Imaging H2O Photofragments in the Predissociation of the HCl− H2O Hydrogen-Bonded Dimer (original) (raw)
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The Journal of Physical …, 2010
The state-to-state vibrational predissociation dynamics of the hydrogen-bonded HCl-H 2 O 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 3 ∆ 2 r X 1 Σ + and V 1 Σ + r X 1 Σ + 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 D 0 ) 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. † Part of the "Reinhard Schinke Festschrift".
The Journal of chemical …, 2011
The bond dissociation energy (D 0 ) 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. H 2 O fragments are detected in the ground vibrational (000) and the first excited bending (010) states by 2 + 1 REMPI via theC 1 B 1 (000) ←X 1 A 1 (000 and 010) transitions. The fragments' velocity and center-of-mass translational energy distributions are determined from images of selected rovibrational levels of H 2 O. An accurate value for D 0 is obtained by fitting both the structure in the images and the maximum velocity of the fragments. This value, D 0 = 1105 ± 10 cm −1 (13.2 ± 0.12 kJ/mol), is in excellent agreement with the recent theoretical value of D 0 = 1103 ± 4 cm −1 (13.2 ± 0.05 kJ/mol) suggested as a benchmark by Shank et al.
Rotational, vibrational, and electronic states of formaldehyde and cishydroxymethylene products generated in the photodissociation of the hydroxymethyl radical are investigated by sliced velocity map imaging (SVMI) following excitation of the radical to its 3p x and 3p z 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 CH 2 OD 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 CH 2 OH 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 cishydroxymethylene depends on the excited Rydberg state of CH 2 OD and is lower in dissociation via the 3p z state than via the lower lying 3p x 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.
Imaging the State-Specific Vibrational Predissociation of the Ammonia-Water Hydrogen-Bonded Dimer
The journal of physical …, 2009
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 resonanceenhanced 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 B 1 E′′ r X 1 A 1 ′ transition. The REMPI spectra show that NH 3 is produced with one and two quanta of the symmetric bend (ν 2 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 ν 2 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 D 0 ) 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 J KaKc 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 (ν 4 ) or water products with bend (ν 2 ) 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. Figure 1. Equilibrium geometry of the ammonia-water dimer (ref. 32). R(N-O) ) 2.989 Å; θ N ) 23.1°; θ O ) 49.
Imaging bond breaking and vibrational energy transfer in small water containing clusters
CPL Frontiers, 2013
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 (D 0 ) of (H 2 O) 2 , (H 2 O) 3, HCl-H 2 O and NH 3 -H 2 O. 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 D 0 values for (H 2 O) 2 and (H 2 O) 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.
Research on Chemical Intermediates, 2007
The vibrationally-mediated H 2 O gas-phase photodissociation was studied at a photolysis wavelength of 248 nm. Single rotational states of the |03 − |2 and |04 − H 2 O overtone vibrations were prepared by laser photoexcitation around 720 nm. H atoms formed in the photodissociation of the H 2 O (|04 − J K a K c = 3 13 ) were detected by Lyman-α laser-induced fluorescence spectroscopy with sub-Doppler resolution to determine their translational energy. The present result confirms that in the dissociation process the major part (ca. 93%) of the available energy is released as relative translational energy of the nascent H + OH photofragments, in agreement with earlier complementary measurements (R. L. Vander Wal, J. L. F. F. Crim, J. Chem. Phys. 94, 1859 (1991)), where the internal excitation of the OH product radical was investigated at different photolysis wavelengths.
J. Phys. Chem. A, 2014
The breaking of hydrogen bonds in molecular 9 systems has profound effects on liquids, e.g., water, biomolecules 10 (e.g., DNA), etc., and so it is no exaggeration to assert the 11 importance of these bonds to living systems. However, despite 12 years of extensive research on hydrogen bonds, many of the 13 details of how these bonds break and the corresponding energy 14 redistribution processes remain poorly understood. Here we 15 report extensive experimental and theoretical insights into the 16 breakup of two or three hydrogen bonds of the dissociation of a 17 paradigm system of a hydrogen-bonded network, the ring HCl trimer. Experimental state-to-state vibrational predissociation 18 dynamics of the trimer following vibrational excitation were studied by using velocity map imaging and resonance-enhanced 19 multiphoton ionization, providing dissociation energies and product state distributions for the trimer's breakup into three 20 separate monomers or into dimer + monomer. Accompanying the experiments are high-level calculations using diffusion Monte 21 Carlo and quasiclassical simulations, whose results validate the experimental ones and further elucidate energy distributions in the 22 products. The calculations make use of a new, highly accurate potential energy surface. Simulations indicate that the dissociation 23 mechanism requires the excitation to first relax into low-frequency motions of the trimer, resulting in the breaking of a single 24 hydrogen bond. This allows the system to explore a critical van der Waals minimum region from which dissociation occurs 25 readily to monomer + dimer.
Vibrational predissociation of the phenol–water dimer: a view from the water
Physical Chemistry Chemical Physics, 2019
The vibrational predissociation (VP) dynamics of the phenol-water (PhOH-H 2 O) dimer were studied by detecting H 2 O fragments and using velocity map imaging (VMI) to infer the internal energy distributions of PhOH cofragments, pair-correlated with selected rotational levels of the H 2 O fragments. Following infrared (IR) laser excitation of the hydrogen-bonded OH stretch fundamental of PhOH (Pathway 1) or the asymmetric OH stretch localized on H 2 O (Pathway 2), dissociation to H 2 O + PhOH was observed. H 2 O fragments were monitored state-selectively by using 2+1 Resonance-Enhanced Multiphoton Ionization (REMPI) combined with time-of-flight mass spectrometry (TOF-MS). VMI of H 2 O in selected rotational levels was used to derive center-of-mass (c.m.) translational energy (E T) distributions. The pair-correlated internal energy distributions of the PhOH cofragments derived via Pathway 1 were well described by a statistical prior distribution. On the other hand, the corresponding distributions obtained via Pathway 2 show a propensity to populate higher-energy rovibrational levels of PhOH than expected from a statistical distribution and agree better with an energy-gap model. The REMPI spectra of the H 2 O fragments from both pathways could be fit by Boltzmann plots truncated at the maximum allowed energy, with a higher temperature for Pathway 2 than that for Pathway 1. We conclude that the VP dynamics depends on the OH stretch level initially excited.
Rotationally mediated vector correlations in the photodissociation of H2O (1,0,0)
Chemical Physics, 1994
The vector correlations following the photodissociation of initially prepared rotational states of the fundamental symmetric stretch of water, Hz0 (l,O,O), are studied in detail. The rovibrationslly excited water molecules are prepared by stimulated Raman excitation and photodissociated at 193 nm via the first electronically excited state. The OH fragments are probed by both broadband and sub-Doppler polarization spectroscopy. It is shown that the degree of rotational alignment (the p-J correlation) obtained when the parent molecule is prepared in an in-plane rotational state (3& is a factor of two higher than in a state containing a mixture of in-plane and out-of-plane rotations (321 + 322 + 4r4). Also, the p-u, v-J and p-u-J correlations following the photodissociation of the 303 state are close to the limiting values expected for an idealized orientation (with minimal influence of the parent rotation) where the transition dipole moment (cc) of the parent is parallel to the fragment angular momentum (J) and perpendicular to its velocity (v). In addition, from the degree of misalignment due to the out-ofplane rotation, an estimate of a lifetime of 40 fs of Hz0 in the first electronically excited state is obtained. These results demonstrate that experiments which prepare the parent molecule in a particular rotational state, before a second laser dissociates it, provide a powerful means for understanding the directional characteristics of dissociation processes.