Striking Isotopologue-Dependent Photodissociation Dynamics of Water Molecules: The Signature of an Accidental Resonance

Abstract

Investigations of the photofragmentation patterns of both light and heavy water at the state-to-state level are a prerequisite for any thorough understanding of chemical processing and isotope heterogeneity in the interstellar medium (ISM). Here we reveal dynamical features of the dissociation of water molecules following excitation to the C (010) state using a tunable vacuum ultraviolet source in combination with the high resolution H(D)-atom Rydberg tagging time-of-flight technique. The action spectra for forming H(D) atoms and the OH(OD) product state distributions resulting from excitation to the C (010) states of H2O and D2O both show striking differences, which are attributable to the effects of an isotopologue-specific accidental resonance. Such accidental resonance induced state mixing may contribute to the D/H isotope heterogeneity in the Solar System. The present study provides an excellent example of competitive state-to-state non-adiabatic decay pathways involving at least five electronic states.

The vibronic state dependent predissociation of H2S: determination of all fragmentation processes

“…The wavelength-dependent photofragmentation dynamics of H 2 O molecules have been studied extensively, both experimentally and theoretically, to the extent that this system now provides several textbook illustrations of the importance of non-adiabatic effects (conical intersections (CIs) between potential energy surfaces (PESs)) in determining molecular photochemistry. 29–42 In contrast, photodissociation dynamics of H 2 S are limited. The UV absorption spectrum of H 2 S shows many similarities to that of H 2 O, but some important differences have already been recognised.…”

Strong isotope effect in the VUV photodissociation of HOD: A possible origin of D/H isotope heterogeneity in the solar nebula

“…In this work, we have recorded the quantum state–resolved photodissociation features of HOD as well as the branching ratios of the H + OD and D + OH channels between 120.8 and 121.7 nm by using tunable VUV sources combined with the high-resolution H (D) atom Rydberg tagging time-of-flight (HRTOF) technique ( 47 ). The experimental results show that the quantum state population distributions of OD products differ markedly from that of OH products, revealing very strong isotope effect.…”

Electronically Excited OH Super-rotors from Water Photodissociation by Using Vacuum Ultraviolet Free-Electron Laser Pulses

“…The HRTOF technique employs a sequential two-step excitation of the H atom. − The first step involves the excitation of the H atom from the n = 1 ground state to the n = 2 state by absorbing one photon at 121.6 nm. The second step concerns the UV laser excitation (365 nm) of the H atom to a high- n Rydberg state from the n = 2 state.…”