Collision Energy Dependence of the Overall Rate Constant for the Reaction NH(a1Δ ) + HN3 (original) (raw)
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The Journal of Physical Chemistry A, 2002
Nascent vibrational distributions of NH 2 (X 2 B 1 , V 2 ′′) 0-4) generated in the 193 nm photolysis of NH 3 at 298 (1 K have been determined by simple kinetic analysis. The vibrational levels of NH 2 (X 2 B 1) were detected by laser-induced fluorescence (LIF) excited via the à 2 A 1-X 2 B 1 system, and time-resolved concentration profiles were recorded. Rate constants for intramode relaxation of ν 2 vibration (V 2 ′′ f V 2 ′′-1) by collisions of He have been determined to be (4.2 (1.0) × 10-14 , (7.5 (0.5) × 10-14 , (1.6 (0.1) × 10-13 , (3.0 (0.1) × 10-13 in units of cm 3 molecule-1 s-1 for V 2 ′′) 1, 2, 3, and 4, respectively. The quoted errors are 2σ. Rate constants for intermode coupling between V 1 ′′) 1 and V 2 ′′) 2 by He have also been determined to be (4.7 (1.6) × 10-12 (V 1 ′′)1 f V 2 ′′) 2) and (1.3 (0.4) × 10-12 cm 3 molecule-1 s-1 (V 1 ′′)1 r V 2 ′′) 2). Relative detectivities of the levels V 2 ′′) 1-4 were obtained by the kinetic analysis, and vibrational distributions immediately after the photolysis have been given to be 2.2 (0.7/1.0/ < 1.3/0.66 (0.3/ < 3.0 for V 2 ′′) 0/1/2/3/4 (normalized by the population of V 2 ′′) 1). To the best of our knowledge, this is the first quantitative report on the initial vibrational populations of NH 2 generated in the photolysis at 193 nm.
Chemical Physics Letters, 1993
The chemiluminiscent reaction following the ultraviolet (266 nm) photolysis of HN, has been studied. A dispersed emission spectrum was recorded, which is, to the best of our knowledge, the fast direct identification of the origin of the chemiluminescence. The emissive species was assigned to vibronically excited NH2. The observed vibrational distribution in the NHz(A'A,) was not consistent with the reaction mechanism previously suggested (NH (a 'A, IJ~ 0) + HN,). A modified mechanism including vibrationally excited NH(a 'A) has been proposed on the basis of energetics and kinetics in terms of the emission.
Photodissociation dynamics of NH2OH from the first absorption band
The Journal of Chemical Physics, 1994
The dynamics of the photofragmentation of hydroxylamine from its lowest excited electronic state, Ã 1A′, have been investigated. The main dissociation channel leads to H+H+HNO with a quantum efficiency of 1.7 for hydrogen atoms. The H atoms have been analyzed by laser induced fluorescence using a frequency tripled dye laser with sub-Doppler resolution. A sequential decay process is proposed where the first ejected H fragment leaves a highly vibrationally excited intermediate which dissociates after intramolecular vibrational redistribution into H+HNO. Another photodissociation channel leads to OH(X 2Π) and NH2(Ã 2A1). NH2(Ã) has been detected by its emission spectrum, Ã 2A1→X̃ 2B1, indicating strong vibrational excitation of the ν2 bending mode. The OH product shows no vibrational excitation, whereas rotational states up to N=20 have been observed. Observation of the product state distributions and of the 〈μ⋅v〉 and 〈v⋅J〉 correlations yield a qualitative picture of the upper pote...
The Journal of Physical Chemistry A, 1997
State-selected photodissociation of hydrozoic acid is investigated by vibrationally exciting the molecule in the region of the second overtone of its N-H stretching motion (3ν 1 ) and then photodissociating it using 532 nm light. Measurement of the resulting NH fragment rotational state distribution and vector correlation reveals that photodissociation from initial nuclear configurations with an extended N-H bond leads to a substantially hotter rotational state distribution as well as a more pronounced 〈V b,j B〉 correlation than from nearly isoenergetic single-photon dissociation at 355 nm. These observations are interpreted as indicating that the region of the excited electronic surface accessed and hence the forces experienced by the molecule are different in the two isoenergetic photodissociation experiments.
Vibrationally mediated photodissociation of NH3 and ND3 in the A band allows the exploration of the excited-state potential energy surface in regions that are not accessible from the ground vibrational state of these polyatomic systems. Using our recently developed coupled ab initio potential energy surfaces in a quasi-diabatic representation, we report here a full-dimensional quantum characterization of the à ← X̃ absorption spectra for vibrationally excited NH3 and ND3 and the corresponding nonadiabatic dissociation dynamics into the NH2(Ã2A1) + H and NH2(X̃2B1) + H channels. The predissociative resonances in the absorption spectra have been assigned with appropriate quantum numbers. The NH2(Ã2A1)/NH2(X̃2B1) branching ratio was found to be mildly sensitive to the initial vibrational excitation prior to photolysis. Implications for interpreting experimental data are discussed.
The Journal of Physical Chemistry A, 2013
Issues such as mode selectivity and Polanyi rules are connected to the effects of vibrational and translational energy in dynamics studies. Using the heavy−light−heavy OH(ν) + NH 3 (ν) gas-phase reaction, these effects were analyzed by performing quasi-classical trajectory calculations, at low and high collision energies (3.0 and 10.0 kcal mol −1 ), based on an analytical potential energy surface developed by our group. While the independent vibrational excitation of the NH 3 (ν) modes increases the reactivity by a factor of ∼1.1−2.8 with respect to the vibrational ground-state at both collision energies, OH(ν) stretching acts as a spectator mode. With respect to mode selectivity, we find a different behavior for both reactants. Thus, while the OH(ν) vibrational excitation is maintained in the products, indicating a certain degree of mode selectivity, the vibrational excitation of the NH 3 (ν) modes is not retained in the products; furthermore, the independent excitation of the N−H asymmetric and symmetric stretch modes leads to similar reaction probabilities, indicating negligible mode selectivity. For this early transition state reaction, translational energy is more effective in driving the reaction than an equivalent amount of energy in vibration, thus extending the validity of Polanyi rules to this polyatomic system. Finally, these results were interpreted on the basis of the existence of little or negligible intramolecular vibrational redistribution in the reactants before collision, while the nonconservation of the zero-point energy has a strong influence.
Theoretical study on the excited states of HCN
The Journal of Chemical Physics, 2005
In the flash-photolysis of oxazole, iso-oxazole, and thiozole a transient band system was observed in the region 2500-3050 Å. This band system was attributed to a meta-stable form of HCN, i.e., either HNC or triplet HCN. Theoretical investigations have been carried out on the ground and excited states of HCN to characterize this and other experimentally observed transitions. The predicted geometries are compared with the experiment and earlier theoretical calculations. The present calculations show that the band system in the region 2500-3050 Å corresponds to the transition 4 3-AЈ ← 1 3-AЈ of HCN.
Photodissociation dynamics of HN3(DN3) + hv → H(D)+ N3
Chemical Physics, 1996
The photolysis of HN3/DN 3 to give H/D and N 3 is investigated at different photolysis wavelengths: 266, 248, 222, 193 and 122 nm. Nascent H/D atoms are characterized via Doppler and polarization spectroscopy using laser-induced fluorescence in the VUV. The following quantum yields have been found: tbH N3(266 nm)= 0.04, ~b u N~(248 nm)= 4~D_N3(248 rim) = 0.20, and qSH_N3(193 nm)= 4~D_N3(193 nm)= 0.14. At a photolysis wavelength of 266 nm most of the available energy goes into product translation, (Eki n) = 5820 cm ~, while the internal energy of the N 3 fragment is fairly low, (Eint(N3)) = 1250 cm-i. At 248 nm the values are 6640 and 3150 cm l, respectively. Thus additional excess energy is preferentially released as internal energy of the N 3 radical. This trend is less pronounced when the excitation wavelength is set to 222 or 193 nm. At 122 nm the kinetic energy of the photofragments is smaller than in the 193 nm experiment. At 266 and 248 nm the spatial distribution of the photofragments is described by a strongly negative anisotropy parameter indicating a definite preference for a perpendicular alignment of the electronic transition moment and the recoil velocity vector. At 222 and 193 nm the anisotropy parameter is close to zero, while the VUV photolysis results in a slightly positive anisotropy parameter. These experimental findings indicate that the access to different electronic states of HN3/DN 3 is gained as the photolysis wavelength is varied from 266 to 122 nm.
Deactivation Rate Constants of OH(X 2 Π i , v = 1−4) by Collisions of NH 3
The Journal of Physical Chemistry A, 2000
Collisional deactivation of OH(X 2 Π, V)1-4) by NH 3 has been studied using pulsed laser photolysis coupled with the pulsed laser probe technique. Mixtures of O 3 /NH 3 /N 2 were photolyzed at 248 nm, and time-resolved OH populations were monitored via the ∆V) 0 and-3 sequences of the A 2 ∑ +-X 2 Π i transition. The following deactivation rate constants at 298 (1 K were obtained: (2.9 (0.2) × 10-11 for V) 1, (1.1 (0.2) × 10-10 for V) 2, (3.4 (0.4) × 10-10 for V) 3, and (4.1 (0.3) × 10-10 for V) 4 in units of cm 3 molecule-1 s-1 (the errors are 2σ). The present study is the first report on the rate constant for OH(V)4) + NH 3 reaction. The deactivation of OH(Ve3) by NH 3 can be elucidated by nonresonant V-V energy transfer caused by long-range interaction; that of V) 4 deviates from the gap law.
Non-adiabatic processes play an important role in photochemistry, but the mechanism for conversion of electronic energy to chemical energy is still poorly understood. To explore the possibility of vibrational control of non-adiabatic dynamics in a prototypical photoreaction, namely, the A-band photodissociation of NH 3(X˜1A1) , full-dimensional state-to-state quantum dynamics of symmetric or antisymmetric stretch excited NH 3(X˜1A1) is investigated on recently developed coupled diabatic potential energy surfaces. The experimentally observed H atom kinetic energy distributions are reproduced. The experimentally observed H atom kinetic energy distributions are reproduced. However, contrary to previous inferences, the NH 2(A˜2A1) /NH 2(X˜2B1) branching ratio is found to be small regardless of the initial preparation of NH 3(X˜1A1) , while the internal state distribution of the preeminent fragment, NH 2(X˜2B1) , is found to depend strongly on the initial vibrational excitation of NH 3(X˜1A1) . The slow H atoms in photodissociation mediated by the antisymmetric stretch fundamental state are due to energy sequestered in the internally excited NH 2(X˜2B1) fragment, rather than in NH 2(A˜2A1) as previously proposed.