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Nuclear motion captured by the slow electron velocity imaging technique in the tunnelling predissociation of the S1 methylamine
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Image of FIG. 1.
FIG. 1.

One-color two-photon (1+1) ionization velocity map photoelectron images with reconstructed photoelectron spectra (upper parts), and two-color two-photon (1+1) ionization SEVI images with corresponding photoelectron spectra (lower parts) of (a) CH3NH2 and (b) CH3ND2 taken via the S19 = 1) intermediate state, respectively. The blue stick spectra represent the Frank-Condon simulation (see the text). Here ν 1 +, ν 4 +, and ν + 9 are NH(D)2 symmetric stretching, scissoring, or wagging mode of the methylamine cation, respectively. In photoelectron images, the ring with the smaller radius corresponds to the peak with the higher ion internal energy in photoelectron spectra. The regions I and II represent the energy regions near ν 9 + and ν 9 ++ν 1  +  modes, respectively.

Image of FIG. 2.
FIG. 2.

Regions I and II of SEVI spectra of (a) CH3NH2 and (b) CH3ND2 indicated in Fig. 1. Open circles via the ν 9 = 1 with no torsional excitation, dashed lines via the ν9 combined with torsional mode excitation of the S1 state. Here, ν 1 +, ν 9 +, and ν 10 + are NH(D)2 symmetric stretching, wagging, or anti-symmetric stretching mode of the methylamine cation, respectively. The (ν 9 + ν 10)+ combination bands are observed at 105 or 160 cm−1 higher than (ν 9 + ν 1)+ bands of CH3NH2 or CH3ND2, respectively. In region II of (b) CH3ND2, a simulated spectrum (red sticks) calculated from the hindered rotor Hamiltonian17 is shown. In order of increasing ion internal energy, red sticks correspond to rotational transitions of 0a1 ← 0a1, 1e ← 1e, 2e ← 1e, 3a1 ← 0a1, and 3a2 ← 0a1.

Image of FIG. 3.
FIG. 3.

(a) Schematic of the calculated potential energy surfaces of the S1 state along two N–H bond elongation coordinates, shown with optimized geometries at the minimum energy point and transition state. (b) Proposed mechanism for tunnelling reactions of CH3NH2 and CH3ND2 on the S1 states. The ultrafast H-atom tunnelling occurs at the frozen geometry whereas the D-atom tunnelling takes place with the significant torsional mode excitation during its longer retention time.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Nuclear motion captured by the slow electron velocity imaging technique in the tunnelling predissociation of the S1 methylamine