Rotational State Selection Research Articles

A Photodissociation dynamics study utilizing spatial orientation control of asymmetric top molecules

AbstractBackgroundThe hexapole state selection had been developed mainly as a measure of rotational state selection and orientation control of symmetric top molecules.ObjectiveThis review aims to reorganize the results that the state selection technique was applied to study the photodissociation dynamics of an asymmetric top molecule 2‐bromobutane.Methods2‐Bromobutane molecules have been state‐selected and spatially oriented using a hexapole state selector. The oriented molecules were then photolyzed to emit Br (2P1/2) photofragments. The photofragments were observed by using the slice ion imaging technique.ResultsThe photofragment scattering distribution was experimentally observed to present a characteristic top‐bottom asymmetry, which allows us to extract geometric information of molecular vector properties related to the photodissociation dynamics. Besides, a theoretical study suggested a possibility of chiral discrimination by the scattering angular distributions.ConclusionThis review introduces the overview of the theoretical formulation of the angular distributions and representative experimental results using the orientational control technique as an effective tool for studies of photodissociation dynamics.

Conformer Selection by Electrostatic Hexapoles: A Theoretical Study on 1-Chloroethanol and 2-Chloroethanol

The electrostatic hexapole is a versatile device that has been used for many years in gas-phase experiments. Its inhomogeneous electric field has been employed for many purposes such as the selection of rotational states, the selection of clusters, the focusing of molecular beams, and molecular alignment as a precursor for molecular orientation. In the last few years, the hexapolar electric field has been demonstrated to be able to control the conformer composition of molecular beams. The key point is that conformers, where the component of the permanent electric dipole moment with respect to the largest of the principal axes of inertia is close to zero, require more intense hexapolar electric fields to be focused with respect to the other conformers. Here, we simulated the focusing curves of the conformers of 1-chloroethanol and 2-chloroethanol under hypothetical beam conditions, identical for all conformers, in a hypothetical and realistic experimental setup with three different hexapole lengths: 0.5, 1, and 2 m. The objective was to characterize this selection process to set up collision experiments on conformer-selected beams that provide information on the van der Waals clusters formed in collision processes.

Photoelectron imaging on vibrational excitation and Rydberg intermediate states in multi-photon ionization process of NH3 molecule**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574116, 11534004, and 10704028).

The ionization processes of NH3 molecule are studied by photoelectron velocity map imaging technique in a linearly polarized 400-nm femtosecond laser field. The two-dimensional photoelectron images from ammonia molecules under different laser intensities are obtained. In the slow electron region, the values of kinetic energy of photoelectrons corresponding to peaks 1, 2, 3, and 4 are 0.27, 0.86, 1.16, and 1.6 eV, respectively. With both the kinetic energy and angular distribution of photoelectrons from NH3 molecules, we can confirm that the two-photon excited intermediate Rydberg state is A∼1 () state for photoelectron peaks 2, 3, 4, and the three peaks are marked as 1223 (2 + 2), 1123 (2 + 2), and 1023 (2 + 2) multi-photon processes, respectively. Then, peak 1 is found by adding a hexapole between the source chamber and the detection chamber to realize the rotational state selection and beam focusing. Peak 1 is labeled as the 1323 (3 + 1) multi-photon process through the intermediate Rydberg state . The phenomena of channel switching are found in the slow electron kinetic energy distributions. Our calculations and experimental results indicate that the stretching vibrational mode of ammonia molecules varies with channels, while the umbrella vibration does not. In addition, we consider and discuss the ac-Stark effect in a strong laser field. Peaks 5 and 6 are marked as (2 + 2 + 1) and (2 + 2 + 2) above threshold ionization processes in the fast electron region.

Slow and velocity-tunable beams of metastableHe2by multistage Zeeman deceleration

We report on the use of multistage Zeeman deceleration to generate beams of ${\mathrm{He}}_{2}$ molecules in the metastable $a\phantom{\rule{0.16em}{0ex}}{}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ state with velocities tunable down to 100 m/s. The metastable molecules are generated by striking a discharge in a supersonic expansion of pure helium gas from a pulsed valve held at cryogenic temperature. The velocity and internal-state distributions of the metastable ${\mathrm{He}}_{2}$ molecules are measured for nozzle temperatures of 300, 77, and 10 K by high-resolution photoelectron and photoionization spectroscopy. The deceleration process does not exhibit any rotational state selectivity with rotational levels up to ${N}^{\ensuremath{'}\ensuremath{'}}=21$ being populated, but eliminates molecules in spin-rotational sublevels with ${J}^{\ensuremath{'}\ensuremath{'}}={N}^{\ensuremath{'}\ensuremath{'}}$ from the beam, where ${J}^{\ensuremath{'}\ensuremath{'}}$ and ${N}^{\ensuremath{'}\ensuremath{'}}$ are the total and the rotational angular momentum quantum number, respectively. The lack of rotational state selectivity is attributed to the fact that the Paschen-Back regime of the Zeeman effect in the rotational levels of ${\mathrm{He}}_{2}$ is already reached at fields of only 0.1 T.

Role of rotational state-selected for nonadiabatic alignment: OCS molecules in femtosecond laser fields

Nonadiabatic alignment by intense nonresonant laser fields is a versatile technique to manipulate the spatial direction of molecules. By solving the time-dependent Schrodinger equation numerically the degree of alignment of the molecules initially in different rotational state are calculated and the results show that the degree of alignment strongly depends on the initial rotational state. Thus, the present study indicates that, for obtaining a high degree of alignment for molecules, appropriate selection of molecular rotational states is necessary.

Electrostatic hexapole state-selection of the asymmetric-top molecule propylene oxide: Rotational and orientational distributions

This theoretical study is complementary to previous experimental work (see D.-C. Che, F. Palazzetti, Y. Okuno, V. Aquilanti, T. Kasai, J. Phys. Chem. A 114 (2010) 3280) on the orientation and rotational state-selection of supersonic molecular beams of the asymmetric-top molecule propylene oxide, both pure and seeded (in He and in Ar) by using an 85-cm length hexapole state-selector. One objective is to obtain an accurate distribution of the rotational states after hexapole selection for the three molecular beams, the most relevant feature consisting in the evaluation of the variation in energy of the manifold of rotational states when an electric field is applied (the Stark forces). Previously, the Stark effect on the effective dipole moment was considered through second order for all rotational states, while in this work the energy derivatives of the rotational states with respect to the applied electric field, are obtained accurately by diagonalizing the very large Stark matrices, whose elements depend on the dipole moment components of the molecule. The Stark energies and the corresponding forces were calculated for values of the electric field between 0 and 80kVcm−1 in steps of 0.5kVcm−1 and then linearly interpolated, covering the whole experimental range of the hexapole. A treatment is given for the intricate pattern of avoided crossings among derivatives of the rotational levels and two limiting cases were considered, corresponding to transitions occurring either adiabatically or diabatically. The two treatments lead to slightly different distributions of the rotational states for the pure and Ar seeded molecular beams, while for the He seeded molecular beam the two distributions are substantially the same. This experimental arrangement is a perspective tool for experiments of photochemistry and scattering of oriented molecules and clusters, and therefore we calculated the orientational distributions in a configuration where a uniform electric field is placed between the hexapole field and the detector.

Electrostatic Hexapole State-Selection of the Asymmetric-Top Molecule Propylene Oxide

Rotational state-selection of the asymmetric-top molecule propylene oxide was carried out using an electrostatic hexapole field of 85-cm length. Molecular beam intensities were monitored by a quadrupole mass spectrometer. It was found that beam intensities of molecular beams for pure propylene oxide and those seeded in He and in Ar increased with increasing hexapole voltages. The hexapole voltage dependence of the beam intensity, which is called the focusing curve, was interpreted by computer simulation of the trajectories of molecules in the hexapolar field due to the Stark effect, as a function of rotational temperatures of molecular beams. The calculated best fit focusing curves, when compared with the experimental results, demonstrated that the rotational temperatures, associated with the distribution of states of a given rotational angular momentum J, are similar to the translational temperatures. It was found that the M = 0 states (where M is the projection of J along the direction of the electrostatic field) and negative values of the pseudoquantum number tau of propylene oxide can be selected using our experimental setup. These results suggest that the hexapole electric field is a tool even for the selection of rotational and orientation states of asymmetric-top molecules.

Molecular dynamics simulations of the diffusion of benzene sub-monolayer films on graphite basal plane surfaces

A classical molecular dynamics study of the diffusive motion of benzene molecules on graphite basal planes has been performed in the sub-monolayer coverage regime. Atomistic calculations were performed using the second generation forcefield COMPASS as well as the general purpose forcefields Dreiding and Universal. The COMPASS based calculations show evidence for a Brownian nature of the diffusion with a very small diffusion activation barrier of 11 ± 2 meV in agreement with recent helium and neutron spin-echo spectroscopy data [Fouquet P, Hedgeland H, Jardine AP, Alexandrowicz G, Allison W, Ellis J. Measurements of molecule diffusion on surfaces using neutron and helium spin echo. Physica B 2006;385–386:269-71]. Reasonable agreement is also found for the general purpose forcefields if screened charges are used in the description of the Coulombic non-bond interaction. The less computationally intensive Dreiding forcefield was shown to give a good qualitative description of the diffusion validating its applicability for future large scale calculation, i.e., for long times or enlarged systems. A potential energy surface (PES) has been established for the translational and rotational diffusive motion. The PES shows a peculiar dependence of the lateral diffusion barriers on the rotation angle of the benzene molecule which leads to a preferential selection of certain rotational states in the MD trajectories.

Sub-Doppler laser absorption spectroscopy of the A2Π i − X2Σ + (1, 0) band of CN: Measurement of the 14N hyperfine parameters in A2Π CN

Measurements of multiple rotational lines in the (1, 0) band of the A 2Π i − X 2Σ + “red” system of the cyanogen radical (CN) at sub-Doppler resolution are reported. The CN radical was produced by 193 nm photodissociation of NCCN (ethane dinitrile) and detected with a Ti:sapphire ring laser operating near 10 900 cm −1. The sample was exposed to a weak, frequency-modulated probe beam and a strong, counterpropagating bleach laser beam. Demodulated probe laser signals display absorption and dispersion features derived from Doppler-free saturation of the hyperfine components as the laser scans across the central region of a Doppler-broadened rotational line spectrum. Hyperfine-resolved transition frequencies were combined with known ground-state X 2Σ hyperfine term values to determine A 2Π state hyperfine term values, which were analyzed in terms of an effective Hamiltonian for the A 2Π state. All the expected hyperfine and 14N quadrupolar parameters were determined and their values analyzed in terms of a simple molecular orbital picture of the bonding in the radical. Higher sensitivity obtained with 400 kHz amplitude modulation of the bleach laser and additional phase-sensitive detection allowed hyperfine splittings in some rotational lines of 13C 14N to be observed in natural abundance. Excited state hyperfine splittings were determined for a selection of rotational states, but not enough to determine the 13C hyperfine parameters.

Speed dependent rotational angular momentum polarization of the O2 (aΔg1) fragment following ozone photolysis in the wavelength range 248–265nm

The translational anisotropy and rotational angular momentum polarization of a selection of rotational states of the O2 (a 1Deltag; v=0) photofragment formed from ozone photolysis at 248, 260, and 265 nm have been determined using the technique of resonance enhanced multiphoton ionization in combination with time of flight mass spectrometry. At 248 nm, the dissociation is well described as impulsive in nature with all rotational states exhibiting similarly large, near-limiting values for the bipolar moments describing their angular momentum alignment and orientation. At 265 nm, however, the angular momentum polarization parameters determined for consecutive odd and even rotational states exhibit clear differences. Studies at the intermediate wavelength of 260 nm strongly suggest that such a difference in the angular momentum polarization is speed dependent and this proposal is consistent with the angular momentum polarization parameters extracted and reported previously for longer photolysis wavelengths [G. Hancock et al., Phys. Chem. Chem. Phys. 5, 5386 (2003); S. J. Horrocks et al., J. Chem. Phys. 126, 044308 (2007)]. The alternation of angular momentum polarization for successive odd and even J states may be a consequence of the different mechanisms leading to the formation of the two O2 (a 1Deltag) Lambda doublets. Specifically, the involvement of out of plane parent rotational motion is proposed as the origin for the observed depolarization for the Delta- relative to the Delta+ state.

Experimental validation of formaldehyde rotational state selection

We present experimental observations for the rotational state selection of formaldehyde. Data were acquired by using an electrostatic hexapole to focus a supersonic beam of formaldehyde seeded in argon into the ionizer of a quadrupole mass spectrometer. Empirical focusing spectra agree with theoretical predictions when a rigorous quantum mechanical treatment is used to calculate the Stark effect for the rotational states of formaldehyde. These results have implications in the production of beams of cold molecules.

Hexapole State-Selection and Beam Focus of Polar Top Molecules

We express a description of the state-selection role for a polarmolecule in a hexapole electrostatic field. By a quantum mechanicaltreatment of the molecular Stark energy and a classical mechanicaltreatment for the molecular trajectory in the field, we present thecalculated results of the different molecular rotational stateselection and beam focus and discuss the influence of the high orderStark effect, the beam speed on the results for the symmetric topmolecule CH3CN, CH3I, and the asymmetric top moleculeCH2F2 in the hexapole field. The method established andresults obtained can be taken as a guide for hexapole state selectionand beam focus of polar top molecules.

Rotationally selected product pair correlation in F+CD(4)-->DF(nu('))+CD(3)(nu=0,N).

The title reaction was studied in a crossed-beam experiment by imaging of state-selected products. The rotational state selection of the CD(3) products was achieved using (2+1) resonance-enhanced multiphoton ionization. The coincident information on the DF coproducts was revealed in a state-resolved manner from time-sliced velocity map images. Significant dependences of both the correlated differential cross sections and the DF vibrational branching ratios on the "tagged" CD(3) rotation states were found. The dynamical implications of one of the major findings are discussed.

Rotational and Torsional Analysis of the OH-Stretch Third Overtone in 13CH3OH

We record double resonance spectra of the 4ν1 band of jet-cooled 13C-methanol using single rotational state selection in the ν1 fundamental and subsequent promotion of the selected molecules to the fourth vibrational level. We then detect transitions to the final excited states by infrared laser assisted photofragment spectroscopy (IRLAPS). The assigned A symmetry transitions reach upper states with K=0 and 1, and J from 0 to 5. For E symmetry, the transitions reach levels with K in the range −3 to 2 and J from 1 to 7. The rotation–torsional analysis determines a value for the torsional tunneling splitting of 2.8±0.4 cm−1 at v1=4. In a previous paper (J. Chem. Phys.110, 11 359–11 367 (1999)), we reported a trend of monotonically decreasing tunneling splittings in 12CH3OH for v1=0, 3, and 6 that we explained by a model that incorporates a linear increase in the torsional barrier height with OH stretch excitation. The 13CH3OH tunneling splitting for the 4ν1 band is in quantitative agreement with the trend found for 12CH3OH.

The product rovibrational and spin–orbit state dependent dynamics of the complex reaction H+CO2→OH(2Π;ν,N,Ω, f)+CO: Memories of a lifetime

The product-state-resolved dynamics of the reaction H+CO2→OH(2Π;ν,N,Ω,f)+CO have been explored in the gas phase at 298 K and center-of-mass collision energies of 2.5 and 1.8 eV (respectively, 241 and 174 kJ mol−1), using photon initiation coupled with Doppler-resolved laser-induced fluorescence detection. A broad range of quantum-state-resolved differential cross sections (DCSs) and correlated product kinetic energy distributions have been measured to explore their sensitivity to spin–orbit, Λ-doublet, rotational and vibrational state selection in the scattered OH. The new measurements reveal a rich dynamical picture. The channels leading to OH(Ω,N∼1) are remarkably sensitive to the choice of spin–orbit state: Those accessing the lower state, Ω=3/2, display near-symmetric forward–backward DCSs consistent with the intermediacy of a short-lived, rotating HOCO (X̃ 2A′) collision complex, but those accessing the excited spin–orbit state, Ω=1/2, are strongly focused backwards at the higher collision energy, indicating an alternative, near-direct microscopic pathway proceeding via an excited potential energy surface. The new results offer a new way of reconciling the conflicting results of earlier ultrafast kinetic studies. At the higher collision energy, the state-resolved DCSs for the channels leading to OH(Ω,N∼5–11) shift from forward–backward symmetric toward sideways–forward scattering, a behavior which resembles that found for the analogous reaction of fast H atoms with N2O. The correlated product kinetic energy distributions also bear a similarity to the H/N2O reaction; on average, 40% of the available energy is concentrated in rotation and/or vibration in the scattered CO, somewhat less than predicted by a phase space theory calculation. At the lower collision energy the discrepancy is much greater, and the fraction of internal excitation in the CO falls closer to 30%. All the results are consistent with a dynamical model involving short-lived collision complexes with mean lifetimes comparable with or somewhat shorter than their mean rotational periods. The analysis suggests a potential new stereodynamical strategy, “freeze-frame imaging,” through which the “chemical shape” of the target CO2 molecule might be viewed via the measurement of product DCSs in the low temperature environment of a supersonic molecular beam.

Intramolecular energy transfer in highly vibrationally excited methanol. III. Rotational and torsional analysis

We report here torsional analysis of rotationally resolved spectra of the 3ν1, 5ν1, and 6ν1 (OH stretch) bands of jet-cooled methanol. The upper states are reached by a double resonance excitation scheme involving the selection of single rotational states in the ν1 fundamental band. Detection of the overtone transitions (nν1←ν1) is by infrared laser assisted photofragment spectroscopy (IRLAPS). The torsional tunneling frequency declines monotonically from 9.1 cm−1 in the vibrational ground state to 1.6 cm−1 at 6ν1. For the available rotational levels at 3ν1 (K=0–3) and 6ν1 (K=0,1), the pattern of torsional energies is approximately regular. To obtain the vibrational dependence of the torsional barrier V3, it was necessary to use the OH radical and HOOH as models for the vibrational dependence of the torsional inertial constant F. The assumed linear dependence of V3 on ν1 accounts for the torsional tunneling splittings at v1=0, 3, and 6 and for the pattern of the torsional energies. V3 increases by 40–45 cm−1 per quantum of OH excitation.

Quantum state-resolved reactive scattering of F+H2 in supersonic jets: Nascent HF(v,J) rovibrational distributions via IR laser direct absorption methods

Supersonically cooled discharge radical atom sources are combined with high-sensitivity IR absorption methods to investigate state-to-state reactive scattering of F+n-H2→HF(v,J)+H in low-density crossed supersonic jets at center-of-mass collision energies of 2.4(6) kcal/mole. The product HF(v,J) is probed with full vibrational and rotational quantum state selectivity via direct absorption of a single mode (Δν≈0.0001 cm−1), tunable F-center laser in the Δv=1 fundamental manifold with near shot noise limited detection levels of 108 molecules/cm3/quantum state per pulse. The high absorption sensitivity, long mean free path lengths, and low-density conditions in the intersection region permit collision-free HF(v,J) rovibrational product state distributions to be extracted for the first time. Summed over all rotational levels, the HF vibrational branching ratios are 27.0(5)%, 54.2(23)%, 18.8(32)%, and <2(2)%, respectively, into vHF=3:2:1:0. The nascent vibrational distributions are in good agreement with rotationally unresolved crossed-beam studies of Neumark et al. [J. Chem. Phys. 82, 3045 (1985)], as well as with full quantum close-coupled calculations of Castillo and Manolopoulos [J. Chem. Phys. 104, 6531 (1996)] on the lowest adiabatic F+H2 potential surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)]. At a finer level of quantum state resolution, the nascent rotational distributions match reasonably well with full quantum theoretical predictions, improving on the level of agreement between theory and experiment from early arrested relaxation studies. Nevertheless, significant discrepancies still exist between the fully quantum state-resolved experiment and theory, especially for the highest energetically allowed rotational levels.

Rotational State Selection and Orientation of OH and OD Radicals by Electric Hexapole Beam-Focusing

An electrostatic hexapole was used to state-select OH and OD radicals in single, low-lying, |JΩMJ〉 rotational states. The radicals were produced in a corona discharge, supersonic molecular beam source by dissociating H2O (D2O) seeded in Ar or He. Beam velocities ranged from 650 to 1850 m s-1, and translational temperatures were less than 10 K for all expansion conditions. Measured beam flux densities, J, of selected states were high (e.g., J > 1013 radicals cm-2 s-1 for the |3/2 ±3/2 ∓3/2〉 states of OH seeded in He). Classical trajectory simulations reproduced the well-resolved rotational state structure of experimental beam-focusing spectra. Simulations were based on a Stark energy analysis of the rotational energy levels, including significant effects due to Λ-doubling and spin−orbit coupling. Orientational probability distribution functions were calculated in the high-field limit for all selectable states and demonstrate exceptional experimental control over collision geometry for scattering experiments.

Rotational energy transfer in vibrationally excited acetylene X̃ 1Σg(ν2″=1,J″):ΔJ propensities

A complete set of state-to-state rotational energy transfer rate constants has been measured for acetylene–acetylene collisions at room temperature under single collision conditions. Initial rotational states (Ji=5,7,...,25) were prepared and final states (Jf=1,...,25) interrogated. The measurements were carried out in a typical gas phase pump and probe arrangement. The initial vibrationally excited state was prepared by stimulated Raman pumping using strong Q-branch transitions. State preparation via this branch produces an isotropic spatial distribution of the excited state which is important for data analysis. Narrow bandwidth lasers ensure single rotational state selectivity. The rotational distribution after collisions is monitored by time-delayed laser-induced fluorescence via the à 1Au(ν3′ = 1)←X̃ 1Σg(ν2″ = 1) transition. In general, the rate constants decrease exponentially with the transferred rotational energy. The complete rate constant matrix can consistently be described by a simple parameter set within the dynamical infinite order sudden power approximation. In addition to this general behavior a significant ΔJ propensity of the rate constants is observed. Using the energy corrected sudden approximation with a power law basis an excellent match, reproducing the ΔJ propensities, to the rate constant matrix is obtained, again with a single set of parameters.

Velocity dependence of collisional alignment of oxygen molecules in gaseous expansions

THE orientational dependence of molecular interactions has long been recognized as central to an understanding of reaction mechanisms and of collisions in the gas phase and at surfaces. Studies of orientation effects have recently become possible owing to the development of techniques for aligning molecules. 'Brute-force' methods using electric or magnetic fields can induce alignment of molecules with dipole moments1,2, and polarized-absorption approaches3 can be used in cases where there are suitable molecular transitions; but one of the simplest and most general methods involves the supersonic expansion of molecular beams seeded with molecules that induce rotational alignment—selection of specific rotational states—by collisions4–12. Here we use such an approach to induce strong rotational alignment of oxygen molecules in a beam seeded with various other gases at close to atmospheric pressure. Most significantly, we find that the degree of alignment depends on the velocity of the molecules in the supersonic expansion—fast molecules are much more highly aligned than slower ones, and the velocity of maximum alignment can be altered by changing the gas mixture. In this way, we can prepare rotationally aligned molecules with well defined velocities, opening up new possibilities for experiments in molecular dynamics.