Kyoung-Seok Lee

State-selective predissociation dynamics of methylamines: The vibronic and H∕D effects on the conical intersection dynamics

The photodissociation dynamics of methylamines (CH(3)NH(2) and CD(3)ND(2)) on the first electronically excited state has been investigated using the velocity map ion imaging technique probing the H or D fragment. Two distinct velocity components are found in the H(D) translational energy distribution, implying the existence of two different reaction pathways for the bond dissociation. The high H(D) velocity component with the small internal energy of the radical fragment is ascribed to the N-H(D) fragmentation via the coupling of S(1) to the upper-lying S(2) repulsive potential energy surface along the N-H(D) bond elongation axis. Dissociation on the ground S(0) state prepared via the nonadiabatic dynamics at the conical intersection should be responsible for the slow H(D) fragment. Several S(1) vibronic states of methylamines including the zero-point level and nnu(9) states (n=1, 2, or 3) are exclusively chosen in order to explore the effect of the initial quantum content on the chemical reaction dynamics. The branching ratio of the fast and slow components is found to be sensitive to the initial vibronic state for the N-H bond dissociation of CH(3)NH(2), whereas it is little affected in the N-D dissociation event of CD(3)ND(2). The fast component is found to be more dominant in the translational distribution of D from CD(3)ND(2) than it is in that of H from CH(3)NH(2). The experimental result is discussed with a plausible mechanism of the conical intersection dynamics.

Direct Observation of the Primary and Secondary C-Br Bond Cleavages from the 1,2-Dibromopropane Photodissociation at 234 and 265 nm Using the Velocity Map Ion Imaging Technique

Photodissociation dynamics of 1,2-dibromopropane has been investigated at 234 and 265 nm by using the velocity map ion imaging method. At both pump energies, a single Gaussian-shaped speed distribution is observed for the Br*((2)P(1/2)) fragment, whereas at least three velocity components are found to be existent for the Br((2)P(3/2)) product. The secondary C-Br bond cleavage of the bromopropyl radical which is energized from the ultrafast primary C-Br bond rupture should be responsible for the multicomponent translational energy distribution at the low kinetic energy region of Br((2)P(3/2)). The recoil anisotropy parameter (beta) of the fragment from the primary C-Br bond dissociation is measured to be 0.53 (0.49) and 1.26 (1.73) for Br((2)P(3/2)) and Br*((2)P(1/2)), respectively, at 234 (265) nm. The beta value of Br((2)P(3/2)) from the secondary C-Br bond dissociation event at 265 nm is found to be 0.87, reflecting the fact that the corresponding Br((2)P(3/2)) fragment carried the initial vector component of the bromopropyl radical produced from the primary bond dissociation event. Density functional theory has been used to calculate energetics involved both in the primary and in the secondary C-Br bond dissociation dynamics.

The 212.8-nm photodissociation of formic acid: Degenerate four-wave mixing spectroscopy of the nascent OH(X 2Πi) radicals

The 212.8-nm photodissociation dynamics of formic acid was investigated utilizing degenerate four-wave mixing spectroscopy. The background-free rotational spectrum of the nascent OH radicals was obtained, and a cold rotational energy distribution peaking at N″=3 was extracted from the DFWM spectrum. The distribution was well approximated by a Boltzmann distribution with a rotational temperature of Trot∼716 K, which corresponds to an average rotational energy of ∼498 cm−1. The observation of a nonstatistical spin–orbit state distribution, with a preference for the low-energy F1 manifold, implies the absence of any interactions with nearby triplet states during dissociation. Preferential population of the Λ-doublet was observed, indicating that the ν7 H–O–C bending vibration in HCOOH(A) and the recoil impulse are the principal sources of the OH rotation.

The dynamics of Br(2Pj) formation in the photodissociation of vinyl and perfluorovinyl bromides

The photodissociation dynamics of vinyl bromide and perfluorovinyl bromide have been investigated at 234 nm using a photofragment ion imaging technique coupled with a state-selective [2+1] resonance-enhanced multiphoton ionization scheme. The nascent Br atoms stem from the primary C-Br bond dissociation leading to the formation of C2H3(X) and Br(2Pj;j=1/2,3/2). The obtained translational energy distributions have been well fitted by a single Boltzmann and three Gaussian functions. Boltzmann component has not been observed in the perfluorovinyl bromide. The repulsive 3A(n,sigma *) state has been considered as the origin of the highest Gaussian components. Middle translational energy components with Gaussian shapes are produced from the 1A(pi,sigma*) and/or 3A(pi,sigma*) which are very close in energy. Low-energy Gaussian components are produced via predissociation from the 3A(pi,pi*) state. The assignments have also been supported by the recoil anisotropy corresponding to the individual components. It is suggested that intersystem crossing from the triplet states to the ground state has been attributed to the Boltzmann component and the fluorination reduces the probability of this electronic relaxation process.

Nonadiabatic dynamics in the photodissociation of ICH2CN at 266 and 304 nm studied by the velocity map imaging

Photodissociation dynamics of iodoacetonitrile (ICH2CN) have been investigated at pump wavelengths of 266 and 304 nm using a photofragment ion image velocity mapping technique. At both wavelengths, the prompt C-I bond rupture takes place on the repulsive excited states to give I(2P3/2) and I*(2P1/2), and their speed and spatial distributions are simultaneously measured. The recoil anisotropy parameter (beta) at 266 nm is determined to be 1.10 and 1.60 for I and I*, respectively, while it is found to be much higher at 304 nm to give beta=1.70 and 1.90 for I and I*, respectively. The branching ratios for I*I channels are measured to be 0.724 and 0.136 at 266 and 304 nm, respectively, giving insights on nonadiabatic transition phenomena and relative oscillator strengths of optically accessible transitions of ICH2CN. Accordingly, relative oscillator strengths of parallel/perpendicular transitions and nonadiabatic transitions among the excited states are quantitatively characterized. A large portion of the available energy (41%-48%) goes into the internal energy of the CH2CN fragment. A modified impulsive model in which the CH2CN fragment is assumed to be rigid predicts the energy disposal quite well. Delocalization of an unpaired electron of the CH2CN radical during the C-I bond cleavage, leading to a large structural change of the CH2CN moiety, may be responsible for internally hot fragments.

Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of SO2 at 222· ·4 nm

The dynamics of formation of oxygen atoms after UV photoexcitation of SO2 in the gas-phase was studied by pulsed laser photolysis-laser-inducedfluorescence ‘pump-and-probe’ technique in a flow reactor. SO2 at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and O(3Pj) photofragments were detected under collision-free conditions by vacuum ultraviolet laser-induced fluorescence. The use of narrow-band probe laser radiation, generated viaresonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent O(3Pj=2,1,0) fine-structure state distribution:nj=2/nj=1/nj=0 = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing NO2photolysis as a reference, a value of Φ0(3P) = 0.13 ± 0.05 for the absolute O(3P) atom quantum yield was determined. The measured O(3P) quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited SO2(~C1B2 (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.

Effectiveness of the scavenging action of NO in vacuum UV photolyses of C2H5Br: 121.6 - 193.1 nm region

Abstract The radical scavenging effect of NO, in the system of C 2 H 5 Br photolyses in the vacuum UV region, has been studied as a function of the energy content of C 2 H 5 radicals and of the irradiation time. C 2 H 6 , one of the principal reaction products, from the primary radical process in the system can be completely suppressed by adding the appropriate amount of a radical scavenger, e.g. NO at greater than 0.1 Torr. However, by adding an unsuitable amount of NO, e.g. 0.02 Torr of NO and 50 Torr of C 2 H 5 Br, the reaction between the C 2 H 5 radical and NO becomes competitive with that of the C 2 H 5 radical and C 2 H 5 Br by the following mechanism: where R represents the C 2 H 5 or C 2 H 4 Br radical. The competitive reaction ratio between the C 2 H 5 Br/C 2 H 5 and NO/C 2 H 5 systems was obtained as a function of the irradiation energies with the variation in irradiation time by observing the production of C 2 H 6 . The values of k NO I / k 1 , deduced by comparing the observed and the theoretical values, were found to be 4.0 × 10 3 at 121.6 nm, 9.5 × 10 3 at 147 nm, 14.0 × 10 3 at 163.3 nm, 17.0 × 10 3 at 174.3 – 174.5 nm and 23.0 × 10 3 at 193.1 nm.