Translational energy study of CH3 photofragments following (n, π*) excitation of acetone

Ultrafast photodissociation studies of acetyl cyanide and acetic acid and unimolecular decomposition rates of the acetyl radical products Ultrafast deep UV mass-resolved photoionization spectroscopy has been used to investigate the photodissociation dynamics of the 3s Rydberg state of acetone. Single photon excitation at 193-195 nm is followed by single photon ͑at 260 nm͒ and two photon ͑at 390 nm͒ ionization and the signal is measured for both the acetone and acetyl photoions. The acetone Rydberg state lifetime determined from both single and two photon detection is surprisingly long, 4.7Ϯ0.2 ps. The higher probe energy for two photon ionization results in a lower minimum acetyl internal energy for ionization, so that part of the measured signal is due to neutral acetyl dissociation dynamics rather than only dissociative ionization of excited state acetone ͑which is the case for single photon ionization at 260 nm͒. The secondary dissociation rate of the neutral acetyl intermediate is measured, clearly establishing that photodissociation via the first Rydberg state of acetone occurs by a sequential dissociation mechanism. The acetyl dissociation occurs with a characteristic time of 3.1Ϯ0.5 ps. Based on RRKM ͑Rice-Ramsperger-Kassel-Marcus͒ calculations, this suggests an average acetyl internal energy of ϳ25 kcal/mole. The long lifetime of the 3s Rydberg state suggests that the dissociation dynamics may be described in terms of a fully statistical dissociation mechanism.

Section: Resultscontrasting

confidence: 85%

Section: Introductionsupporting

confidence: 83%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Ultrafast studies of the photodissociation of the acetone 3s Rydberg state at 195 nm: Formation and unimolecular dissociation of the acetyl radical

Owrutsky

Baronavski

1998

“…[1][2][3][4][5][6][7][8][9][10] Zewail and co-workers are the first to use a femtosecond time-resolved mass spectrometry to establish the nonconcerted nature of the dissociation of the acetone molecule upon photoexcitation. 4 Recently Castleman, 1 Baronavski 2 and their co-workers also performed femtosecond pump-probe experiments to study the dissociation dynamics of the excited states of the acetone molecule and acetone clusters.…”

Section: Introductionmentioning

confidence: 99%

Ultrafast dissociation dynamics of acetone: A revisit to the S1 state and 3s Rydberg state

Zhong

Poth

Castleman

1999

Because of the dispute in the literature over the dissociation rate and energy partitioning of the acetone molecule upon photoexcitation to the S 1 state (*←n) and 3s Rydberg state (3s←n), we have remeasured the lifetime of acetone ͑also d 6 -acetone͒ on the S 1 and 3s surfaces by a femtosecond time-resolved multiphoton ionization technique, coupled with a reflectron time-of-flight mass spectrometer. The measured dissociation rate of acetone on the S 1 surface is prompt, and the acetyl radical is long lived. The lifetime of acetone on the 3s surface is measured to be 3.2Ϯ0.4 ps ͑6.0Ϯ0.5 ps for d 6 -acetone͒. The dissociation rate of acetyl is approximately 1.7 ps ͑2.5 ps for d 3 -acetyl͒ from the curve fitting. This agrees well with the Rice-Ramsperger-Kassel-Marcus theory predicted lifetime of 1.0 ps ͑1.9 ps for d 3 -acetyl͒ when the internal excitation energy of the acetyl radical is treated by a statistical-adiabatic-impulsive model.

“…Ultraviolet ͑UV͒ pohotodissociation of moderate sized carbonyl compounds [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] has been extensively studied by both experimental and theoretical methods. The ␣ bond cleavage of the carbonyl to give alkyl and acyl radicals is one of the most important photochemical processes of aliphatic ketones, and is denoted as Norrish type I dissociation.…”

Section: Introductionmentioning

confidence: 99%

Dissociative excitation of acetyl cyanide by ultraviolet multiphoton absorption

Aoyama

Sugihara

Tabayashi

et al. 2003

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.