Ultrafast Rydberg-state dynamics of N-methyl morpholine excited by 160 nm femtosecond laser pulses

The ultrafast Rydberg-state dynamics of N-methyl morpholine following photoexcitation at 160 nm is investigated using the femtosecond time-resolved photoelectron imaging method. The 3d → 3p → 3s sequential decay mechanism is proposed, as supported by the present time-, energy-, and angle-resolved photoelectron spectroscopic study. The lifetime of the initially prepared 3d Rydberg manifold is found to be 80 ± 10 fs, while the subsequently populated 3p Rydberg series has a lifetime of 160 ± 20 fs and serves as an intermediate state that further decays to the lower-lying 3s Rydberg state (the S₁ state). Notably, the 3s state contains substantial excess vibrational energy and relaxes over a potential barrier with a lifetime of 2.1 ± 0.2 ps. In addition, it is evident that the coherent structural dynamics occurs on the 3s state potential energy surface in N-methyl morpholine. These observed coherent oscillations with a period of approximately 600 fs dephase rapidly with a damping time constant of 200-300 fs.