1E10
Unveiling electron momentum spectroscopy of Acetone upon 195nm laser
excitation by nonadiabatic ab initio molecular dynamics simulation○Le Yu1,2, Masahiko Takahashi1
1IMRAM, Tohoku University, Sendai, Japan
2
College of Chemistry & Materials Science, Northwest University, Xi’an, P. R. China[Abstract] The photodissociation of deuterated acetone upon 195nm laser excitation wereinvestigated by employing the trajectory-based on-the-fly nonadiabatic ab initio moleculardynamics (AIMD) simulations with the potential energies and gradients calculated atCASSCF/6-31+G** level. The primary methyl radical dissociation take place in S2 state andyields to the linear 3s acetyl radical. When the C-C bond length increase to ~3.0 Å, the 3s
acetyl radical splits into two distinct pathways, immediate cascade S2→S1→S0 hopping
within 100 fs or stay in the 3s Rydberg state for a certain time (0.3-6.0 ps) before that. The
secondary methyl dissociation occur in the ground state of acetyl radical, and due to the rather
high kinetic energies, the ground state lifetime is short. The dynamical properties of
deuterated acetone, such as lifetimes of intermediates, correlation between vibration modes
and competition between different reaction pathways were analyzed.
[Introduction] Acetone photodissociation is a classical prototype of the α-CC bond cleavage
reaction, which plays fundamental importance in understanding the dynamics of the entire
class of carbonyl compounds. [1] Although the photodissociation dynamics of acetone upon
195nm laser excitation (S0→S2) have been extensively studied, debates among threemechanisms still exists. [2] The first commonly adopted one is “T1 dissociation mechanism”.
The S2 population decay to the T1 state via fast internal conversion and intersystem crossing,then the primary dissociation take place by overcoming a low barrier and yield to the groundstate acetyl radical. Later, the linear acetyl radical was observed in its first excited state,
CH3CO(Ã), which relates to a much lower dissociation barrier compare with those leading to
the bent form CH3CO(Ã). This result supports that the direct dissociation in S1 state may be
more favorable than the ISC to T1 state, and the mechanism is namely as “S1 dissociationmechanism”. Similar process can also take place in S2 state with a barrier of ~0.7 eV, then thelinear 3s acetyl radical, CH3CO(3s) could arrive the S2/S1/S0 conical intersection along thebasically barrierless potential energy surface. This “S2 dissociation mechanism” was further
confirmed by high level ab initio calculations. [3] In this study, the on-the-fly trajectorysurface hopping dynamic simulations were performed to detecting the acetone dissociation
mechanisms started at S2 state and the preliminary results are presented.
[Methods] The on-the-fly trajectories are propagated by numerically integrating theNewtonian equation of motion on the basis of the velocity-Verlet method with potential
energies and the corresponding analytical gradients calculated at CASSCF/6-31+G** level byemploying the MOLPRO program. [4] As the primary reaction coordinate is a high frequency
C-C stretching mode, the time step for the trajectory simulation is set as 0.2 fs. Along thetrajectory propagation, the improved Zhu-Nakamura formulas [5,6] were utilized to calculate
the global nonadiabatic switching probability at the avoided crossing points, which weredetected by locating minimum separation between two adjacent adiabatic potential energy
surfaces in three consecutive time steps. The surface hopping detection along trajectory isactivated when the potential energy gap is smaller than 0.5 eV. For the detail description ofthis surface hopping method, please see ref.5.

[Results and Discussion]
Within the simulated trajectories, the primary α-CC cleavage take place prior to the S2/S1
surface hopping, which agree with the “S2 dissociation mechanism”. The direct dissociation
precedes via joint contribution of OCC bending and C-C stretching modes. Accompanied withC-C bond breaking, the acetyl part become linear and the methyl radical lie in theperpendicular direction to the central carbon atom. As shown in Fig. 1, the S2/S1 hops can beclassified into two categories with CC bond lengths of ~1.5 (A) or ~1.9 Å (B). For the latter
case, the CO bond may be triple or double and which distributed in wider region up to 1.3 Å.
For the consecutive S1/S0 hop, the CC bond length of category A ones stay in similar region
but the CO length evidently increased. For category B, the CC bond distribution move tosmaller region, however the CO region slightly increased. The OCC bending and HHHC
umbrella motion were enhanced at S1/S0 hops, which is due to the larger kinetic energies thatconversed from potential energies. In between the acetone primary α-CC breaking and S2/S1
surface hopping, the lifetime of 3s acetyl radical span in a very wide region. About 40% ofacetyl radicals stay in 3s state for less than 0.1 ps and the rest ones possess much longerlifetimes up to 6.0 ps. The entire decay procedure of 3s acetyl radical to ground state isultrafast with a time constant of sub 0.1 ps. The secondary methyl dissociation occur in theacetyl radical ground state, where the equilibrium OCC moiety is highly bent and the methyl
radical leaves in semi perpendicular direction to the CO. Due to the high excitation energy of
195 nm laser, the ground state acetyl radical possesses rather large kinetic energy, which will
result in short lifetime.

Fig. 1. Selected internal coordinates for conical intersections. Category A and B S2/S1 hops are in blue andgreen, respectively, and the corresponding consecutive S1/S0 hops are in cyan and red.

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