POPTARTS: A New Method to Determine Quantum Yields in a Molecular Beam

Area of Science:

• Chemical Physics
• Photochemistry
• Molecular Spectroscopy

Background:

• Determining absolute photodissociation quantum yields is crucial for understanding chemical reaction dynamics.
• Existing methods often require external calibration, limiting their applicability and accuracy.
• Molecular beam techniques provide a controlled environment for studying photodissociation processes.

Purpose of the Study:

• To develop and validate a novel, calibration-free technique for measuring absolute photodissociation quantum yields.
• To demonstrate the technique's efficacy using the well-characterized photochemistry of formaldehyde (HCHO).
• To investigate the pressure dependence of HCHO quantum yields at specific photolysis wavelengths.

Main Methods:

• Utilizing a molecular beam apparatus with a molecule possessing two photolytic product channels (A and B).
• Measuring the relative decrease of species in channel A and the relative increase of species in channel B.
• Applying the sum rule (ϕ_A + ϕ_B = 1) to convert relative yields to absolute quantum yields.
• Employing formaldehyde photolysis at ~310 nm, leading to H + HCO (channel A) or H₂ + CO (channel B), with subsequent HCO photolysis.

Main Results:

• The technique provides direct, calibration-free measurement of relative product yields.
• Absolute quantum yields for HCO from HCHO photolysis were determined to be 0.66 and 0.74 for specific excited states.
• These results align excellently with established atmospheric pressure quantum yields, indicating no significant pressure dependence.

Conclusions:

• The developed technique is a reliable and efficient method for determining absolute photodissociation quantum yields.
• The study confirms the lack of pressure dependence for HCHO quantum yields at the investigated photolysis energies.
• This approach has broad applicability for studying the photochemistry of various molecules.