Time-resolved studies of NO2 photoinitiated unimolecular decomposition: step-like variation of κuni(E)

Angular distributions of state-selected NO and O products in the photoinitiated unimolecular decomposition of jet-cooled NO 2 have been measured by using both the photofragment ion imaging technique with velocity map imaging and ion time-of-flight translational spectroscopy. The recoil anisotropy parameter of the photofragments, ␤, depends strongly on the rotational angular momentum of the photoproduct. O( 3 P jϭ2,0 ) angular distributions are recorded at photolysis wavelengths 371.7, 354.7, and 338.9 nm. At these wavelengths, respectively, vibrational levels v ϭ0, vϭ0,1 and vϭ0 -2 of NO are generated. In addition, ␤ values for NO(vϭ2) in specific high rotational levels are determined at ϳ338 nm. The experimental observations are rationalized with a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. The model is general and applicable in cases where fragment angular momentum is large, i.e., a classical treatment is justified. It is applied here both to the experimental NO 2 results, and results of quantum calculations of the vibrational predissociation of the Ne-ICl van der Waals complex. It is concluded that deviations from the limiting ␤ values should be prominent in fast, barrierless unimolecular decomposition, and in certain dissociation processes where a large fraction of the available energy is deposited in rotational excitation of the diatom. The application of the model to NO 2 dissociation suggests that the nuclear dynamics leading to dissociation involves a decrease in bending angle at short internuclear separations followed by a stretching motion. This interpretation is in accord with recent theoretical calculations.

Section: Introductionmentioning

confidence: 57%

Section: Saturation Behaviormentioning

confidence: 72%

Product quantum-state-dependent anisotropies in photoinitiated unimolecular decomposition

Dem'yanenko

Dribinski

Reisler

et al. 1999

“…3 Thus, rotational energy is available for dissociation. Also, the average dissociation rate constant as a function of photon energy has been measured for expansion cooled ͑i.e., almost non-rotating͒ molecules [4][5][6][7][8] and has been found to be smaller than for room temperature samples. 5,9,10 Though such experiments demonstrate unequivocally that rotational energy, on average, increases the dissociation rate constant, statespecific information cannot be gleaned.…”

Section: Introductionmentioning

confidence: 83%

“…Moreover, experiments have been limited to either small angular momenta [3][4][5][6][7][8]16,17,24,25,[31][32][33][34] or high rotational temperatures. 2,5,9 In the latter, the data are averaged over a number of rotational states.…”

Section: Introductionmentioning

confidence: 99%

Photoinitiated unimolecular decomposition of NO2: Rotational dependence of the dissociation rate

Bezel

Ionov

Wittig

1999

Self Cite

“…Let us briefly mention among these techniques those based on pulsed sources: photofragment excitation, 35,36 socalled PHOFEX, 7,20,37,38 grating-PHOFEX, 39 photofragment yield (PHOFRY, similar to PHOFEX), 40,41 zero kinetics energy detection (ZEKE), 42 REMPI/PHOMPI 43 (a PHOFEX variant), and population depletion. 44 Time-revolved fluorescence techniques (in a pump-probe scheme) have been implemented by Wittig's group (using picosecond sources) [45][46][47] and by Troe's group (using sub-picosecond sources) 48,49 to access directly to the photodissociation times. Recently, a femtosecond pumpprobe technique has been set up to probe the dissociation dynamics of the vibronic states imprinted in the transient gratings.…”

Section: Introductionmentioning

confidence: 99%

Photodissociation resonances of jet-cooled NO2 at the dissociation threshold by CW-CRDS

Dupré

2015

Around 398 nm, the jet-cooled-spectrum of NO2 exhibits a well identified dissociation threshold (D0). Combining the continuous-wave absorption-based cavity ringdown spectroscopy technique and laser induced fluorescence detection, an energy range of ∼25 cm(-1) is analyzed at high resolution around D0. In addition to the usual molecular transitions to long-lived energy levels, ∼115 wider resonances are observed. The position, amplitude, and width of these resonances are determined. The resonance width spreads from ∼0.006 cm(-1) (i.e., ∼450 ps) to ∼0.7 cm(-1) (∼4 ps) with large fluctuations. The identification of at least two ranges of resonance width versus the excess energy can be associated with the opening of the dissociation channels NO2→NO(X(2)Π1/2, v=0, J=1/2+O((3)P2) and NO2→NO(X(2)Π1/2, v=0, J=3/2)+O((3)P2). This analysis corroborates the existence of loose transition states close to the dissociation threshold as reported previously and in agreement with the phase space theory predictions as shown by Tsuchiya's group [Miyawaki et al., J. Chem. Phys. 99, 254-264 (1993)]. The data are analyzed in the light of previously reported frequency- and time-resolved data to provide a robust determination of averaged unimolecular dissociation rate coefficients. The density of reactant levels deduced (ρreac ∼ 11levels/cm(-1)) is discussed versus the density of transitions, the density of resonances, and the density of vibronic levels.