Multiphoton ionization vibrational state selection of H2O+, D2O+ and HDO+

Rotational state specific dissociation dynamics of D2O via the C̃(010) state: The effect of bending vibrational excitation

The rotational state resolved photodissociation dynamics of D2O via the [Formula: see text](010) state has been investigated by using the D-atom Rydberg tagging time-of-flight technique combined with a tunable vacuum ultraviolet light source. The D-atom action spectrum of the [Formula: see text](010) ← [Formula: see text](000) band and the corresponding time-of-flight (TOF) spectra of D-atom photoproducts formed following the excitation of D2O to individual rotational transition have been measured. By comparison with the action spectrum of the [Formula: see text](000) ← [Formula: see text](000) band, the bending vibrational constant of the [Formula: see text] state for D2O can be determined to be v2 = 1041.37 ± 0.71 cm−1. From the TOF spectra, the product kinetic energy spectra, the vibrational state distributions of OD products, and the state resolved anisotropy parameters have been determined. The experimental results indicate a dramatic variation in the OD product state distributions for different rotational excitations. This illuminates that there are two distinctive coupling channels from the [Formula: see text](010) state to the low-lying electronic states: the homogeneous electronic coupling to the Ã1B1 state, resulting in vibrationally hot OD(X) products, and the Coriolis-type coupling to the [Formula: see text]1A1 state, producing vibrationally cold but rotationally hot OD(X) and OD(A) products. Furthermore, the three-body dissociation channel is confirmed, which is attributed to the [Formula: see text] → 1A2 or [Formula: see text] → Ã pathway. In comparison with the previous results of D2O photolysis via the [Formula: see text](000) state, it is found that the v2 vibration of the parent molecule enhances both the vibrational and rotational excitations of OD products.

Strong isotope effect in the VUV photodissociation of HOD: A possible origin of D/H isotope heterogeneity in the solar nebula

The deuterium versus hydrogen (D/H) isotopic ratios are important to understand the source of water on Earth and other terrestrial planets. However, the determinations of D/H ratios suggest a hydrogen isotopic diversity in the planetary objects of the solar system. Photochemistry has been suggested as one source of this isotope heterogeneity. Here, we have revealed the photodissociation features of the water isotopologue (HOD) at λ = 120.8 to 121.7 nm. The results show different quantum state populations of OH and OD fragments from HOD photodissociation, suggesting strong isotope effect. The branching ratios of H + OD and D + OH channels display large isotopic fractionation, with ratios of 0.70 ± 0.10 at 121.08 nm and 0.49 ± 0.10 at 121.6 nm. Because water is abundant in the solar nebula, photodissociation of HOD should be an alternative source of the D/H isotope heterogeneity. This isotope effect must be considered in the photochemical models.

Maximal kinetic energy and angular distribution analysis of spatial map imaging: Application to photoelectrons from a single quantum state of H2O

Dynamical or spatial properties of charged species can be obtained using electrostatic lenses by velocity map imaging (VMI) or spatial map imaging (SMI), respectively. Here, we report an approach for extracting dynamical and spatial information from patterns in SMI images that map the initial coordinates, velocity vectors, and angular distributions of charged particles onto the detector, using the same apparatus as in VMI. Deciphering these patterns required analysis and modeling, involving both their predictions from convolved spatial and velocity distributions and fitting observed images to kinetic energies (KEs) and anisotropy parameters (βs). As the first demonstration of this capability of SMI, the ensuing photoelectrons resulting from (2 + 1) resonant ionization of water in a selected rotational state were chosen to provide a rigorous basis for comparison to VMI. Operation with low acceleration voltages led to a measured SMI pattern with a unique vertical intensity profile that could be least-squares fitted to yield KE and β, in good agreement with VMI measurement. Due to the potential for improved resolution and the extended KE range achievable by this new technique, we expect that it might augment VMI in applications that require the analysis of charged particles and particularly in processes with high KE release.

Effects of translational and vibrational excitation on the reaction of HOD+ with C2H2 and C2D2: mode- and bond-specific effects in exoergic proton transfer

Reactions of mode-selectively excited HOD(+) with C2H2 and C2D2 were studied over the center-of-mass collision energy (Ecol) range from 0.15 to 2.9 eV. HOD(+) was prepared in each of its fundamental vibrational states: ground state (000), bend (010), OD stretch (100), and the OH stretch (001). Charge transfer is the dominant reaction at all energies, although it is inhibited by increasing Ecol, and is accompanied by hydrogen exchange. The total charge transfer cross section is similar for C2H2 and C2D2, however, the tendency toward charge transfer with hydrogen exchange (CTHE) is significantly greater for C2D2 compared to C2H2. Charge transfer shows no significant effects of HOD(+) vibrational excitation, however, CTHE is significantly enhanced by vibration at Ecol < 0.62 eV. Both H(+) and D(+) transfer reactions (HT, and DT, respectively) are observed for both C2H2 and C2D2, with little dependence on collision energy, but with mode- and bond-specific enhancements from excitation of the OH and OD stretches. Recoil velocity measurements show that all channels are direct, except perhaps at the lowest collision energies. Mode-specific effects on the recoil velocity distributions are also observed, revealing how vibrational excitation affects reaction at different collision impact parameters.

Vibrationally enhanced charge transfer and mode/bond-specific H+ and D+ transfer in the reaction of HOD+ with N2O

The reaction of HOD(+) with N2O was studied over the collision energy (E(col)) range from 0.20 eV to 2.88 eV, for HOD(+) in its ground state and in each of its fundamental vibrational states: bend (010), OD stretch (100), and OH stretch (001). The dominant reaction at low E(col) is H(+) and D(+) transfer, but charge transfer becomes dominant for E(col) > 0.5 eV. Increasing E(col) enhances charge transfer only in the threshold region (E(col) < 1 eV), but all modes of HOD(+) vibrational excitation enhance this channel over the entire energy range, by up to a factor of three. For reaction of ground state HOD(+), the H(+) and D(+) transfer channels have similar cross sections, enhanced by increasing collision energy for E(col) < 0.3 eV, but suppressed by E(col) at higher energies. OD stretch excitation enhances D(+) transfer by over a factor of 2, but has little effect on H(+) transfer, except at low E(col) where a modest enhancement is observed. Excitation of the OH stretch enhances H(+) transfer by up to a factor of 2.5, but actually suppresses D(+) transfer over most of the E(col) range. Excitation of the bend mode results in ~60% enhancement of both H(+) and D(+) transfer at low E(col) but has little effect at higher energies. Recoil velocity distributions at high E(col) are strongly backscattered in the center-of-mass frame, indicating direct reaction dominated by large impact parameter collisions. At low E(col) the distributions are compatible with mediation by a short-lived collision complex. Ab initio calculations find several complexes that may be important in this context, and RRKM calculations predict lifetimes and decay branching that is consistent with observations. The recoil velocity distributions show that HOD(+) vibrational excitation enhances reactivity in all collisions at low E(col), while for high E(col) with enhancement comes entirely from the subset of collisions that generate strongly back-scattered product ions.

Effects of collisional and vibrational velocity on proton and deuteron transfer in the reaction of HOD+ with CO

Reaction of HOD(+) with CO was studied over the collision energy (E(col)) range between 0.18 and 2.87 eV, for HOD(+) in its ground state and with one quantum in each of its vibrational modes: (001)--predominantly OH stretch; (010)--bend, and (100)--predominately OD stretch. In addition to integral cross sections, product recoil velocity distributions were also measured for each initial condition. The dominant reactions are near-thermoneutral proton and deuteron transfer, generating HCO(+) and DCO(+) product ions by a predominantly direct mechanism. The HCO(+) and DCO(+) channels occur with a combined efficiency of 76% for ground state HOD(+) at our lowest E(col), increasing to 94% for E(col) around 0.33 eV, then falling at high E(col) to ~40%. The HCO(+) and DCO(+) channels have a complicated dependence on the HOD(+) vibrational state. Excitation of the OH or OD stretch modes enhances H(+) or D(+) transfer, respectively, and inhibits D(+) or H(+) transfer. Bend excitation preferentially enhances H(+) transfer, with no effect on D(+) transfer. There is no coupling of energy initially in any HOD(+) vibrational mode to recoil velocity of either of the product ions, even at low E(col) where vibrational excitation doubles or triples the energy available to products. The results suggest that transfer of H or D atoms is enhanced if the atom in question has a high vibrational velocity, and that this effect offsets what is otherwise a general inhibition of reactivity by added energy. HOCO(+) + D and DOCO(+) + H products are also observed, but as minor channels despite being barrierless and exoergic. These channels appear to be complex mediated at low E(col), essentially vanish at intermediate E(col), then reappear with a direct reaction mechanism at high E(col).

H+ versus D+) transfer from HOD+ to N2: mode- and bond-selective effects

Reactions of HOD(+) with N(2) have been studied for HOD(+) in its ground state and with one quantum of excitation in each of its vibrational modes: (001)--predominately OH stretch, 0.396 eV, (010)--bend, 0.153 eV, and (100)--predominately OD stretch, 0.293 eV. Integral cross sections and product recoil velocities were recorded for collision energies from threshold to 4 eV. The cross sections for both H(+) and D(+) transfer rise slowly from threshold with increasing collision energy; however, all three vibrational modes enhance reaction much more strongly than equivalent amounts of collision energy and the enhancements remain large even at high collision energy, where the vibration contributes less than 10% of the total energy. Excitation of the OH stretch enhances H(+) transfer by a factor of ∼5, but the effect on D(+) transfer is only slightly larger than that from an equivalent increase in collision energy, and smaller than the effect from the much lower energy bend excitation. Similarly, OD stretch excitation strongly enhances D(+) transfer, but has essentially no effect beyond that of the additional energy on H(+) transfer. The effects of the two stretch vibrations are consistent with the expectation that stretching the bond that is broken in the reaction puts momentum in the correct coordinate to drive the system into the exit channel. From this perspective it is quite surprising that bend excitation also results in large (factor of 2) enhancements of both H(+) and D(+) transfer channels, such that its effect on the total cross section at collision energies below ∼2 eV is comparable to those from the two stretch modes, even though the bend excitation energy is much smaller. For collision energies above ∼2 eV, the vibrational effects become approximately proportional to the vibrational energy, though still much larger than the effects of equivalent addition of collision energy. Measurements of the product recoil velocity distributions show that reaction is direct at all collision energies, with roughly half the products in a sharp peak corresponding to stripping dynamics and half with a broad and approximately isotropic recoil velocity distribution. Despite the large effects of vibrational excitation on reactivity, the effects on recoil dynamics are small, indicating that vibrational excitation does not cause qualitative changes in the reaction mechanism or in the distribution of reactive impact parameters.

Reaction of HOD+ with NO2: effects of OD and OH stretching, bending, and collision energy on reactions on the singlet and triplet potential surfaces

Integral cross sections and product recoil velocity distributions were measured for the reaction of HOD(+) with NO(2), in which the HOD(+) reactant was prepared in its ground state and with mode-selective excitation in the 001 (OH stretch), 100 (OD stretch), and 010 (bend) modes. In addition, we measured the 300 K thermal kinetics in a selected ion flow tube reactor and report product branching ratios different from previous measurements. Reaction is found to occur on both the singlet and triplet surfaces with near-unit efficiency. At 300 K, the product branching indicates that triplet → singlet transitions occur in about 60% of triplet-coupled collisions, which we attribute to long interaction times mediated by complexes on the triplet surface. Because the collision times are much shorter in the beam experiments, the product distributions show no signs of such transitions. The dominant product on the singlet surface is charge transfer. Reactions on the triplet surface lead to NO(+), NO(2)H(+), and NO(2)D(+). There is also charge transfer, producing NO(2)(+) (a(3)B(2)); however, this triplet NO(2)(+) mostly predissociates. The NO(2)H(+)/NO(2)D(+) cross sections peak at low collision energies and are insignificant above ~1 eV due to OH/OD loss from the nascent product ions. The effects of HOD(+) vibration are mode-specific. Vibration inhibits charge transfer, with the largest effect from the bend. The NO(2)H(+)/NO(2)D(+) channels are also vibrationally inhibited, and the mode dependence reveals how energy in different reactant modes couples to the internal energy of the product ions.

H+ versus D+ transfer from HOD+ to CO2: bond-selective chemistry and the anomalous effect of bending excitation

Reactions of HOD(+) with CO(2) have been studied for HOD(+) in its ground state, and with one quantum of excitation in each of its vibrational modes: (001)--predominantly OH stretch, 0.396 eV; (010)--bend, 0.153 eV; and (100)--predominantly OD stretch, 0.293 eV. Integral cross sections and product recoil velocities were recorded for collision energies from threshold to 3 eV. The cross sections for both H(+) and D(+) transfer rise with increasing collision energy from threshold to ∼1 eV, then become weakly dependent of the collision energy. All three vibrational modes enhance the total reactivity, but quite mode specifically. The H(+) transfer reaction is enhanced by OH stretch excitation, whereas OD stretch excitation has little effect. Conversely, the D(+) transfer reaction is enhanced by OD stretch excitation, while the OH stretch has little effect. Excitation of the bend strongly enhances both channels. The effects of the stretch excitations are consistent with previous studies of neutral HOD mode-selective chemistry, and can be at least qualitatively understood in terms of a late barrier to product formation. The fact that bend excitation produces the largest overall enhancement is surprising, because this is the lowest energy excitation, and is not obviously connected with the reaction coordinates for either H(+) or D(+) transfer. A rationalization in terms of the effects of water distortion on the potential surface is proposed.

REMPI spectroscopy and predissociation of the C(1)B(1)(v = 0) rotational levels of H(2)O, HOD and D(2)O

Rotational analysis of the (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectrum of the C(1)B(1) Rydberg state of the water isotopomers H(2)O, HOD and D(2)O is reported. Spectroscopic parameters for the v = 0 vibrational level of the C(1)B(1) state of the mixed isotopomer HOD are derived and its spectra are accurately simulated for the first time using the PGOPHER program. Simulation of two photon spectra of the C(1)B(1)-X(1)A(1) transition of HOD requires two transition moments, and the ratio of these is determined and explained by a simple geometrical model. Optimal transitions for state-selective detection of low energy rotational states are identified for all three molecules. Analysis of the linewidths in the present work, combined with previous work [H. H. Kuge and K. Kleinermanns, J. Chem. Phys., 1989, 90, 46-52; K. J. Yuan et al., Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 19148-19153; M. N. R. Ashfold et al., Chem. Phys., 1984, 84, 35-50; G. Meijer et al., J. Chem. Phys., 1986, 85, 6914-6922.], suggests that while a simple ⟨J(a)'(2)〉-dependent model for heterogeneous predissociation of the C(1)B(1) Rydberg state accounts for much of the quantum number dependence, it is not sufficient for describing the predissociation in any of the three isotopomers. The component of the linewidth due to the homogeneous predissociation attributed to predissociation of the C(1)B(1) by the Ã(1)B(1) state was found to be significantly narrower than in previous work, indicating a longer lifetime of the C(1)B(1) Rydberg state. The current work provides the basis for on-going studies of rotational energy transfer in the mixed isotopomers of water using the velocity map imaging technique.