Laser pulses that act on fragile samples often alter them irreversibly, motivating single-pulse data collection. Amorphous solid water (ASW) is a good example. In addition, neither well-defined paths for molecules to travel through ASW nor sufficiently small samples to enable molecular dynamics modeling have been achieved. Combining nanoimprint lithography and photoinitiation overcomes these obstacles. An array of gold nanoparticles absorbs pulsed (10 ns) 532 nm radiation and converts it to heat, and doped ASW films grown at about 100 K are ejected from atop the irradiated nanoparticles into vacuum. The nanoparticles are spaced from one another by sufficient distance that each acts independently. Thus, a temporal profile of ejected material is the sum of about 106 “nanoexperiments,” yielding high single-pulse signal-to-noise ratios. The size of a single nanoparticle and its immediate surroundings is sufficiently small to enable modeling and simulation at the atomistic (molecular) level, which has not been feasible previously. An application to a chemical system is presented in which H/D scrambling is used to infer the presence of protons in films composed of D2O and H2O (each containing a small amount of HDO contaminant) upon which a small amount of NO2 has been deposited. The pulsed laser heating of the nanoparticles promotes NO2/N2O4 hydrolysis to nitric acid, whose protons enhance H/D scrambling dramatically.

This Review summarizes recent research on vibrational predissociation (VP) of hydrogen-bonded clusters. Specifically, the focus is on breaking of hydrogen bonds following excitation of an intramolecular vibration of the cluster. VP of the water dimer and trimer, HCl clusters, and mixed HCl–water clusters are the major topics, but related work on hydrogen halide dimers and trimers, ammonia clusters, and mixed dimers with polyatomic units are reviewed for completion and comparison. The theoretical focus is on generating accurate potential energy surfaces (PESs) that can be used in detailed dynamical calculations, mainly using the quasiclassical trajectory approach. These PESs have to extend from the region describing large amplitude motion around the minimum to regions where fragments are formed. The experimental methodology exploits velocity map imaging to generate pair-correlated product translational energy distributions from which accurate bond dissociation energies of dimers and trimers and energy disposal in fragments are obtained. The excellent agreement between theory and experiment on bond dissociation energies, energy disposal in fragments, and the contributions of cooperativity demonstrates that it is now possible, with state-of-the-art experimental and theoretical methods, to make accurate predictions about dynamical and energetic properties of dissociating clusters.

The vibrational predissociation of the HCl–(H2O)3 tetramer, the largest HCl–(H2O)n cluster for which HCl is not predicted to be ionized, is reported. This work focuses on the predissociation pathway giving rise to H2O + HCl–(H2O)2 following IR laser excitation of the H-bonded OH stretch fundamental. H2O fragments are monitored state selectively by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) combined with time-of-flight mass spectrometry (TOF-MS). Velocity map images of H2O in selected rotational levels are used to determine translational energy distributions from which the internal energy distributions in the pair-correlated cofragments are derived. From the maximum translational energy release, the bond dissociation energy, D0 = 2400 ± 100 cm–1, is determined for the investigated channel. The energy distributions in the fragments are broad, encompassing the entire range of allowed states. The importance of cooperative (nonpairwise) interactions is discussed.

Molecular transport and morphological change were examined in films of amorphous solid water (ASW). A buried N2O4 layer absorbs pulsed 266 nm radiation, creating heated fluid. Temperature and pressure gradients facilitate the formation of fissures through which fluid travels to (ultrahigh) vacuum. Film thickness up to 2400 monolayers was examined. In all cases, transport to vacuum could be achieved with a single pulse. Material that entered vacuum was detected using a time-of-flight mass spectrometer that recorded spectra every 10 μs. An ASW layer insulated the N2O4 layer from the high-thermal-conductivity MgO substrate; this was verified experimentally and with heat-transfer calculations. Laser-heated fluid strips water from fissure walls throughout its trip to vacuum. Experiments with alternate H2O and D2O layers reveal efficient isotope scrambling, consistent with water reaching vacuum via this mechanism. It is likely that ejected water undergoes collisions just above the film surface due to the high density of material that reaches the surface via fissures, as evidenced by complex temporal profiles extending past 1 ms. Little material enters vacuum after cessation of the 10 ns pulse because cold ASW near the film surface freezes material that is no longer being heated. A proposed model is in accord with the data.

We achieve excellent agreement on D0 between theory and experiments for all of the clusters that we have compared, as well as for cooperativity in ring trimers of water and HCl. We also show that both the long-range and the repulsive parts of the potential must be involved in bond breaking. We explain why H-bonds are so resilient and hard to break, and we propose that a common motif in the breaking of cyclic trimers is the opening of the ring following transfer of one quantum of stretch excitation to form open-chain structures that are weakly bound. However, it still takes many vibrational periods to release one monomer fragment from the open-chain structures. Our success with water and HCl dimers and trimers led us to embark on a more ambitious project: studies of mixed water and HCl small clusters. These clusters eventually lead to ionization of HCl and serve as prototypes of acid dissociation in water. Measurements and calculations of such ionizations are yet to be achieved, and we are now characterizing these systems by adding monomers one at a time. We describe our completed work on the HCl–H2O dimer and mention our recent theoretical results on larger mixed clusters.

The breaking of hydrogen bonds in molecular systems has profound effects on liquids, e.g., water, biomolecules, e.g., DNA, etc., and so it is no exaggeration to assert the importance of these bonds to living systems. However, despite years of extensive research on hydrogen bonds, many of the details of how these bonds break and the corresponding energy redistribution processes remain poorly understood. Here we report extensive experimental and theoretical insights into the breakup of two or three hydrogen bonds in the dissociation of a paradigm system of a hydrogen-bonded network, the ring HCl trimer. Experimental state-to-state vibrational predissociation dynamics of the trimer following vibrational excitation were studied by using velocity map imaging and resonance-enhanced multiphoton ionization, providing dissociation energies and product state distributions for the trimer’s breakup into three separate monomers or into dimer + monomer. Accompanying the experiments are high-level calculations using diffusion Monte Carlo and quasiclassical simulations, whose results validate the experimental ones and further elucidate energy distributions in the products. The calculations make use of a new, highly accurate potential energy surface. Simulations indicate that the dissociation mechanism requires the excitation to first relax into low-frequency motions of the trimer, resulting in the breaking of a single hydrogen bond. This allows the system to explore a critical van der Waals minimum region from which dissociation occurs readily to monomer + dimer.

Rotational, vibrational, and electronic states of formaldehyde and cis-hydroxymethylene products generated in the photodissociation of the hydroxymethyl radical are investigated by sliced velocity map imaging (SVMI) following excitation of the radical to its 3px and 3pz Rydberg states. SVMI of H and D photofragments is essential in these studies because it allows zooming in on low-velocity regions of the images where small threshold signals can be identified. With CH2OD precursors, formaldehyde and hydroxymethylene products are examined separately by monitoring D and H, respectively. Whereas the main dissociation channels lead to formaldehyde and cis-hydroxymethylene in their ground electronic states, at higher excitation energies the kinetic energy distributions (KEDs) of H and D photofragments exhibit additional small peaks, which are assigned as triplet states of formaldehyde and hydroxymethylene. Results obtained with deuterated isotopologs of CH2OH demonstrate that the yield of the triplet state of formaldehyde decreases upon increasing deuteration, suggesting that the conical intersection seams that govern the dynamics depend on the degree of deuteration. The rotational excitation of cis-hydroxymethylene depends on the excited Rydberg state of CH2OD and is lower in dissociation via the 3pz state than via the lower lying 3px and 3s states. Vibrational excitation of cis-HCOD, which spans the entire allowed internal energy range, consists mostly of the CO-stretch and in-plane bend modes. When the internal energy of cis-HCOD exceeds the dissociation threshold to D + HCO, slow D and H photofragments deriving from secondary dissociation are observed. The yields of these H and D fragments are comparable, and we propose that they are generated via prior isomerization of cis-HCOD to HDCO.

•State-to-state vibrational predissociation dynamics of hydrogen bonded clusters.•Accurate bond dissociation energies and pair-correlated product energy distributions.•Contribution of cooperative interactions to the H-bonding network.•Comparisons with quasi-classical trajectory calculations and phase space theory.This letter presents a brief overview of our recent experimental studies of state-to-state vibrational predissociation (VP) dynamics of small hydrogen bonded (H-bonded) clusters following vibrational excitation. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) are used to determine accurate bond dissociation energies (D0) of (H2O)2, (H2O)3, HCl–H2O and NH3–H2O. Pair-correlated product energy distributions from the VP of these complexes are also presented and compared to theoretical models. Further insights into mechanisms are obtained from the recent quasi-classical trajectory (QCT) calculations of Bowman and coworkers. The D0 values for (H2O)2 and (H2O)3 are in very good agreement with recent calculated values, and the results are used to estimate the contributions of cooperative interactions to the H-bonding network.

The photodissociation dynamics of the hydroxymethyl radical (CH2OH, CH2OD, and CD2OD) following excitation to the 3s and 3px Rydberg states is studied using time-sliced velocity map imaging of hydrogen photofragments. Dissociation takes place on the ground potential energy surface reached via conical intersections from the excited states, and formaldehyde and hydrxymethylene are identified as reaction products. The major product, formaldehyde, has a bimodal internal energy distribution. The largest fraction has high kinetic energy (KE), modest rotational excitation, and vibrational excitation mainly in the CO stretch and the CH(D)2 deformations modes (scissors, wag, and rock). The minor fraction has lower KEs and a higher rovibrational excitation that is unresolved. A bimodal internal energy distribution in the formaldehyde fragment has been predicted by Yarkony [J. Chem. Phys. 2005, 122, 084316] for a conical intersection along the O–H bond coordinate. The hydroxymethylene product state distributions depend strongly on the nature of the excited state. In dissociation via the 3s state, the hydroxymethylene products have broad rovibrational state distributions and are produced at low yield. As suggested by Yarkony, they may be produced in the same dissociation events that give rise to low KE formaldehyde. In these events, the bound region of the PES is sampled following the conical intersection along O–H(D). The hydroxymethylene yield is low near its threshold and increases slowly with excitation energy to the 3s state, but its internal energy distribution remains broad and the contributions of the cis- and trans-isomers cannot be resolved. The mechanism changes markedly when exciting to the 3px state. The hydroxymethylene products have less rotational excitation and show separate contributions of cis- and trans-isomers. The trans-isomer is found to be a minor product relative to the higher-energy cis-isomer, as predicted by Yarkony for conical intersections along the C–H coordinate. It appears that the efficiency of dissociation via conical intersections along the O–H and C–H coordinates depends on the initial excited state. While the O–H conical intersection seam (vertical cone) provides an efficient route to the ground state following excitation via the 3s or the 3px Rydberg states, conical intersections along the C–H bond coordinate (tilted cone) are sampled more efficiently via 3px excitation and proceed through different dynamics. The energy separations between formaldehyde and hydroxymethylene and between the cis- and trans-isomers of hydroxymethylene are determined experimentally for all the investigated isotopologs and are in good agreement with theory.

We report a joint experimental-theoretical study of the predissociation dynamics of the water trimer following excitation of the hydrogen bonded OH-stretch fundamental. The bond dissociation energy (D0) for the (H2O)3 → H2O + (H2O)2 dissociation channel is determined from fitting the speed distributions of selected rovibrational states of the water monomer fragment using velocity map imaging. The experimental value, D0 = 2650 ± 150 cm–1, is in good agreement with the previously determined theoretical value, 2726 ± 30 cm–1, obtained using an ab initio full-dimensional potential energy surface (PES) together with Diffusion Monte Carlo calculations [Wang; Bowman. J. Chem. Phys. 2011, 135, 131101]. Comparing this value to D0 of the dimer places the contribution of nonpairwise additivity to the hydrogen bonding at 450–500 cm–1. Quasiclassical trajectory (QCT) calculations using this PES help elucidate the reaction mechanism. The trajectories show that most often one hydrogen bond breaks first, followed by breaking and re-forming of hydrogen bonds (often with different hydrogen bonds breaking) until, after many picoseconds, a water monomer is finally released. The translational energy distributions calculated by QCT for selected rotational levels of the monomer fragment agree with the experimental observations. The product translational and rotational energy distributions calculated by QCT also agree with statistical predictions. The availability of low-lying intermolecular vibrational levels in the dimer fragment is likely to facilitate energy transfer before dissociation occurs, leading to statistical-like product state distributions.

The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (D0) of (D2O)2. The value obtained, 1244 ± 10 cm–1 (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm–1 (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H2O)2. In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H2O)2 and (D2O)2 were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking. A minor channel is also observed in which both fragments are in the ground vibrational state and are highly rotationally excited. The theoretical calculations reveal equal probability of bending excitation in the donor and acceptor subunits, which is a result of interchange of donor and acceptor roles. The rotational distributions associated with the major channel, in which one water fragment has one quantum of bend, and the minor channel with both water fragments in the ground vibrational state are calculated and are in agreement with experiment.

Thin films composed of 400–500 monolayers (ML) of either amorphous solid water (ASW) or ASW/CO2 mixtures are grown atop a MgO(100) substrate under ultrahigh vacuum conditions. Samples are irradiated at an infrared frequency of 3424 cm–1, which lies within the broad OH stretch band of condensed water. Ablation is achieved using 10 ns pulses whose energy (<2.7 mJ) is focused to a beam waist of approximately 0.5 mm. By using a time-of-flight mass spectrometer to monitor ablated material, excellent single-shot detection is demonstrated. This capability is essential because, in general, the first infrared pulse can induce irreversible changes throughout the irradiated volume. With ASW/CO2 samples, CO2 is released preferentially. This is not surprising in light of the metastability of the samples. Indeed, repeated irradiation of the same spot can rid the sample of essentially all of the CO2 in as little as a few pulses, whereas only 10–20 ML of H2O are removed per pulse. The influence of the substrate is profound. It cools the sample efficiently because the characteristic time for heat transfer to the substrate is much less than the infrared pulse duration. This creates temperature gradients, thereby quenching processes such as explosive boiling (phase explosion) and the heterogeneous nucleation of cavities that take place at lower depths in significantly thicker samples, i.e., with sufficient inertial confinement. This efficient quenching accounts for the fact that only 10–20 ML of H2O are removed per pulse. The presence of small protonated water cluster ions in the mass spectra is interpreted as evidence for the trivial fragmentation mechanism examined assiduously by Lewis and co-workers. Mixed samples such as ASW/CO2, where species segregation plays a pivotal role, add interesting and potentially useful dimensions to the ablation phenomenon.

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded HCl−H2O dimer was studied following excitation of the dimer’s HCl stretch by detecting the H2O fragment. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, H2O fragments were detected by 2 + 1 REMPI via the C̃ 1B1 (000) ← X̃ 1A1 (000) transition. REMPI spectra clearly show H2O from dissociation produced in the ground vibrational state. The fragments’ center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational states of H2O and were converted to rotational state distributions of the HCl cofragment. The distributions were consistent with the previously measured dissociation energy of D0 = 1334 ± 10 cm−1 and show a clear preference for rotational levels in the HCl fragment that minimize translational energy release. The usefulness of 2 + 1 REMPI detection of water fragments is discussed.

Experimental observations of D fragments from the predissociation of rovibrationally excited partially deuterated 2-hydroxyethyl radicals, CD2CD2OH, are reported, and possible dissociation channels are analyzed by theory. The radicals are produced by photolysis of 2-bromoethanol at 202−215 nm, and some of them have sufficient internal energy to predissociate. D fragments are detected by 1 + 1′ REMPI and their TOF distributions are determined. They can be associated with vinyl alcohol and/or acetaldehyde cofragments. From analysis of the maximum velocities and kinetic energies of the observed D fragments it is concluded that they originate from the decomposition of CD2CD2OH, but the experimental resolution is insufficient to distinguish between the two possible channels leading to D products. Theoretical analysis and RRKM calculations of microcanonical dissociation rates and branching ratios for the range of available excess energies (up to 5000−8000 cm−1 above the OH + C2D4 threshold) indicate that the D-producing channels are minor (about 1%) compared to the predominant OH + C2D4 channel, and the branching ratio for D production is more favorable when the reactant radicals have low rotational energy. The vinyl alcohol channel is strongly favored over the acetaldehyde channel at all excess energies, except near the threshold of these channels.

The state-to-state vibrational predissociation dynamics of the hydrogen-bonded HCl−H2O dimer were studied following excitation of the HCl stretch of the dimer. Velocity-map imaging and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, HCl fragments were detected by 2 + 1 REMPI via the f 3Δ2 ← X 1Σ+ and V 1Σ+ ← X 1Σ+ transitions. REMPI spectra clearly show HCl from dissociation produced in the ground vibrational state with J′′ up to 11. The fragments’ center-of-mass translational energy distributions were determined from images of selected rotational states of HCl and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well when using a dimer dissociation energy of D0 = 1334 ± 10 cm−1. The rotational distributions in the water cofragment pair-correlated with specific rotational states of HCl appear nonstatistical when compared to predictions of the statistical phase space theory. A detailed analysis of pair-correlated state distributions was complicated by the large number of water rotational states available, but the data show that the water rotational populations increase with decreasing translational energy.

Multiphoton ionization and dissociation processes in diazirine have been studied experimentally via 304−325 nm two-photon absorption and theoretically by using the EOM-CCSD and B3LYP methods. The electronic structure calculations identified two excited valence states and four Rydberg states in the region 4.0−8.5 eV. In one-photon excitation, the strongest absorption is to the 21A1(3px ← n) Rydberg state, whereas in two-photon absorption at comparable energies the first photon excites the low-lying 11B2 (π* ← n) valence state, from which the strongest absorption is to the dissociative valence 11A2 (π* ← σNN) state. The diazirine ion is calculated to be rather unstable, with a binding energy of only 0.73 eV and a geometry that resembles a weakly bound CH2+···N2 complex. In the experimental studies, resonance-enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH2+ ions from dissociative photoionization. It is proposed that weak one-photon absorption to the 11B2 state is immediately followed by more efficient absorption of another photon to reach the 11A2 state from which competition between ionization and fast dissociation takes place. Strong signals of CH+ ions are also detected and assigned to 2 + 1 REMPI via the D2Π (v′ = 2) ← ← X2Π (v′′ = 0) two-photon transition of CH fragments. Velocity map CH+ images show that CH(X, v′′ = 0, N′′) fragments are born with substantial translational energy, indicating that they arise from absorption of two photons in diazirine. It is argued that two-photon processes via the 11B2 intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the 11A2 state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN2. In agreement with other theoretical papers, we recommend revisions of the heats of formation of diazirine and diazomethane.

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia−water dimer were studied following excitation of the bound OH stretch. Velocity-map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the bound OH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B̃1E′′ ← X̃1A1′ transition. The REMPI spectra show that NH3 is produced with one and two quanta of the symmetric bend (ν2 umbrella mode) excitation, as well as in the ground vibrational state. Each band is quite congested, indicating population in a large number of rotational states. The fragments’ center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with zero, one, or two quanta in ν2 and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well by using a dimer dissociation energy of D0 = 1538 ± 10 cm−1. The rotational state distributions in the water cofragment pair-correlated with specific rovibrational states of ammonia are broad and include all the JKaKc states allowed by energy conservation. The rotational populations increase with decreasing c.m. translational energy. There is no evidence for ammonia products with significant excitation of the asymmetric bend (ν4) or water products with bend (ν2) excitation. The results show that only restricted pathways lead to predissociation, and these do not always give rise to the smallest possible translational energy release, as favored by momentum gap models.

The state-to-state predissociation dynamics of the HCl–acetylene dimer were studied following excitation in the asymmetric C–H (asym-CH) stretch and the HCl stretch. Velocity map imaging (VMI) and resonance enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Different vibrational predissociation mechanisms were observed for the two excited vibrational levels. Following excitation in the of the asym-CH stretch fundamental, HCl fragments in υ = 0 and j = 4–7 were observed and no HCl in υ = 1 was detected. The fragments’ center-of-mass (c.m.) translational energy distributions were derived from images of HCl (j = 4–7), and were converted to rotational state distributions of the acetylene co-fragment by assuming that acetylene is generated with one quantum of C–C stretch (ν2) excitation. The acetylene pair-correlated rotational state distributions agree with the predictions of the statistical phase space theory, restricted to acetylene fragments in 1ν2. It is concluded that the predissociation mechanism is dominated by the initial coupling of the asym-CH vibration to a combination of C–C stretch and bending modes in the acetylene moiety. Vibrational energy redistribution (IVR) between acetylene bending and the intermolecular dimer modes leads to predissociation that preserves the C–C stretch excitation in the acetylene product while distributing the rest of the available energy statistically. The predissociation mechanism following excitation in the Q band of the dimer’s HCl stretch fundamental was quite different. HCl (υ = 0) rotational states up to j = 8 were observed. The rovibrational state distributions in the acetylene co-fragment derived from HCl (j = 6–8) images were non-statistical with one or two quanta in acetylene bending vibrational excitation. From the observation that all the HCl(j) translational energy distributions were similar, it is proposed that there exists a constraint on conversion of linear to angular momentum during predissociation. A dimer dissociation energy of D0 = 700 ± 10 cm−1 was derived.