Dissociative excitation of acetyl cyanide by ultraviolet multiphoton absorption

Abstract

Dissociative excitation of CH 3 COCN to produce CN͑B-X͒ and CN͑A-X͒ fluorescence was studied by resonance enhanced multiphoton excitation at 292 nm. The laser power dependence of the CN͑B-X͒ fluorescence intensity and the lifetime of the one-photon excited S 1 state showed that CN͑B͒ formation takes place in the direct two-photon and two-body dissociation mechanism, CH 3 COCNϩ2h →CH 3 CO(X )ϩCN(B). Vibrational and rotational energy distributions of the nascent CN͑B͒ fragment were determined by a simulation analysis of the dispersed fluorescence spectrum. The vibrational distribution was found to be of the relaxed type and rotational distribution in each vibrational state could be approximated by a Boltzmann distribution. The best-fit vibrational distribution of CN͑B͒ was N v Ј ϭ0 : N v Ј ϭ1 :N v Ј ϭ2 ϭ1.00: 0.25: 0.07 with the respective rotational temperatures of T r (vЈϭ0)ϭ2600 K, T r (vЈϭ1)ϭ1000 K, and T r (vЈϭ2)ϭ900 K. The internal state distributions were found to be hotter than those predicted by the statistical model with complete energy randomization within the excited molecule. The results indicate a dissociation mechanism where both the vibrational energy deposition in the photoexcitation and available energy redistribution before the bond breakage are limited within the modes of the skeletal CCOCN structure. Possible decay channels other than the CN͑B͒ production, upon two-photon excitation at 292 nm, are also discussed based on the potential surfaces previously predicted. The formation of CN͑A͒ presently observed in the direct two-photon excitation can be interpreted as the dissociation of the electronic excited intermediate states, populated competitively via internal conversion͑s͒ from the upper electronic states. To obtain a deeper understanding of higher excited states of acetyl cyanide, the vacuum UV absorption cross section was also determined in the 110-200 nm region, using a synchrotron radiation source.

Gas-phase photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy

“…Thus, the photodissociation of CH 3 COCN occurs on the energetic ground state potential surface, different from the reaction mechanisms reported previously. [1][2][3][4][5][6][7][8][9][10] The ro-vibrational energy disposals in the HCN and CO fragments are determined by analyzing the corresponding high-resolution spectra. The measurements of O 2 dependence exclude the production possibility of these fragments via intersystem crossing.…”

“…However, when the excitation energy is increased to 34 247 cm −1 (292.0 nm), the lifetimes decreases to 0.53 ± 0.02 μs. 7 The behavior was caused by the internal vibrational redistribution process. In this work, an additional flow cell was set up to measure the excited state lifetime of CH 3 COCN and the Ar quenching rate constant at the room temperature.…”

“…They proposed a mechanism for the CN(X) preference to be attributed to the completion between adiabatic and nonadiabatic dynamics on the intermediate S 2 surface. Aoyama et al 7 are the only group investigating the photodissociation by twophoton absorption at 292 nm, yielding dissociation products of CH 3 CO(X) + CN(B). Thus far, the single-photon or multi-photon dissociation of acetyl cyanide is initiated from the higher excitation state such as S 3 and the subsequent dissociation mechanism and product analysis are mostly related to CN and CH 3 .…”

Quantum chemical study of mechanisms of dissociation and isomerization reactions in some molecules and radicals of astrophysical significance: Cyanides and related molecules

“…These calculations are particularly useful in understanding and rationalizing some of the experimental findings about the photodissociation processes in acetyl cyanide [8,24] and carbonyl cyanide [1] for which experimental results are available. Such a study does not seem to exist in the literature.…”

VUV photoionization and dissociative photoionization of the prebiotic molecule acetyl cyanide: Theory and experiment

“…This decay time is interpreted there using Rice-Ramsperger-Kassel-Marcus theory. Photodissociation at λ = 193 nm was further studied by Lee et al 11(a), 11(b) In 2004, Aoyama et al 12 used resonance enhanced multiphoton excitation at λ = 292 nm. The nascent photofragment CN is formed in excited states A 2 i and B 2 + when resonantly absorbing two of the 292 nm photons.…”