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We investigated the photodissociation dynamics of vibrationally excited HBr molecules and clusters. The species were generated in a molecular beam and excited with an IR laser to a υ = 1 vibrational state. A subsequent ultraviolet (UV)-pulse with 243 nm radiation photolysed the molecules to yield H-fragments, which were resonantly ionized by the same UV-pulse (2 + 1 REMPI) and detected in a velocity map imaging (VMI) experiment. We performed action spectroscopy to distinguish between two expansion regimes: (i) expansion leading to isolated HBr molecules and (ii) generation of large (HBr)n clusters. Photodissociation of isolated HBr (υ = 1) molecules in particular J ro-vibrational states yielded faster H-fragments (by approximately 0.3 eV) with respect to the photodissociation of the ground state HBr (υ = 0). On the contrary, the IR excitation of molecules in (HBr)n clusters enhanced the yield of the H-fragments UV-photodissociated from the ground-state HBr (υ = 0) molecules. Our findings showthat these molecules are photodissociated within clusters, and they are not free molecules evaporated from clusters after the IR excitation. Nanosecond IR-UV pump-probe experiments show that the IR-excitation enhances the H-fragment UV-photodissociation yield up to ~100 ns after the IR excitation. After these long IR-UV delays, excitation of HBr molecules in clusters does not originate from the IR-excitation but from the UV-photodissociation and subsequent caging of HBr molecules in υ > 0 states.We show that even after ~100 ns the IR-excited larger (HBr)n clusters do not decay to individual molecules, and the excitation is still present in some form within these clusters enhancing their UV-photodissociation. [ABSTRACT FROM AUTHOR]

We demonstrate that the vacuum ultraviolet (VUV) photodissociation dynamics of N2 and CO2 can be studied using VUV photoionization with time-sliced velocity-mapped ion imaging (VUV-PI-VMI) detection. The VUV laser light is produced by resonant sum frequency mixing in Kr. N2 is used to show that when the photon energy of the VUV laser is above the ionization energy of an allowed transition of one of the product atoms it can be detected and characterized as the wavelength is varied. In this case a β parameter = 0.57 for the N(2D°) was measured after exciting N2(o1Πu, v′ = 2, J′ = 2) ← N2(X1Σg+, v″ = 0, J″ = 1). Studies with CO2 show that when there is no allowed transition, an autoionization resonance can be used for the detection of a product atom. In this case it is shown for the first time that the O(1D) atom is produced with CO(1Σ+) at 92.21 nm. These results indicate that the VUV laser photodissociation combined with the VUV-PI-VMI detection is a viable method for studying the one-photon photodissociation from the ground state of simple molecules in the extreme ultraviolet and VUV spectral regions. [ABSTRACT FROM AUTHOR]

IR photodissociation (IRPD) spectra of (N2O)2+•Ar and (N2O)n+ with n=3–8 are measured in the 1000–2300 cm-1 region. The (N2O)2+•Ar ion shows an IRPD band at 1154 cm-1, which can be assigned to the out-of-phase combination of the ν1 vibrations of the N2O components in the N4O2+ ion; the positive charge is delocalized over the two N2O molecules. The geometry optimization and the vibrational analysis at the B3LYP/6-311+G* level show that the N4O2+ ion has a C2h structure with the oxygen ends of the N2O components bonded to each other. The IRPD spectra of the (N2O)n+(n=3–8) ions show three prominent bands at ∼1170, ∼1275, and ∼2235 cm-1. The intensity of the ∼1170 cm-1 band relative to that of the other bands decreases with increasing the cluster size. Therefore, the ∼1170 cm-1 band is ascribed to the N4O2+ dimer ion core and the ∼1275 and ∼2235 cm-1 bands are assigned to the ν1 and ν3 vibrations of solvent N2O molecules, respectively. Since the band of the N4O2+ ion core is located at almost the same position for all the (N2O)n+(n=2–8) clusters, the C2h structure of the dimer ion core does not change so largely by the solvation of N2O molecules, which is quite contrastive to the isoelectronic (CO2)n+ case. [ABSTRACT FROM AUTHOR]

The photofragmentation dynamics of Ar2+ and Ar3+ clusters has been investigated at a 527 nm wavelength (2.35 eV) using a setup that allows simultaneous detection of the ionic and neutral fragments in a coincidence experiment. Measurement of positions and times of flight enables in principle a complete description of the fragmentation dynamics. The photofragmentation dynamics of Ar3+ clusters is similar to that of Ar2+ with, in addition, the ejection of a third fragment that can be neutral or ionized via a resonant electron capture. This is attributed to the triangular geometry of the Ar3+ ion. [ABSTRACT FROM AUTHOR]

Thermal HCl and HBr molecules were photodissociated using circularly polarized 193 nm light, and the speed-dependent spin polarization of the H-atom photofragments was measured using polarized fluorescence at 121.6 nm. Both polarization components, described by the a01(⊥) and Re[a11(∥,⊥)] parameters which arise from incoherent and coherent dissociation mechanisms, are measured. The values of the a01(⊥) parameter, for both HCl and HBr photodissociation, are within experimental error of the predictions of both ab initio calculations and of previous measurements of the polarization of the halide cofragments. The experimental and ab initio theoretical values of the Re[a11(∥,⊥)] parameter show some disagreement, suggesting that further theoretical investigations are required. Overall, good agreement occurs despite the fact that the current experiments photodissociate molecules at 295 K, whereas previous measurements were conducted at rotational temperatures of about 15 K. [ABSTRACT FROM AUTHOR]

Infrared photodissociation (IRPD) spectra of carbon dioxide cluster ions, (CO2)n+ with n=3–8, are measured in the 1000–3800 cm–1 region. IR bands assignable to solvent CO2 molecules are observed at positions close to the vibrational frequencies of neutral CO2 [1290 and 1400 cm–1 (ν1 and 2ν2), 2350 cm–1 (ν3), and 3610 and 3713 cm–1 (ν1+ν3 and 2ν2+ν3)]. The ion core in (CO2)n+ shows several IR bands in the 1200–1350, 2100–2200, and 3250–3500 cm–1 regions. On the basis of previous IR studies in solid Ne and quantum chemical calculations, these bands are ascribed to the C2O4+ ion, which has a semicovalent bond between the CO2 components. The number of the bands and the bandwidth of the IRPD spectra drastically change with an increase in the cluster size up to n=6, which is ascribed to the symmetry change of (CO2)n+ by the solvation of CO2 molecules and a full occupation of the first solvation shell at n=6. [ABSTRACT FROM AUTHOR]

Transient grating (TG) spectroscopy has been applied to the photoinduced hydrogen-abstraction reaction of benzophenone (BP) in various kinds of room temperature ionic liquids (RTILs). After the photoexcitation of BP in RTILs, the formation of a benzophenone ketyl radical (BPK) was confirmed by the transient absorption method, and the TG signal was analyzed to determine the diffusion coefficients of BPK and BP. For comparison, diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) in various RTILs were determined by the TG method using the photodissociation reaction of DPCP. While the diffusion coefficients of the stable molecules BP, DPA, and DPCP were always larger than those predicted by the Stokes–Einstein (SE) relation in RTILs, that of BPK was much smaller than those of the stable molecules and relatively close to that predicted by the SE relation in all solvents. For the smallest molecule CO, the deviation from the SE relation was evident. The diffusion coefficients of stable molecules are better represented by a power law of the inverse of the viscosity when the exponent was less than unity. The ratios of the diffusion coefficient of BP to that of BPK were larger in RTILs (2.7–4.0) than those (1.4–2.3) in conventional organic solvents. The slow diffusion of BPK in RTILs was discussed in terms of the fluctuation of the local electric field produced by the surrounding solvent ions. [ABSTRACT FROM AUTHOR]

Infrared photodissociation spectra of (CS2)n+ and (CS2)n- with n=3–10 are measured in the 1100–2000 cm-1 region. All the (CS2)n+ clusters exhibit three bands at ∼1410, ∼1490, and ∼1540 cm-1. The intensity of the 1540 cm-1 band relative to those of the other bands increases with increasing the cluster size, indicating that the band at 1540 cm-1 is assignable to the antisymmetric CS stretching vibration of solvent CS2 molecules in the clusters. On the basis of density functional theory calculations, the 1410 and 1490 cm-1 bands of (CS2)n+ are assigned to CS stretching vibrations of the C2S4+ cation core with a C2 form. The (CS2)n- clusters show two bands at around 1215 and 1530 cm-1. Similar to the case of cation clusters, the latter band is ascribed to the antisymmetric CS stretching vibration of solvent CS2 molecules. Vibrational frequency analysis of CS2- and C2S4- suggests that the 1215 cm-1 band is attributed to the antisymmetric CS stretching vibration of the CS2- anion core with a C2v structure. [ABSTRACT FROM AUTHOR]

Comparing the recoil energy distributions of the fragments from one-photon dissociation of phenol-d5 with those from vibrationally mediated photodissociation shows that initial vibrational excitation strongly influences the disposal of energy into relative translation. The measurements use velocity map ion imaging to detect the H-atom fragments and determine the distribution of recoil energies. Dissociation of phenol-d5 molecules with an initially excited O–H stretching vibration produces significantly more fragments with low recoil energies than does one-photon dissociation at the same total energy. The difference appears to come from the increased probability of adiabatic dissociation in which a vibrationally excited molecule passes around the conical intersection between the dissociative state and the ground state to produce electronically excited phenoxyl-d5 radicals. The additional energy deposited in electronic excitation of the radical reduces the energy available for relative translation. [ABSTRACT FROM AUTHOR]

We report on the first observation of the organoxenon HXeCCH molecule in the gas phase. This molecule has been prepared in a molecular beam experiment by 193 nm photolysis of an acetylene molecule on Xen clusters (n≈390). Subsequently the molecule has been oriented via the pseudo-first-order Stark effect in a strong electric field of the polarized laser light combined with the weak electrostatic field in the extraction region of a time-of-flight spectrometer. The experimental evidence for the oriented molecule has been provided by measurements of its photodissociation. For comparison, photolysis of C2H2 on Arn clusters (n≈280) has been measured. Here the analogous rare gas molecule HArCCH could not be generated. The interpretation of our experimental findings has been supported by ab initio calculations. In addition, the experiment together with the calculations reveals information on the photochemistry of the HXeCCH molecule. The 193 nm radiation excites the molecule predominantly into the 2 1Σ+ state, which cannot dissociate the Xe–H bond directly, but the system evolves along the Xe–C coordinate to a conical intersection of a slightly nonlinear configuration with the dissociative 1 1Π state, which then dissociates the Xe–H bond. [ABSTRACT FROM AUTHOR]

The 248 and 193 nm photodissociations of submonolayer quantities of CH3Br and CH3I adsorbed on thin layers of n-hexane indicate that the dissociation is caused by dissociative electron attachment from subvacuum level photoelectrons created in the copper substrate. The characteristics of this photodissociation-translation energy distributions and coverage dependences show that the dissociation is mediated by an image potential state which temporarily traps the photoelectrons near the n-hexane–vacuum interface, and then the charge transfers from this image state to the affinity level of a coadsorbed halomethane which then dissociates. [ABSTRACT FROM AUTHOR]

The photodissociation of rotationally state-selected methyl bromide is studied in the wavelength region between 213 and 235 nm using slice imaging. A hexapole state selector is used to focus a single (JK=11) rotational quantum state of the parent molecule, and a high speed slice imaging detector measures directly the three-dimensional recoil distribution of the methyl fragment. Experiments were performed on both normal (CH3Br) and deuterated (CD3Br) parent molecules. The velocity distribution of the methyl fragment shows a rich structure, especially for the CD3 photofragment, assigned to the formation of vibrationally excited methyl fragments in the ν1 and ν4 vibrational modes. The CH3 fragment formed with ground state Br(2P3/2) is observed to be rotationally more excited, by some 230–340 cm-1, compared to the methyl fragment formed with spin-orbit excited Br(2P1/2). Branching ratios and angular distributions are obtained for various methyl product states and they are observed to vary with photodissociation energy. The nonadiabatic transition probability for the 3Q0+→1Q1 transition is calculated from the images and differences between the isotopes are observed. Comparison with previous non-state-selected experiments indicates an enhanced nonadiabatic transition probability for state-selected K=1 methyl bromide parent molecules. From the state-to-state photodissociation experiments the dissociationenergy for both isotopes was determined, D0(CH3Br)=23 400±133 cm-1 and D0(CD3Br)=23 827±94 cm-1. [ABSTRACT FROM AUTHOR]

Time-of-flight (TOF) spectra of photofragment H atoms from the photodissociation of water ice films at 193 nm were measured for amorphous and polycrystalline water ice films with and without dosing of hydrogen chloride at 100–145 K. The TOF spectrum is sensitive to the surface morphology of the water ice film because the origin of the H atom is the photodissociation of dimerlike water molecules attached to the ice film surfaces. Adsorption of HCl on a polycrystalline ice film was found to induce formation of disorder regions on the ice film surface at 100–140 K, while the microstructure of the ice surface stayed of polycrystalline at 145 K with adsorption of HCl. The TOF spectra of photofragment Cl atoms from the 157 nm photodissociation of neutral HCl adsorbed on water ice films at 100–140 K were measured. These results suggest partial dissolution of HCl on the ice film surface at 100–140 K. [ABSTRACT FROM AUTHOR]

The sequential photodissociation dynamics of (HI)2 is studied by means of a nonadiabatic wave packet treatment starting from the I*-HI complex. The model reproduces the main experimental findings for photolysis with 266 nm radiation. The results confirm that some of the H atoms dissociated from the I*-HI complex deactivate the I* atom through a H/I* intracluster collision which induces an I*→I electronically nonadiabatic transition. As a consequence, these H fragments become very fast by acquiring nearly all the I* excitation energy, equivalent to the I*/I spin-orbit splitting. A most interesting result is the high production of bound I2 fragments in highly excited rovibrational states in the photolysis, indicating that the H dissociation is mainly direct. [ABSTRACT FROM AUTHOR]

We develop a method to improve the population transfer and final-channel control of multichannel photodissociation reactions. The method is applied to the photodissociation of methyl iodide, CH3(v)+I*(2P1/2)←CH3I→CH3(v)+I(2P3/2). Our method is based on simultaneously exciting many two-photon pathways that lead to the same final outcome, each proceeding via a different intermediate bound state. The selectivity of the final product state(s) is a result of coherently controlled interference between the quantum pathways. The improvement in the population transfer yield from the ground state to the selected dissociative channel(s) is made possible by executing the process in an adiabatic fashion. [ABSTRACT FROM AUTHOR]

Quantum mechanical calculations of photofragment angular distributions have been performed as a function of the frequency of excitation, the lifetime of the dissociative state, the rotational level, and the rotational constant. In the limit of high J values and white, incoherent excitation, the general results are found to agree exactly with both those of Mukamel and Jortner [J. Chem. Phys. 61, 5348 (1974)] and those of Jonah [J. Chem. Phys. 55, 1915 (1971)]. Example calculations describe how the anisotropy is dependent on the degree of broadening, the rotational constant, the initial rotational level, and the frequency of excitation. Applications are also made to interpret experimental results on the photodissociation of ClO via the 11-0, 10-0, and 6-0 bands of the A 2Π3/2–X 2Π3/2 transition and on the photodissociation of O2 via the 0-0 band of the E 3Σu-–X 3Σg- transition. [ABSTRACT FROM AUTHOR]

The dynamics of the 193 nm photodissociation of the CCl2 molecule have been investigated in a molecular beam experiment. The CCl2 parent molecule was generated in a molecular beam by pyrolysis of CHCl3, and both CCl2 and the CCl photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section was estimated from the reduction of the CCl2 signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A 2Δ-X 2Π band system. Most of the energy available to the CCl(X 2Π)+Cl fragments appears as translational energy. The CCl fragment rotational energy is much less than predicted in an impulsive model. The excited electronic state appears to dissociate indirectly, through coupling with a repulsive state arising from the ground-state CCl(X 2Π)+Cl asymptote. The identity of the initially excited electronic state is discussed on the basis of what is known about the CCl2 electronic states. [ABSTRACT FROM AUTHOR]

The results are presented of three-dimensional model studies of the photodissociation of the water dimer following excitation in the first absorption band. Diabatic potential-energy surfaces are used to investigate the photodissociation following excitation of the hydrogen bond donor molecule and of the hydrogen bond acceptor molecule. In both cases, the degrees of freedom considered are the two OH-stretch modes of the molecule being excited, and the dimer stretch vibration. The diabatic potentials are based on adiabatic potential surfaces computed with the multireference configuration-interaction method, and the dynamics of dissociation was studied using the time-dependent wave-packet method. The dynamics calculations yield a donor spectrum extending over roughly the same range of frequencies as the spectrum of the water monomer computed at the same level of theory. The acceptor spectrum has the same width as the monomer spectrum, but is shifted to the blue by 0.4–0.5 eV. The dimer spectrum obtained by averaging the donor and the acceptor spectrum is broader than the monomer spectrum, with the center of the dimer first absorption band shifted to the blue by about 0.2 eV relative to the monomer band. Our reduced dimensionality calculations do not find the red tail predicted for the dimer first absorption band by Harvey et al. [J. Chem. Phys. 109, 8747 (1998)]. This conclusion also holds if preexcitation of the dimer stretch vibration with one or two quanta is considered. [ABSTRACT FROM AUTHOR]

We report the photodissociation of laboratory oriented OCS molecules. A molecular beam of OCS molecules is hexapole state-selected and spatially oriented in the electric field of a velocity map imaging lens. The oriented OCS molecules are dissociated at 230 nm with the linear polarization set at 45° to the orientation direction of the OCS molecules. The CO(ν=0,J) photofragments are quantum state-selectively ionized by the same 230 nm pulse and the angular distribution is measured using the velocity map imaging technique. The observed CO(ν=0,J) images are strongly asymmetric and the degree of asymmetry varies with the CO rotational state J. From the observed asymmetry in the laboratory frame we can directly extract the molecular frame angles between the final photofragment recoil velocity and the permanent dipole moment and the transition dipole moment. The data for CO fragments with high rotational excitation reveal that the dissociation dynamics is highly nonaxial, even though conventional wisdom suggests that the nearly limiting β parameter results from fast axial recoil dynamics. From our data we can extract the relative contribution of parallel and perpendicular transitions at 230 nm excitation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ni+(CO2)n, Ni+(CO2)nAr, Ni+(CO2)nNe, and Ni+(O2)(CO2)n complexes are generated by laser vaporization in a pulsed supersonic expansion. The complexes are mass-selected in a reflectron time-of-flight mass spectrometer and studied by infrared resonance-enhanced photodissociation (IR-REPD) spectroscopy. Photofragmentation proceeds exclusively through the loss of intact CO2 molecules from Ni+(CO2)n and Ni+(O2)(CO2)n complexes, and by elimination of the noble gas atom from Ni+(CO2)nAr and Ni+(CO2)nNe. Vibrational resonances are identified and assigned in the region of the asymmetric stretch of CO2. Small complexes have resonances that are blueshifted from the asymmetric stretch of free CO2, consistent with structures having linear Ni+–O==C==O configurations. Fragmentation of larger Ni+(CO2)n clusters terminates at the size of n=4, and new vibrational bands assigned to external ligands are observed for n>=5. These combined observations indicate that the coordination number for CO2 molecules around Ni+ is exactly four. Trends in the loss channels and spectra of Ni+(O2)(CO2)n clusters suggest that each oxygen atom occupies a different coordination site around a four-coordinate metal ion in these complexes. The spectra of larger Ni+(CO2)n clusters provide evidence for an intracluster insertion reaction assisted by solvation, producing a metal oxide-carbonyl species as the reaction product. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photodissociation experiments were carried out at 193 nm for single HCl molecules which are adsorbed on the surface of large Arn clusters and small (HCl)m complexes which are embedded in the interior of these clusters. For the surface case the size dependence is measured for the average sizes =140–1000. No cage exit events are observed in agreement with the substitutional position of the molecule deeply buried in the outermost shell. This result is confirmed by a molecular dynamics simulation of the pickup process under realistic conditions concerning the experiment and the interaction potentials. The calculations of the dissociation process employ the surface hopping model. For the embedded case the average sizes covered are =3 and 6 and =8–248. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and around 2.0 eV which mark completely caged and unperturbed fragments, respectively. The ratio of theses peaks strongly depends on the cluster size and agrees well with theoretical predictions for one and two closed icosahedral shells, in which the nonadiabatic coupling of all states was accounted for. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An experimental technique based on a scheme of vibrationally mediated photodissociation has been developed and applied to the spectroscopic study of highly excited vibrational states in HCN, with energies between 29 000 and 30 000 cm-1. The technique consists of four sequential steps: in the first one, a high power laser is used to vibrationally excite the sample to an intermediate state, typically (0,0,4), the ν[sub 3] mode being approximately equivalent to the C–H stretching vibration. Then a second laser is used to search for transitions between this intermediate state and highly vibrationally excited states. When one of these transitions is found, HCN molecules are transferred to a highly excited vibrational state. Third, a ultraviolet laser photodissociates the highly excited molecules to produce H and CN radicals in its A [sup 2]Π electronic state. Finally, a fourth laser (probe) detects the presence of the CN(A) photofragments by means of an A→B→X laser induced fluorescence scheme. The spectra obtained with this technique, consisting of several rotationally resolved vibrational bands, have been analyzed. The positions and rotational parameters of the states observed are presented and compared with the results of a state-of-the-art variational calculation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the production in the gas phase of ionically bound HXeI molecules. The molecules are generated by the photodissociation of HI molecules in large Xe[SUBn] clusters and are identified from the asymmetry of the detected H atom fragments arising from the dissociation of oriented HXeI. The orientation, resulting from a synergistic action of a pulsed laser field with a weak electrostatic field, is quite pronounced, due to a large ratio of the polarizability anisotropy to the rotational constant of HXeI. [ABSTRACT FROM AUTHOR]

With the preparing of SO[SUB2SUP+](&Xtilde;[SUP2] A[SUB1](000)) by [3+1] multiphoton ionization of the neutral SO[SUB2] molecules at 380.85 nm, the photodissociation process and the symmetry of the excited states &Dtilde; of SO[SUB2SUP+] molecular ions has been investigated by measuring the photofragment SO[SUP+] excitation (PHOFEX) spectrum in ultraviolet (282-332 nm) and in visible (562-664 nm) wavelength ranges, respectively. The &Dtilde;(ν[SUB1]00),&Dtilde;(0ν[SUB2])←&Xtilde;[SUP2] A[SUB1](000) transition of SO[SUB2SUP+] assigned in the PHOFEX spectrum in the UV range. By comparing the discernible PHOFEX spectrum in UV range with the continuous PHOFEX spectrum in visible range, it is deduced that (i) SO[SUB2SUP+](&Dtilde;) there exists a repulsive state α[SUP2]A[SUB2] converging to the dissociation limit of SO[SUP+](X[SUP2]Π)+O([SUP3P[SUBg], (ii) the coupling between SO[SUB2SUP+](&Dtilde;) and SO[SUB2SUP+] and (α[SUP2] A[SUB2]) leads to the dissociation to SO[SUP+](X[SUP2]Π)+O([SUP3P[SUBg] the symmetry of &Dtilde;state is &Dtilde;[SUP2][SUB1]. [ABSTRACT FROM AUTHOR]

Orientation and alignment parameters have been computed from first principles for the photodissociation of the HF and DF diatomic molecules. The calculations are entirely ab initio and the break-up dynamics of the molecule is treated rigorously taking account of the electronically nonadiabatic dynamics on three coupled adiabatic electronic potential energy curves. The potential energy curves and spin-orbit interactions, which have been previously reported [J. Chem. Phys. 113, 1870 (2000)], are computed using ab initio molecular electronic structure computer codes. These are then used to compute photofragmentation T matrix elements using a time-dependent quantum mechanical wave packet treatment and from these a complete set of anisotropy parameters with rank up to K = 3 is computed. The predicted vector correlations and alignment parameters are presented as a function of energy for HF and DF initially in both their ground and first excited vibrational states. The parameters predicted for the molecules which are initially in their excited vibrational states display a pronounced sharp energy dependence arising from the nodal structure of the initial vibrational wavefunction. The theoretical results are analyzed using a simple model of the dynamics and it is demonstrated how the magnitude and relative phases of the photofragrnentation T matrix elements can be deduced from the experimentally measured alignment parameters. No experimental measurements have yet been made of alignment parameters for hydrogen halide diatomics and the present results provide the first predictions of these quantities which may be compared with future experimental observations. [ABSTRACT FROM AUTHOR]

Photodissociation of jet-cooled, vibrationally excited CHF[sub 2]Cl molecules was studied in a time-of-flight mass spectrometer to elucidate bond rupture and intramolecular dynamics. The molecules were first excited with infrared photons to the N=3, N=7/2, and N=4 C–H stretch-bend polyad components, representing stretch-bend mixed states. They were then dissociated via promotion to excited electronic states by ∼235 or 243.135 nm photons, which also tagged [sup 35]Cl([sup 2]P[sub j]) and [sup 37]Cl([sup 2]P[sub j]) or H photofragments, respectively, by (2+1) resonantly enhanced multiphoton ionization. Comparison of the photofragment yield spectra to the simultaneously measured room-temperature infrared absorption spectra revealed significant narrowing of the former due to the reduction of rotational inhomogeneous structure. These spectra, and particularly the band contraction, afforded observation of resonance splitting in the vicinity of the 3[sub 1], 3[sub 4], 4[sub 1], and 4[sub 4] components, reflecting redistribution times in the range of 1–18 ps. These times manifest the vibrational redistribution of the mixed states to other states of the molecule and are longer than those for the coupling of the stretch-bend. The initial vibrational excitation enhanced C–Cl and C–H bond cleavage with the former producing both ground-, Cl [sup 2]P[sub 3/2][Cl], and excited-, Cl [sup 2]P[sub 1/2][Cl[sup *]], spin-orbit states. The branching ratio of Cl*/Cl was ∼0.5 and of H/[Cl*+Cl]∼0.1, independent on the initially prepared state, signifying preferential production of Cl photofragments and energy flow from the initially excited bond. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An examination of the dependence of total flux from competing photodissociation channels on the photolysis field parameters and initial vibrational states for IBr and HI molecules reveals that, for a range of field attributes, considerable selectivity and yield may be obtained by using only the ground or the ground and the first excited vibrational states in the optimal linear combination constituting the field optimized initial state (FOIST). The new simplifications obviate the need for overtone excitations or multicolor photolysis fields making it easier to implement FOIST experimentally. Concrete specifications of field attributes for achieving selective control of IBr photodissociation products is provided. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the photodissociation of the hydrogen sulfide molecule, a model system for unimolecular dissociation dynamics. Vibrational and rotational distribution of the hydrogen sulfide photofragment; H-atom and time-of-flight spectrum and total kinetic energy distribution; Hydrogen sulfide product internal energy distribution.

Discusses the photodissociation of a molecule using a three-dimensional, hybrid quantum dynamical method. Diabatic potential energy surfaces used in the dynamics; Computation of a small product fraction; Ranges of resonances that affect the photodissociation.

A formalism for the analysis of detailed photofragmentation experiments involving oriented/aligned parent molecules is presented. It is demonstrated for the photofragmentation of an oriented/aligned parent molecule that both the magnitudes and the relative-phases of the transition dipole matrix elements can be determined from angle integrated cross sections or the product orientation/alignment moments derived from them. Formulas are presented for extracting transition dipole matrix elements from the photodissociation of an aligned diatomic molecule to form aligned and/or oriented atomic products using linearly or circularly polarized photolysis lasers. While these detailed measurements are ideally performed using inhomogeneous field state-selected parent molecules, they can be also readily carried out by doing double-resonance photodissociation experiments with polarized lasers. The analysis presented here can, therefore, be used to extract the transition dipole matrix elements from these double-resonance photofragmentation experiments as well. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We calculate the alignment and orientation of the CN fragment of the photodissociation of ICN using a time-dependent method and the diabatic potentials and transition moments of Morokuma and co-workers [J. Chem. Phys. 100, 4894 (1994)]. The time-dependent Schrödinger equation is solved using a Chebyshev method evaluating sums required to do the matrix-vector products sequentially. To prevent the wave packet from drifting off the edge of our grid we propagate in successive steps. The ground state wave function, from which the molecule is dissociated, is computed using a three-dimensional variational method. We calculate alignment and orientation for a Ji=0→Jf=1 transition. We use parity-adapted angular basis functions. Our calculated alignment and orientation are qualitatively close to experimental results. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The H(D) Rydberg atom photofragment translational spectroscopy technique has been applied to a further detailed investigation of the photodissociation dynamics of NH3 and ND3 molecules following excitation to the lowest two (v2=0 and 1) vibrational levels of the first excited (A 1A2″) singlet electronic state. Analysis of the respective total kinetic energy release spectra, recorded at a number of scattering angles Θ [where Θ is the angle between the ε vector of the photolysis photon and the time-of-flight (TOF) axis], enables quantification of a striking, quantum state dependent, μ-v correlation in the NH2(ND2) products. The spatial distribution of the total flux of H(D) atom photofragments is rather isotropic (βlab∼0). However, more careful analysis of the way in which the TOF spectra of the H(D) atom photofragments vary with Θ reveals that each H+NH2(D+ND2) product channel has a different ‘‘partial’’ anisotropy parameter, βlab(v2,N), associated with it: The H(D) atom ejected by those molecules that dissociate to yield NH2(ND2) fragments with little rotational excitation largely appear in the plane of the excited molecule (i.e., perpendicular to the transition moment and the C3 axis of the parent, with β tending towards -1). Conversely, the H(D) atoms formed in association with the most highly rotationally excited partner NH2(ND2) fragments tend to recoil almost parallel to this C3 axis (i.e., β→+2). Such behavior is rationalized in the context of the known potential energy surfaces of the A and X states of ammonia using a classical, energy and angular momentum conserving impact parameter model in which we assume that all of the product angular momentum is established at the ‘‘point’’ of the conical intersection in the H–NH2(D–ND2) dissociation coordinate. We conclude by reemphasizing the level of care needed in interpreting... [ABSTRACT FROM AUTHOR]

The experiments presented here investigate the competing photodissociation pathways for allyl chloride upon excitation of the nominally ππ*(C=C) transition at 193 nm. The measured photofragment velocity distributions evidence C–Cl bond fission and HCl elimination. The recoil kinetic energy distribution for the HCl products is bimodal, indicating two primary processes for HCl elimination. The experimental measurements show C–Cl bond fission dominates, giving an absolute branching ratio of HCl:C–Cl=0.12±0.03 when the parent molecule is expanded through a nozzle at 200 °C. The branching ratio depends on the nozzle temperature; at 475 °C, the absolute branching ratio measured is HCl:C–Cl=0.24±0.03. We analyze the experimental results along with supporting ab initio calculations and earlier photodissociation studies of vinyl chloride in order to examine the potential influence of nonadiabaticity along the C–Cl fission reaction coordinate and its dependence on molecular conformation. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

HOCl is a closed shell system in its ground electronic state. In common with many molecules it is photodissociated to give open shell fragments. In this paper the photodissociation process HOCl(X 1A′)+hν→HOCl(2 1A′)→OH+Cl is investigated. A model is proposed to predict the OH product spin–orbit and Λ-doublet state distributions as well as the distribution of the Cl atom spin–orbit states. The model considers both the electronic structure of the system and the nuclear motion. Predictions are made not only of the individual fragment state distributions and of their dependence on total initial angular momentum and photon frequency, but also of the degree of correlation between the production of the different states of the two open shell fragments. The computed results agree with the available experiments, in particular the current theory is the first to agree with experimental findings in predicting a smoothly varying OH rotational state distribution. It is demonstrated that the model reproduces the frequency dependence of the absorption line shape as computed using more exact procedures. In the simple treatment presented here rotational transitions during the dissociation process are ignored, as is the detailed coupling of the electronic states in the asymptotic region. Some of the remaining disagreements between the current predictions and the experimental results are ascribed to these approximations in the model. It is hoped that the predictions of the correlated product state distributions will stimulate coincidence experiments on the photodissociation process. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Coherent phase control of the photodissociation of CH3I has been achieved by quantum mechanical interference between competing paths. The control was accomplished by exciting the parent molecules with three UV photons of frequency ω1 and one VUV photon of frequency ω3=3ω1. Varying the phase difference between the two laser beams resulted in a modulation of the I+ and CH+3 signals, without affecting the parent ion signal. We propose a mechanism in which control occurs over the photodissociation step to produce CH3+I*, followed by ionization of the neutral fragments by additional UV photons. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper describes an analytical method to the solution of semiclassical first-order, time-dependent coupled equations in the case of a three states process. The method is applied to the study of the photodissociation of the HCl molecule. The results of the semiclassical instantaneous probabilities as function of the interparticle distance are compared with quantum–quantum flux redistribution calculations [M. H. Alexander, B. Pouilly, and T. Duhoo, J. Chem. Phys. 99, 1752 (1993)]. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The results of time-dependent quantum mechanical calculations on the photodissociation of initially nonrotating HOCl and DOCl molecules are presented. Two photodissociation processes, 1 1A‘←X 1A’ and 2 1A’←X 1A’, are considered. The dynamics are treated in a two dimensional model with a fixed O–H distance and the dissociation process is followed in a rotating body-fixed frame with total angular momentum of J’=1. The total photodissociation spectra and product rotational distributions resulting from dissociation of HOCl and DOCl originating in a number of different vibrational states are presented over a continuous range of incident photon wavelengths in the range 140–440 nm. It is found that, for HOCl, the physically more correct treatment with J’=1 leads to a qualitatively different OH product rotational distribution than that resulting from the more usual model treatment in which both initial and excited states are assumed to possess zero total angular momentum. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report strong site-selective ionic fragmentation of the core-excited bromo-chloro-alkanes [Br(CH2)nCl] (n=1,...,3), using time-of-flight mass spectrometry in the multicoincidence mode and monochromatized synchrotron radiation to excite the vicinity of the bromine 3d (65–150 eV) and chlorine 2p edge (150–220 eV), respectively. We observe strong differences between the fragmentation after excitation of the two edges, where for all three molecules the bond between the excited halogen atom and the rest of the molecule is broken preferentially. This site selectivity is discussed in terms of a fast two- and three-body fragmentation which takes place after the primary Auger decay of the core vacancy but on the same time scale as further autoionization. [ABSTRACT FROM AUTHOR]

We use photoassociation of ultracold Na to study transitions from free atoms to bound molecules. We obtain rovibrational spectra of the 1g, the 0+u, and, for the first time, the 0-g ‘‘purely long-range’’ state of Na2, all of which dissociate to Na(3 2S1/2)+Na(3 2P3/2). [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of ICN to CN+I(2P3/2) are investigated by state selective one-dimensional photofragmentation translation spectroscopy at 304.67 nm. Translational energy release, laboratory anisotropy factors, and energy distributions are obtained from analysis of the velocity and spatial distributions of the photodissociated iodine atoms. Two velocity distributions peaks are deconvoluted which are found to be separated by 2000 cm-1, which is the CN stretching vibration of the CN radical. The high intensity velocity peak is assigned to dissociation to I+CN(X 2Σ+) in v=0 (channel I), while the weak lower velocity peak is attributed to dissociation to I+CN(X 2Σ+) in v=1 (channel II). More than 80% of the iodine are produced from channel I and are found to have a relatively small anisotropy parameter, β, that is independent of velocity, suggesting a mixed absorption polarization leading to rapid dissociation. The weak shoulder, representing less than 20% of the photodissociated iodine, is formed via channel II and found to have a β value that decreases with velocity and produces CN with more of the available excess energy appearing in rotation, suggesting longer dissociation time that allows for more energy redistribution prior to dissociation. The dissociation mechanisms involved in these two channels are discussed in terms of these results, the theoretically predicted properties of the 3Π+0 and 3Π1 surfaces of ICN, our previous conclusion that suggests that ICN bends prior to dissociation via channel II, the laser wavelength used, and curve crossing between the 3Π+0 and 1Π1 surfaces. [ABSTRACT FROM AUTHOR]

In this paper, we examine how vector properties of photodissociation with a coherent laser pulse evolve in time during and after the excitation process. In particular, we define and explore the transient anisotropy parameter β(t). We apply our formulation to the case of the predissociation of the NaI molecule. We predict that under certain conditions, β(t) displays a beating phenomenon, in which the system oscillates periodically between a parallel-type and a perpendicular-type angular distribution. Depending on the excitation pulse, this ‘‘vectorial’’ beating may be accompanied by a faster beating similar to the ‘‘scalar’’ vibrational beating observed in the past in the NaI system. It is shown that the transient anisotropy beating may be used to obtain experimentally the time-dependent average interfragment separation, and that different J states are characterized by different interfragment separation curves. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical treatment within the Franck–Condon and impulse approximations of direct photodissociation of polyatomic molecules sparsely physisorbed on static and UV transparent crystals. A separable local mode model is adopted for the ground state adsorbate and for the initially excited molecule in order to evaluate the multidimensional Franck–Condon excitation and impulse fragmentation amplitudes. Since physisorption forces are weak, the adsorbate excitation and decomposition processes remain intramolecular in nature. The photodissociation of surface physisorbed molecules thus differs from the corresponding gas phase process mainly because of substrate confinement on adsorbate rotation and translation degrees of freedom, which are all described by our theory. One result is that the photodissociation cross section depends on the laboratory frame observation angles. Another is the availability of fragment–substrate rearrangement channels which involve combined dissociation, fragment adsorption, and surface migration excitations. In particular, the excitation may lead to adsorbate desorption and/or migration in addition to adsorbate photofragmentation. Accordingly, the cross section contains six contributions depending on the excitation process and the fragment–substrate rearrangements. [ABSTRACT FROM AUTHOR]

We solve numerically the time-dependent Schrödinger equation to study the behavior of a molecule interacting with two phase-locked ultrashort laser pulses. The two-dimensional model used in the calculations mimics the properties of the CH3ONO molecule. The two pulses are identical except for their relative phase and are tuned to excite an upper electronic state of the molecule. After excitation the molecule predissociates and we calculate the dependence of the NO yield and of the NO vibrational population on the delay time between the pulses. Because the molecular wave functions representing the excited states created by each pulse interfere, the total product population created by the joint action of the pulses differs from the total population obtained when the two pulses act independently on the molecule. This type of experiment provides a means of using quantum interference for a modest control of photodissociation. [ABSTRACT FROM AUTHOR]

Mg+–H2O ion–molecule complexes are produced in a pulsed supersonic nozzle cluster source. These complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as 2B2←X 2A1 is observed with an origin at 28 396 cm-1. The spectrum has a prominent progression in the metal-H2O stretching mode with a frequency (ω’e) of 518.0 cm-1. An extrapolation of this progression fixes the excited state dissociation energy (D’0) at 15 787 cm-1. The corresponding ground state value (D‘0) is 8514 cm-1 (24.3 kcal/mol). The solvated bending mode, and symmetric and asymmetric stretching modes of water are also active in the complex, as are the magnesium bending modes. A second electronic transition assigned as 2B1←X 2A1 is observed with an origin at 30 267 cm-1 and a metal stretch frequency for Mg+–H2O of 488.5 cm-1 (ΔG1/2). Spectra of both excited states are also observed for Mg+–D2O. Partially resolved rotational structure is analyzed for both isotopes, leading to the conclusion that the complex has a structure with C2v symmetry. This study was guided by ab initio calculations by Bauschlicher and co-workers, which provide accurate predictions of the electronic transition energies, vibrational constants, and dissociation energies. [ABSTRACT FROM AUTHOR]

A new method proposed recently by Alexander for studying the mechanisms of inelastic collisions is adapted to the study of molecular photodissociation. This adaptation involves the determination of the current density associated with a driven photodissociation wavefunction at each fixed scattering energy, and yields a picture of how, as a function of the dissociation coordinate, the outgoing photofragment flux rises in the Franck–Condon region on absorption of the photon and how it redistributes between the available internal channels as the photofragments move apart. This picture complements the usual time-dependent picture of photodissociation, allowing one in particular to analyze the mechanism of the photodissociation in detail at each individual excitation laser frequency. A study of flux redistribution in a simple two-state model for the electronically nonadiabatic photodissociation of methyl iodide is presented as a first illustration of the approach. [ABSTRACT FROM AUTHOR]

A new probe of surface structure is presented which is based on the photodissociation of hydrogen from an adsorbate molecule. The event creates an atomic hydrogen fragment, positioned between the adsorbate layer and the solid surface. Due to its light mass, the hydrogen dynamics is quantum mechanical in nature. A useful image is of the hydrogenic wave function behaving like a liquid able to fill all cracks. The coherent character of the hydrogenic wave function is crucial in the ability of the photodissociation experiment to act as a probe. A series of case studies has been carried out whose aim is to reveal the relation between the structure of the surface and the asymptotic energy resolved angular distribution of the hydrogen fragment. The dynamics of the hydrogen atom motion was modeled by the time dependent Schrödinger equation. The cases studied include the dissociation of a single HBr adsorbate on flat and corrugated surfaces. A broad specular peak was observed, in addition to diffraction peaks which can be correlated with the corrugation. Moreover, selective adsorption peaks, which can be correlated with the attractive part of the surface potential, have been identified. Systems in which the hydrogenic wave function scatters from several adsorbates were also investigated. It was found that the scattering is dominated by the trapping of the wave function by unstable periodic orbits. The quantization rules of these periodic orbits have been identified, creating a link between the structure of the adsorbates and the asymptotic angular distributions. [ABSTRACT FROM AUTHOR]

We apply a simple model for the photodissociation absorption spectra of bent symmetric triatomic molecules to the Wulf–Chappuis band system of ozone (10 000–22 000 cm-1 ) to assign the electronic states and the diffuse vibrational bands involved. The conical intersection between the two lowest 1A‘ states is treated in an approximate way, and the role of the lowest excited triplet states is explored. The results indicate that the Wulf band is probably due to the 3A2 state of ozone which gains intensity through spin–orbit coupling. The 1 1A‘ (1A2) state gives rise to the featureless red wing of the Chappuis band. Most of the structure in the Chappuis band is reproduced in the model and is due to the 2 1A‘ (1B1) state as was previously supposed. A more complete treatment of the conical intersection and nonadiabatic effects will be necessary to quantitatively reproduce all features of the experimental spectra. [ABSTRACT FROM AUTHOR]

The influence of parent molecule bending motion on nonadiabatic transitions during photodissociation is investigated using a model involving two linear excited-state surfaces. The two dissociative states are strongly coupled near the so-called conical intersection. Due to symmetry constraints, the two adiabatic surfaces are not allowed to interact in linear configurations and the coupling is only switched on when the molecule is bent. Therefore, electronic transitions from one adiabatic surface to another are only possible when the dissociation process involves bending motion. A quantum-mechanical model including both the dissociation and bending degrees of freedom is established to assess the importance of parent bending motion on nonadiabatic transition dynamics. The coupling between the two electronic states is explicitly taken into account and the dissociation dynamics is described by a time-dependent wave packet. Although the approach is applicable to any triatomic molecular systems, we choose to model a pseudotriatomic system, i.e., methyl iodide CX3I (X=H, D, etc.). In our calculations, the three X atoms are treated as a single pseudoatom and the C–X3 umbrella bending coordinate is frozen at its equilibrium geometry. The two dynamically active coordinates are the I–CX3 stretch and the I–C–X3 bend. Both the ground and two excited states are represented by linear potential-energy surfaces and the coupling of the two dissociative surfaces is a conical intersection in nature. Several dissociation processes are modeled with different initial bending wave functions and different isotopic substitutions. It is found that parent bending motion has a significant effect on the final electronic branching of dissociation fragments. The calculation generates a larger I* yield from the CD3I dissociation than that from CH3I, in agreement with experimental observations which could not be reproduced by previous theoretical calculations. Our... [ABSTRACT FROM AUTHOR]

The two step photodissociation (1 1Σ+g→1 1Σ+u→1 1Πg) transition in Li2 is studied by a coupled equations method. The first transition is assumed weak and selects particular rovibrational states as intermediate states. The second transition is induced by a high intensity infrared laser. The photodissociation yield for different isotopes is investigated as well as the coherence effects in the excited intermediate states produced by the intense field. Stabilization of dressed molecular states and laser induced alignment effects are shown to occur in the photodissociation at high intensities of the second laser. The effect of the polarization of the lasers on the differential cross sections is also discussed. [ABSTRACT FROM AUTHOR]

A variational approach for calculating the cross section of the photodissociation process of triatomic molecules is put forth as a generalization of a formulation used previously for fully three-dimensional calculations of transition probabilities for the reaction H2+H→H+H2 and the rovibrational spectrum of H+3. It is based upon the generator coordinate method and the hyperspherical coordinates and the evaluation of the scattering wave function employs the R-matrix theory. [ABSTRACT FROM AUTHOR]