Mode-specific infrared photodissociation of nitric oxide dimers: High-resolution infrared spectroscopy of (14NO)2 and (15NO)2

Abstract

The high-resolution infrared absorption spectra of the symmetric (ν1) and the antisymmetric NO stretching (ν4) bands of nitric oxide dimer (NO)2 have been measured for 14NO and 15NO in supersonic free jets. The ν1 and ν4 bands exhibit a dramatic difference in linewidth: approximately 200 MHz [full width at half-maximum (FWHM)] for the ν1 band and approximately 5 GHz (FWHM) for the ν4 band. The predissociation lifetimes deduced from the linewidths are in excellent agreement with those reported in the recent time-resolved measurement for 14NO [Casassa et al., J. Chem. Phys. 89, 1966 (1988)]. There is no systematic dependence of the linewidth on the rotational states of (NO)2. Isotope substitution does not influence the linewidths significantly. However, the ν4 band structure of (15NO)2 is very different from that of (14NO)2, a difference that may be explained by a perturbation from a low-lying singlet vibronic state. All of the experimental results obtained to date may be accounted for if it is assumed that the predissociation of (NO)2 is enhanced by an electronically nonadiabatic transition to a repulsive triplet surface. Vibrational potential coupling between the NO stretching and intermolecular modes, particularly an in-plane NO bending mode, appears to play a key role in the mode specificity.

Infrared combination and difference bands of the NO dimer

“…In the mid-infrared region, there are two strong fundamental N-O stretching bands, m 1 at 1868.252 and m 5 at 1789.098 cm À1 . The former exhibits lines which are only slightly broadened (0.006 cm À1 ) by predissociation, and so it can be analyzed in detail [7][8][9][10][11]. The latter has greater predissociation broadening (0.13 cm À1 ), meaning that only an approximate profile analysis is possible [7,10,[12][13][14][15][16], as opposed to a line-by-line fit.…”

“…The former exhibits lines which are only slightly broadened (0.006 cm À1 ) by predissociation, and so it can be analyzed in detail [7][8][9][10][11]. The latter has greater predissociation broadening (0.13 cm À1 ), meaning that only an approximate profile analysis is possible [7,10,[12][13][14][15][16], as opposed to a line-by-line fit. The overtone region of the N-O stretch has also been characterized [12], most reliably for the 2m 5 and m 1 + m 5 bands which were observed under supersonic jet conditions in a study of NO dimer photodissociation dynamics [2].…”

The Far Infrared Spectrum of the NO Dimer

“…The high-frequency intramolecular vibrations of (NO) 2 are known to lie at 1868.252 (4,12) ( 1 symmetric N-O stretch) and 1789.098 cm Ϫ1 (11) ( 5 antisymmetric N-O stretch). However, there has been a long standing mystery as to the locations of the low-frequency intermolecular vibrations of gas-phase (NO) 2 , since the only data on these modes has come from condensed-phase experiments (13,14).…”

The NO Dimer: Jet-Cooled Study of ν1and ν5Transitions

“…The pioneering works were mainly devoted to solid state and matrix isolation studies, while more recent papers present gas phase spectra, thus allowing a precise analysis of the fundamental vibrational state (17,18) as well as the symmetric and antisymmetric vibrations (19 -22). Only two spectroscopic studies were previously based, partly or totally, upon jet-cooled spectra (23,24). In reference (24) a first evaluation of the 5 predissociation time was given (20 -40 ps).…”