Rotational State Distributions Research Articles

Fringe fields are important when examining molecular orientation in a cold ammonia beam

External fields have been widely adopted to control and manipulate the properties of gas-phase molecular species. In particular, electric fields have been shown to focus, filter and decelerate beams of polar molecules. While there are several well-established approaches for controlling the velocity and quantum-state distribution of reactant molecules, very few of these methods have examined the orientation of molecules in the resulting beam. Here we show that a buffer gas cell and three-bend electrostatic guide (coupled to a time-of-flight set-up) can be configured such that 70% of ammonia molecules in the cold molecular beam are oriented to an external electric field at the point of detection. With a minor alteration to the set-up, an approximately statistical distribution of molecular orientation is seen. These observations are explained by simulations of the electric field in the vicinity of the mesh separating the quadrupole guide and the repeller plate. The combined experimental apparatus therefore offers control over three key properties of a molecular beam: the rotational state distribution, the beam velocity, and the molecular orientation. Exerting this level of control over the properties of a molecular beam opens up exciting prospects for our ability to understand what role each parameter plays in reaction studies.

Time-Resolved Pair-Correlated Imaging of the Photodissociation of Acetaldehyde at 267 nm: Pathway Partitioning.

Photodissociation of acetaldehyde (CH3CHO) by UV excitation involves interwoven multiple reaction pathways, including nonradiative decay, isomerization, transition-state pathway, roaming, and other dissociation mechanisms. Recently, we employed picosecond time-resolved, pair-correlated product imaging in a study of acetaldehyde photodissociation at 267 nm to disentangle those competing mechanisms and to elucidate the possible roaming pathways (Yang, C. H.; Chem. Sci. 2020, 11, 6423-6430). Here, we complement the pair-correlated product speed distribution of CO(v = 0) at the high-j side of the CO rotational state distribution in the CO + CH4 channel and detail the two-dimensional data analysis of the time-resolved images. As a result, extensive comparisons with other studies can be made and the branching fractions of the previously assigned TScc(S0), non-TScc(S0), and CI(S1/S0) pathways for the CO(v = 0) + CH4 molecular channel are evaluated to be 0.74 ± 0.08, 0.15 ± 0.02, and 0.11 ± 0.02, respectively. Together with the macroscopic branching ratio between the molecular (CO + CH4) and radical (CH3 + HCO) channels at 267 nm from the literature, a global view of the microscopic pathways can then be delineated, which provides invaluable insights and should pave the way for further studies of this interesting system.

Full-dimensional MRCI-F12 potential energy surface and dynamics of the F(2P3/2) + C2H6 → HF + C2H5 reaction.

We report a detailed quasi-classical dynamics study on a new full-dimensional multireference spin-orbit-corrected potential energy surface (PES) for the F(2P3/2) + C2H6 → HF + C2H5 reaction. For the PES development, the Robosurfer program package is applied and the MRCI-F12+Q(5,3)/aug-cc-pVDZ energy points are fitted using the monomial symmetrization approach of the permutationally invariant polynomial method. Our simulations provide substantial reaction probabilities and sharply increasing cross sections with an increase in collision energy for this early- and negative-barrier reaction. A direct rebound/stripping mechanism is preferred at low/high collision energies, and the initial translational energy turns out to convert mostly into product recoil, whereas the reaction energy excites the HF vibration. Vibrational and vibrationally resolved rotational state distributions of the HF product obtained from our computations agree well with the single-collision experimental data for the vHF = 1, 2, and 3 states.

Origin of the “odd” behavior in the ultraviolet photochemistry of ozone

The origin of the even-odd rotational state population alternation in the 16O2(a1Δg) fragments resulting from the ultraviolet (UV) photodissociation of 16O3, a phenomenon first observed over 30 years ago, has been elucidated using full quantum theory. The calculated 16O2(a1Δg) rotational state distribution following the 266-nm photolysis of 60 K ozone shows a strong even-odd propensity, in excellent agreement with the new experimental rotational state distribution measured under the same conditions. Theory indicates that the even rotational states are significantly more populated than the adjacent odd rotational states because of a preference for the formation of the A' Λ-doublet, which can only occupy even rotational states due to the exchange symmetry of the two bosonic 16O nuclei, and thus not as a result of parity-selective curve crossing as previously proposed. For nonrotating ozone, its dissociation on the excited B1A' state dictates that only A' Λ-doublets are populated, due to symmetry conservation. This selection rule is relaxed for rotating parent molecules, but a preference still persists for A' Λ-doublets. The A''/A' ratio increases with increasing ozone rotational quantum number, and thus with increasing temperature, explaining the previously observed temperature dependence of the even-odd population alternation. In light of these results, it is concluded that the previously proposed parity-selective curve-crossing mechanism cannot be a source of heavy isotopic enrichment in the atmosphere.

Quantum dynamics studies of isotope effects in the Mg+(3p) + HD \u2192 MgH+/MgD+ + D/H insertion reaction

The time-dependent wave packet quantum dynamics studies for the Mg+(3p) + HD → MgH+/MgD+ + D/H diabatic reaction are carried out for the first time on recently developed diabatic YHWCH potential energy surfaces [Phys. Chem. Chem. Phys., 2018, 20, 6638–6647]. The results of reaction probabilities and total integral cross sections show a dramatic preference to the formation of MgD+ over MgH+ owing to the insertion reaction mechanism in the title reaction. The MgD+/MgH+ branching ratio witnesses a monotonic decrease from 10.58 to 3.88 at collision energy range of 0.01 to 0.20 eV, and at the collision energy of 0.114 eV, it is close to the experimental value of 5. The rovibrational state-resolved ICSs of the two channels show the products MgD+ have higher vibrational excitation and hotter rotational state distributions. The opacity function P(J) suggests that the MgH+ + D channel and MgD+ + H channel are dominated by high-b and low-b collisions, respectively. Both forward and backward scattering peaks are found in the differential cross section curves, whereas the angle distributions of products are not strictly forward-backward symmetric because of the short lifetime of the complex in the reaction.

State-to-state quantum dynamics of the H + LiF → Li + HF reaction on an accurate ab initio potential energy surface

Quantum state-to-state dynamics calculations are reported for the H + LiF(vi, ji) → HF(vf, jf) + Li reaction on a recently reported ground state X̃2A′ potential energy surface. It was found that at low collision energies the title reaction is dominated by long-lived resonances, while the lifetimes of resonances become shorter at higher energies. The existence of resonances is also supported by the differential cross section, which has mostly forward and sideways bias. The integral cross section rises with collision energy slowly, and the opacity function suggests that the reaction is dominated by small impact parameter collisions. The HF product has cold vibrational state distributions in range of the collision energy studied here, although its rotational state distributions peak near the highest accessible rotational states. This early-barrier reaction shows some enhancement by translational energy, but internal excitations of the LiF reactant have more complex behaviors.

Imaging Isocyanic Acid Photodissociation at 193 nm: the NH(a1Δ)+CO(X1Σ+) Channel

The photodissociation dynamics of isocyanic acid (HNCO) has been studied by the time-sliced velocity map ion imaging technique at 193 nm. The NH(a1Δ) products were measured via (2+1) resonance enhanced multiphoton ionization. Images have been accumulated for the NH(a1Δ) rotational states in the ground and vibrational excited state (v=0 and 1). The center-of-mass translational energy distribution derived from the NH(a1Δ) images implies that the CO vibrational distributions are inverted for most of the measured 1NH(v|j) internal states. The anisotropic product angular distribution observed indicates a rapid dissociation process for the N−C bond cleavage. A bimodal rotational state distribution of CO(v) has been observed, this result implies that isocyanic acid dissociates in the S1 state in two different pathways.

Signatures of a Conical Intersection in Adiabatic Dissociation on the Ground Electronic State.

Conical intersections are known to cause nonadiabatic transitions, but their effects on adiabatic dynamics are often ignored. Using the overtone-induced dissociation of the hydroxymethyl radical as an example, we demonstrate that ground-state O-H bond rupture is significantly affected by a conical intersection with an electronically excited state along the dissociation path, despite the much lower energy of the dissociating state than that of the conical intersection. In addition to lifetime differences, the geometric phase leads to a different H2CO rotational state distribution compared with that obtained using the standard single-state adiabatic model, which constitutes a signature of the conical intersection.

Possible effect of static surface disorder on diffractive scattering of H2 from Ru(0001): Comparison between theory and experiment.

Specific features of diffractive scattering of H2 from metal surfaces can serve as fingerprints of the reactivity of the metal towards H2, and in principle theory-experiment comparisons for molecular diffraction can help with the validation of semi-empirical functionals fitted to experiments of sticking of H2 on metals. However, a recent comparison of calculated and Debye-Waller (DW) extrapolated experimental diffraction probabilities, in which the theory was done on the basis of a potential energy surface (PES) accurately describing sticking to Ru(0001), showed substantial discrepancies, with theoretical and experimental probabilities differing by factors of 2 and 3. We demonstrate that assuming a particular amount of random static disorder to be present in the positions of the surface atoms, which can be characterized through a single parameter, removes most of the discrepancies between experiment and theory. Further improvement might be achievable by improving the accuracy of the DW extrapolation, the model of the H2 rotational state distribution in the experimental beams, and by fine-tuning the PES. However, the question of whether the DW model is applicable to attenuation of diffractive scattering in the presence of a sizable van der Waals well (depth ≈ 50 meV) should also receive attention, in addition to the question of whether the amount of static surface disorder effectively assumed in the modeling by us could have been present in the experiments.

Photoassociation driven by a short laser pulse at millikelvin temperature

We investigate theoretically the photoassociation process of $^{85}\mathrm{Rb}$ atoms driven by a short laser pulse at several millikelvins. At this temperature, the actions of the rotational barrier, weights of initial scattering states, and shape resonance emerge and have an influence on the thermally averaged photoassociation probability. The calculated thermal weighted population distribution of rotational states agrees well with photoassociation spectrum experiment. The thermally averaged photoassociation probability decreases with an increase in temperature.

Translational and Rotational Energy Distributions of NO Photodesorbed from Au(100)

We report velocity and internal state distributions of nitric oxide photodesorbed from an Au(100) single crystal using 355 and 266 nm photons. The velocity distributions were measured in all three dimensions independently using our novel 3D-velocity map imaging setup. Combined with the internal energy distributions we reveal two distinct desorption mechanisms for the photodesorption of NO from gold dependent on the photon wavelength. The 355 nm desorption is dominated by a nonthermal mechanism due to excitation of an electron from the gold substrate to the adsorbed NO; this leads to a superthermal and noticeably narrow velocity distribution and a rotational state distribution that positively correlates with the velocity distribution and can be described by a rotational temperature appreciably above the surface temperature. Desorption with 266 nm photons leads to a slower average speed and wider angular distribution and rotational temperatures not too far off the surface temperature. We conclude that in th...

Optical emission in the dissociation of ammonia by low energy He+ ions

Molecular bands associated with the ND(A-X), ND(c-a), ND+(B-X), and ND+(C-X) transitions, as well as atomic lines of Balmer series and several He and He+ lines, were observed in the collisions of He+ with ND3 gas target at energies between 17 eV and 833 eV in the center-of-mass frame. Absolute luminescence cross sections, excitation functions, and the ND(c-a)/ND(A-X) branching ratios (BR) were determined. The sum of all luminescence cross sections in the 200–600 nm spectral window is below 1 × 10-20 m2 at 833 eV. The ND(c-a)/ND(A-X) luminescence BR increases with collision energy from BR = 0.20 at 20 eV to BR = 0.30 at 150 eV and up to 833 eV. Computer simulations of the spectra were used to estimate rotational and vibrational temperatures of the products. The population distributions of rotational and vibrational states of ND∗(A, c) do not vary appreciably with collision energy.Graphical abstract

Internal and translational energy partitioning of the NO product in the S2 photodissociation of methyl nitrite

State-resolved scattering distributions of the NO product in the photodissociation of CH3ONO were measured at 213nm with resonantly-enhanced multiphoton ionization spectroscopy and ion-imaging. The spectra of the NO product displayed the vibrational population up to the v=3 state having the rotational state-distribution with a Gaussian-like function. The scattering data of the NO (v=1) product indicate that the rotational excitation of the NO fragment and the translational energy release are fairly well compensated. This result is explained as being an outcome of the strong repulsion in the CH3ONO bond in the S2 state.

Thermometry of Guided Molecular Beams from a Cryogenic Buffer-Gas Cell.

A comprehensive characterisation of cold molecular beams from a cryogenic buffer-gas cell, providing insight into the physics of buffer-gas cooling, is presented. Cold molecular beams are extracted from a cryogenic cell by electrostatic guiding, which is also used to measure their velocity distribution. The rotational-state distribution of the molecules is probed by radio-frequency resonant depletion spectroscopy. With the help of complete trajectory simulations, yielding the guiding efficiency for all of the thermally populated states, it is possible to determine both the rotational and the translational temperature of the molecules at the output of the buffer-gas cell. This thermometry method is demonstrated for various regimes of buffer-gas cooling and beam formation as well as for molecular species of different sizes (CH3 F and CF3 CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH3 F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.

The influence of carrier envelope phase of single-cycle THz pulse on field-free orientation of NaI molecules

The influence of the carrier envelope phase (CEP) on the field-free orientation of the NaI molecule is investigated theoretically. The molecular orientation steered by a single-cycle terahertz (THz) pulse is studied by solving exactly the time-dependent Schrodinger equation including the rovibrational degrees of freedom. We find that the field-free orientation is dependent on the central frequency and CEP of the single-cycle pulse. The CEP of single-cycle THz pulse affects the field-free orientation mainly via changing the population distributions of rotational states. By choosing the CEP of single-cycle THz pulse at the appropriate central frequency, the maximum value of the degree of field-free orientation, ⟨cosθ⟩, can reach 0.80. Moreover, we find that the duration of effective orientation (|⟨cosθ⟩| ≥ 0.5) is relatively long in certain regions of the laser central frequency and the CEP.

Rotational and angular distributions of NO products from NO-Rg (Rg = He, Ne, Ar) complex photodissociation

We present the results of an investigation into the rotational and angular distributions of the NO à state fragment following photodissociation of the NO-He, NO-Ne, and NO-Ar van der Waals complexes excited via the à ← X̃ transition. For each complex, the dissociation is probed for several values of Ea, the available energy above the dissociation threshold. For NO-He, the Ea values probed were 59, 172, and 273 cm(-1); for NO-Ne they were 50 and 166 cm(-1); and for NO-Ar they were 44, 94, 194, and 423 cm(-1). The NO à state rotational distributions arising from NO-He are cold, with most products in low angular momentum states. NO-Ne leads to broader NO rotational distributions but they do not extend to the maximum possible given the energy available. In the case of NO-Ar, the distributions extend to the maximum allowed at that energy and show the unusual shapes associated with the rotational rainbow effect reported in previous studies. This is the only complex for which a rotational rainbow effect is observed at the chosen Ea values. Product angular distributions have also been measured for the NO à photodissociation product for the three complexes. NO-He produces nearly isotropic fragments, but the anisotropy parameter, β, for NO-Ne and NO-Ar photofragments shows a surprising change in sign from negative to positive as Ea increases within the unstructured excitation profile. Franck-Condon selection of a broader distribution of geometries including more linear geometries at lower excitation energies and more T-shaped geometries at higher energies can account for the changing recoil anisotropy. Two-dimensional wavepacket calculations are reported to model the rotational state distributions and the bound-continuum absorption spectra.

Full-dimensional quantum molecular dynamics calculations of the rovibrationally mediated X 1A′ → 2 1A′′ transition of nitrous oxide

A full dimensional time-dependent quantum wavepacket approach is used to study the photodissociation dynamics of nitrous oxide for the X 1A′ → 2 1A′′ bound–bound transition based on new highly accurate potential energy and transition dipole moment surfaces. The computed 2 1A′′ absorption spectra at room temperature are characterized by sharp vibrational structures that contribute slightly to the diffuse vibrational structures around the maximum peak at 180 nm of the first ultraviolet absorption band (from the contribution of 2 1A′, 1 1A′′, and 2 1A′′ states) of N2O. Transitions from different initial rovibrational states reveal that the sharp structures arise mainly from N2O bending vibrations, whereas, at higher temperatures, the N2O and NNO stretching vibrations are responsible for enhancing the intensity of the structures. At absorption wavelengths 166 nm and 179 nm, vibrational quantum state distributions of N2 product fragments decrease monotonically with increasing vibrational quantum number v = 0, 1, 2. At 166 nm, rotational quantum state distributions of N2 at fixed v = 0 and v = 1 display multimodal profiles with maximum peaks at j = 77 and j = 75, respectively, whereas, the distributions at the 179 nm absorption wavelength display bimodal profiles with maximum peaks at j = 73 and j = 71, respectively. Accordingly, the presence of rotationally hot N2 from previous experimental and theoretical works in the first band strongly implies a significant influence of the 2 1A′′ state in determining the final dissociation pathway of N2 + O. © 2016 Wiley Periodicals, Inc.

Final rotational state distributions from NO(vi = 11) in collisions with Au(111): the magnitude of vibrational energy transfer depends on orientation in molecule-surface collisions.

When NO molecules collide at a Au(111) surface, their interaction is controlled by several factors; especially important are the molecules' orientation with respect to the surface (N-first vs. O-first) and their distance of closest approach. In fact, the former may control the latter as N-first orientations are attractive and O-first orientations are repulsive. In this work, we employ electric fields to control the molecules' incidence orientation in combination with rotational rainbow scattering detection. Specifically, we report final rotational state distributions of oriented NO(vi = 11) molecules scattered from Au(111) for final vibrational states between vf = 4 and 11. For O-first collisions, the interaction potential is highly repulsive preventing the close approach and scattering results in high-J rainbows. By contrast, these rainbows are not seen for the more intimate collisions possible for attractive N-first orientations. In this way, we reveal the influence of orientation and the distance of closest approach on vibrational relaxation of NO(vi = 11) in collisions with a Au(111) surface. We also elucidate the influence of steering forces which cause the O-first oriented molecules to rotate to an N-first orientation during their approach to the surface. The experiments show that when NO collides at the surface with the N-atom first, on average more than half of the initial vibrational energy is lost; whereas O-first oriented collisions lose much less vibrational energy. These observations qualitatively confirm theoretical predictions of electronically non-adiabatic NO interactions at Au(111).

Spectroscopic measurements of the electron number density, electron temperature and OH(A) rotational distribution in a liquid electrode dielectric barrier discharge

The electron temperature and number density as well as the OH(A) rotational distribution of a discharge with flowing liquid electrode and dielectric barrier coupling (a liquid electrode dielectric barrier discharge, LE-DBD) were investigated by means of optical emission spectroscopy. By using the Stark broadening of three Strontium lines, the electron number density Ne and the lower bound of the electron temperature Te can be simultaneously measured. The values obtained were Ne=(0.8−1.6)×1016cm−3 and Te>1.1eV, respectively. The OH(A) rotational distribution deviates from equilibrium and can be described by a superposition of two Boltzmann distributions with T1=(3230±90)K for K'≤15 and T2=(7300±300)K for K'≥16. Consideration of the formation mechanisms of OH(A) and reaction rates suggests that the dissociative recombination of H2O+ and H3O+ is responsible for the higher rotational state distribution, where these ions can only be produced in the LE-DBD through an electrospray-like process.

State to State Photodissociation Dynamics of Vibrationally Excited D2O in B Band

The state-to-state photodissociassion dynamics for the B band of D2O have been explored from quantum dynamical calculations including the electronic ̃X and ̃B states. The calculations were carried out using a Chebyshev real wave packet method. The calculated absorption spectra, product state distributions, and branching ratios from different initial vibrational states show different dynamic features, due to the different shapes of the vibrational wavefunctions. The initial bending mode (0,1,0) generates two lobes with a shallow minimum on the absorption spectrum and a slight inverted vibrational population of OD(̃X) product at high total energies. The rotational state distributions of OD(̃X, v=0) product are highly inverted and depend weakly on the initial state and total energy. On the other hand, the ro-vibrational distributions of OD(̃A) product strongly oscillate with the total energy, which are dominated by the long-living resonances and depend sensitively on the potential surfaces. The antisymmetric stretching mode (0,0,1) has large OD(̃A)/OD(̃X) branching ratios at high total energies, which indicates that the B band dissociation proceeds mainly via the adiabatic pathway in some cases.