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•The ion-imaging study was performed for photodissociation of dimethylamine.•Two kinetic energy components of the methyl radical product were measured.•The calculation revealed the ...potential energy curves for N–CH3 and N–H dissociations.
State-resolved ion-imaging of the CH3 product of dimethylamine photolysis in the 3s and 3p Rydberg bands (200–235 nm) revealed bimodal CH3 velocity distributions with a fast portion disappearing at the longer photolysis wavelength. The fast component is assigned to direct dissociation based on the calculated potential energy curves. The slow component is assigned to indirect dissociation via N–H channel conical intersection with subsequent CH3 dissociation. The origin of the threshold wavelength near 225 nm was attributed to the N–CH3 bond rupture energy barrier on the S1(3s) state, implying an increasing contribution of direct dissociation at the shorter photolysis wavelengths.
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•Bromine atoms have been detected by vacuum UV emission after two-photon excitation.•Twenty lines of 25 allowed two-photon transitions were observed.•Five missing lines did not show ...infrared emission.•Detection limit of the number density of Br atoms is ≈109 cm−3.•The present method can be applied to the measurements under high pressure conditions.
Two ultraviolet photons (250–282 nm) excited atomic bromine, Br(4p52PJ;J=1/2,3/2), to the terms built from the 4p45p electronic configuration. Through visible and infrared (VIS–IR) transitions and/or collisions with ambient gases, the terms transfer to the 2,4DJ and 2,4PJ states in the 4p44d and 4p45s electronic configurations. The vacuum ultraviolet (VUV) emission (125–163 nm) from the 4p44d and 4p45s states to the 4p52PJ state was detected. Twenty in 25 allowed two-photon transitions were observed; however, no VUV or VIS–IR emission from the other five transitions was detected. The findings differ from those by the previous reports on the two-photon resonance-enhanced ionization.
Ultraviolet photochemistry of iron pentacarbonyl, Fe(CO)5, was investigated with resonantly enhanced multiphoton ionization (REMPI) spectroscopy and ion imaging. The REMPI spectrum of CO ...photofragments, generated by ultraviolet irradiation of Fe(CO)5, showed the generation in the highly vibrationally excited states with v = 11–15. Analysis of the band intensities observed in the 213–235 nm region indicated that the high-v CO generation was maximized at around 220 nm. Generation yields of the coordinatively unsaturated intermediates, Fe(CO) n=1–4, were measured as a function of the photolysis wavelength using a nonresonant detection scheme. The yield spectrum of FeCO was correlated with that of the high-v CO fragments, suggesting high-v CO generation in the photodissociation of FeCO. The density functional theory calculations of the excited states of FeCO showed an intense photoabsorption to the metal-centered state near 220 nm. The theoretical results were consistent with the interpretation of FeCO + hν → Fe + high-v CO, which was experimentally indicated. The momentum distribution obtained from the velocity distributions of Fe, Fe(CO)4, and CO fragments further supported that Fe is the counter-product of the high-v CO fragment. The present results provided selective observation of the photochemistry of the unsaturated iron carbonyl complexes, which has not been well elucidated in laser-based experiments because of the uncontrollable sequential photodissociation producing mixed Fe(CO) n intermediates.
Versatile transformations of azo compounds are utilized not only in synthetic organic chemistry but also in materials science. In this study, a hitherto unknown stereoselectivity was observed by ...low-temperature in situ NMR spectroscopy for the photochemical denitrogenation of a cyclic azoalkane (2,3-diazabicyclo2.2.1heptane) derivative. Direct (singlet) photodenitrogenation at 188 K afforded two products, the configurationally retained ring-closed compound (ret-CP) and the inverted compound (inv-CP), in a ratio of 82/18 (±3) (ret-CP/inv-CP), with an overall yield of >95%. Triplet-sensitized denitrogenation at 199 K using benzophenone (3BP*) or xanthone (3Xan*) selectively produced inv-CP, with a ret-CP/inv-CP ratio of 7/93 (±3). Thermal isomerization of inv-CP into ret-CP was observed by low-temperature NMR spectroscopy. Transient absorption spectroscopy revealed that two distinct singlet diradicals are involved in the formation of CP during direct photodenitrogenation, that is, puckered puc-1 DR and planar pl-1 DR diradicals. The former produces ret-CP, whereas the latter affords inv-CP. Kinetic analysis using the integrated profiles method was used to determine the molecular absorption coefficient of pl-1 DR (ε560 = 4900 ± 250 M–1 cm–1) for the first time. The involvement of the puckered singlet diradical resolves the mechanistic puzzle of stereoselective denitrogenation of diazabicycloheptane-type azoalkanes.
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•The product state distributions in trimethylamine photodissociation were measured.•The CH3 photofragments showed dual ring-like scattering distributions.•The slow CH3 fragment was ...ascribed to the production of the excited state N(CH3)2.•The energy threshold of CN fission was located between the S2 and S1 state origins.
The photodissociation dynamics of trimethylamine, N(CH3)3, was studied using ion-imaging. The photolysis wavelength was scanned over the 200–236 nm region, where the S1(3s) and S2(3p) states were excited with varying relative populations. The dissociation threshold of CN bond fission was found at 42,500 cm−1, which is located between the S1(3s) and S2(3p) origins. The final-state distributions were qualitatively insensitive to the photoinitiated states, indicating a dissociation mechanism following ultrafast electronic dynamics. The CH3 photofragments showed dual ring-like scattering distributions, which were ascribed to branching to the CH3 + N(CH3)2 (A∼2A1) and CH3 + N(CH3)2 (X∼2B1) pathways.
Ion-imaging and dispersed fluorescence spectroscopy are employed for the photodissociation dynamics study of methylamine in the photolysis wavelength range 205–213 nm. The methyl radical product is ...found to populate a wide range of ro-vibrational states, among which the CH3 fragment generated in the v = 0 state shows a bimodal kinetic energy distribution. The internal energy analysis of the NH2 counterproduct indicates that a lower kinetic energy component, which was observed only with the CH3(v=0) fragment, energetically matches the electronically excited Ã2A1 state. The dispersed fluorescence spectrum, whose band structure is assigned to the Ã2A1 → X̃2B1 transition, provides evidence of the CH3(v=0) + NH2(Ã2A1) pathway. The branching mechanism of the product pathway is discussed in terms of nuclear dynamics in the long-range region, where the conical intersection between the excited- and ground-state potential energy surfaces can play a significant role.
The collision complex formed from a vibrationally excited reactant undergoes redissociation to the reactant, intramolecular vibrational relaxation (randomization of vibrational energy), or chemical ...reaction to the products. If attractive interaction between the reactants is large, efficient vibrational relaxation in the complex prevents redissociation to the reactants with the initial vibrational energy, and the complex decomposes to the reactants with low vibrational energy or converts to the products. In this paper, we have studied the branching ratios between the intramolecular vibrational relaxation and chemical reaction of an adduct HO(v)–CO formed from OH(X2Πi) in different vibrational levels v = 0–4 and CO. OH(v = 0–4) generated in a gaseous mixture of O3/H2/CO/He irradiated at 266 nm was detected with laser-induced fluorescence (LIF) via the A2Σ+–X2Πi transition, and H atoms were probed by the two-photon excited LIF technique. From the kinetic analysis of the time-resolved LIF intensities of OH(v) and H, we have found that the intramolecular vibrational relaxation is mainly governed by a single quantum change, HO(v)–CO → HO(v–1)–CO, followed by redissociation to OH(v–1) and CO. With the vibrational quantum number v, chemical process from the adduct to H + CO2 is accelerated, and vibrational relaxation is decelerated. The countertrend is elucidated by the competition between chemical reaction and vibrational relaxation in the adduct HOCO.
•Vibrational relaxation between O2 and N2.•Potential energy surface using an interpolating moving least-square method.•Energy transfer and the scattering processes were investigated.
Vibrational ...relaxation processes in the collision of highly vibrationally excited O2 (v=15–20) and ground-state N2 (v=0) have been studied using a quasiclassical trajectory method. The potential energy surface of the system is obtained using the interpolating moving least-squares method. The energy transfer and the scattering processes in the collision were investigated. We found that vibrational–vibrational (V–V) transfer was dominant under the conditions of our calculations.
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