Velocity Mapping Studies of Vibrational Energy Disposal Following Methyl Iodide Photodissociation

In this paper previous results are compared for two different types of velocity mapping studies which probe vibrational energy disposal following the A-band photodissociation of methyl iodide, CH₃I + hv → CH₃ (v) + 1(²P_3/2), 1*(²P_1/2). Full three-dimensional state-specific speed and angular distributions of the nascent fragments have been recorded for the photoelectrons, iodine atoms, and methyl radicals, using state- and mass-selective (2+1) resonance-enhanced multi-photon ionization (REMPI)/time-of-flight spectrometry. Two sources of information on the vibrational energy disposal are available from velocity mapping: (a) the photoelectron images, which give information on the initial stages of vibrational excitation in electronically excited CH₃I, and (b) methyl radical images, which indicate the final energy disposal channels. Even though the two signals are believed to probe very different time-scales of the dissociation process, good agreement between the two is found for the vibrational energy disposal trends. Several trends found in the data for methyl iodide photodissociation indicate that readjustment of the ab initio multi-dimensional potential energy surfaces calculated for this molecule appears to be needed.     

Laser-induced photodissociation of He+2

The momentum distributions of He⁺ fragments from photodissociation of He⁺₂ ions have been recorded in a crossed-beams experiment. The discrete values of the kinetic energy releases can be predicted from the vibrational spacings in the ground state of the primary ions.     

The photodissociation reaction dynamics of CF(3)I at 304 nm ((3)Q(0+), (1)Q(1)<--(3)Q(0+), and (3)Q(1))

The photodissociation of CF(3)I at 304 nm has been studied using long time-delayed core-sampling photofragment translational spectroscopy. Due to its capability of detecting the kinetic energy distribution of iodine fragments with high resolution, it is able to directly assign the vibrational state distribution of CF(3) fragments. The vibrational state distributions of CF(3) fragments in the I(*)((2)P(12)) channel, i.e., (3)Q(0+) state, have a propensity of the nu(2) (') umbrella mode with a maximum distribution at the vibrational ground state. For the I((2)P(32)) channel, i.e., (1)Q(1)<--(3)Q(0+), the excitation of the nu(2) (') umbrella mode accounts for the majority of the vibrational excitation of the CF(3) fragments. The 1 nu(1) (') (symmetric CF stretch) +nnu(2) (') combination modes, which are associated with the major progression of the nu(2) (') umbrella mode, are observed for the photodissociation of CF(3)I at the I channel, i.e., (3)Q(1) state. The bond dissociation energy of the CI bond of CF(3)I is determined to be D(0)(CF(3)-I)相似文献   

State-to-state correlated study of CD3I photodissociation at 266 and 304 nm

High-resolution photofragment translational spectroscopy is used in this work to measure the translational and internal energy distributions in the CD3 and iodine fragments produced from the photodissociation of CD3I at 266 and 304 nm. Channel selected detection, via resonantly enhanced multiphoton ionization, combined with one-dimensional core sampling provides detailed information about vibrational state distributions of the CD3 fragments. The vibrational state distributions of CD3 fragments in the I*(2P12) channel have a propensity of nu2 ' umbrella bending mode with a maximum at nu2 ' = 1 for 266 nm photodissociation. For I*(2P12) channel at 304 nm photodissociation, vibrational state distributions of CD3 fragment have a maximum in the vibrational ground state. For the I(2P32) channel (1Q1 <-- 3Q(0+)), nu2 ' umbrella bending vibrational distribution is measured as the predominant vibrational mode but has a much broader distribution when compared to that of the I* channel. The vibrational state distributions of the CD3 fragment produced from the perpendicular transition, i.e., 3Q1, which was determined at 304 nm photodissociation, has a maximum at nu2 ' = 1. The curve crossing possibility between the 1Q1 and 3Q(0+) adiabatic potentials is determined as 0.19 for 266 and 0.85 for 304 nm. The trend in reaction dynamics in 266 and 304 nm photodissociation of CD3I is compared with theoretical calculations. A bond dissociation energy D0(C-I) = 56.60+/-0.5 kcal/mol was derived by applying laws of energy conservation.     

Theoretical study on the formation mechanism of iso‐CH2I‐Cl

The dissociation and isomerization reaction mechanism on the ground‐state potential energy surface for CH₂ClI are investigated by ab initio calculations. It is found that the isomer iso‐CH₂I‐Cl can be produced from either the recombination of the photodissociation fragments or the isomerization reaction of CH₂ClI, rather than from isomerization reaction of iso‐CH₂Cl‐I. Further explanations of experimental results are also presented. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Dynamics of CO formation in the photodissociation of HNCO and CH2CO at 193nm

The photodissociation of isocyanic acid (HNCO) and ketene (CH₂CO) at 193 nm was investigated using an ArF laser to dissociate the carbonyl compound and a CO laser to probe the resulting vibrationally excited CO. The dissociation of HNCO at 193 nm produces CO with an average vibrational energy of 4.6 ± 0.3 kcal/mol. The dissociation Gf CH₂CO at 193 nm produces CO with an average vibrational energy of 6.4 ± 0.8 kcal/mol. The observed CO vibrational energy distributions were found to be in close agreement with those predicted statistically assuming NH(a ¹Δ) + CO and CH₂(¹A₁) + CO were the photodissociation products.     

Picosecond laser-induced fluorescence study of the collisionless photodissociation of nitrocompounds at 266 nm

The photodissociation of nitroalkanes and dimethylnitramine by picosecond laser pulses at 266 nm has been investigated by observing fluorescence from electronically excited NO*₂ formed directly in the UV photodissociation process and also by laser-induced fluoresence (LIF) probing of NO₂ formed in the electronic ground state. The formation of the ground state fragment is a monophotonic process, and follows closely the integrated laser pulse shape, implying that the NO₂ is formed within 6 ps after absorption of a 266 nm photon by CH₃NO₂ or (CH₃)₂NNO₂. Formation of NO*₂ from dimethylnitramine was monophotonic; for the nitroalkanes the observed NO*₂ formation was much less efficient and increased faster than linearly with increasing energy in the UV photolysis pulse. In the RNo₂ nitroalkanes under study (R  Ch₃, C₂H₅, n-C₃H₇ and i-C₃H₇), the quantum efficiency of NO₂ formation does not depend on the nature of the alkyl group. An estimation of the quantum yields of photodissociation is discussed.     

Fragment state distribution,energy partitioning and rotational alignment in the state-to-state photodissociation of methylnitrite

The photodissociation of methylnitrite, CH₃ONO, generates vibrationally, rotationally, and translationally excited NO and CH₃O fragments. Following selective excitation via the localizedS ₁(nπ*) ←S ₀ transition into different overtones (v′=1, 2, 3) of theN=0 stretching modeν ₃, the complete state distribution and the energy partitioning of the NO fragment was determined. For the CH₃O fragment the complete energy, the translational energy and the sum of the rotational and vibrational energy was obtained. Owing to strong exit channel interactions between the initialν ₃ mode and the translational and rotational motions, the fragment energy redistribution is highly selective with respect to the vibrational excitation and alignment of NO. Energywise the CH₃O moiety behaves almost like a spectator. Furthermore the rotational alignment \(\overline {A_0^{(2)} } \) of NO(v″) in the populated vibrational states (v″ = 0, 1, 2, 3) was measured as a function of the initial overtone excitation. In accord with theoretical predictions based on an ab initioS ₁-potential surface, the highest geometrical selectivity of the dissociation process is obtained when the vibrational quantum numbersv′ ofν ₃ andv″ of NO are the same. With increasing mismatch the deviation from a planar dissociation process is increasing.     

Unimolecular dissociations of excited C3H6O+: A comparative photoelectron—photoion coincidence study of 1,2-epoxypropane and acetone

The unimolecular fragmentation of internal energy selected 1,2-epoxypropane cations has been studied by fixed-wavelength photoelectron—photoion coincidence spectroscopy. Branching ratios for the prominent fragment ions are reported up to an ionization energy of I = 14 eV. It is shown that 1,2-epoxypropane cations initially formed with none or only little vibrational excitation in the electronic ground state do not dissociate, though their excess energy with respect to the lowest energetic fragmentation pathway is 1.25 eV. As the internal energy is increased, slow fragmentation into several dissociation channels is observed. This is used to explain a comparably slow dissociation process observed in the case of acetone molecular ions initially excited to their electronic à state. CH₂C(OH)CH₃⁺ and/or CH₃CHCHOH⁺ are proposed as precursors for these low-rate unimolecular reactions.     

Photodissociation of molecules of benzene and some of its derivatives in the liquid phase

The effect of intramolecular interactions on photodissociation has been studied. The quantum yields have been determined for the primary free radicals of benzene, its polymethyl-substituted derivatives Ph-(CH₃), and its monosubstituted Ph-(CH₂)-X derivatives, where X is a substituent containing a heteroatom. The conclusion made previously on the importance of singlet-singlet deactivation in nonradiative transitions of an excited molecule has been confirmed. The compounds Ph-(CH₂)-X have different photolytic stability on excitation by different normal vibrations within the limits of a single electron absorption band, A comparison of the spectral data with the photodissociation data for Ph-(CH₂)n-X enable us to propose that, depending on the nature of the intramolecular interactions, a transfer of both electron and vibrational excitation energy may take place from the ring to the substituent. This process is exceeded in the latter case by the conversion of electron excitation to vibrational excitation.We thank Professor Kh. S. Bagdasar'yan for his interest in the investigation and for discussion of the results.     

Photodissociation of acryloyl chloride in the gas phase

The 193 nm photodissociation dynamics of CH2 CHCOCl in the gas phase has been examined with the technique of time-resolved Fourier transform infrared emission (TR-FTIR) spectroscopy.Vibrationally excited photofragments of CO (≤ 5),HCl (≤ 6),and C2H2 were observed and two photodissociation channels,the C-Cl fission channel and the HCl elimination channel have been identified.The vibrational and rotational state distributions of the photofragments CO and HCl have been acquired by analyzing their fully rotationally resolved v→v-1 rovibrational progressions in the emission spectra,from which it has been firmly established that the mechanism involves production of HCl via the four-center molecular elimination of CH2 CHCOCl after its internal conversion from the S1 state to the S0 state.In addition to the dominant C-Cl bond fission along the excited S1 state,the S1→S0 internal conversion has also been found to play an important role in the gas phase photolysis of CH2 CHCOCl as manifested by the considerable yield of HCl.     

Selectivity in resonant vibrational excitation by e− impact (0.3–4.5 eV) on formaldehyde,acetaldehyde and acetone

Vibrational excitation by e⁻ impact via low-energy resonances has been investigated in acetaldehyde and acetone and compared with similar results in formaldehyde. Despite the large number of vibrational modes involved, the three systems exhibit a selective excitation of only several modes. Besides an important excitation of the CO stretch modes (dominant in H₂CO), excitation of CH stretch, CH₃ stretch and CH₃ deformation are also observed in CH₃CHO and (CH₃)₂CO. Interpretation of the energy loss spectra is given in terms of recently developed symmetry considerations together with the character of the LUMO occupied by the extra electron to form the transitory negative ion (resonance). Differential cross sections versus electron energy are presented for elastic and several inelastic processes. Weak oscillations (of the “boomerang” type) are observed on the inelastic cross sections for acetaldehyde, whereas no structure appears for acetone. This is in contrast with the pronounced oscillations observed for H₂CO, and reveals a shorter lifetime for the CH₃CHO⁻ and (CH₃)₂CO⁻ resonant states, compared to H₂CO⁻.     

Imaging Isocyanic Acid Photodissociation at 193 nm: the NH(a1Δ)+ CO(X1Σ+) Channel

The photodissociation dynamics of isocyanic acid (HNCO) has been studied by the timesliced velocity map ion imaging technique at 193 nm. The NH(aΔ) products were measured via (2+1) resonance enhanced multiphoton ionization. Images have been accumulated for the NH(aΔ) rotational states in the ground and vibrational excited state (v=0 and 1). The center-of-mass translational energy distribution derived from the NH(aΔ) images implies that the CO vibrational distributions are inverted for most of the measured NH(v|j) internal states. The anisotropic product angular distribution observed indicates a rapid dissociation process for the N-C bond cleavage. A bimodal rotational state distribution of CO(v) has been observed, this result implies that isocyanic acid dissociates in the S₁ state in two different pathways.     

State determination of atoms formed by dye-laser induced photodissociation of KI in the near UV

By means of a time of flight technique, using high temperature surface ionization, we have been able to measure the internal energy of atoms formed in the photodissociation of KI at wavelengths between 265 and 335 nm. At all wavelengths only ground state K(²S_{$\text{1}\text{2}$}) is formed. Between 265 and 295 nm iodine is solely formed in the excited I(²P_{$\text{1}\text{2}$}) state, above 305 nm only in the ground state I(²P_{$\text{3}\text{2}$}).     

Multiphoton vacuum ultraviolet laser dissociation of CH3Br — analysis of chemiluminescent products

An ArF laser (193 nm) is used to induce the multiphoton dissociation of CH₃Br. Electronically excited CH fragments (A²Δ and B²Σ^?) are monitored to determine the excited state branching ratio. Possible photodissociation mechanisms are discussed.     

Picosecond laser study of the collisionless photodissociation of dimethylnitramine at 266 nm

We report a picosecond pump-probe study of the UV photolysis of gaseous dimethylnitramine. After photolysis by picosecond laser pulses at 266 nm, efficient monophotonic collision-free photodissociation occurs within 6 ps. NO₂ fragments are formed in the ground electronic state and in a fluorescent excited state; the quantum yields for both of these channels are estimated.     

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas‐phase radical–radical dynamics of the reaction of ground‐state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃)₂CH→C₃H₆ (propene)+OH, is examined by high‐resolution laser‐induced fluorescence spectroscopy in crossed‐beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Π) products with low‐ and high‐N′′ components in a ratio of 1.25:1. No significant spin–orbit or Λ‐doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS‐QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential‐energy surface: direct abstraction, giving the dominant low‐N′′ components, and formation of short‐lived addition complexes that result in hot rotational distributions, giving the high‐N′′ components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast‐flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O(³P) with primary and tertiary hydrocarbon radicals allows molecular‐level discussion of the reactivity and mechanism of the title reaction.     

Photofragment translational spectroscopy at 304 nm from C-H symmetric stretch excited CH3I [v 1 = 1]

The 304 nm photodissociation of the C-H symmetric stretch excited CH3I[v1=1,v2=0](v1 denotes the C-H symmetric stretch mode,and v2 denotes the umbrella mode)is studied with our simple photofragment translational spectrometer.An IR laser is used to excite the ground state CH3I[0,0]to the C-H symmetric stretch excited CH3I[1,0].With IR laser OFF and ON,the fractions of photofragments CH3(ν1,ν2)from the 304 nm photodissociation of CH3I[1,0]have been determined through the photofragment translational spectra(PTS)from measuring I and I*and also through the PTS from measuring CH3(0,0)(1,0)(0,1)and(1,1).The experimental results show that the C-H symmetric stretch vibration(v1=1)in parent molecules is about 66%retained in the photofragments in the I channel,but only 24%in the I*channel.The populations of photofragments CH3(0,2)and(0,3)are higher than CH3(0,0)and(0,1),showing strong inverted population both in I and I*channels.     

CEPA calculations of potential energy surfaces for open-shell systems.: IV. Photodissociation of H2O in the A1B1 state

A three-dimensional potential energy surface for the photodissociation of H₂O in its lowest excited singlet state $\text{A}$¹B₁ in C_2v or $\text{A}$¹A″ in C₃ symmetry, respectively, has been calculated with quantum-chemical ab initio methods including electron correlation. The main features of the surface are discussed and qualitative explanations are given for the experimentally observed vibrational and rotational excitations of the product OH(²Π) radicals. The surface will be used in subsequent investigations of the dynamics of the H₂O photodissociation process.