Electronic relaxation and dissociation dynamics in formaldehyde: pump wavelength dependence

Area of Science:

• Chemical Physics
• Molecular Dynamics
• Photochemistry

Background:

• Formaldehyde photodissociation is a fundamental process in atmospheric chemistry.
• Understanding excited-state dynamics is crucial for predicting reaction pathways.
• Previous studies have explored formaldehyde dissociation, but wavelength-dependent effects require further investigation.

Purpose of the Study:

• To investigate the influence of UV pump wavelength on formaldehyde's excited-state dynamics and dissociation.
• To correlate specific vibronic transitions with dissociation rates and yields.
• To compare theoretical predictions with experimental observations.

Main Methods:

• First principles quantum chemistry simulations were employed.
• Coulomb explosion imaging (CEI) experiments were conducted.
• Tunable UV pulses (304, 314, 329, 337 nm) were used to excite formaldehyde.

Main Results:

• Excited state decay and dissociation rates show a strong dependence on the pump wavelength.
• Simulations predict over a 100-fold increase in non-adiabatic transition rates and dissociation yields from 337 nm to 304 nm.
• Experimental and theoretical results show broad agreement, with dissociation plateauing after ~2 ps.

Conclusions:

• The prepared vibronic transition critically affects formaldehyde's excited-state decay and dissociation.
• Shorter UV pump wavelengths lead to significantly faster dissociation.
• The study highlights the importance of precise wavelength control in photochemical studies.