Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal...

Area of Science:

• Physical Chemistry
• Photochemistry
• Molecular Spectroscopy

Background:

• Previous studies suggested a specific ion fragment (m/z = 84) in uracil photolysis indicated ultrafast ring-opening.
• This interpretation was based on nanosecond laser pulses at excitation wavelengths ≤232 nm.

Purpose of the Study:

• To investigate the ultrafast dynamics of uracil excited-state relaxation.
• To determine if the m/z = 84 fragment is a signature of ultrafast ring-opening.
• To explore non-adiabatic dynamics in uracil and related compounds.

Main Methods:

• Time-resolved ion-yield measurements using a novel ultra-high vacuum spectrometer with laser-based thermal desorption.
• Excitation at pump wavelengths of 267 nm, 220 nm, and 200 nm.
• Complementary measurements on 2-thiouracil at 267 nm.

Main Results:

• Confirmed rapid (<200 fs) relaxation of uracil's bright S3(1ππ*) state at 220 nm and 200 nm.
• Found no evidence for the m/z = 84 fragment within picoseconds of excitation at these wavelengths.
• Results at 267 nm agreed with previous cold molecular beam studies, indicating negligible internal energy effects.

Conclusions:

• The m/z = 84 fragment is not a direct product of ultrafast ring-opening in uracil.
• The previously observed fragment likely arises from slower excited-state processes.
• This study refines our understanding of uracil photodynamics and excited-state reaction pathways.