Near ultraviolet photolysis of ammonia and methylamine studied by H Rydberg atom photofragment translational spectroscopy

MNR Ashfold*, Richard N Dixon, M Kono, DH Mordaunt, CL Reed

*Corresponding author for this work

Research output: Chapter in Book/Report/Conference proceedingConference Contribution (Conference Proceeding)

Abstract

H(D) Rydberg atom photofragment translational spectroscopy has been used to provide new insights into the primary photochemistry of methylamine, ammonia and various of their respective isotopomers following excitation at wavelengths in the near ultraviolet (UV). The bimodal appearance of the total kinetic energy release (TKER) spectra associated with H atom production in the near UV photolysis of methylamine is consistent with there being both 'dynamical' thigh TKER) and 'statistical' (slower) contributions to the total H + CH3NH dissociation yield. Both contributions arise as a result of one H atom tunnelling through (or passing over) an earlier barrier in the N-H dissociation coordinate of the (A) over tilde state potential energy surface and then evolving into the region of the conical intersection connecting the (A) over tilde state and ground-state surfaces. 'Dynamical' energy disposal is associated with those molecules which pass directly through this conical intersection en route to the ground-state (H + CH3NH((X) over tilde)) asymptote, whilst the 'statistical' contribution is attributed to those molecules that 'miss' the conical intersection on the first traversal ;and only make the (A) over tilde --> (X) over tilde transfer on a later encounter. This interpretation has inspired further consideration of the form of the TKER spectra derived from TOF measurements of the H/D atom products arising in the dissociation of various isotopomers of ammonia following excitation to the 0(0) and 2(1) levels of their respective (A) over tilde states. A similar model which associates 'dynamical' energy disposal with those molecules that pass through the (A) over tilde/(X) over tilde conical intersection during bond extension, and 'statistical' kinetic energy release with those that transfer during N-H(D) bond compression, appears to provide a qualitative explanation for the way the observed H and/or D atom yields and their associated TKER spectra vary with excitation wavelength (00 versus 2(0)(1) band excitation) and isotopic composition.

Original languageEnglish
Title of host publicationMOLECULAR RYDBERG DYNAMICS
EditorsMS Child
Place of PublicationCOVENT GARDEN
PublisherImperial College Press
Pages195-212
Number of pages18
ISBN (Print)1-86094-094-3
Publication statusPublished - 1999
EventRoyal-Society Discussion Meeting on Molecular Rydberg Dynamics - LONDON, United Kingdom
Duration: 6 Nov 19967 Nov 1996

Conference

ConferenceRoyal-Society Discussion Meeting on Molecular Rydberg Dynamics
CountryUnited Kingdom
Period6/11/967/11/96

Keywords

  • SELECTIVE PHOTODISSOCIATION DYNAMICS
  • STATE AMMONIA
  • (A)OVER-TILDE STATE
  • AB-INITIO
  • MOLECULES
  • NM
  • DISSOCIATION
  • EXCITATION
  • ABSORPTION
  • ABINITIO

Cite this

Ashfold, MNR., Dixon, R. N., Kono, M., Mordaunt, DH., & Reed, CL. (1999). Near ultraviolet photolysis of ammonia and methylamine studied by H Rydberg atom photofragment translational spectroscopy. In MS. Child (Ed.), MOLECULAR RYDBERG DYNAMICS (pp. 195-212). Imperial College Press.