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.     

IRPD spectroscopy and ensemble measurements: Effects of different data acquisition and analysis methods

Three different commonly used infrared photodissociation (IRPD) spectroscopy acquisition and analysis methods are described, and results from these methods are compared using the same dataset for an extensively hydrated metal cation, La³⁺(H₂O)₃₆. Using the first-order laser-induced photodissociation rate constant as an IRPD intensity has several advantages over photodissociation yield and depletion/appearance methods in that intensities can be more directly compared with calculated infrared absorption spectra, and the intensities can be readily corrected for changes in laser power or irradiation times used for optimum data acquisition at each frequency. Extending IRPD spectroscopy to large clusters can be complicated when blackbody infrared radiative dissociation competes strongly with laser-induced photodissociation. A new method to obtain IRPD spectra of single precursor ions or ensembles of precursor ions that is nearly equivalent to the photodissociation rate constant method for single precursor ions is demonstrated. The ensemble IRPD spectra represent the “average” structure of clusters of a given size range, and this method has the advantage that spectra with improved signal-to-noise ratios can be obtained with no increase in data acquisition time. Results using this new method for a precursor ensemble consisting of La³⁺(H₂O)_35–37 are compared with results for La³⁺(H₂O)₃₆.     

Polarization of thallium atoms produced in molecular photodissociation: experiment and theory

A study has been made of the oriented ground state Tl(6² P _1/2) atoms produced in the photodissociation of TlBr molecules by circularly polarized 266-nm laser light. A significant degree of atomic orientation (15%) has been measured in the experiment which corresponds to the initial degree of orientation of 37%. A high value of depolarization cross section (210 Ų) for the oriented Tl atoms colliding with TlBr molecules has been also observed. The obtained experimental results have been treated theoretically. We present a general quantum mechanical theory of the orientation phenomenon in which all possible nonadiabatic interactions as well as molecular rotation are properly treated. The application of the theory to the case of TlBr photodissociation allowed to understand the obtained experimental results and to evaluate the probability of the earlier unknown radial nonadiabatic transition in the decaying molecule.     

Resonant and Off-resonance ionization of laser-excited sodium vapor

Electrons produced in a sodium effusive beam irradiated by pulsed laser light have been energy-analyzed. For resonant excitation of the atoms to the 3p state, we have studied associative ionization of excited Na(3p) (AI), Penning ionization (PI) and photoionization of highly-excited Na(n l),n l=3d, 4p, 5s, 4d/4f, by measurements of electron energy spectra. Our interpretation of the spectra is supported by a numerical analysis of the data which leads to estimates of the cross sections compatible with values previously measured in other experiments. Off-resonance ionization is considered at a particular wavelength. It is presumed to involve laser-excited dimers Na ₂ ^** . Spectra show that highly-excited atoms are produced by photodissociation of Na ₂ ^** or from Na ₂ ^** -Na collisional energy transfer, and that these atoms support the dominant off-resonance ionization channel in our laser excitation conditions.     

Photofragmentation of mass resolved carbon cluster ions

The results of a detailed study of the photodissociation of carbon cluster ions, C ₃ ⁺ to C ₂₀ ⁺ , are presented and discussed. The experiments were performed using internally cold cluster ions derived from pulsed laser evaporation of a graphite target rod in a helium buffer gas followed by supersonic expansion. The mass selected clusters were photodissociated using 248 nm and 351 nm light from an excimer laser. Photofragment branching ratios, photodissociation cross sections and data on the laser fluence dependence of photodissociation are reported. For almost all initial clusters, C _n ⁺ , the dominant photodissociation pathway was observed to be loss of a C₃ unit to give a C _n?3 ⁺ ion. This observation is interpreted as indicating that dissociation occurs by a statistical unimolecular process rather than by direct photodissociation. The photodissociation was found to be linear with laser fluence forn>5 with 248 nm and 351 nm light; quadratic forn=5 for 248 nm and 351 nm; and linear forn=4 at 248 nm. Dissociation energies for the carbon cluster ions implied by these results are discussed. The photodissociation cross sections were found to change dramatically with cluster size and with the wavelength of the photodissociating light.     

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.     

H-atom photofragments from H2O2 dissociated at 193 nm

Hydrogen atoms produced by photodissociation of H₂O₂ using 193 nm laser light have been observed for the first time. The H atoms are detected by VUV spectroscopy. The yield is 12 ± 1%. The H atoms are produced by a one-photon process.     

Laser-induced fluorescence study of the hydrogen atom formation dynamics in the 248 nm gas-phase photodissociation of vibrational state selected water (H<Subscript>2</Subscript>O (|04<Superscript>−</Superscript>〉))

The vibrationally-mediated H₂O gas-phase photodissociation was studied at a photolysis wavelength of 248 nm. Single rotational states of the |03⁻〉|2〉 and |04⁻〉 H₂O overtone vibrations were prepared by laser photoexcitation around 720 nm. H atoms formed in the photodissociation of the H₂O (|04⁻〉 = 3₁₃) were detected by Lyman-α laser-induced fluorescence spectroscopy with sub-Doppler resolution to determine their translational energy. The present result confirms that in the dissociation process the major part (ca. 93%) of the available energy is released as relative translational energy of the nascent H + OH photofragments, in agreement with earlier complementary measurements (R. L. Vander Wal, J. L. Scott and F. F. Crim, J. Chem. Phys. 94, 1859 (1991)), where the internal excitation of the OH product radical was investigated at different photolysis wavelengths.     

Rotational anisotropy and product state distribution in the photodissociation of deuterium peroxide

Nascent OD(X ²Π_i) radicals, generated in the photodissociation of D₂O₂ at 266 nm, have been probed by laser-induced fluorescence. The observed line intensities exhibit a marked dependence on excitation-detection geometry and laser polarisation, indicating rotational alignment (A_o⁽²⁾ ⩽ 0.28). The analysed product rotational distributions and Doppler width measurements indicate that the fragment energy partitioning (f_kin = 0.86,f_rot = 0.14) is too different from that reported for H₂O₂ photodissociation to explain the isotopic process by energy arguments alone.     

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.     

A Study on the Reaction of O(1D) and N2O by Trajectory Calculation and Time-Resolved FTIR Spectroscopy

O(¹D) atoms, generated via the laser photodissociation of N₂O at 193 nm, reacting with N₂O was studied by Time-Resolved Fourier Transform Infrared (TR-FTIR) Spectroscopy. The IR emission from NO(v? 1–11.) was collected at 30 μs after the laser was fired. Several instrumental corrections were made to obtain reliable experimental results. The vibrational temperature of the nearly-nascent NO product was estimated to be Tv ? 9,000 K. A Quasi-Classical Trajectory (QCT) calculation of the reaction was performed. The calculated results, corresponding to a head-on attack mechanism, agree with the experimental data. Additional reaction channel with side-on attack producing N₂ and O₂ was also studied by QCT, where vibrationally hot O₂(a ¹ Δ_g) and cold N₂(X¹∑_g⁺) products are predicted.     

Fragmentation dynamics and energy disposal in photodissociation of (N2O)2+ in the 458–660 nm wavelength range

The nitrous oxide dimer cation (N₂O)₂⁺ has been studied in the visible wavelength range by photodissociation of a mass-selected high-energy ion beam followed by energy analysis of the charged photofragments. Information on the angular anisotropy of the fragmentation process has been obtained by rotating the polarization direction of the laser light. The results allow conclusions to be drawn about the lifetime of the optically accessed excited electronic state and on the energy disposal in the photofragmentation event.     

Multiphoton dissociation of molecules with low power cw infrared lasers: collisional enhancement of dissociation probabilities

Multiphoton dissociation of C₃F⁺₆ is observed using low intensity cw CO₂ laser radiation. Ion cyclotron resonance (ICR) techniques are used to store ions for irradiation. Ion storage times up to 2 s are used. Multiphoton dissociation is observed at laser intensities below 100 W cm^?2 and at pressures below 10^?5 Torr. Only the lowest energy decomposition of C₃F⁺₆, to give C₂⁺₄ and CF₂, is observed. Multiphoton dissociation probabilities show a sharp wavelength dependence in contrast to typical pulsed laser multiphoton dissociation experiments. The photodissociation spectrum of C₃F⁺₆ is similar to the infrared absorption spectrum of neutral C₃F₆ at both low and high resolution. Collisions between C₃F⁺₆ and unreactive buffer gases (Ar, N₂, SF₆) are seen to enhance multiphoton dissociation, while collisions with C₃F₆ deactive the laser excited species. The results are discussed in terms of mechanisms for slow multiphoton dissociation.     

Theoretical study on structures and infrared spectroscopy of Cu<Superscript>2+</Superscript>(H<Subscript>2</Subscript>O)Ar<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1–4)

The binding energy of Cu²⁺(H₂O) is computed to be 98.4 kcal/mol and thus one-photon photodissociation is not possible in the 3400–3800 cm^–1 (9.7–10.9 kcal/mol) region. To study whether the infrared photodissociation processes of Cu²⁺(H₂O) can occur by multiple argon atoms tagging technique, density functional and CCSD(T) methods are used to investigate the geometries, OH stretching frequencies and the argon atom binding energies of Cu²⁺(H₂O)Ar _n (n = 1–4) complexes. Various isomers are found resulting from the different coordination sites of argon atoms. The OH stretches in these complexes are shifted to lower frequencies than those of the free water molecule, and the corresponding vibrational red shifts are progressively smaller as more argon atom is added to Cu²⁺ while binding an argon atom to an OH site should lead to additional sizable red shift to the OH stretching vibrations.     

Acetone Formation from Photolysis of 2-Propanol on Anatase-TiO2(101)

Photocatalysis of 2-propanol on A-TiO₂(101) has been investigated using a temperature programed desorption method with 266 nm laser light. A clear mechanism is proposed for photodissociation of 2-propanol on A-TiO₂(101). Acetone product on five coordinate Ti⁴⁺ sites is formed in a stepwise manner in which the O-H dissociation proceeds first and then followed by secondary C-H dissociation of 2-propanol while H atoms are transferred to the adjacent bridge bond oxygen (BBO) sites. Low temperature water is formed in a thermally driven process via H-atom on BBO in exchange with isopropyl groups of molecule 2-propanol, while isopropyl radical desorbs at high temperature during the TPD process. The observation demonstrates the prospect of TiO₂ as a photocatalyst for degradation of organics.     

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.     

UV Photodissociation Action Spectroscopy of Haloanilinium Ions in a Linear Quadrupole Ion Trap Mass Spectrometer

UV–vis photodissociation action spectroscopy is becoming increasingly prevalent because of advances in, and commercial availability of, ion trapping technologies and tunable laser sources. This study outlines in detail an instrumental arrangement, combining a commercial ion-trap mass spectrometer and tunable nanosecond pulsed laser source, for performing fully automated photodissociation action spectroscopy on gas-phase ions. The components of the instrumentation are outlined, including the optical and electronic interfacing, in addition to the control software for automating the experiment and performing online analysis of the spectra. To demonstrate the utility of this ensemble, the photodissociation action spectra of 4-chloroanilinium, 4-bromoanilinium, and 4-iodoanilinium cations are presented and discussed. Multiple photoproducts are detected in each case and the photoproduct yields are followed as a function of laser wavelength. It is shown that the wavelength-dependent partitioning of the halide loss, H loss, and NH₃ loss channels can be broadly rationalized in terms of the relative carbon-halide bond dissociation energies and processes of energy redistribution. The photodissociation action spectrum of (phenyl)Ag₂ ⁺ is compared with a literature spectrum as a further benchmark.

Kinetic energy spectra of N2+ photofragments in the laser photodissociation of N2O+

Using a ZAB-2F double-focusing mass spectrometer together with an argon-ion laser, the kinetic energy spectra of N₂⁺ photofragments from the photodissociation of N₂O⁺ have been measured at wavelengths 514.5, 496.5, 488.0 and 476.5 nm in the visible region of the spectrum. Energies released in the centre-of-mass frame of reference are given. From the results it is deduced that the states involved in the absorption and dissociation processes ar probably N₂O⁺(B̃²Π) v ⩾ 3 and N₂O⁺ (C̃²Σ⁺) v ⩾ 0, respectively.     

Vibrational population of H 2 + after electroionization of thermal H2

In an ion trap experiment we have determined the vibrational population of the lowest 9 vibrational levels of H ₂ ⁺ . We used photodissociation of the trapped molecules by 248 nm light from an excimer laser and the dependence of the photodissociation cross section from the vibrational state. Our results are in good agreement to calculations, which are based on the Franck-Condon principle, but include a variation of the internuclear distance in the transition matrix element.