Photodissociation of 2-Bromobutane at ～265 nm by Ion-velocity Map Imaging Technique

The photodissociation dynamics of 2-bromobutane has been investigated at 264.77 and 264.86 nm by ion-velocity map imaging technique coupled with resonance-enhanced multi-photon ionization. The speed and angular distributions have been derived from the velocity map images of Br and Br*. The speed distributions of Br and Br* atoms in the photodis-sociation of 2-bromobutane at ～265 nm can be fitted using only one Gaussian function indicating that bromine fragments were produced via direct dissociation of C-Br bond. Thecontributions of the excited ³Q₀, ³Q₁, and ¹Q₁ states to the products (Br and Br*) were discussed. It is found that the nonadiabatic ¹Q₁←³Q₀ transition plays an important role for Br photofragment in the dissociation of 2-C₄H₉Br at ～265 nm. Relative quantum yield of 0.621 for Br(²P_3/2) at ～265 nm in the photodissociation of 2-bromobutane is derived. By comparing the photodissociation of 2-C₄H₉Br at ～265 nm and that that at ～234 nm, the anisotropy parameter β(Br) and β(Br*), and relative quantum yield ?(Br) decrease with increasing wavelength, the probability of curve crossing between ³Q₀ and ¹Q₁ decreases with increasing laser wavelength.     

Multiphoton dissociative ionization of tert-pentyl bromide near 265 nm

We report on the photodissociation dynamics of tert-pentyl bromide near 265 nm investigated by time-sliced velocity map imaging. The speed and angular distributions have been analyzed for both the ground-state Br((2)P(3∕2)) atom (denoted Br) and the spin-orbit excited-state Br((2)P(1∕2)) atom (denoted Br*). The speed distributions of Br and Br* atoms are all found to consist of three Gaussian components, which correlate to three independent dissociation pathways on the excited potential energy surfaces: (1) the high translational energy (E(T)) component from the prompt dissociation along the C-Br stretching mode, (2) the middle E(T) component from the repulsive mode along the C-Br stretching coupled with some bending motions, and (3) the low E(T) component from the repulsive mode along the C-Br stretching coupled with more bending motions. More interestingly, we have also observed the tert-C(5)H(11)(+) ions in 263-267 nm. The near-zero kinetic energy distributions extracted from the three tert-C(5)H(11)(+) images near 265 nm show the typical characteristics that are attributable to multiphoton dissociative ionization, suggesting the existence of a neutral superexcited state of the parent tert-pentyl bromide molecule. The contribution of bromine atoms formed in this dissociative ionization channel adds in the total relative distribution of low E(T) component in the Br*(Br) formation channel, which reasonably explains the abnormal distributions observed in between the middle and low E(T) components in the Br*(Br) formation channel.     

Br(2Pj) atom formation dynamics in ultraviolet photodissociation of tert-butyl bromide and iso-butyl bromide

The photodissociation dynamics of tert-C(4)H(9)Br and iso-C(4)H(9)Br has been studied at 234 and 265 nm using two-dimensional velocity map imaging technique. The translational energy and angular distributions have been analyzed for Br, Br(*), and tert-C(4)H(9) radical. The energy distribution of Br atom in the photodissociation of tert-C(4)H(9)Br is found to consist of two Gaussian components. The two components are correlated to two independent reaction paths on the excited potential energy surfaces: (1) the high-energy component from the prompt dissociation along the C-Br stretching mode and (2) the low-energy component from the repulsive mode along the C-Br stretching, coupled with some bending motions. For the energy distribution of Br(*) atom in the photodissociation of tert-C(4)H(9)Br, a third multiphoton dissociative ionization channel is observed at 265 nm in addition to the two energy components corresponding to channels (1) and (2). The energy distributions of Br and Br(*) atoms in the photodissociation of iso-C(4)H(9)Br can be fitted using only one Gaussian function indicating a single formation channel. Relative quantum yields for Br((2)P(32)) at 234 and 265 nm in the photodissociation of tert-C(4)H(9)Br are measured to be 0.76 and 0.65, respectively. For iso-C(4)H(9)Br, the measured value is Phi(234 nm)(Br)=0.81. The contribution of bending modes to Br and Br(*) is much more obvious in the photodissociation of tert-C(4)H(9)Br than in iso-C(4)H(9)Br.     

Ultraviolet Photodissociation Dynamics of m-Bromofluorobenzene at around 240 nm?

The photodissociation dynamics of m-bromofluorobenzene has been experimentally investigated at around 240 nm using the DC-slice velocity map imaging technique. The kinetic energy release spectra and the recoiling angular distributions of fragmented Br(²P_3/2) and Br(²P_1/2) atoms from photodissociation of m-bromofluorobenzene have been measured at different photolysis wavelengths around 240 nm. The experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying ¹ππ^* excited states dominate the production process of the ground state Br(²P_3/2) atoms. Because of the weak spin-orbit coupling effect among the low-lying triplet and singlet states, the spin-orbit excited Br(²P_1/2) atoms are mainly produced via singlet-triplet state coupling in the dissociation step. The similarity between the present results and that recently reported for o-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.     

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.     

Photofragmentation of CH3Br in the A band

CH₃Br is photodissociated in the first continuum. Dissociation takes place into ground state CH₃ and Br [ = Br(²P_{$\text{3}\text{2}$}] or Br* [ = Br(*P_{$\text{1}\text{2}$})]. Time of flight and angular distributions of the CH₃ fragments are measured. The Br*/Br ratios upon excitation at 222 and 193 nm are found to be 1.00 and 0.20 respectively. The anisotropy parameters at these wavelengths are β = 0.28±0.04 and β = ?0.23±0.02, respectively. The total absorption cross section is decomposed into partial absorption cross sections of the ¹Q, ³Q₀ and ³Q₁ states. It appears that excitation at 222 nm takes place to the ³Q₀ and ³Q ₁ states whereas at 193 nm the ¹Q and ³Q₀ states are excited. Contrary to CH₃I, the adiabatic curve crossing between the ³Q₀ and the ¹Q states in Ch₃Br is not important. The dissociation energy of the CBr bond is determined to be D₀(CH₃Br) = 2.87±0.02 eV.     

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.     

Photodissociation Dynamics of Dichlorodifluoromethane (CF2Cl2) around 235 nm using Time-Sliced Velocity Map Imaging Technology

Photodissociation dynamics of dichlorodifluoromethane (CF₂Cl₂) around 235 nm has been studied using the time-sliced velocity map imaging technology in combination with the resonance enhanced multi-photon ionization technology. By measuring the raw images of chlorine atoms which are formed via one-photon dissociation of CF₂Cl₂, the speed and angular distributions can be directly obtained. The speed distribution of excited-state chlorine atoms consists of high translation energy (E_T) and low E_T components, which are related to direct dissociation on ³Q₀ state and predissociation on the ground state induced by internal conversion, respectively. The speed distribution of ground-state chlorine atoms also consists of high E_T and low E_T components which are related to predissociation between ³Q₀ and ¹Q₁ states and predissociation on the ground state induced by internal conversion, respectively. Radical dissociation channel is confirmed, nevertheless, secondary dissociation and three-body dissociation channels are excluded.     

二溴代烷烃的紫外光解动力学研究

利用飞行时间质谱装置研究了234和267nm激光作用下二溴甲烷、二溴乙烷、二溴丙烷和二溴丁烷分子的光解离过程．研究表明二溴代烷烃分子在紫外激光的作用下主要是断裂C—Br键解离出一个Br原子,并且存在两种可能的布居：基态Br(^2P3/2^0)和激发态Br^*(^2P1／2^0)．通过共振增强多光子电离技术探测两种光解产物布居的分支比．对比得到了分子构型对称性不同的二溴代烷烃的分支比,提出了两种假设的光解离模型．     

C-Br Bond Dissociation Mechanisms of 2-Bromothiophene and 3-Bromothiophene at 267 nm

C-Br bond dissociation mechanisms of 2-bromothiophene and 3-bromothiophene at 267 nm were investigated using ion velocity imaging technique. Translational energy distributions and angular distributions of the photoproducts, Br(²P_3/2) and Br^*(²P_½), were obtained and the possible dissociation channels were analyzed. For these two bromothiophenes, the Br fragments were produced via three channels: (i) the fast predissociation following the intersystem crossing from the excited singlet state to repulsive triplet state; (ii) the hot dissociation on highly vibrational ground state following the internal conversion of the excited singlet state; and (iii) the dissociation following the multiphoton ionization of the parent molecules. Similar channels are involved for photoproduct Br^* of the 2-bromothiophene dissociation at 267 nm; whereas for the photoproduct Br^* of 3-bromothiophene, the dissociation channel via internal conversion from the excited singlet state to highly vibrational ground state became dominating and the fast predissociation channel via the excited triplet state almost disappeared. Informations about the relative contribution, energy disposal, and the anisotropy of each channel were quantitatively given. It was found that with the position of Br atom in thienyl being far from S atom, the relative ratios of products from channels (i) and (ii) decreased obviously and the anisotropies corresponding to each channel became weaker.     

Ultraviolet photodissociation of bromopropane: Energy partitioning and curve potential crossing analysis

The photodissociation mechanism of bromopropane (C₃H₇Br) molecule in the UV region is treated within time-dependent numerical methods. The results and our ab initio calculations indicated that the absorption of C₃H₇Br in the investigated region primarily originated from the excitations to the lowest three repulsive states, as assigned as 2A ₂, 4A ₁, and 3A ₁ in C _s symmetry. Dissociations occurred on the PES surfaces of the three states, giving C₃H₇ + Br (² P _3/2) or C₃H₇ + Br* (² P _1/2) as two final products, and being impacted by an avoided crossing between the PES surfaces of the 3A ₁ and 4A ₁ states. The transition dipole to the 2A ₂ state was perpendicular to the symmetry plane, so perpendicular to the C-Br bond. The transitions to the 3A ₁ state was polarized parallel to the symmetry plane, and also parallel to the C-Br bond. We have also determined the avoided crossing probabilities, which affected the relative fractions of the individual pathways and angular distributions of photofragments, for the photolysis of C₃H₇Br in the UV region.     

Quantum yield of Cl* (2P1/2) production in the gas phase photolysis of CCl4 in the ultraviolet

In this paper, we have probed the dynamics of chlorine atom production from the gas phase photodissociation of carbon tetrachloride at 222 and 235 nm. The quantum yield, φ* of Cl* (²P_1/2) production has been determined by probing the nascent concentrations of both excited (²P_1/2) and ground state (²P_3/2) chlorine atoms by suitable resonance-enhanced multiphoton ionization (REMPI) detection schemes. Although at the photolysis wavelengths the absorption of carbon tetrachloride is weak, significant amounts of Cl* are produced. Surprisingly, the quantum yield of Cl* production does not follow the absorption spectrum closely, which gives rise to the possibility of an indirect dissociation mechanism present in CCl₄ along with direct dissociation at these ultraviolet wavelengths     

A two-dimensional wavepacket study of the nonadiabatic dynamics of CH2BrCl

The nonadiabatic photodissociation dynamics of CH2BrCl into CH2Br + Cl or CH2Cl + Br is studied using two-dimensional wavepacket propagations on ab initio multiconfigurational MS-CASPT2 potential energy surfaces. Using a three-state diabatic model, we investigate the electronic states responsible for the two competing fragmentation channels and how the conical intersection present between the two lowest excited states affects the dissociation rate. Within this model, we find that the Br/Cl branching ratio depends on the irradiation wavelength. Predominant C-Br fragmentation occurs for wavelengths longer than 200 nm, while nonadiabatic C-Cl dissociation with a constant branching ratio of 0.4 is predicted upon absorption of photons in the range of 170-180 nm. Additionally, we observe complete nonadiabatic population transfer in less than 100 fs, that is, before the wavepacket can reach the conical intersection. As a consequence, there is no three-body CH2 + Br + Cl dissociation.     

Spin-orbit ab initio investigation of the photodissociation of C2H5Br

The photodissociation of ethyl bromide (C₂H₅Br) has been investigated by spin-orbit (SO) ab initio calculations. The vertical excitation energies of some excited states for C₂H₅Br were calculated. The potential energy curves of C₂H₅Br along the C–Br dissociation coordinate were calculated by multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit (SO) interaction through complete active space state interaction (MS-CASPT2/CASSI-SO). The calculated results clearly assigned the experimentally observed photodissociation channels leading to C₂H₅ + Br (²P_3/2) and C₂H₅ + Br*(²P_1/2).     

Halogen Effect on the Photodissociation Mechanism for Gas‐Phase Bromobenzene and Iodobenzene

The velocity imaging technique combined with (2+1) resonance‐enhanced multiphoton ionization (REMPI) is used to detect the halogen fragments in the photodissociation of bromobenzene and iodobenzene at 266 nm. With the aid of potential energy curve calculations by Lunell (Y. J. Liu, P. Persson, S. Lunell, J. Phys. Chem. A 2004 , 108, 2339–2345.), the Br fragmentation is proposed to stem from excitation of the lowest excited singlet state followed by predissociation along a repulsive triplet state. The slowed dissociation rate leads to production of the isotropic Br fragments and 93 % internal energy deposition. Only the ground state Br(²P_3/2) is detectable. In contrast, when iodine is substituted, the iodine effect stabilizes the repulsive states associated with the I? C₆H₅ bond rupture and the subsequent dissociation channels become more complicated. 84 % of the iodobenzene molecules obtained follow a direct dissociation channel, while the remaining undergo a predissociative process. Both routes result in rapid dissociation with anisotropy parameters of 0.7±0.2 and 0.9±0.2 as well as 70 % and 26 % in the fractions of translational energy deposition, respectively. The relative quantum yields of I* and I are 0.35 and 0.65 and their related photodissociation pathways are discussed in detail.     

Ion imaging studies of ClONO2 photodissociation: Primary branching ratios and secondary dissociation

The photodissociation dynamics of ClONO₂ at 235 nm has been reinvestigated using velocity map ion imaging. We report branching ratios for the Cl + NO₃ and ClO + NO₂ channels to be 0.49:0.51 with anisotropy parameters of β = 0.5 ± 0.1 and β = −0.1 ± 0.3 for the Cl and ClO production channels, respectively. Photodissociation at 248 nm and 262 nm results in similar branching ratios and dynamics as observed at 235 nm. Measured O(³P₂) images arising from ClONO₂ dissociation at 226 nm suggest that oxygen atoms result from the spontaneous dissociation of metastable NO₃. The quantum yield of O atoms arising from the spontaneous dissociation of NO₃ varies from 0.09 at 262 to 0.38 at 235 nm based on the derived internal energy distributions of the NO₃ fragments. We also describe a Monte-Carlo forward-convolution fitting of imaging data which permits detailed analysis of both spontaneous secondary dissociation and secondary photodissociation.     

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.     

Ab initio potential energy surface for Ar 3 +

The potential energy surface (PES) of linear Ar ₃ ⁺ is calculated at the MP4/6-31G* level including all single, double, triple and quadruple excitations. The results show that the PES of the linear Ar ₃ ⁺ has a very flat valley along the asymmetric stretching vibration normal mode, ν₃. A higher level quadratic configuration interaction calculation including single, double and triple substitutions QCISD (T) along this flat valley suggests that an asymmetric geometry energy minimum reported earlier based on MP2 [1] is due to symmetry breaking in UHF. The global minimum of the PES is found to be for the symmetric geometry atR _ab =R _bc =2.66±0.01 Å, which is in good agreement with the MRD-CI calculation [2] and expectations from our earlier photodissociation experiments [3]. The calculational results are compared with other theoretical calculations, and are discussed in the context of the photodissociation and dynamics of dissociation experiments conducted on Ar ₃ ⁺ .     

Photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol and 2-(bromomethyl) hexafluoro-2-propanol at 234 nm: resonance-enhanced multiphoton ionization detection of Br (2P(j))

The photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol (BTFP) and 2-(bromomethyl) hexafluoro-2-propanol (BMHFP) have been studied at 234 nm, and the C-Br bond dissociation investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS). Br formation is a primary process and occurs on a repulsive surface involving the C-Br bond of BTFP and BMHFP. Polarization dependent time-of-flight profiles were measured, and the translational energy distributions and recoil anisotropy parameters extracted using forward convolution fits. A strong polarization dependence of time-of-flight profiles suggest anisotropic distributions of the Br((2)P(3/2)) and Br((2)P(1/2)) fragments with anisotropy parameter, β, of respectively 0.5 ± 0.2 and 1.2 ± 0.2 for BTFP, and 0.4 ± 0.1 and 1.0 ± 0.3 for BMHFP. The measured velocity distributions consist of a single velocity component. The average translational energies for the Br((2)P(3/2)) and Br((2)P(1/2)) channels are 9.2 ± 1.0 and 7.4 ± 0.9 kcal/mol for BTFP, and 15.4 ± 1.8 and 15.1 ± 2.0 kcal/mol for BMHFP. The relative quantum yields of Br((2)P(3/2)) and Br((2)P(1/2)), which are 0.70 ± 0.14 and 0.30 ± 0.06 in BTFP and 0.81 ± 0.16 and 0.19 ± 0.04 in BMHFP, indicate that the yield of the former is predominant. The measured anisotropy parameters for the Br((2)P(3/2)) and Br((2)P(1/2)) channels suggest that the former channel has almost equal contributions from both the parallel and the perpendicular transitions, whereas the latter channel has a significant contribution from a parallel transition. Non-adiabatic curve crossing plays an important role in the C-Br bond dissociation of both BTFP and BMHFP. The estimated curve crossing probabilities suggest a greater value in BTFP, which explains a greater observed value of the relative quantum yield of Br((2)P(1/2)) in this case.     

2-溴噻吩和3-溴噻吩在267 nm的C-Br键解离机理

利用离子速度影像技术, 研究了2-溴噻吩和3-溴噻吩两种同分异构体在267 nm激光作用下的C—Br键解离机理, 获得了光解产物Br(2P3/2)和Br*(2P1/2)的能量和角度分布, 分析了两异构分子在267 nm 的C—Br键解离通道. 对于2-溴噻吩和3-溴噻吩, 产物Br来源于三个通道: (i) 从单重激发态系间窜跃到排斥的三重激发态的快速预解离; (ii)单重激发态内转化到高振动基态的热解离; (iii) 母体分子多光子电离后的解离. 2-溴噻吩的产物Br*具有类似的产生机制; 但对于3-溴噻吩, 从激发态内转换到高振动基态发生热解离成为产物Br*的主导通道, 而来自激发三重态的快速预解离通道则几乎消失. 定量地给出了各个通道的相对贡献、能量分配及各向异性分布信息. 实验发现, 随着溴原子在噻吩上取代位置远离硫原子, 来自通道(i)和(ii)产物之间的比例明显减小, 相应的各向异性分布有变弱趋势.