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2-Bromoethanol and 2-chloroethanol were photodissociated in a molecular beam at 193 nm. Only one primary reaction channel was observed, elimination of the halogen atom, with an average translational energy release of 33 kcal/mol. In the case of 2-bromoethanol, some of the C2 H4 OH partner fragment survived and some underwent secondary dissociation to produce C2 H4 and OH. The surviving C2 H4 OH contained up to 43 kcal/mol of internal energy, far more than the expected C2 H4 -OH bond energy of ∼28 kcal/mol. The initial C–Br recoil occurs with a large exit impact parameter and leaves most of the internal energy in C2 H4 OH rotation, creating rotationally metastable fragments. The angular distributions of the secondary C2 H4 and OH products were strongly forward–backward peaked with respect to the primary (C2 H4 OH) velocity vector, consistent with the decay of a long-lived complex in which the total angular momentum is perpendicular to the velocity vector and mainly carried away as orbital angular momentum. This effect is analogous to that observed in the decay of similar long-lived complexes in crossed molecular beam experiments. [ABSTRACT FROM AUTHOR]

A molecular beam of zinc–diethyl (ZnEt2) is photodissociated at 248 and 193 nm and the velocity distributions of the photofragments are measured by time-of-flight techniques. One and two photon processes are observed. The dominant one photon process at both wavelengths is the dissociation of (ZnEt2)2 to form two ZnEt2 monomers. The absence of secondary dissociation of the ZnEt2 photofragments at both excitation wavelengths and the small fraction of the available energy partitioned to product translation implicates dissociation to an excited electronic potential energy surface correlating to one electronically excited ZnEt[ATOTHER]@B|[/ATOTHER]2 monomer. The mass spectrum of the ZnEt2 photofragments is the same as measured for ‘‘cold’’ ZnEt2 monomers in the molecular beam, suggesting that the electronically excited ZnEt[ATOTHER]@B|[/ATOTHER]2 monomers have fluoresced prior to ionization in the mass spectrometer. A small photodissociation signal of uncomplexed ZnEt2 is observed only at low expansion pressures. The sensitive dependence of this monomeric photodissociation signal to the Ar pressure of the adiabatic expansion suggests that ground state vibrational excitation is required for monomeric photodissociation at 248 nm.In contrast to the dimer single photon photodissocation channel, when ZnEt2 monomers are photodissociated, a significant fraction of the available energy appears as product translation. A qualitative molecular orbital analysis can explain the observed fast photoproduct velocity if dissociation occurs via a repulsive triplet state which correlates to electronic ground state products. The two photon process observed is assigned to single photon photodissociation of the electronically excited ZnEt[ATOTHER]@B|[/ATOTHER]2 monomers produced in the dimer photodissociation step. The photofragment velocity distributions for the two photon channel can be quantitatively modeled by sequential ethyl eliminations on... [ABSTRACT FROM AUTHOR]

Photodissociation of Cr(CO)6 induced by a KrF* excimer laser (248 nm) was examined under molecular beam conditions, using a quadrupole mass spectrometer with electron impact ionization (11–19 eV) to detect the products as their respective ions. The ions of all Cr(CO)x products (x=0–5) were produced upon photodissociation of Cr(CO)6 at laser fluences from 1 to 10 mJ cm-2. The quantitative behavior of the Cr(CO)+x product ion yields vs laser fluence as well as statistical rate calculations indicate that Cr(CO)4 is the major product formed upon single-photon dissociation of Cr(CO)6 under collisionless conditions at 248 nm. The additional Cr(CO)+x (x<4) product ions observed at low fluences are formed primarily by electron-impact fragmentation of Cr(CO)4 in the mass spectrometer ionizer. At laser fluences >=5 mJ cm-2, secondary photodissociation of Cr(CO)4 becomes important. Cr(CO)2 is assigned as the major photoproduct formed via secondary photodissociation of Cr(CO)4. [ABSTRACT FROM AUTHOR]

A method is presented for computing the effective center-of-mass velocity distribution for photofragments produced by sequential bond ruptures using Fourier transforms. The method has the advantage that energy and linear momentum conservation are correctly accounted for while remaining computationally feasible. To illustrate the method, the one- and two-photon photodissociation of Fe(CO)5 at 248 nm has been experimentally investigated using the crossed laser-molecular beam method and the measured velocity distributions compared to the prediction of various statistical models for the photodissociation process calculated by the Fourier transform method. The strength of the Fourier transform method is illustrated by the two-photon channel which involves five sequential bond ruptures. The main conclusion regarding the photodissociation mechanism is that a modified form of the separate statistical ensemble theory developed by Wittig and co-workers can quantitatively explain the observed velocity distributions for the one-photon process. Fluxional interchange of the CO ligands is shown not to be important on the CO elimination time scale. The two-photon products have a faster than statistical translational energy distribution which is rationalized by both a dynamical constraint on CO rotational excitation and on the change in orbital– and spin–coupling configuration of the Fe atom as the last two CO ligands are removed. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of CH3I and CD3I have been examined by using multiphoton ionization to probe the CH3, CD3, I(≡5 2P3/2) and I*(≡I 5 2P1/2) photoproducts. The parent compounds were cooled in a supersonic expansion, collimated into a molecular beam, and dissociated at 266 nm. For the CD3I dissociation, the ratio of CD3(v=0)/(v=2) was estimated to be about 1.1, with multiple determinations ranging from 0.47–2.1. The quantum number v here denotes the nascent excitation of the ν2 ‘‘umbrella’’ mode. Measurements of the CD3(v=1) and (v=3) vibronic bands indicated that the (v=1)/(v=3) ratio is greater than unity, with some measurements suggesting values as large as 10. A value for the CH3(v=0)/(v=2) ratio from dissociation of CH3I could not be estimated, although it was clearly larger than that for CD3. The CH3(v=0) and CD3(v=0) products from this dissociation are fit by 120±30 K and 105±30 K rotational distributions, respectively. The dissociation mechanism produces alignment in the molecular frame such that there is a strong preference for K=0 (rotation perpendicular to the top axis). Assuming that the relative velocity vector lies along the CH3 C3 axis, the velocity and rotation vectors tend to be perpendicular. It is likely that K=0 molecular frame alignment is produced in photodissociation through both the I and I* channels. [ABSTRACT FROM AUTHOR]

Photodissociation of O2 is observed by direct detection of coincident neutral photofragments following laser excitation of a fast molecular beam. The O2 beam was prepared by charge-transfer neutralization of keV energy O+2 and irradiated by a linearly polarized, line selected Kr+ ion laser. Kinetic-energy resolved photofragments were observed at eight laser lines between 752.5 and 476.2 nm. Correlation of the observed fragment kinetic energies with laser wavelength shows that the observed photodissociations arise from O2 B 3σ-u(v’=1–18)←X 3σ-g (v‘=22–37) Schumann–Runge transitions. [ABSTRACT FROM AUTHOR]

A crossed laser-molecular beam study of the photodissociation mechanism of Fe(CO)5 has been performed at 193 nm where time-of-flight measurements of the primary iron containing photofragments have been recorded under collision free conditions. The center-of-mass velocity distributions derived from the TOF data by the method of moments show that Fe(CO)2 accounts for >99% of all photoproducts formed after absorption of one photon. The only mechanism which quantitatively reproduces the measured velocity distributions is a sequence of three, uncorrelated, statistical CO eliminations. At high photon flux, a second photon can be absorbed by the Fe(CO)2 photofragment which decomposes by an uncorrelated sequential elimination of the remaining two CO ligands. [ABSTRACT FROM AUTHOR]

Picosecond and nanosecond lasers and pulsed molecular beam techniques have been used to measure the infrared photodissociation spectra, the product state distributions, and the predissociation lifetimes of vibrationally excited nitric oxide dimer (NO)2 . Results for the ν1 (v=1) symmetric NO stretching mode and the ν4 (v=1) antisymmetric NO stretching mode are presented. Predissociation lifetimes are determined by time-resolved laser induced fluorescence probing of the NO monomer product appearance rate. A dramatic mode dependence of the predissociation lifetimes is observed with the higher energy ν1 mode decaying in approximately 1 ns, and the lower energy ν4 mode decaying in approximately 40 ps. The mode dependence is independent of which product state is probed. The product state distributions show that 75% to 80% of the available energy is channeled into relative translational energy of the fragments for both modes. Rotational state distributions are Boltzmann-like with temperatures ranging from 71 to 112 K depending on both the initially excited mode and on the NO product spin–orbit state. Predissociation from ν1 produces NO fragments in the 2Π1/2 and 2 Π3/2 states with equal probability. Predissociation from ν4 exhibits a propensity for producing the lower energy 2 Π1/2 spin–orbit state. The observations are discussed in terms of various vibrational predissociation mechanisms, including vibrational potential coupling and electronically nonadiabatic predissociation. [ABSTRACT FROM AUTHOR]

A semiclassical calculation is presented that fully accounts for the angular momentum disposal in photodissociation of triatomic molecules. Rotational distributions are calculated for O2(3Σ-g) produced by the visible photolysis of ozone, O2(1Δg) by the UV photolysis of ozone, and OH by the 157 nm dissociation of water, to illustrate the effects of parent internal motion on fragment rotational distributions in the dissociation of C2v geometry molecules. A simple, but realistic, impulsive model of the energy release is used to describe the dissociation dynamics. The calculations are carried out for parent molecules at room temperature, as well as at the low temperatures characteristic of molecular beams. The contributions to the diatomic fragment rotational distribution from both parent triatom rotation and zero-point bending vibration are computed. Comparison of the calculated distributions with experimentally measured distributions indicates that the spread in rotational and bending vibrational angular momenta of the parent molecule can account for all or nearly all of the spread in final J of the diatomic photofragment. However, the rotational distributions of the diatomic photofragment reveal a strong vector correlation between the diatom angular momentum produced by the dissociative energy release, and the angular momentum associated with the in-plane rotation. The correlation is such that only half of all the photofragment states allowed by energy and angular momentum conservation are actually produced with appreciable probability. Of two energy degenerate photofragment states, corresponding to breaking of one or the other nominally equivalent bonds in the AB2 molecule, the one with the smaller orbital angular momentum/recoil linear momentum is strongly favored. This is explained by larger Franck–Condon overlap in the photoexcitation for the state of lower recoil angular momentum. The correlation involves selection of which of the two... [ABSTRACT FROM AUTHOR]

The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser-molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin-orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr. Analysis of the product translational energy distributions for the dissociation products indicates that the IBr is formed electronically excited and that the halogen atom products are spin-orbit excited. Excitation at 210 nm, of the transition assigned as n(Br)→σ*(C–Br) based on comparison with CH3Br, results in selective breaking of the stronger C–X bond in the molecule, the C–Br bond, and no fission of the C–I bond. Some concerted elimination of IBr also occurs; the IBr velocity distribution indicates it is probably formed electronically excited as in photolysis at 193.3 nm. The selective breaking of the C–Br bond over the weaker C–I bond is discussed in contrast to previous photolysis studies of polyhalomethanes. [ABSTRACT FROM AUTHOR]

M(CH3)2 (M=Zn,Cd) vapor was photodissociated at 193 nm into CH3 and MCH3 (A 2E) radicals and also into CH3 radicals and M atoms. The kinetic energy distribution of the CH3 was measured by a molecular beam time of flight technique. The emission spectrum of the MCH3(A 2E) was recorded in the visible region (for the first time) and it was shown that the radicals were vibrationally cold. An excitation spectrum for visible emission was determined through the VUV region down to 125 nm. Two photon production of M(n 3P01 ) states was easily observable but shown to be negligible compared to one photon processes. The methyl radicals released in this dissociation are slower but vibrationally hotter than those dissociated from CH3I. [ABSTRACT FROM AUTHOR]

Laser-based spectroscopies coupled with molecular beam techniques facilitated the monitoring of H fragments released in ultraviolet photodissociation of pre-excited isoenergetic vibrational levels of pyrrole. Most noticeably, there was an order of magnitude larger reactivity for an eigenstate primarily consisting of two quanta of ring deformation than for another with one quantum of symmetric C-H stretch. The dynamics, the intramolecular interactions controlling the energy flow, and the mode-selectivity within a medium-sized, ten atom molecule, is discussed. [ABSTRACT FROM AUTHOR]

The [1+1] A˜ [sup 2]Π[sub u]-state resonance enhanced two-photon dissociation process of CS[sub 2][sup +] molecular ions has been investigated by measuring the photofragment S[sup +] excitation (PHOFEX) spectrum in the wavelength range of 424-482 nm, where the CS[sub 2][sup +] molecular ions were prepared purely by [3+1] multiphoton ionization of the neutral CS[sub 2] molecules at 483.2 nm. The PHOFEX spectrum was assigned essentially to the CS[sub 2][sup +](A˜ [sup 2]Π[sub u])←CS[sub 2][sup +](X˜ [sup 2]Π[sub g]) transition, and the dissociation mechanism of CS[sub 2][sup +] was preliminarily attributed to (i) CS[sub 2][sup +](X˜ [sup 2]Π[sub g])→CS[sub 2][sup +](A˜ [sup 2]Π[sub u]) through one-photon excitation, (ii) CS[sub 2][sup +](A˜ [sup 2]Π[sub u])→CS[sub 2][sup +](X˜[sup †]) via internal conversion process due to the vibronic coupling between the A˜ and X˜ states, (iii) CS[sub 2][sup +](X˜[sup †])→CS[sub 2][sup +](B˜ [sup 2]Σ[sub u][sup +]) through the second photon excitation, and (iv) CS[sub 2][sup +](B˜ [sup 2]Σ[sub u][sup +])→S[sup +]+CS owing to the potential curve crossing with the repulsive [sup 4]Σ[sup -] state correlated with the first dissociation limit. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]