational mode on the repulsive S1 part during N−D(H) bond elongation, and the manipulated passage through the CI that leads to radicals with high Eint(R) • Theoretical calculations required

  15. Acknowledgements Michael Epshtein Portnov Alexander Yair Yifrach Yaakov Monsa Afik Shahar Tuval Ben Dosa Yuval Ganot Zion Hazan Gyora Gal $$$ISF, BSF Thank You !

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