Rotational Distributions Of CN Research Articles

Inverted Vibrational and Rotational Excitation of CN(B2Σ+) Produced through Superexcited Ion-Pair States of MCN (M = Na, K, Rb).

This paper reports an energy-partition mechanism in dissociative excitation of alkali cyanide molecules, MCN (M = Rb, K, Na), to produce CN(B2Σ+) and M(ns2S) (n = 5, 4, and 3 for Rb, K, and Na, respectively) in collision with Kr metastable atoms, Krm(3P2,0). Both the vibrational and rotational distributions of CN(B2Σ+) produced in the reactions of RbCN and KCN were inverted as being peaked at v' = 1 and N' = 35, respectively, where v' and N' are the vibrational and the rotational quantum numbers of CN(B2Σ+), respectively. According to a state crossing model, it was derived that CN(B2Σ+) is produced by predissociation through a superexcited ion-pair state, CN-(31Σ+)·M+(1S), followed by an adiabatic transition to a repulsive state correlating to the dissociation limit of CN(B2Σ+) + M(ns2S). The inverted distributions are driven by structural changes during the excitation and the adiabatic transition. The maximum vibrational population at v' = 1 originates from a large Franck-Condon overlap between the vibrational wavefunctions of CN-(31Σ+) and CN(B2Σ+) at v' = 1. The rotational excitation of the CN(B2Σ+) product is explained with changing from a T-shape geometry of MCN in the ground state to a linear one in the superexcited ion-pair state.

Dissociative excitation of benzonitrile by ultraviolet multiphoton absorption

Dissociative excitation of benzonitrile, C 6H 5CN, for generating excited photofragments, CN( A) and CN( B), was studied with resonantly enhanced multiphoton excitation at 266 nm. Laser power dependence of CN( A– X) and CN( B– X) intensities demonstrated that the CN( A, B) formation takes place via the two-photon absorption process, C 6H 5CN + 2 hν → C 6H 5( X ˜ ) + CN( A, B). Vibrational and rotational energy distributions of the nascent CN( A, B) fragments from photoexcited benzonitrile in the VUV energy region were determined by simulation analysis of dispersed fluorescence spectra. For both CN( A) and CN( B), the vibrational distributions were found to be of the uninverted type and the rotational distribution in each vibrational state could be approximated by a Boltzmann distribution. The best-fit vibrational distribution of CN( A) was determined to be N v ′ = 2 : N v ′ = 3 : N v ′ = 4 : N v ′ = 5 : N v ′ = 6 = 1.0:0.45:0.16:0.06:0.03 and the rotational temperature in each vibrational state was T r( v ′ = 2) = 3500 K, T r( v ′ = 3) = 3200 K, T r( v ′ = 4) = 2200 K, T r( v ′ = 5) = 2000 K, and T r( v ′ =6) = 2000 K, respectively. The vibrational distribution of CN( B) was N v ′ = 0 : N v ′ = 1 : N v ′ = 2 = 1.00:0.43:0.18 with respective rotational temperatures of T r( v ′ = 0) = 2000 K, T r( v ′ = 1) = 1300 K, and T r( v ′ = 2) = 1300 K. The vibrational and rotational distributions of CN( A) were found to be hotter than those predicted by the statistical model with complete energy randomization within the excited molecule. These results indicate that the dissociation to C 6H 5( X ˜ ) + CN( A) proceeds via the mechanism where both vibrational energy deposition in the photoexcitation and available energy redistribution before the dissociation are limited within the internal modes of skeletal C 6–CN structure. The vibrational distribution of CN( B) was significantly hotter than the statistical distribution and can be attributed to a fast decay of the photoexcited Rydberg state via an intermediate state onto a repulsive dissociation surface leading to C 6H 5( X ˜ ) + CN( B).

Photodissociation of HCN at 157 nm: Energy disposal in the CN(A 2Π) fragment

Photodissociation of HCN at 157 nm was found to produce only H and CN(A 2Π) through analysis of laser-induced fluorescence experiments. The CN(A 2Π) vibrational populations decrease monotonically from v=0 to 3, represent 19±1% of the available energy and are in good agreement with a prior distribution of 9500 cm−1 of energy. The rotational distributions of the four vibrational levels are very similar, represent 6.5% of the available energy and are approximately Boltzmann with temperatures near 900 K. The rotational distribution of CN(A 2Π,v=0) is compared to several highly simplified reference distributions and approximate agreement is found with a phase space calculation.

Vacuum ultraviolet photodissociation dynamics of acetonitrile

Vacuum ultraviolet photodissociation dynamics of CH3CN and CD3CN has been studied at wavelength near the Lyman-α transition (121.6 nm). Two products, CN(B 2Σ) and H or D, were observed. The CN(B 2Σ–X 2Σ) chemiluminescence spectrum was analyzed to determine vibrational and rotational distributions of CN(B). The vibrational distribution was Boltzmann-like and agreed with a statistical distribution. Although the rotation of the v=0–3 states were also characterized by the Boltzmann distribution, the average rotational energy was higher than the statistical energy disposal. The line shape of the Lyman-α laser-induced fluorescence of H and D showed a Gaussian Doppler profile, which was ascribed to the statistical translational energy distribution of H and D. Isotope effects were determined for both products. While no isotope effect was observed for energy partition into the vib-rotation of CN(B) and the translation of H and D, relative yields of two products showed an isotope effect, i.e., CN(B) formation was favored for CD3CN over CH3CN and the reverse was true for H(D) atom formation. These results suggest a dissociation mechanism where fast internal conversion occurs after the primary photoexcitation and the vibrationally hot molecule leads to the H(D) atom elimination which competes with C–C bond dissociation.

Violet emission of cyanogen produced in the reaction of argon(3P0,2) with cyanogen bromide. 1. Nascent vibrational and rotational distributions of CN(B2.SIGMA.+)

The CN(B 2 Σ + -X 2 Σ + ) emission spectrum, produced in the dissociative excitation reaction of BrCN with Ar( 3 P 0,2 ) at thermal collision energy, was observed by using the flowing afterglow method. The effect of collisional relaxation of the excited electronic states of CN by the ground-state Ar atoms was minimized by lowering the ambient argon pressure to less than 10 mTorr. The «nascent» rovibrational distribution of CN(B 2 Σ + ) was determined by spectral simulation. A surprisal analysis of the nascent rovibrational distribution of CN(B 2 Σ + ) indicated that the dominant pathway of the reaction of BrCN with Ar( 3 P 0,2 ) was energy transfer, instead of argon complex formation of ion-pair exciplex formation

ICN photodissociation at 193 nm; CN(B 2Σ) rotational distributions and fluorescence polarization

The rotational distribution of CN(B 2Σ, v=0) dissociated from ICN at 193 nm is well represented by a thermal rotational distribution for T=250 K, giving an average energy of 175 cm −1. The energy of the highest rotational state detected ( N=28) suggests that some or all of the molecules dissociated have appreciable internal energy. We discuss recent values of the available energy. Polarization of the R(8) line of the CN(B) fluorescence agrees with other experiments in this absorption feature, and gives the same upper-state symmetry observed at shorter wavelengths. CN(A 2Π) was also detected promptly after the fragmentation.

Photodissociation of BrCN and ICN in the a continuum: Vibrational and rotational distributions of CN(X 2Σ+)

We report results of experiments on photodissociation of ICN and BrCN in the long-wavelength (A) continuum in which primary rotational and vibrational distributions of CN in its electronic ground state are determined. For the CN fragment from ICN, our observations at 222 and 249 nm, reported here, together with our earlier results at longer wavelengths, show that the fraction of energy which goes into rotation (in the ground-state channel) is very nearly independent of exciting wavelength in the range 222–351 nm. This holds for each of CN vibrational levels 0, 1, and 2, where the fractions are 15, 25, and 33% respectively. The vibrational distributions decrease monotonously with υ, but show little change as a function of exciting wavelength. CN distributions are also presented for the photodissociation of BrCN at 193, 222, 249, and 308 nm. Results for both parent molecules are briefly discussed in relation to other recent experimental and theoretical work.

Vibrational and rotational state distributions of CN(B 2Σ+) produced in collision of Xe(3P2) with CICN and BrCN

Abstract The dynamics of the dissociative excitation of CICN and BrCN in collision with metastable Xe atoms, Xe m , is studied by the vibrational and rotational distributions of the CN(B 2 Σ + ) fragment produced in a Xe afterglow. The distributions determined by a simulation analysis of the CN(B 2 Σ + −X 2 Σ + ) violet emission spectra are compared with those produced in the dissociation of CICN and BrCN by photons of nearly equal excess energies, reported by Simons et al. A dominant mechanism for CN(B 2 Σ + ) formation is found to be the resonance excitation of the cyanides to the α-continuum or the B 3 Π state followed by dissociation. More vibrational and rotational energy is found to be partitioned to the CN(B 2 Σ + ) fragment produced by Xe m -impact dissociation, especially from BrCN. This is ascribed to a lengthening of the CN bond in XCN caused by interaction with the Xe m atom.

Rotational distribution of CN from the 266 nm photodissociation of ICN

Rotational energy disposal in the 266 nm à state continuum photodissociation of ICN has been measured using pulsed laser photolysis wilh laser-induced fluorescence probing of the CN fragment. The rotational distribution is dominated by a cold part peaking at N = 1 or 2, and it has a high-temperature tail Possible dissociation mechanisms are discussed and a crude energy balance suggested.

The influence of laser intensity on the rotational distribution of CN(X 2Σ +) produced via IR multiple-photon dissociation

The rotational distribution of CN(X 2Σ +) produced via laser-induced IR multiple-photon dissociation, has been measured while varying intensity by a factor of ⩾ 6 at constant fluence. The higher-intensity pulses produced a “hotter” Boltzmann distribution than those at lower intensity (780 = 20 versus 570 = 15 K respectively).