Display omitted
•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.
Display omitted
•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.
Display omitted
•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.
We performed He I ultraviolet photoelectron spectroscopy (UPS) of jet-cooled aromatic molecules using a newly developed photoelectron imaging (PEI) spectrometer. The PEI spectrometer can measure ...photoelectron spectra and photoelectron angular distributions at a considerably higher efficiency than a conventional spectrometer that uses a hemispherical energy analyzer. One technical problem with PEI is its relatively high susceptibility to background electrons generated by scattered He I radiation. To reduce this problem, we designed a new electrostatic lens that intercepts background photoelectrons emitted from the repeller plate toward the imaging detector. An energy resolution (ΔE/E) of 0.735% at E = 5.461 eV is demonstrated with He I radiation. The energy resolution is limited by the size of the ionization region. Trajectory calculations indicate that the system is capable of achieving an energy resolution of 0.04% with a laser if the imaging resolution is not limited. Experimental results are presented for jet-cooled benzene and pyridine, and they are compared with results in the literature.
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.
The vibrational relaxation of OH(X2Π) by collisions with rare gases is very slow due to small molecular interactions. No measurement of the rate coefficients has been made for relaxation of ...relatively low vibrational levels v ≤ 4 of OH by He, and there is only one report of the upper limit for v = 2, <1 × 10–14 cm3 molecule–1 s–1. In this article, we have studied vibrational relaxation of the levels v = 1–4 of OH(X2Π) by collisions with He. A gaseous mixture of O3 and H2 in a carrier gas at 70–130 Torr of He was irradiated at 266 nm, and OH(X2Π, v ≤ 4) was generated in the reaction O(1D) + H2. A single vibrational level of OH was detected with laser-induced fluorescence (LIF) via the A2Σ+–X2Π transition. Time-resolved LIF intensities of OH(v) were recorded, and kinetic analysis was made by an originally developed integrated profiles method (IPM). On the basis of the evaluation of the pressure-dependent rate coefficients of diffusion loss and the effect of impurities on the kinetics, the rate coefficients of vibrational relaxation for OH(X2Π, v = 1–4) by He have been determined to be (2.9 ± 1.5) × 10–17, (1.4 ± 0.4) × 10–16, (5.2 ± 0.5) × 10–16, and (1.6 ± 0.2) × 10–15 cm3 molecule–1 s–1 for v = 1, 2, 3, and 4, respectively (the confidence limits are 2σ). The rate coefficients are larger at higher vibrational levels and smoothly correlate to those reported previously for v = 10–12.
The internal energy distributions of reaction products are important information in clarifying the mechanism of chemical reactions. There are few reports of the nascent vibrational energy ...distribution of CS(X1Σ+) generated in the S(1D) + CS2 reaction. As long as S(1D) is produced by photodissociation of CS2, CS(X1Σ+), as a product of the chemical reaction and as a photoproduct of CS2 is indistinguishable. In this study, S(1D) was generated by the photolysis of OCS at 248 nm, where CS2 hardly dissociates, and CS(X1Σ+) was generated only by the S(1D) + CS2 reaction. The vibrational levels v″ = 0–6 of CS(X1Σ+) were detected with laser-induced fluorescence (LIF) via the A1Π–X1Σ+ transition. The identical time profiles of the LIF intensities showed that all the vibrational levels were produced by the S(1D) + CS2 reaction. The relative nascent vibrational populations of CS(X1Σ+) determined from the area intensities of the excitation spectra are 1.00 ± 0.11/0.58 ± 0.06/0.31 ± 0.03/0.078 ± 0.009/0.013 ± 0.001/<0.002/<0.002 (the values for v″ = 5 and 6 are the upper limits) for v″ = 0/1/2/3/4/5/6. The distribution agrees well with the statistical (prior) distribution.
