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The cyclic HCl–(H2O)3 tetramer is the largest observed neutral HCl–(H2O)n cluster. The vibrational predissociation of HCl–(H2O)3 is investigated by theory, quasiclassical trajectory (QCT) calculations, and experiment, following the infrared excitation of the hydrogen-bonded OH-stretch fundamental. The energetically possible dissociation pathways are HCl + (H2O)3 (Pathway 1) and H2O + HCl–(H2O)2 (Pathway 2). The HCl and H2O monomer fragments are observed by 2 + 1 resonance enhanced multiphoton ionization combined with time-of-flight mass spectrometry, and their rotational energy distributions are inferred and compared to the theoretical results. Velocity map images of the monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions obtained by experiment and QCT calculations are broad and structureless, encompassing the entire range of allowed speeds for each pathway. Bond dissociation energies, D0, are estimated experimentally from the endpoints of the speed distributions: 2100 ± 300 cm−1 and 2400 ± 100 cm−1 for Pathway 1 and Pathway 2, respectively. These values are lower but in the same order as the corresponding calculated D0: 2426 ± 23 cm−1 and 2826 ± 19 cm−1. The differences are attributed to contributions from vibrational hot bands of the clusters that appear in the high-speed tails of the experimental pair-correlated distributions. Satisfactory agreement between theory and experiment is achieved when comparing the monomer fragments’ rotational energies, the shapes of the speed distributions, and the average fragment speeds and center-of-mass translational energies. Insights into the dissociation mechanism and lifetime are gained from QCT calculations performed on a previously reported many-body potential energy surface. It is concluded that the dissociation lifetime is on the order of 10 ps and that the final trimer products are in their lowest energy cyclic forms. [ABSTRACT FROM AUTHOR]

The state-specifc vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-acetylene dimer were studied. Ammonia fragments were detected following vibrational excitation of Ch stretch, the vibrational state distributions show distinct state specificity among channels with low translational energy release and the calculations show that dissociation takes place from bent geometries.

Core sampling time-of-flight method and infrared ultraviolet (IR+UV) double resonance ionization spectroscopy were used to investigate the photodissociation of the hydroxymethyl radical in excited vibrational levels of the Rydberg 3p(sub z) state. Lifetime broadening was observed for all levels in the 3p(sub z) state and the linewidth increased gradually with excitation energy from 11 cm for the origin band to 35 cm for the first overtone of the OH stretch.

Time-sliced velocity map imaging (SVMI), a high-resolution method for measuring kinetic energy distributions of products in scattering and photodissociation reactions, is challenging to implement for atomic hydrogen products. We describe an ion optics design aimed at achieving SVMI of H fragments in a broad range of kinetic energies (KE), from a fraction of an electronvolt to a few electronvolts. In order to enable consistently thin slicing for any imaged KE range, an additional electrostatic lens is introduced in the drift region for radial magnification control without affecting temporal stretching of the ion cloud. Time slices of ∼5 ns out of a cloud stretched to >=50 ns are used. An accelerator region with variable dimensions (using multiple electrodes) is employed for better optimization of radial and temporal space focusing characteristics at each magnification level. The implemented system was successfully tested by recording images of H fragments from the photodissociation of HBr, H2S, and the CH2OH radical, with kinetic energies ranging from <0.4 eV to >3 eV. It demonstrated KE resolution <=1%-2%, similar to that obtained in traditional velocity map imaging followed by reconstruction, and to KE resolution achieved previously in SVMI of heavier products. We expect it to perform just as well up to at least 6 eV of kinetic energy. The tests showed that numerical simulations of the electric fields and ion trajectories in the system, used for optimization of the design and operating parameters, provide an accurate and reliable description of all aspects of system performance. This offers the advantage of selecting the best operating conditions in each measurement without the need for additional calibration experiments. [ABSTRACT FROM AUTHOR]

The bond dissociation energy (D0) of the water dimer is determined by using state-to-state vibrational predissociation measurements following excitation of the bound OH stretch fundamental of the donor unit of the dimer. Velocity map imaging and resonance-enhanced multiphoton ionization (REMPI) are used to determine pair-correlated product velocity and translational energy distributions. H2O fragments are detected in the ground vibrational (000) and the first excited bending (010) states by 2 + 1 REMPI via the C 1B1 (000) ← X 1A1 (000 and 010) transitions. The fragments' velocity and center-of-mass translational energy distributions are determined from images of selected rovibrational levels of H2O. An accurate value for D0 is obtained by fitting both the structure in the images and the maximum velocity of the fragments. This value, D0 = 1105 ± 10 cm-1 (13.2 ± 0.12 kJ/mol), is in excellent agreement with the recent theoretical value of D0 = 1103 ± 4 cm-1 (13.2 ± 0.05 kJ/mol) suggested as a benchmark by Shank et al. [J. Chem. Phys. 130, 144314 (2009)]. [ABSTRACT FROM AUTHOR]

Molecular orbital theory and calculations are used to describe the ultraviolet singlet excited states of NO dimer. Qualitatively, we derive and catalog the dimer states by correlating them with monomer states, and provide illustrative complete active space self-consistent field calculations. Quantitatively, we provide computational estimates of vertical transition energies and absorption intensities with multireference configuration interaction and equations-of-motion coupled-cluster methods, and examine an important avoided crossing between a Rydberg and a valence state along the intermonomer and intramonomer stretching coordinates. The calculations are challenging, due to the high density of electronic states of various types (valence and Rydberg, excimer and charge transfer) in the 6–8 eV region, and the multiconfigurational nature of the ground state. We have identified a bright charge-transfer (charge-resonance) state as responsible for the broadband seen in UV absorption experiments. We also use our results to facilitate the interpretation of UV photodissociation experiments, including the time-resolved 6 eV photodissociation experiments to be presented in the next two papers of this series. [ABSTRACT FROM AUTHOR]

The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical, CH2OH, are reported in the region of the second and third overtones, which is above the thermochemical threshold to dissociation to H+CH2O (D0=9600 cm-1). The second overtone spectrum at 10 484 cm-1 is obtained by double resonance IR-UV resonance enhanced multiphoton ionization (REMPI) spectroscopy via the 3pz electronic state. It is rotationally resolved with a linewidth of 0.4 cm-1 and displays properties of local-mode vibration. No dissociation products are observed. The third overtone spectra of CH2OH and CD2OH are observed at ∼13 600 cm-1 by monitoring H-atom photofragments while scanning the excitation laser frequency. No double resonance REMPI spectrum is detected, and no D fragments are produced. The spectra of both isotope analogs can be simulated with a linewidth of 1.3 cm-1, indicating dissociation via tunneling. By treating the tunneling as one dimensional and using the calculated imaginary frequency, the barrier to dissociation is estimated at about 15 200 cm-1, in good agreement with theoretical estimations. The Birge-Sponer plot is linear for OH-stretch vibrations 1ν1–4ν1, demonstrating behavior of a one-dimensional Morse oscillator. The anharmonicity parameter derived from the plot is similar to the values obtained for other small OH containing molecules. Isomerization to methoxy does not contribute to the predissociation signal and the mechanism appears to be direct O–H fission via tunneling. CH2OH presents a unique example in which the reaction coordinate is excited directly and leads to predissociation via tunneling while preserving the local-mode character of the stretch vibration. [ABSTRACT FROM AUTHOR]

Photodissociation studies of the CH[sub 2]OD radical in the region 28 000–41 000 cm-1 (357–244 nm), which includes excitation to the 3s, 3p[sub x], and 3p[sub z] states, are reported. H and D photofragments are monitored by using resonance-enhanced multiphoton ionization (REMPI) from the onset of H formation at ∼30 500 cm-1 to the origin band region of the 3p[sub z]([sup 2]A[sup ″])←1 [sup 2]A[sup ″] transition at 41 050 cm-1. Kinetic energy distributions P(E[sub T]) and recoil anisotropy parameters as a function of kinetic energy, β[sub eff](E[sub T]), are determined by the core sampling technique for the channels producing H and D fragments. Two dissociation channels are identified: (I) D+CH[sub 2]O and (II) H+CHOD. The contribution of channel II increases monotonically as the excitation energy is increased. Based on the calculations of Hoffmann and Yarkony [J. Chem. Phys. 116, 8300 (2002)], it is concluded that conical intersections between 3s and the ground state determine the final branching ratio even when initial excitation accesses the 3p[sub x] and 3p[sub z] states. The different β[sub eff] values obtained for channels I and II (-0.7 and ∼0.0, respectively) are attributed to the different extents of out-of-plane nuclear motions in the specific couplings between 3s and the ground state (of A[sup ′] and A[sup ″] symmetry, respectively) that lead to each channel. The upper limit to the dissociation energy of the C–H bond, determined from P(E[sub T]), is D[sub 0](C–H)=3.4±0.1 eV (79±2 kcal/mol). Combining this value with the known heats of formation of H and CH[sub 2]OD, the heat of formation of CHOD is estimated at ΔH[sub f][sup 0](CHOD)=24±2 kcal/mol. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation of the deuterated hydroxymethyl radical CH[SUB2]OD is investigated on the lowest excited state, the 3s Rydberg state, in the wavelength region 365-318 nm where the D atom is the only significant product. The translational energy distribution and kinetic energy-dependent anisotropy parameter of the D channel are determined by the core-sampling time-of-flight technique at 352.5 nm. The negative recoil anisotropy parameter β[SUBeff)=-0.7±0.1 is consistent with the perpendicular nature of the transition from the ground state. The fraction of the available energy partitioned into the translational degree of freedom is 0.69. Ground state products CH[SUB2]O (1 [SUP1]A[SUB1]) +D constitute the main photodissociation channel, and no significant H product is detected at these wavelengths. Comparison with the conical intersection calculations of Hoffman and Yarkony suggests that O-D bond rupture involves crossing from the 3s potential energy surface to a repulsive region of the ground state surface at a large O-D bond distance. Isomerization of CH[SUB2]OD to CH[SUB2]DO is not competitive with the fast O-D dissociation. [ABSTRACT FROM AUTHOR]

A strong enhancement of absorption to the lowest [SUP2]A[SUB1] state is observed for vibrationally excited)chloromethyl radicals. It is demonstrated that this enhancement is due to a significant increase in both electronic and vibrational Franck-Condon factors. Electronic structure calculations of potential energy surfaces (PESs) and transition dipole moments for the ground and the two lowest excited states of A[SUB1] (symmetry, the 1 [SUP2]A[SUB1] valence and 2 [SUP2]A[SUB1] Rydberg states, reveal the origin of this effect.)The shelflike shape of the 1 [SUP2]A[SUB1] PES in the Franck-Condon region and the strong dependence of) the electronic transition dipole moment on C-Cl distance are responsible for the enhancement. Analysis of the shape of the electron density distribution demonstrates that Rydberg-valence interaction in the two lowest excited states causes the changes in the shape of PESs and transition dipoles with C-Cl distance. [ABSTRACT FROM AUTHOR]

The absorption spectrum and photodissociation dynamics of the hydroxymethyl radical via its two lowest excited electronic states, 3s and 3p[sub χ], are investigated in a supersonic molecular beam by the depletion, resonance enhanced multiphoton ionization, and photofragrnent yield spectroscopy methods. The measured origins of the electronic transitions to the 3s and 3p[sub χ] states agree with the most recent ab initio calculations. The vibronic bands of the 2 ²A'(3p[sub χ]) ← 1 ²A' transition are much broader than those of the transition terminating in the 2 ²A' (3p[sub z]) state, while the transition to the 1 ²A'(3s) state appears structureless. The investigation of the deuterated analog CH[sub 2]OD shows that near the onset of the transition to the 3s state, only the O-D bond fission pathway is important, while both H and D products are detected following excitation to the 3p[sub χ] state. The progressive broadening of the absorption features from the uppermost 3p[sup z] to the lowest 3s excited state is explained based on recent calculations of surface couplings to lower electronic states. These couplings also control the photodissociation dynamics and the reaction outcomes. [ABSTRACT FROM AUTHOR]

The electronic spectroscopy and photodissociation dynamics of the 1-hydroxyethyl, C[H.sub.3]CHOH radical in the region of 515-270 nm is described. The isotopologs of 1-hydroxyethyl leads to two major product channels of acetaldehyde and the vinyl alcohol (enol).

The vibronic structure and normal-mode frequencies of the ground and excited Rydberg states of diazomethane and its isotopologs and of the corresponding cations are examined. Many differences are observed in the behavior of the in-plane and out-of-plane modes within the CNN framework and the C[H.sub.2] moiety.

The electronic states of diazomethane are characterized by ab initio calculations and the electronic transitions are examined by using a combination of 2+1 REMPI spectroscopy and photoelectron imaging in a molecular beam. The analysis of the photoelectron spectra of the excited states has described the interactions between the Rydberg electrons with the ion core and has disclosed the mixings with valence states.

The observations of rotationally resolved OH- and CH-stretch vibrational transitions in CH2OH obtained in a molecular beam by using double resonance ionization detected IR (DRID-IR) spectroscopy are reported. Rotationally resolved IR spectra are obtained in the region of the fundamental CH symmetric stretch (v3), CH asymmetric stretch (v2), OH stretch(v1) and the OH stretch overtone (2v1).

We develop a procedure to determine excited states potentials from photofragmentation data using adiabatic energy curves. The adiabatic curves, obtained from empirically derived diabatic channel potentials, are inverted to yield a polyatomic potential energy surface which is expressed as a piecewise collection of diatomic cuts. The procedure is applied to obtaining a two-dimensional potential of the FNO molecule in the S1 excited state. The potential is more successful than available ab initio surfaces in reproducing the observed asymmetric (Fano) line shapes of the FNO S1←S0 state-specific photofragment yield spectra. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Unimolecular decay processes are studied in the regime of overlapping resonances with the goal of elucidating how unimolecular reaction rates depend on resonances widths (the imaginary part of the Siegert eigenvalues). As illustrated analytically for one-dimensional models and numerically for a more general random matrix version of Feshbach’s optical model, transition state theory (TST, Rice–Ramsperger–Kassel–Marcus) provides the correct average unimolecular decay rate whether the resonances are overlapping or not. For all studied cases, the explicit ‘‘universal’’ dependence of the TST average rate on the average resonance width (for a given energy, or an energy interval) is that of a saturation curve: in the regime of nonoverlapping resonances (i.e., weak coupling) the standard relation ‘‘unimolecular decay rate=resonance width /h’’ holds, but as the resonance overlap increases (strong coupling) the rate saturates, becoming practically independent of the average resonance width in the strong overlapping limit. On the basis of these conclusions, a discussion of what has been or can be measured in experiments of unimolecular decay that relates to the average decay rate and to the resonance widths is given. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study experimentally and theoretically reflection-type structures in the rotational distributions of NO following the photodissociation of FNO via excitation of the S1 state. Exciting quasibound states with zero quanta of bending vibration in the FNO(S1) state yields Gaussian-type rotational distributions, while excitation of states with one bending quantum leads to bimodal distributions. In the latter case, the ratio of the two intensity maxima depends on the number of NO stretching quanta in the S1 state. The accompanying calculations employing a three-dimensional ab initio potential energy surface for the S1 state of FNO are performed in the time-dependent wave packet approach. They reproduce the main features of the experimental distributions, especially the bimodality. The analysis of two-dimensional calculations for a frozen NO bond distance shows that the final rotational state distributions can be explained as the result of a dynamical mapping of the stationary wave function on the transition line onto the fragment rotational quantum number axis. Here the transition line is defined as the line which separates the inner part of the FNO(S1) potential energy surface from the strongly repulsive F+NO product channel. [ABSTRACT FROM AUTHOR]

Final state-selected spectra in unimolecular decomposition are obtained by a random matrix version of Feshbach’s optical model. The number of final states which are independently coupled to the molecular quasibound states is identified with the number of states at the dividing surface of transition state theory (TST). The coupling of the transition state to the molecular complex is modeled via a universal random matrix effective Hamiltonian which is characterized by its resonance eigenstates and provides the correct average unimolecular decay rate. The transition from nonoverlapping resonances which are associated with isolated Lorentzian spectral peaks, to overlapping resonances, associated with more complex spectra, is characterized in terms of deviations from a χ2-like distribution of the resonance widths and the approach to a random phase-distribution of the resonance scattering amplitudes. The evolution of the system from a tight transition state to reaction products is treated explicitly as a scattering process where specific dynamics can be incorporated. Comparisons with recently measured final state-selected spectra and rotational distributions for the unimolecular reaction of NO2 show that the present model provides a useful new approach for understanding and interpreting experimental results which are dominated by overlapping resonances. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]