Photodissociation dynamics of CBr4 at 267 nm by means of ion velocity imaging

The photodissociation dynamics of CBr4 at 267 nm has been studied using time of flight (TOF) mass spectrometry and ion velocity imaging techniques. The photochemical products are detected with resonance enhanced multiphoton ionization (REMPI) as well as single-photon vacuum ultraviolet ionization at 118 nm. REMPI at 266.65 and 266.71 nm was used to detect the ground Br(2P32) and spin-orbit excited Br(2P12) atoms, respectively. The translational energy and angular distributions are consistent with direct dissociation from an excited triplet state and indirect dissociation from high vibrational levels on the singlet ground state surface. Br2+ ions are also observed in the TOF spectra with a focused 267 nm laser. The counter fragment, CBr2+, is observed when this photolysis laser is unfocused, and photons at 118 nm are used to ionize the radical products. The translational energy distributions of the CBr2+ and Br2+ products can be momentum matched, which indicates that molecular Br2 elimination is one of the primary dissociation channels.     

Photodissociation of cyclobutyl bromide at 234 nm studied using velocity map imaging

This study investigates the 234 nm photodissociation dynamics of cyclobutyl bromide using a two-dimensional photofragment velocity imaging technique. The spin-orbit ground- and excited-state Br(2P) atoms are state-selectively detected via [2+1] resonance enhanced multiphoton ionization (REMPI), whereas the cyclobutyl radicals are ionized using 157 nm laser light. The Br(2P(3/2)) and the Br(2P(1/2)) atoms and their c-C4H7 radical cofragments evidence a single-peaked, Gaussian-shaped translational energy distribution ranging from approximately 14 to approximately 39 kcal/mol and angular distributions with significant parallel character. The Br(2P(1/2))/ Br(2P(3/2)) spin-orbit branching ratio is determined to be 0.11 +/- 0.07 by momentum match between the Br(2P) photofragments and the recoiling c-C4H7 fragments, assuming a uniform photoionization probability of the c-C4H7 radicals with an internal energy range of 10-35 kcal/mol. The REMPI line strength ratio for the detection of Br(2P(3/2)) and Br(2P(1/2)) atoms at 233.681 and 234.021 nm, respectively, is therefore derived to be 0.10 +/- 0.07. The measured recoil kinetic energies of the c-C4H7 radicals, and the resulting distribution of internal energies, indicates some of the radicals are formed with total internal energies above the barrier to isomerization and subsequent dissociation, but our analysis indicates they may be stable due to the substantial fraction of the internal energy which is partitioned to rotational energy of the radicals.     

Photodissociation study of 1,3-dibromopropane at 234 nm via an ion velocity imaging technique

The photodissociation of 1,3-dibromopropane has been studied at 234 nm using a 2D photofragment ion velocity imaging technique coupled with a [2 + 1] resonance-enhanced multiphoton ionization scheme. The velocity distributions for the Br (2P1/2) (denoted Br) and Br (2P3/2) (denoted Br) fragments are determined, and each can be fitted by a narrow single-peaked Gaussian curve, suggesting that bromine fragments are generated as a result of direct dissociation via repulsive potential energy surfaces. The recoil anisotropies were measured to be beta = 0.80 for Br and 1.31 for Br, and the product relative quantum yields at 234 nm is Phi234 nm(Br) = 0.21.     

Photodissociation of the BrO radical using velocity map ion imaging: excited state dynamics and accurate D0(0)(BrO) evaluation

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278-281.5 nm) and below (355 nm) the A (2)Pi(3/2) state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O((1)D(2)) threshold the contribution of the direct excitation to states other than the A (2)Pi(3/2) state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O((1)D(2)) threshold provides an accurate and direct determination of the A (2)Pi(3/2) state dissociation threshold of 35418+/-35 cm(-1), leading to a ground state bond energy of D(0)(0)(BrO)=55.9+/-0.1 kcal/mol.     

Photodissociation dynamics of the ethoxy radical investigated by photofragment coincidence imaging

The photodissociation dynamics of the ethoxy radical (CH3CH2O) have been studied at energies from 5.17 to 5.96 eV using photofragment coincidence imaging. The upper state of the electronic transition excited at these energies is assigned to the C2A'state on the basis of electronic structure calculations. Fragment mass distributions show two photodissociation channels, OH + C2H4 and CH3 + CH2O. The presence of an additional photodissociation channel, identified as D + C2D4O, is revealed in time-of-flight distributions from the photodissociation of CD3CD2O. The product branching ratios and fragment translational energy distributions for all of the observed mass channels are nonstatistical. Moreover, the significant yield of OH + C2H4 product suggests that the mechanism for this channel involves isomerization on the excited-state surface. Photodissociation at a much lower yield is seen following excitation at 3.91 eV, corresponding to a vibronic band of the B2A' <-- X2A' transition.     

Photodissociation dynamics of small aromatic molecules studied by multimass ion imaging

The photodissociation dynamics of various aromatic molecules, studied using multimass ion imaging techniques, is reviewed. The experimental data reveals new isomerization and dissociation mechanisms. Our investigation of benzene, pyridine, and pyrimidine finds that H-atom elimination thresholds remain the same for the three molecules. We also notice that ring-opening dissociation thresholds decrease rapidly with the increase of the number of nitrogen atoms in the aromatic ring. Hydrogen atom elimination is the sole dissociation channel for benzene at 193 nm. Along with H-atom elimination, we observe five distinct ring-opening dissociation channels for pyridine at 193 nm. No dissociation channels were observed for benzene and pyridine at 248 nm. Ring-opening dissociation channels are the major channels for pyrimidine, which dissociates at 193 nm and also at 248 nm. A six-membered to seven-membered ring isomerization was observed for photodissociation processes involving toluene, m-xylene, aniline, 4-methylpyridine, alpha-fluorotoluene, and 4-fluorotoluene, indicating a general isomerization mechanism for all such aromatic molecules. What is significant, is that during the isomerization, atoms (i.e., carbon, nitrogen, fluorine, and hydrogen) belonging to respective alkyl or amino groups are involved in an exchange with atoms within the aromatic ring. This type of isomerization is not observed in other aromatic isomerization mechanisms. For small tyrosine chromophores, such as phenol, 4-methylphenol, and 4-ethylphenol, H-atom elimination from a repulsive excited state plays a key role. However, dissociation is quenched in large chromophores like 4-(2-aminoethyl)-phenol. Our work demonstrates the capability and high sensitivity of multimass ion imaging techniques in the study of aromatic compounds.     

Photodissociation dynamics of pyrimidine

Photodissociation of pyrimidine at 193 and 248 nm was investigated separately using vacuum ultraviolet photoionization at 118.4 and 88.6 nm and multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C4N2H4 --> C4N2H3 + H and five ring opening dissociation channels, C4N2H4 --> C3NH3 + HCN, C4N2H4 --> 2C2NH2, C4N2H4 --> CH3N + C3NH, C4N2H4 --> C4NH2 + NH2, and C4N2H4 --> CH2N + C3NH2. Only the first four channels were observed at 248 nm. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion at both wavelengths. The dissociation rates were found to be >5 x 10(7) and 1 x 10(6) s(-1) at 193 and 248 nm, respectively. Comparison with the potential energies from ab initio calculations have been made.     

Photodissociation dynamics of pyridine

Photodissociation of pyridine, 2,6-d2-pyridine, and d5-pyridine at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C5NH5 --> C5NH4 + H (10%) and five ring opening dissociation channels, C5NH5 --> C4H4 + HCN, C5NH5 --> C3H3 + C2NH2, C5NH5 --> C2H4 +C3NH, C5NH5 --> C4NH2 + CH3 (14%), and C5NH5 --> C2H2 + C3NH3. Extensive H and D atom exchanges of 2,6-d2-pyridine prior to dissociation were observed. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion. The dissociation rate of pyridine excited by 248-nm photons was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 2x10(3) s(-1). Comparisons with potential energies obtained from ab initio calculations and dissociation rates obtained from the Rice-Ramsperger-Kassel-Marcus theory have been made.     

Photodissociation dynamics of phenol

The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H(2)O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or H(2)O occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 相似文献   

Photodissociation dynamics of fluorobenzene

Photodissociation of both fluorobenzene and d(5)-fluorobenzene at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF and DF eliminations were found to be the major dissociation channels. Small amounts of photofragments, C(6)H(4)F and C(6)D(4)F, corresponding to H and D atom eliminations were also observed. Dissociation rate and fragment translational energy distribution suggest that HF (DF) and H (D) atom elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF and DF eliminations. A comparison with photodissociation of benzene has been made.     

The UV photodissociation dynamics of ClO radical using velocity map ion imaging

We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical from 235 to 291 nm using velocity map ion imaging. We find that Cl(2P(3/2))+O(1D(2)) is the dominant channel above the O(1D(2)) threshold with minor contributions from the Cl(2P(J))+O(3P(J)) and Cl(2P(1/2))+O(1D(2)) channels. We have measured the photofragment angular distributions for each dissociation channel and find that the A 2pi state reached via a parallel transition carries most of the oscillator strength above the O(1D(2)) threshold. The formation of O(3P(J)) fragments with positive anisotropy is evidence of curve crossing from the A 2pi state to one of several dissociative states. The curve crossing probability increases with wavelength in good agreement with previous theoretical calculations. We have directly determined the O(1D(2)) threshold to be 38,050+/-20 cm(-1) by measuring O(1D(2)) quantum yield in the wavelength range of 260-270 nm. We also report on the predissociation dynamics of ClO below the O(1D(2)) threshold. We find that the branching ratio of Cl(2P(3/2))/Cl(2P(1/2)) is 1.5+/-0.1 at both 266 and 291 nm. The rotational depolarization of the anisotropy parameters of the Cl(2P(3/2)) fragments provides predissociation lifetimes of 1.5+/-0.2 ps for the 9-0 band and 1.0+/-0.4 ps for the 8-0 band, in reasonable agreement with previous spectroscopic and theoretical studies.     

Photodissociation dynamics of hydroxybenzoic acids

Aromatic amino acids have large UV absorption cross-sections and low fluorescence quantum yields. Ultrafast internal conversion, which transforms electronic excitation energy to vibrational energy, was assumed to account for the photostability of amino acids. Recent theoretical and experimental investigations suggested that low fluorescence quantum yields of phenol (chromophore of tyrosine) are due to the dissociation from a repulsive excited state. Radicals generated from dissociation may undergo undesired reactions. It contradicts the observed photostability of amino acids. In this work, we explored the photodissociation dynamics of the tyrosine chromophores, 2-, 3- and 4-hydroxybenzoic acid in a molecular beam at 193 nm using multimass ion imaging techniques. We demonstrated that dissociation from the excited state is effectively quenched for the conformers of hydroxybenzoic acids with intramolecular hydrogen bonding. Ab initio calculations show that the excited state and the ground state potential energy surfaces change significantly for the conformers with intramolecular hydrogen bonding. It shows the importance of intramolecular hydrogen bond in the excited state dynamics and provides an alternative molecular mechanism for the photostability of aromatic amino acids upon irradiation of ultraviolet photons.     

Photodissociation dynamics of the HCNN radical

The photodissociation dynamics of the diazomethyl (HCNN) radical have been studied using fast radical beam photofragment translational spectroscopy. A photofragment yield spectrum was obtained for the range of 25,510-40,820 cm(-1), and photodissociation was shown to occur for energies above 25,600 cm(-1). The only product channel observed was the formation of CH and N2. Fragment translational energy and angular distributions were obtained at several energies in the range covered by the photofragment yield spectrum. The fragment translational energy distributions showed at least two distinct features at energies up to 4.59 eV, and were not well fit by phase space theory at any of the excitation energies studied. A revised C-N bond dissociation energy and heat of formation for HCNN, D0(HC-NN)=1.139+/-0.019 eV and DeltafH0(HCNN)=5.010+/-0.023 eV, were determined.     

Photodissociation dynamics of ionic argon pentamer

Photodissociation of the ionized argon pentamer, Ar(5)(+), is studied using an extended diatomics-in-molecules interaction model with the inclusion of the spin-orbit coupling and various dynamical approaches. A thorough comparison with the experimental data available in the literature is presented, including photofragment abundances and their kinetic and internal energy distributions. New predictions are reported for ultraviolet photoexcitation energies, a range that has not been studied before either experimentally or theoretically.     

Photodissociation dynamics of nitrobenzene and o-nitrotoluene

Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO(2) elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO(2) elimination channels were determined to be NONO(2)=0.32+/-0.12 (193 nm), 0.26+/-0.12 (248 nm), and 0.4+/-0.12(266 nm) for nitrobenzene and 0.42+/-0.12(193 nm) and 0.3+/-0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results.     

Photodissociation studies of CBr(4) (+) and CBr(3) (+) at 267 nm using ion velocity imaging

Time-of-flight (TOF) mass spectroscopy and ion velocity imaging were employed to study the formation and photodissociation of CBr(4) (+) and CBr(3) (+) ions that were observed in the TOF spectrum when a CBr(4) beam was irradiated with 118 nm and 355 nm lasers. Energy dependence measurements show that both CBr(4) (+) and CBr(3) (+) ions depend on the fourth power of the 355 nm laser energy, which indicates that direct ionization and dissociative ionization of CBr(4) have low probabilities from the state initially excited at 118 nm. This is likely due to the large geometry change in the CBr(4) (+) ion. Two ionic fragments Br(+) and CBr(2) (+) were observed from the dissociation of CBr(4) (+) and CBr(3) (+) ions when another laser at 267 nm was introduced to the interaction region at a delayed time. The possible dissociation pathways and the angular and translational distributions are discussed in the paper.     

Photochemistry of bromofluorobenzenes

The photochemistry of low lying excited states of six different fluorinated bromobenzenes has been investigated by means of femtosecond laser spectroscopy and high level ab initio CASSCF/CASPT2 quantum chemical calculations. The objective of the work was to investigate how and to what extent light substituents, position on the benzene ring and number, would influence the dissociation mechanism of bromobenzene. In general, the actual position of a fluorine atom affects the dissociation rate to a less extent than the number of fluorine atoms. A clear connection between a lowering of a repulsive pisigma relative to a bound pipi state and the number of fluorine substituents exists, and the previously suggested model of coupling between dissociation rate and relative location of bound and repulsive state still holds for these molecules. A more elaborate examination of the electronic structure of the excited states in bromobenzenes than previously reported is presented.     

Photodissociation of pyrrole-ammonia clusters by velocity map imaging: mechanism for the H-atom transfer reaction

The photodissociation dynamics of pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-6) has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. The excited state hydrogen-atom transfer mechanism (ESHT) is evidenced through delayed ionization and presents a threshold around 236.6 nm, in agreement with previous reports. A high resolution determination of the kinetic energy distributions (KEDs) of the products reveals slow (～0.15 eV) and structured distributions for all the ammonia cluster masses studied. The low values of the measured kinetic energy rule out the existence of a long-lived intermediate state, as it has been proposed previously. Instead, a direct N-H bond rupture, in the fashion of the photodissociation of bare pyrrole, is proposed. This assumption is supported by a careful analysis of the structure of the measured KEDs in terms of a discrete vibrational activity of the pyrrolyl co-fragment.     

Photodissociation dynamics of ClCN at different wavelengths

The photodissociation dynamics of small molecules in the gas and condensed phase is an important source of information for better characterizing intermolecular interactions. Herein, classical molecular dynamics simulations with provisions to follow reactive processes between different electronic states are used to probe the wavelength dependence of product state distributions after laser excitation of ClCN. It is found that the maximum of the rotational excitation distribution P(j) of the CN product shifts to lower j-values with increasing wavelength and the width of the distribution narrows. Both observations are in accord with earlier experiments and provide improvements over previous theoretical treatments of the process with the same interaction potentials. For the reaction in a water droplet, strong quenching of rotational excitation is found.