Rydberg state of an open shell species: Characterization and photophysics of the 3p(z) state of hydroxymethyl radical

Abstract

The 3p_z Rydberg state of the hydroxymethyl radical has been investigated by time-of-flight mass spectrometry (TOF-MS) combined with the techniques of resonance enhanced multiphoton ionization (REMPI) spectroscopy, Doppler spectroscopy, photofragment ionization spectroscopy, and core sampling in the molecular beam.

An apparatus dedicated to the study of radicals in molecular beams was designed and constructed with the goal of maximizing sensitivity and using a variety of diagnostic techniques. A modular design was adopted, which employed TOF-MS detection, yet supplied provision for later implementation of the ion imaging technique.

Two aspects of the 3p_z state were investigated: the assignment of the electronic transition at ∼243.5 nm, and its subsequent intramolecular dynamics. When combined with model calculations, the spectroscopy of the radical led to the conclusion that the excitation is achieved via a parallel transition to the 3p_z state and that the excited state lifetime is ∼½ ps.

Photofragment ionization studies of isotopically substituted hydroxymethyl radicals excited to the Rydberg origin established the CH₂OD → CH₂O + D channel as a major pathway. The same technique showed that CH₂OD ↔ CH₂DO isomerization and the channels that produce CO (v = 0) radical were of no importance.

A mathematical transform for the extraction of photofragment kinetic energy distributions from observed core sample TOF distributions was derived and compared to an existing treatment.

The kinetic energy distribution of photofragments from CH₂OD radicals excited to the 3p_z state was measured using the core sampling technique. It was concluded that the formaldehyde co-fragment is formed in its electronic ground state with considerable internal excitation, and was verified by Doppler spectroscopy. Angular distributions show that the dissociation rate is prompt, in agreement with linewidth measurements. Core sampling of CD₂OH confirmed that isomerization was unimportant.

It was concluded that the prompt predissociation CH₂OH(3p _z) → CH₂O([special characters omitted]) + H involving direct O–H bond scission is a major channel, terminating on the ground potential surface. Exit channel dynamics were implicated as the cause of the internal excitation of the formaldehyde fragment.