Excited Bromine Research Articles

Evidence for concerted ring opening and C-Br bond breaking in UV-excited bromocyclopropane.

Photodissociation of gaseous bromocyclopropane via its A-band continuum has been studied at excitation wavelengths ranging from 230 nm to 267 nm. Velocity-map images of ground-state bromine atoms (Br), spin-orbit excited bromine atoms (Br(∗)), and C3H5 hydrocarbon radicals reveal the kinetic energies of these various photofragments. Both Br and Br(∗) atoms are predominantly generated via repulsive excited electronic states in a prompt photodissociation process in which the hydrocarbon co-fragment is a cyclopropyl radical. However, the images obtained at the mass of the hydrocarbon radical fragment identify a channel with total kinetic energy greater than that deduced from the Br and Br(∗) images, and with a kinetic energy distribution that exceeds the energetic limit for Br + cyclopropyl radical products. The velocity-map images of these C3H5 fragments have lower angular anisotropies than measured for Br and Br(∗), indicating molecular restructuring during dissociation. The high kinetic energy C3H5 signals are assigned to allyl radicals generated by a minor photochemical pathway which involves concerted C-Br bond dissociation and cyclopropyl ring-opening following single ultraviolet (UV)-photon absorption. Slow photofragments also contribute to the velocity map images obtained at the C3H5 radical mass, but the corresponding slow Br atoms are not observed. These features in the images are attributed to C3H5 (+) from the photodissociation of the C3H5Br(+) molecular cation following two-photon ionization of the parent compound. This assignment is confirmed by 118-nm vacuum ultraviolet ionization studies that prepare the molecular cation in its ground electronic state prior to UV photodissociation.

Dynamics of C⿿Br bond dissociation in methyl 2-bromopropionate at 235 nm: A resonance-enhanced multiphoton ionization study

The dynamics of the C⿿Br bond dissociation on UV excitation of methyl 2-bromopropionate mainly to the 1(nϿ*) state, repulsive in the C⿿Br bond, has been investigated, employing resonance-enhanced multiphoton ionization. Both the ground state and spin-orbits excited bromine atoms were detected, with the former being the major channel. Bromine fragments show bimodal translational energy distributions, with slow and fast (major) bromine atoms arising mainly from the ground and excited electronic states, respectively. The measured recoil anisotropy suggests isotropic angular distributions of bromine atoms. Molecular orbital calculations reveal an important role of avoided curve crossing on C⿿Br bond dissociation dynamics.

Tracing dissociation dynamics of CH3Br in the 3Q0 state with femtosecond extreme ultraviolet ionization

The ultrafast photodissociation dynamics of gas phase CH3Br molecules in the red wing of the A-band, i.e. the first molecular adsorption continuum, is investigated by pump–probe spectroscopy. The experiment employs femtosecond laser pulses at 266nm in the ultraviolet to dissociate the molecule and high order harmonic extreme ultraviolet pulses in conjunction with time-of-flight mass spectrometry to ionize and detect the fragments. A dissociation time of 116±25fs is obtained from the pump–probe risetime of the Br+ ion signal, which originates from formation of either or both Br(2P3/2) or Br∗(2P1/2). The timescale is in a good agreement with the previously calculated A-band dissociation time of CH3Br using the anisotropy parameter β deduced from angular photodissociation experiments. Based on classical molecular dynamics simulations and previous spectroscopic information, the most likely pathway is dissociation of the CH3Br molecule via the 3Q0 state of the A-band, which correlates with the formation of the spin–orbit excited bromine fragment.

Photodissociation of carbonic dibromide at 267 nm: Observation of three-body dissociation and molecular elimination of Br2

The photodissociation of Br2CO around 267 nm has been studied by time-of-flight mass spectroscopy and ion velocity imaging. The atomic (Br and Br*) and molecular products (Br2 and BrCO) are detected via multiphoton ionization with the same laser. The results show that the molecule dissociates into (1) Br(fast)+Br(slow)+CO via an asynchronously concerted three-body decay process for both ground and spin–orbit excited bromine atoms, (2) BrCO(A)+Br, and (3) Br2+CO, the molecular elimination channel. The translational energy distributions of bromine atoms from reaction (1) are bimodal. For both spin–orbit states the anisotropy parameters differ clearly for slow and fast bromine atoms, where the β values for slow bromine atoms decrease relative to those for fast atoms. The β values for the Br2 elimination channel almost reach the low limit of −1. Taking into account the translational energy and angular distributions of these reactions, an asynchronously concerted decay mechanism could be proposed for the three-body dissociation. It is concluded that the transition dipole moment is in the direction perpendicular to the C=O bond in the initial excitation, which is also consistent with all the observations for reactions (2) and (3).

Optimal control of IBr curve-crossing reactions

Control of IBr photodissociation branching ratios is explored using optimal control theory to design the requisite pulses which produce ground or spin—orbit excited bromine atoms. To conform to experimental pulse shaping limitations, these pulses are confined to a limited bandwidth and further modified by inspection of the power spectra from which extraneous frequency components are removed. Branching ratios using the filtered fields are shown to still provides appreciable selectivity, but with the added benefit of more feasible pulse shapes.

Br*+CO2 revisited: An interrogation of E–V energy transfer with time-resolved diode laser spectroscopy

High resolution (0.0003 cm−1) time-resolved diode laser absorption spectroscopy has been used to reinvestigate the electronic–vibrational energy transfer from spin–orbit excited bromine, Br(2P1/2), to carbon dioxide. The experiments are carried out by generating Br* atoms with pulsed 193 nm laser photolysis of CF3Br and monitoring the subsequent energy transfer by following directly the temporal evolution of selected vibrational states of CO2 (1001, 0201, 0221 and 0001). By comparing the temporal profile of the 1001 state with that predicted by various kinetic models it has been established that quenching of Br* by CO2 occurs via the nearly resonant 1001 state. The E–V rate measured in the present study, (4.8±0.6)×105 Torr−1 s−1, agrees well with that reported previously using infrared fluorescence probes. It has also been determined that the efficiency of this energy transfer is 0.87±0.15; that is, on average, 87% of the spin–orbit excited energy of the Br* atoms will show up as CO2 vibrational energy. The rate constant for the vibrational relaxation of the 1001 state is found to be (4.1±0.5)×106 Torr−1 s−1, also in good agreement with previous low resolution fluorescence measurements.

Infrared fluorescence study of E–V energy transfer from Br*(4 2P1/2) to nitrosyl bromide

Electronic-to-vibration (E–V) energy transfer from excited Br(4 2P1/2) atoms to nitrosyl bromide has been investigated by an infrared fluorescence technique. The excited bromine atoms are generated by pulsed laser photolysis of Br2 at 500 nm. Approximately 50% of the quenching collisions of Br*+BrNO yield BrNO molecules excited in the 1800 cm−1 v1 stretching mode. Of these, six times as many BrNO molecules are produced in the v1=1 state as in v1=2, despite the fact that the latter is nearly resonant with the electronic energy of Br*. The total rate coefficient for removal of Br* by BrNO is determined to be (1.5±0.2)×10−11 cm3 molecule−1 s−1. Indirect comparison of our results with previous work suggests that quenching proceeds primarily by inelastic collisions; reactive quenching to form Br2+NO is probably a minor channel.

Bromine-bromine energy transfer by binary collisions

The collisional energy transfer between a highly excited bromine molecule and a non-reactive monatomic (Ar or Br) or diatomic (Br 2) medium has been investigated by binary trajectory calculations over a wide range of medium temperatures and internal energies of the reactant molecule. The efficiency of the energy transfer is compared with expectations based on simple statistical approximations used in unimolecular reaction rate theory. Large deviations are found, particularly with respect to the contributions made by different types of degrees of freedom and to the dependence on the internal energy of the reactant molecule. Transfer to or from vibrational degrees of freedom appears to be very inefficient. Translational energy is most readily transferred. The rate coefficient for energy transfer appears to decrease with increasing internal energy in most cases.

Chemiluminescent reactions of oxygen fluoride. 2. Reactions producing excited bromine fluoride and iodine fluoride

Chemiluminescent reactions of oxygen fluoride. 2. Reactions producing excited bromine fluoride and iodine fluoride

Simulation of microscopic chemical processes. III. Deactivation and dissociation of bromine in argon at high pressure

Previous numerical simulation of the vibrational relaxation of highly excited bromine in argon at different pressures is here extended in several ways.

Simulation of microscopic chemical processes.: II. De-energization of bromine in argon at high pressures

The collisional de-energization of very highly vibrationally excited bromine in an argon medium has been studied by molecular dynamics calculation at 295 K and pressures up to 1200 atm. This note is concerned, in particular, with the validity of the scaling laws for the relaxation rate obtained on the basis of the independent binary collision theory. The relaxation rate is found to increase faster than predicted by independent binary collision theory so that the deviation at 1200 atm amounts to a factor of two.

Simulation of microscopic chemical processes.: I. De-energization of bromine in argon

This is the first in a series of articles devoted to the study of fundamental chemical processes by numerical simulation. The main aim of this work is to facilitate the development of accurate chemical reaction rate theories. We report here the results of an investigation of the collisional de-energization of highly vibrationally excited bromine by a heatbath of argon atoms at 295 K. A molecular dynamics calculation has been carried out for a cell containing one bromine molecule and 106 argon atoms at pressures of about 20 and 100 atmospheres. On the average the bromine molecule loses about 0.4 kcal/mole per collision but the relaxation is dominated by collisions removing about 3 kcal/mole. Detailed statistics on the energy transfer including collision lifetimes and their correlation with the strength of the collision is provided. Comparing the relaxation rate for the two pressures we find a 20% deviation from the binary collision scaling law.

Excited state bromine atom and molecule reactions

Excitation of bromine at wavelengths near the dissociation limit of the B 3Π+0u electronic state is shown to produce substitution and extraction reactions of excited bromine with olefins. These reactions are produced by drastically shortening free-radical chains in contrast to more conventional photochemical experiments which yield addition products only. Shortening of radical chains is accomplished by irradiating a flowing gas mixture in a capillary optical reactor. Wavelength selective photochemical reactions of excited 2P1/2 bromine atoms are found to occur simultaneously with gas phase ground state 2P3/2 atom reactions and with surface reactions. Evidence for reactions of electronically excited molecules of bromine in the B 3Π+0u state is also found.

Trajectory studies of atomic recombination reactions. III. Energy transfer in bromine-inert gas systems

Energy transfer in collisions of helium, argon, and xenon atoms with highly vibrationally excited bromine molecules is studied by computing 3D classical trajectories. Dissociation probabilities Pdiss are obtained as a function of the energy barrier Δ E/kT to be overcome. They vary approximately as Pdiss=P0(1+Δ E/kT)−4.5, and P0∝μ0.6 where μ is the reduced collision mass. Mean vibrational and rotational energy changes are calculated for systems in which the bromine molecules are initially in thermal rotational equilibrium but have varying degrees of vibrational excitation. The relative importance of vibration-rotation coupling at different vibrational energies is demonstrated. The order of collision numbers for rotational relaxation is He>Ar>Xe. The relative efficiencies of He, Ar, and Xe atoms in producing vibrational relaxation of Br2 vary with the initial degree of vibrational excitation.

Reversible Photodissociative Laser System

Laser action at 2.7 μ from atomic bromine was produced by flash photolysis of gaseous iodine monobromide. The output consists of a series of sharp pulses having peak powers up to 50 W and a total duration of 5 μsec. Optical gain was estimated to be ∼1 dB/m. Chemical reversibility of the system is very good; regeneration of the starting material occurs spontaneously and requires about 3 msec. The fact that excited bromine rather than excited iodine atoms are formed leads to clarification of previous conflicting spectroscopic assignments.

Selective Laser Photocatalysis of Bromine Reactions

The results have shown that selective excitation obtained with a tunable monochromatic laser is a useful technique for studying photochemical and energy transfer processes. A new phenomenon in the photochemistry of bromine was observed, in which bound excited molecules, and not atoms, were formed in the primary process. The mechanism of the subsequent reaction consists of collisional dissociation of the excited molecules into atoms, which then initiated free-radical chains. A quantitative estimate of the collisional electronic relaxation rate for excited bromine molecules was obtained, and a new upper limit to the continuous absorption strength at 14,400 cm(-1) was determined.

Primary Photochemical Process in Bromine

A kinetic investigation of the photobromination of ethylene has led to results which are in general consistent with the earlier work of Schumacher et al. The existence of an induced oxidation reaction was demonstrated in mixtures containing traces of oxygen, which is a very strong inhibitor of the ethylene-bromine chain reaction. The relative quantum yields of the photobromination reaction were measured at six wavelengths using narrow pass Baird interference filters. The quantum yields were shown to be independent of wavelength between the limits of 4800 and 6800A. Radiation of 7150A wavelength is absorbed but is totally ineffective photochemically. These results were interpreted with the aid of spectroscopic information to mean that the photochemically effective transitions of bromine molecules in radiation of wavelengths longer than the convergence limit, 5107A, lead to the continum of the 3π1u state, which then dissociates in one elementary act to form two normal atoms. It is unnecessary to invoke the assumption of collisional dissociation of excited bromine molecules to explain their photochemical reactivity below the convergence limit.

HALOGEN SENSITIZED PHOTOREACTIONS

Spectroscopic evidence indicates that reactions photosensitized by fluorine would take place by way of fluorine atoms. Similarly, the sensitizing action of chlorine will practically always be due to chlorine atoms. For bromine, when the wave length is less than 5107 Å, reaction will be by way of bromine atoms. Experiments using wave lengths 5107–6290 Å also probably involve bromine atoms although the possibility of reaction due to excited bromine molecules cannot be wholly ruled out. Iodine atoms are produced in photosensitization experiments involving iodine and wave lengths less than 4989 Å. Experimental evidence also favors the production of iodine atoms using wave lengths up to 6200 Å. Mechanisms in accordance with the above are advanced for a number of chlorine sensitized oxidation reactions. In the oxidation reactions an intermediate compound with a trivalent carbon is formed which reacts with oxygen forming a peroxide, giving rise to a chain reaction. Reactions sensitized by bromine and iodine are also discussed. It appears that all the halogen sensitized gaseous photoreactions thus far known can be explained by chemical mechanisms involving halogen atoms.

Absorption Spectrum of Iodine Bromide

It is shown by means of a vibrational analysis that the absorption spectrum of iodine bromide is analogous to that of iodine chloride and that an interpretation similar to that recently proposed by the writer and Gibson for ICl is applicable. The infrared bands of IBr discovered by Badger and Yost are classified as a $^{3}\ensuremath{\Pi}_{1}\ensuremath{\leftarrow}^{\ensuremath{'}}\ensuremath{\Sigma}$ transition. The $^{3}\ensuremath{\Pi}_{{0}^{+}}\ensuremath{\leftarrow}^{\ensuremath{'}}\ensuremath{\Sigma}$ system is observed as a faint set of bands in the red, not hitherto reported, which exhibit the same type of predissociation as the corresponding bands of ICl. Transitions to a state which originates at the maximum of the $^{3}\ensuremath{\Pi}_{{0}^{+}}$ state and which dissociates to yield normal iodine and excited bromine atoms are observed as a strong system of partially diffuse bands. Cordes' assignment of these bands to two systems is not confirmed. In the present work the vibrational quantum numbering for each system is deduced from measurements of the isotope effect due to bromine. The heat of dissociation of IBr is 1.808\ifmmode\pm\else\textpm\fi{}0.001 volts. For the four molecules ${\mathrm{I}}_{2}$, IBr, ICl, and ${\mathrm{Br}}_{2}$ it is shown that the separation of the $^{3}\ensuremath{\Pi}_{{0}^{+}}$ and $^{3}\ensuremath{\Pi}_{1}$ states is roughly equal to two-thirds of the mean $^{2}P$ multiplet widths of the constituent atoms. The total $^{3}\ensuremath{\Pi}$ widths are then probably greater and the $^{3}\ensuremath{\Pi}_{2}$ state somewhat lower than has been supposed.