Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

Detection of OH Radical in the Photodissociation of p-Aminobenzoic Acid at 266 nm

Photodissociation of p-aminobenzoic acid at 266 nm was investigated by probing the nascent OH photoproduct employing the laser-induced fluorescence technique. It was found that the nascent OH radical was vibrationally cold and its rotational state distribution conformed to be a Boltzmann behavior, characterized by a rotational temperature of 1040±110 K. The rotational energy of OH was determined to be 8.78±0.84 kJ/mol. Between the two spinorbit states of OH, ^2Ⅱ3/2 and ^2Ⅱ1/2, the former was found to be preferentially populated. The distribution of the II（A＇） state for the A-doublet was dominant. Finally, a probable mechanism for the formation of OH produced from the photodissociation of p-aminobenzoic acid is discussed.     

OH rotational quantum state distributions and relaxation efficiencies for the reaction system O(1D) + H2O → 2OH

The OH rotational distribution from O(¹D) + H₂O → 2OH is presented. The v' = 0 distribution corresponds to two Boltzmann distributions. ≈500 K (k = 1–6) and ≈2500 K (k ? 6). Rotational relaxation efficiencies for N₂, He, O₂ were ?0.1, ?0.1, ?0.4. More limited data are presented for the v' = 1 and 2 levels.     

Molecular beam study of methyl nitrite photodissociation after excitation into the second absorption band at 248 and 193 nm

The photodissociation dynamics of methyl nitrite in the second UV absorption band (B band) has been studied by molecular beam photofragment spectroscopy at 248 and 193 nm. Angular distributions were measured at both wavelengths yielding anisotropy parameters β=1.36±0.10 at 248 nm and β=1.08±0.07 at 193 nm which indicates a predominantly in plane transition. Measurements performed atT _rot<10 K and atT _rot≧70 K show that within the experimental error β is independent of the rotational temperature of the molecular beam implying that the dissociation process is considerably faster than a typical rotational period. The anglex between the electronic transition moment and the dissociation direction is estimated to be 28° at 248 nm and 34° at 193 nm. The difference ofx at these two wavelengths is proposed to be the effect of an admixture of a perpendicular transition at 193 nm. No contribution attributable to the electronically excited methoxy fragment was found in the TOF spectrum after excitation at 193 nm. It is concluded that the production quantum yield for electronically excited methoxy is smaller than 7%.     

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   

Dynamics of N-OH bond dissociation in cyclopentanone and cyclohexanone oxime at 193 nm: laser-induced fluorescence detection of nascent OH (v', J')

Cyclohexanone oxime (CHO) and cyclopentanone oxime (CPO) in the vapor phase undergo N-OH bond scission upon excitation at 193 nm to produce OH, which was detected state selectively employing laser-induced fluorescence. The measured energy distribution between fragments for both oximes suggests that in CHO the OH produced is mostly vibrationally cold, with moderate rotational excitation, whereas in CPO the OH fragment is also formed in v' = 1 (~2%). The rotational population of OH (v' = 0, J') from CHO is characterized by a rotational temperature of 1440 ± 80 K, whereas the rotational populations of OH (v' = 0, J') and OH (v' = 1, J') from CPO are characterized by temperatures of 1360 ± 90 K and 930 ± 170 K, respectively. A high fraction of the available energy is partitioned to the relative translation of the fragments with f(T) values of 0.25 and 0.22 for CHO and CPO, respectively. In the case of CHO, the Λ-doublet states of the nascent OH radical are populated almost equally in lower rotational quantum levels N', with a preference for Π(+) (A') states for higher N'. However, there is no preference for either of the two spin orbit states Π(3/2) and Π(1/2) of OH. The nascent OH product in CPO is equally distributed in both Λ-doublet states of Π(+) (A') and Π(-) (A') for all N', but has a preference for the Π(3/2) spin orbit state. Experimental work in combination with theoretical calculations suggests that both CHO and CPO molecules at 193 nm are excited to the S(2) state, which undergoes nonradiative relaxation to the T(2) state. Subsequently, molecules undergo the N-OH bond dissociation from the T(2) state with an exit barrier to produce OH (v', J').     

Vibrational energy transfer from C2H4 to CO2 studied at low temperatures in a jet by infrared double resonance

In a mixed CO₂-C₂H₄ jet the populations of rotational levels in the ground andv ₂ (667 cm^?1) vibrational state of CO₂ are probed by color center laser absorption. The rotational distributions are well described by a rotational temperature. On the timescale of the expansion thev ₂ state does only relax through resonant deexcitation by CO₂ clusters. A 10% fraction of C₂H₄ molecules is excited into thev ₇ (949 cm^?1) level by CW CO₂ laser absorption, subsequently releasing energy into the expansion due to relaxation to lower C₂H₄ vibrational states and to thev ₂ mode of CO₂. Observed are the induced rise of rotational temperature and the increase ofv ₂ level population. By probing at several distances from the nozzle the temperature range from 30–155 K is covered. The vibrational transfer of the excited C₂H₄ to thev ₂ level of CO₂ has a near quadratic inverse temperature dependence, increasing by a factor of 40 when the temperature is lowered from 155 K to 30 K.     

Energy partitioning in the reaction O(1D) + H2O → OH + OH. V. Rotational relaxation of OH(X2Π, ν″, J″)

The rotational relaxation of OH(X²Π, ν″, J″) in ν″, = 0, 1, and 2 produced from the reaction of O (¹D) with H₂O has been studied as a function of H₂O vapor pressure and added argon. Water molecules are extremely efficient in bringing about relaxation and the experiments performed indicate that, on the average, the high temperature distribution is relaxed to nearly room temperature at a gas kinetic rate. This observation is rationalized by assumming a collision complex between OH and H₂O having a quasichemical interaction similar to weak hydrogen bonding. The nascent OH internal energy distribution does not depend upon the translational energy of the O(¹D) reactant. Translational relaxation of the nascent OH product by H₂O is fast, as fast as rotational relaxation.     

Experimental determination of the dissociation energies D0 and De of H2O…HF

The zero-point and equilibrium dissociation energies of the hydrogen-bonded dimer H₂O…HF have been determined as D₀ = 34.3(3) kJmol⁻¹ and D_e = 42.9 (8) kJmol⁻¹ from absolute intensities of rotational transitions in an equilibrium mixture of H₂O, HF and H₂O…HF.     

Step-scan Fourier transform infrared absorption spectroscopy of liquid benzene cluster in a pulsed supersonic jet

We have obtained the infrared absorption spectrum of the jet-cooled C₆H₆ in the 800–5000 cm⁻¹ region using step-scan Fourier transform infrared spectrometer combined with a pulsed supersonic jet system. The experimental spectrum was in good agreement with the convolution spectrum of the monomer band with the rotational temperature of ∼37 K and the broadbands of liquid C₆H₆, indicating the first spectroscopic evidence for the formation of supercooled liquid C₆H₆ cluster in the supersonic jet.     

Photodissociation dynamics of methyl nitrate at 193 nm: energy disposal in methoxy and nitrogen dioxide products

The photodissociation dynamics of methyl nitrate, CH(3)ONO(2), has been investigated at 193 nm by examining the products from the primary dissociation channel, namely CH(3)O and NO(2). The CH(3)O (X (2)E) photoproducts were probed by laser-induced fluorescence (LIF) on the A (2)A(1)-X (2)E transition under both nascent and jet-cooled conditions. The 3 and 3 bands originating from the vibrationless and C-O stretch (nu(3)) levels, respectively, were characterized to obtain the internal energy distribution of the CH(3)O products. Only a small fraction of the CH(3)O products (< or =10%) were produced with one quantum of C-O stretch excitation as determined from the relative intensities of the bands in combination with transition probabilities derived from dispersed fluorescence measurements and/or calculated Franck-Condon factors. The CH(3)O products also had minimal rotational excitation: those produced in the ground vibrational state had a rotational temperature of 238 +/- 7 K, corresponding to less than 1% of the available energy. Products with C-O stretch excitation were found to have a higher rotational temperature, but still a small fraction of the total energy. Combining the CH(3)O internal energy findings with previous photofragment translational energy measurements [X. Yang, P. Felder and J. R. Huber, J. Phys. Chem., 1993, 97, 10903] indicates that most of the available energy is deposited in the NO(2) fragment. This is verified through dispersed fluorescence measurements which show that the NO(2) fragment is produced electronically excited with internal energies extending to the NO + O dissociation limit. Ab initio calculations confirm that the dominant initial excitation is strongly localized on the NO(2) moiety. The calculations are also used to reveal the forces that give rise to internal excitation of the CH(3)O fragment upon electronic excitation.     

Dynamic aspects of the reaction O(1D) + H2S → OH + SH

The energy distribution of nascent OH(²Π, υ, J) produced in the reaction of O(¹D) with H₂S has been measured by laser-induced fluorescence. The rotational distributions in υ″ = 0 and υ″ = 1 are Boltzmannian with temperature parameters T_r″-0 = 2300 ± 100 K and T_r⁻-1 = 2650 ± 150 K. A population ratio N(υ″ = 1)/N(υ″ = 0) = 0.17 is observed. The product-state distribution over the different spin and A components is statistically within the experimental uncertainty of 20%. A comparison of the OH product populations from the title reaction with the well known OH yield from the O(¹D)+H₂O reaction shows that 25% of the reactive encounters follow the reaction channel which produces OH in υ″ = 0 and υ″ = 1.     

Stability of molecular form of HCl in water vapor: A computer simulation

The free energy and entropy of the dissociation of HCl molecule into ions in water vapor, HCl(H₂O) _n + mH₂O → H₃O + (H₂O) _n+m -1Cl^?, were calculated. The dependences of various parameters on the interionic distance at 273 K and various vapor pressures were obtained. A detailed model taking into account unpaired covalent-type interactions, polarization interactions, charge transfer effect, and hydrogen bonds was applied. The numerical values of the parameters were reconstructed from the experimental data on the free energy and enthalpy of the first reactions of addition of vapor molecules to ions, and also from the results of quantum-chemical calculations of the energy and geometry of locally stable configurations of clusters HCl(H₂O) _n . Despite lower internal energy of the dissociated state, the molecular form is absolutely stable in clusters of water molecules. The dissociated state is relatively stable. Accumulation of unrecombined ion pairs in clusters is possible with a decrease in the temperature to 200 K.     

Measurement of the rotational energy distribution of ground state N+2 ions from H+2N2 collisions by laser-induced fluorescence

The rotational energy distribution of ground state N⁺₂ ions produced by interaction of a principally H⁺₂ ion beam with N₂ has been measured at projectile ion energies of 10 and 2.5 keV by observation of the laser induced fluorescence in the N⁺₂ first negative system (0, 0) band. At 10 keV, the rotational energy distribution is Boltzmann (432 K), due to heating of the gas by the intense ion beam. At 2.5 keV, rotational excitation is definitely observed.     

The three-body dissociation dynamics of Cl2O at 248 and 193 nm

The photodissociation of Cl₂O has been studied at 248 and 193 nm using photofragment translational spectroscopy (PTS) experiments with tunable VUV photoionization detection. The sole products observed were Cl, O and ClO fragments. Based on the derived translational energy distributions for the ClO and Cl photofragments we conclude that at 248 nm 15% of Cl₂O excitation results in three-body dissociation. At 193 nm no Cl₂ fragments are observed and we conclude that the oxygen atoms arise solely from three-body dissociation. Dissociation geometries derived from forward convolution fitting suggest two qualitatively distinct three-body dissociation pathways: asymmetric concerted dissociation and symmetric concerted dissociation in agreement with recent theoretical predictions.     

Temperature dependence of the probabilities of VV energy exchange between H2 and DCl

The temperature dependence of the removal of the vibrational energy of H₂ by DCl in H₂(1) + DCl(0) has been investigated over the range of 300–3000 K. The energy transfer probability of H₂(1) + DCl(0) → H₂(0) + DCl(1), where the vibrational energy of H₂(1) is removed by both the vibrational and rotational motions of DCl(0), is found to be strongly temperature dependent and increases with temperature closely following the relation log P α T^1/3. Over the temperature range it changes by two orders of magnitude. The probability of the near-resonant process H₂ (1) + DCl(O) → H₂(0) + DCl(2) is very close to that of the former at 300 K, but it increases only slightly as the temperature is raised to 3000 K. The sum of the probabilities of these two processes at 300 K is 3.4 × 10^?5, which agrees with the experimental value of 3.95 × 10^?5.     

193 nm photolysis of CHCl3: Probe of the CH product by CRDS

The CH radical production induced by 193 nm two-photon photolysis of CHCl₃ has been measured for the first time via the cavity ring-down absorption spectroscopy of its A–X bands, using a commercial nanosecond pulsed dye laser. The range of pressure and laser intensity, as well as the time window detection, have been carefully chosen to ensure a constant CH number density during the measurement and to avoid post-photolysis reactivity. Internal energy distribution of the CH(X²II) fragment has been derived from population distribution simulations, leading to an average vibrational temperature T_vib = 1900 ± 50 K and rotational temperature T_rot = 300 ± 20 K. Two competing mechanisms can be invoked for the CH production channel: either two-photon absorption via resonant excited states of CHCl₃ leading to dissociation of excited CHCl₃, or two-photon sequential dissociation via the formation of the vibrationally excited CHCl₂ fragment. The latter mechanism is proposed to be the prominent process for CH formation.     

Energy disposal in the photodissociation HCN(Ã1A″) → H + CN(X 2Σ) at 193 nm

Rotational energy disposal has been measured in CN(X ²Σ, υ″ = 0, 1, 2) following 193 nm dissociation of HCN vapor at 295 K. The fractional populations for the three vibrational states are 0.56 ± 0.08, 0.33 ± 0.13, and 0.11 ± 0.03 for υ″ = 0, 1 and 2 respectively. This distribution is fit well by a Poisson distribution with an average vibrational quantum number of 0.55 corresponding to the average vibrational energy of 1128 ± 294 cm⁻¹. This energy represents (10 ± 3)% of the available energy. The rotational distributions in all three vibrational states can be represented by a single surprisal which depends linearly on N″/N″_max where N″_max is the maximum value of the rotational quantum number permitted by energy conservation and the prior distribution is similarly constrained. The average energy in rotation is 1055 ± 373 cm⁻¹ which represents (9 ± 3)% of that available for disposal. Time-dependence measurements indicate that product CN(X ²Σ, υ = 0) is formed in both a fast (τ < 80 ns) and a slow process. No evidence is found for the production of CN(A ²Π, υ ≈ 0).     

The rotational excitation and population distribution of OH(A2Σ+) produced by electron impact on water

The rotational excitation and population distributions in OH(A²Σ⁺, υ′ = 0) have been determined by analyzing the OH(A²Σ⁺ → X²Π_i) emission spectrum. The spectrum results from the impact of mon-energetic electrons (0–100 eV) on water vapour. It is shown that these rotational distributions of the OH(A²Σ⁺) state depend on the electron impact energy and have contributions from singlet and triplet states of water. The contribution from each dissociative state of water can be described by a Boltzmann distribution, both in the case of rotational excitation and population.Three distribution parameters (“temperatures”) for rotational excitation are obtained, namely 13800 K and 2900 K for the singlet contributions and 4000 K for the triplet contribution. The corresponding distribution parameters for the rotational population are 30000, 3300, and 4800 K, respectively. The results are discussed in view of recent theoretical calculations on water energy levels.     

Two-photon absorption spectroscopy of the Na2 molecule in the 4 eV region

By means of two-photon sequential absorption via real intermediate rotational levels of the A state, vibrational levels of three new excited electronic states of the sodium dimer have been observed in the 4 eV region. These states are identified as F¹ ∑⁺_g, G¹ Π_g and H¹ Π_g. Their vibrational and rotational constants have been determined.