S. A. Reid

The monoenergetic unimolecular reaction of expansion‐cooled NO2: NO product state distributions at excess energies 0–3000 cm−1

We report detailed vibrational, rotational, and electronic (V,R,E) distributions of nascent NO(X 2Π1/2,3/2) deriving from monoenergetic unimolecular reactions of expansion‐cooled NO2. Near UV excitation above dissociation threshold (25 130.6 cm−1) prepares molecular eigenstates which are admixtures of the optically active 1 2B2 state and the ground X 2A1 electronic state. The strong mixings among the vibronic states result in vibrational predissociation from states of predominantly ground state character, and the NO product state distributions (PSDs) are compared with the predictions of several statistical theories. The PSDs are combined with previously measured O(3PJ) distributions and unimolecular reaction rates, thereby providing a complete description of the decomposition of NO2 at these excess energies. All the rotational distributions show prominent fluctuations and structures, but tend on average to follow the statistical distributions predicted by phase space theory (PST). This behavior is observ...

Fluctuations in the unimolecular decomposition of jet‐cooled NO2: Implications for overlapping resonances and the transition state

We report a detailed examination of fluctuations in the internal state distributions of the NO (2II) fragment formed in the monoenergetic unimolecular decomposition of jet‐cooled NO2, utilizing photofragment yield (PHOFRY) spectroscopy. The NO(v,J,Ω,Λ) PHOFRY spectra at E°<3000 cm−1 are highly structured, and we examine correlations among these spectra on the basis of the angular momentum, spin–orbit, parity, and vibrational labels of the monitored NO rovibrational levels. We find that levels of similar total angular momentum in the same vibrational manifold are more strongly correlated with specific resonances in the activated molecule, as well as levels of the same N in the two spin–orbit states of NO. The observed PHOFRY correlations, as well as those found in the NO(2ΠΩ; Ω=1/2,3/2) rotational state distributions, are interpreted in terms of projections of coherently excited overlapping molecular eigenstates onto the manifold of final states via levels of the transition state. The implications of the r...

Double resonance infrared–visible photofragment yield spectroscopy of NO2: Interferences among overlapping quasibound levels

We examine fluctuations in the unimolecular decomposition of state‐selected NO2 using double resonance infrared (IR)–visible and visible–IR photofragment yield (PHOFRY) spectroscopy. Spectra for specific NO(v,J,Λ,Ω) states at excess energies (E°)=2000–2525 cm−1 exhibit marked differences in the shapes, positions, and widths of the resonance structures. We interpret this effect as arising from interferences among coherently excited overlapping quasibound levels. Comparisons of IR–visible PHOFRY spectra with one‐photon spectra in the same energy region and IR–visible spectra at lower E° are used to assess the importance of thermal averaging (i.e., incoherent excitations) and to examine the transition from mild to more severe level overlap. The experimental results are compared qualitatively with results from sample calculations to shed further light on the interplay between dynamics and statistics in the decomposition and the role of overlapping quasibound levels.

Experimental probes of dissociative states: Fano profiles in the state‐specific photodissociation of FNO

We have observed strongly asymmetric features in the S1←S0 state‐specific photofragment yield spectrum of jet‐cooled FNO. We interpret these as Fano profiles, arising from interference between direct and indirect dissociation following excitation to a region near the top of a barrier on the S1 potential energy surface.

Fluctuations in state‐selected unimolecular decomposition: Double‐resonance infrared visible photofragment yield spectroscopy of NO2

Fluctuations in the NO(2Π) state distributions from the state‐selected simple bond fission unimolecular reaction of NO2 are probed using double‐resonance photofragment yield (PHOFRY) spectroscopy. These spectra are sensitive probes of the correlation of selected NO rovibronic levels with specific resonances in the excited molecule.

State-specific photofragment yield spectroscopy of jet-cooled methyl nitrite

Abstract We obtained state-specific photofragment yield (PHOFRY) spectra of jet-cooled CH3ONO in the S1 ← S0 absorption system by monitoring selected rotational levels in NO v = 0, 1 and 2. The spectra exhibit bands which are assigned to both the syn- and anti-rotamers with origins at 380 nm and 388 nm, respectively. All the bands are broad (500–800 cm−1) suggesting that the dissociation lifetimes are shorter than those obtained in dynamical calculations on model potential energy surfaces. The widths do not depend on the monitored NO rotational level. The results support a mechanism where nonadiabatic vibrational predissociation is preferred.

A crossed beam study of the reaction C(3P) + N2O: energy partitioning between the NO(X 2Π) and CN(X 2Σ+) products

Abstract The reaction of C( 3 P) with N 2 O is studied at a collision energy of 0.9 ± 0.4 eV in a crossed, pulsed molecular beam arrangement using laser ablation for carbon atom production. Nascent CN(X 2 Σ + ) and NO(X 2 Π) products are detected by laser-induced fluorescence. In accord with previous studies, the CN(X 2 Σ + ) product is found to be vibrationally inverted (maximum at ν = 3) and possesses substantial rotational excitation. Product NO(X 2 Π) is formed vibrationally “cold” (maximum at ν = 0) but rotationally “hot”, with a maximum population found at J = 49.5 for both spin—orbit states. The implications of these results for the reaction dynamics are assessed.

Resonances and fluctuations in the unimolecular reaction of NO2

Fluctuations and oscillations in the unimolecular reaction of NO2, and their manifestations in photofragment yield (PHOFRY) spectra, NO rotational state distributions and decomposition rates are examined. Comparisons between experimental and simulated PHOFRY spectra show that extraction of rates from linewidths in state-selected spectra is unjustified in the regime of overlapping resonances. Measurements of the alignment parameter of the NO product in the excess energy range EE= 200–500 cm–1 evidence the existence of fluctuations in the decay rate. Changes in the patterns of fluctuations and oscillations in the NO rotational state distributions reveal the progressive tightening of the transition state (TS) as the excess energy increases and the importance of exit-channel interactions beyond the TS. Distributions well fit on the average by phase-space calculations can be obtained even when the transition state is tight.