Photodissociation of CH3CHO at 248 nm by time-resolved Fourier-transform infrared emission spectroscopy: Verification of roaming and triple fragmentation

Hung, Kai-Chan; Tsai, Po‐Yu; Li, Hou-Kuan; Lin, King‐Chuen

doi:10.1063/1.4862266

Cited by 24 publications

(7 citation statements)

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“…They obtained branching ratios of 45.4%, 14.6%, 19.5%, and 9.9% for channels (R1a), (R1e), (R1b), and (R1c), respectively. An overall yield of 60% for CH 3 radicals has thus been predicted, in total contradiction with absence of any CH 3 signal in the experiments of Rubio-Lago et al 9,10 In a very recent work, Hung et al 14 applied time-resolved FTIR emission spectroscopy to investigate the roaming and triple fragmentation pathways following the 248 nm photolysis of CH 3 CHO. From time-resolved measurements of the HCO fragment as well as the rotational distribution of CO they conclude to have the first experimental evidence of triple fragmentation following the 248 nm photolysis of CH 3 CHO.…”

Section: Introductionmentioning

confidence: 92%

Photolysis of CH3CHO at 248 nm: Evidence of triple fragmentation from primary quantum yield of CH3 and HCO radicals and H atoms

Morajkar

Bossolasco

Schoemaecker

et al. 2014

The Journal of Chemical Physics

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Radical quantum yields have been measured following the 248 nm photolysis of acetaldehyde, CH3CHO. HCO radical and H atom yields have been quantified by time resolved continuous wave Cavity Ring Down Spectroscopy in the near infrared following their conversion to HO2 radicals by reaction with O2. The CH3 radical yield has been determined using the same technique following their conversion into CH3O2. Absolute yields have been deduced for HCO radicals and H atoms through fitting of time resolved HO2 profiles, obtained under various O2 concentrations, to a complex model, while the CH3 yield has been determined relative to the CH3 yield from 248 nm photolysis of CH3I. Time resolved HO2 profiles under very low O2 concentrations suggest that another unknown HO2 forming reaction path exists in this reaction system besides the conversion of HCO radicals and H atoms by reaction with O2. HO2 profiles can be well reproduced under a large range of experimental conditions with the following quantum yields: CH3CHO + hν(248nm) → CH3CHO*, CH3CHO* → CH3 + HCO ϕ(1a) = 0.125 ± 0.03, CH3CHO* → CH3 + H + CO ϕ(1e) = 0.205 ± 0.04, CH3CHO*[Formula: see text]CH3CO + HO2 ϕ(1f) = 0.07 ± 0.01. The CH3O2 quantum yield has been determined in separate experiments as ϕ(CH₃) = 0.33 ± 0.03 and is in excellent agreement with the CH3 yields derived from the HO2 measurements considering that the triple fragmentation (R1e) is an important reaction path in the 248 nm photolysis of CH3CHO. From arithmetic considerations taking into account the HO2 and CH3 measurements we deduce a remaining quantum yield for the molecular pathway: CH3CHO* → CH4 + CO ϕ(1b) = 0.6. All experiments can be consistently explained with absence of the formerly considered pathway: CH3CHO* → CH3CO + H ϕ(1c) = 0.

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Section: Introductionmentioning

confidence: 92%

Photolysis of CH3CHO at 248 nm: Evidence of triple fragmentation from primary quantum yield of CH3 and HCO radicals and H atoms

Morajkar

Bossolasco

Schoemaecker

et al. 2014

The Journal of Chemical Physics

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“…Time-resolved FTIR emission spectroscopy [51][52][53][54][55] with some modification was employed in this work, as illustrated in Fig. S5.…”

Section: Methodsmentioning

confidence: 99%

“…The sample formic acid (Acros, 97%) was used after several freeze-pump-thaw cycles, and a pressure of 1.0 Torr was then injected in the center of a six-way anodized aluminum cube for reaction. Ar (purity > 99.999%) at a pressure of 4.0 Torr added in the chamber may play two roles: first, to prevent the windows from contamination by the photolyzed species, and second, to enhance the rates of collision-induced IC or ISC to increase the emission intensity 55 .…”

Section: Methodsmentioning

confidence: 99%

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193 nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

Tso

Kasai

Lin

2020

Sci Rep

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In photodissociation of trans-formic acid (HCOOH) at 193 nm, we have observed two molecular channels of co + H 2 o and co 2 + H 2 by using 1 μs-resolved fourier-transform infrared emission spectroscopy. With the aid of spectral simulation, the co spectra are rotationally resolved for each vibrational state (v = 1-8). Each of the resulting vibrational and rotational population distributions is characteristic of two Boltzmann profiles with different temperatures, originating from either transition state pathway or OH-roaming to form the same CO + H 2 o products. the H 2 o roaming co-product is also spectrally simulated to understand the interplay with the CO product in the internal energy partitioning. Accordingly, this work has evaluated the internal energy disposal for the CO and H 2 o roaming products; especially the vibrational-state dependence of the roaming signature is reported for the first time. Further, given a 1 μs resolution, the temporal dependence of the co/co 2 product ratio at v ≥ 1 rises from 3 to 10 of study, thereby characterizing the effect of conformational memory and well reconciling with the disputed results reported previously between absorption and emission methods.Roaming is recognized as one of the hottest topics in reaction dynamics for the past decade. For the most studied roaming cases of H 2 CO and CH 3 CHO 1-11 , following photodissociation a recoiling atom or radical fragment on the ground-state surface can roam and then abstract intramolecularly another H atom to generate the H 2 + CO and CH 4 + CO molecular products. This pathway is expected to open within the roaming window, in which the dissociation threshold of such a radical channel and the transition state (TS) of a molecular channel are close to each other. However, the roaming dynamics turns out to be more complicated to render studies oriented in varied directions 12-25 . As an example using carbonyl compounds in the photodissociation, aliphatic aldehydes showed a loose roaming saddle point along the roaming dissociation coordinate 26,27 , whereas the roaming signature in methyl formate (HCOOCH 3 ) was verified to involve multiple energy states via a conical intersection 17,18 . These two types of roaming dynamics apparently behave differently.Formic acid (HCOOH) belongs to the family of carbonyl compounds, and its photodissociation dynamics has been popularly investigated for more than two decades [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] . When its electronic band S 1 in the range 220 ∼ 250 nm is excited, the dissociation channel mainly contains HCO + OH with a quantum yield of 0.7∼0.8 30 . Because the excited surface is dissociative, the produced fragments bear very large translational energy 30,33 . Kim and co-workers 33 photolyzed the HCOOH molecule at 193 nm, and obtained a fraction of available energy into translational energy of OH as large as 0.82, whereas the fraction into internal energy of HCO was only 0.13. The fragments are mainly from the decomposition along the S 1 surface and the res...

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“…of the product molecules. For example, particle imaging techniques [5] and especially the velocity map imaging (VMI) version, combined with advanced spectroscopic methods, such as multidimensional resonance-enhanced multiphoton ionization [6] and fast Fourier transform infrared spectroscopy [7], have revealed unprecedented details of chemical reactions at the microscopic level.…”

Section: Introductionmentioning

confidence: 99%

“…With this mechanism, an intriguing second peak in the distribution of the rotational states of the product CO molecule was explained. The impact of roaming effect in the research of reaction dynamics is really noteworthy, as is demonstrated by the plethora of studies of a series of aliphatic aldehydes [7] and other molecules by photodissociation, bimolecular collisions [10,11] and multistate non-adiabatic reactions [12].…”

Section: Introductionmentioning

confidence: 99%

Hamiltonian flow over saddles for exploring molecular phase space structures

Farantos

2018

Phil. Trans. R. Soc. A.

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Despite using potential energy surfaces, multivariable functions on molecular configuration space, to comprehend chemical dynamics for decades, the real happenings in molecules occur in phase space, in which the states of a classical dynamical system are completely determined by the coordinates and their conjugate momenta. Theoretical and numerical results are presented, employing alanine dipeptide as a model system, to support the view that geometrical structures in phase space dictate the dynamics of molecules, the fingerprints of which are traced by following the Hamiltonian flow above saddles. By properly selecting initial conditions in alanine dipeptide, we have found internally free rotor trajectories the existence of which can only be justified in a phase space perspective.This article is part of the theme issue 'Modern theoretical chemistry'.

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Photodissociation of CH3CHO at 248 nm by time-resolved Fourier-transform infrared emission spectroscopy: Verification of roaming and triple fragmentation

Cited by 24 publications

References 41 publications

Photolysis of CH3CHO at 248 nm: Evidence of triple fragmentation from primary quantum yield of CH3 and HCO radicals and H atoms

Photolysis of CH3CHO at 248 nm: Evidence of triple fragmentation from primary quantum yield of CH3 and HCO radicals and H atoms

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193 nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

Hamiltonian flow over saddles for exploring molecular phase space structures

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