Rotational State Populations Research Articles

Ternary Association Reaction of Vibrationally Cold N2+ Ions in Ambient Helium.

The rate coefficients for the ternary association reaction of vibrationally cold N2+ ions with N2 and He were measured in an afterglow plasma in the temperature range of 140-250 K: kN4+ = (1.02 ± 0.39)(300/T)(1.28±0.08) × 10-29 cm6 s-1. The rotational and vibrational state populations of N2+ ions were probed in situ by time-resolved cavity ring-down spectroscopy. The rate coefficient for Penning ionization of N2 by helium metastable atoms was also determined: km = [(2.70 ± 0.17) + (84.3 ± 5.9)e-(877±36)/T] × 10-11 cm3 s-1.

Near-complete chiral selection in rotational quantum states

Controlling the internal quantum states of chiral molecules for a selected enantiomer has a wide range of fundamental applications from collision and reaction studies, quantum information to precision spectroscopy. Achieving full enantiomer-specific state transfer is a key requirement for such applications. Using tailored microwave fields, a chosen rotational state can be enriched for a selected enantiomer, even starting from a racemic mixture. This enables rapid switching between samples of different enantiomers in a given state, holding great promise, for instance, for measuring parity violation in chiral molecules. Although perfect state-specific enantiomeric enrichment is theoretically feasible, achieving the required experimental conditions seemed unrealistic. Here, we realize near-ideal conditions, overcoming both the limitations of thermal population and spatial degeneracy in rotational states. We achieve over 92% enantiomer-specific state transfer efficiency using enantiopure samples. This indicates that 96% state-specific enantiomeric purity can be obtained from a racemic mixture, in an approach that is universally applicable to all chiral molecules of C1 symmetry. Our work integrates the control over internal quantum states with molecular chirality, thus expanding the field of state-selective molecular beams studies to include chiral research.

Multiple Dissociation Pathways in HNCO Decomposition Governed by Potential Energy Surface Topography.

The exquisite features of molecular photochemistry are key to any complete understanding of the chemical processes governed by potential energy surfaces (PESs). It is well established that multiple dissociation pathways relate to nonadiabatic transitions between multiple coupled PESs. However, little detail is known about how the single PES determines reaction outcomes. Here we perform detailed experiments on HNCO photodissociation, acquiring the state-specific correlations of the NH (a1Δ) and CO (X1Σ+) products. The experiments reveal a trimodal CO rotational distribution. Dynamics simulations based on a full-dimensional machine-learning-based PES of HNCO unveil three dissociation pathways exclusively occurring on the S1 excited electronic state. One pathway, following the minimum energy path (MEP) via the transition state, contributes to mild rotational excitation in CO, while the other two pathways deviating substantially from the MEP account for relatively cold and hot CO rotational state populations. These peculiar dynamics are unambiguously governed by the S1 state PES topography, i.e., a narrow acceptance cone in the vicinity of the transition state region. The dynamical picture shown in this work will serve as a textbook example illustrating the importance of the PES topography in molecular photochemistry.

Quantum state-resolved methane scattering from Ni(111) and NiO(111) by bolometer infrared laser tagging: The effect of surface oxidation.

We describe a novel ultrahigh vacuum state-to-state molecule/surface scattering apparatus with quantum state preparation of the incident molecular beam and angle-resolved quantum state detection of the scattered molecules. State-resolved detection is accomplished using a tunable mid-infrared laser source combined with a cryogenic bolometer detector and is applicable to any molecule with an infrared-active vibrational transition. Results on rotationally inelastic scattering of CH4 methane from a Ni(111) surface and NiO(111)/Ni(111) oxide film, obtained by the new apparatus, are presented. Molecules scattering from the oxidized surface, compared to those scattering from the bare nickel surface, are more highly excited rotationally and scatter into a broader distribution of angles. The internal alignment of molecular rotation is in addition found to be stronger in molecules scattering from the bare surface. Furthermore, the maxima of the state-resolved angular distributions shift toward and away from surface normal with increasing rotational quantum number J for the oxidized and bare surface, respectively. Finally, the rotational state populations produced in scattering from the oxidized surface are well-described by a Boltzmann distribution, while those produced in scattering from the bare surface exhibit large deviations from their best-fit Boltzmann distributions. These results point toward a marked enhancement in molecule-surface collisional energy exchange induced by oxidation of the nickel surface.

Understanding the temperatures of H3 + and H2 in diffuse interstellar sightlines

The triatomic hydrogen ion H 3 + is one of the most important species for the gas phase chemistry of the interstellar medium. Observations of H 3 + are used to constrain important physical and chemical parameters of interstellar environments. However, the temperatures inferred from the two lowest rotational states of H 3 + in diffuse lines of sight – typically the only ones observable – appear consistently lower than the temperatures derived from H 2 observations in the same sightlines. All previous attempts at modelling the temperatures of H 3 + in the diffuse interstellar medium failed to reproduce the observational results. Here we present new studies, comparing an independent master equation for H 3 + level populations to results from the Meudon PDR code for photon dominated regions. We show that the populations of the lowest rotational states of H 3 + are strongly affected by the formation reaction and that H 3 + ions experience incomplete thermalisation before their destruction by free electrons. Furthermore, we find that for quantitative analysis more than two levels of H 3 + have to be considered and that it is crucial to include radiative transitions as well as collisions with H2. Our models of typical diffuse interstellar sightlines show very good agreement with observational data, and thus they may finally resolve the perceived temperature difference attributed to these two fundamental species.

The CH-3Σ+ Anion: Inelastic Rate Coefficients from Collisions with He at Interstellar Conditions.

We present accurate ab initio calculations on several properties of a gas-phase system of interest in the interstellar medium (ISM), where the title molecular anion has been often surmised but not yet confirmed by observations. The CH-3Σ+ constitutes the smallest term in the series of longer anionic polyynes which have been observed in the ISM (e.g., C4H- and several others). Hence, its dynamical behavior in collision with He atoms, one of the most abundant atoms in that environment, can provide quantitative indicators on the changes which can occur in the rotational state population of the title anion when driven by this collision dynamics. We therefore report an accurate evaluation of the full potential energy surface (PES) which acts between the molecular anion in its ground vibrational state and the He atom. The relevant inelastic scattering cross sections and the corresponding inelastic rate coefficients are then computed within a quantum treatment of the collisions. We find that the fairly small values of the final inelastic rate coefficients indicate state-changing processes by collisions to be inefficient paths for modifying the rotational state populations of this anion and therefore to aid its possible observation from direct radiative emission in the microwave region.

Revival structure of rotational wave packets in the process of the field-free molecular orientation

We investigate theoretically the relationship between the revival structure of rotational wave packets and the orientation of the LiH molecule induced by the half-cycle THz laser pulse (HCP). The revival structure of oriented rotational wave packets can be further controlled through the choice of the delay time and the phase difference between two HCPs. We found the higher absolute values of the orientation corresponding to the higher asymmetries and more centralization of the time-dependent probability density ψθ,t2. We also found a similar population of the adjacent rotational states gives a higher ceiling for the absolute value of molecular orientation. Finally, the effect of the system temperature is discussed, the higher temperature makes a lower degree of orientation steered by two HCPs.

Ozone Photodissociation in the Singlet Channel at 226 nm.

We report the rotational state distribution and vector correlations of the O2(a 1Δg, v = 0) fragments arising from the 226 nm photodissociation of jet-cooled O3. Consistent with previously reported trends, the rotational distribution is shifted to higher rotational states with decreasing wavelength. We observe highly suppressed odd rotational state populations due to a strong Λ-doublet propensity. The measured rotational distribution is in agreement with classical trajectory calculations for the v = 0 products, although the distribution is slightly narrower than predicted. The spatial anisotropy follows the previously observed trend of decreasing β with increasing photon energy with β = 0.72 ± 0.14 for v = 0, j = 38. As expected for a triatomic molecule, the v-j correlation is consistent with v perpendicular to j, but the measured correlation is nonlimiting due, in part, to rotational and translational depolarization. The j-dependent line width of the O2(a 1Δg) REMPI spectrum is also discussed in connection with the lifetime of the resonant O2(d 1Πg) state due to predissociation via the II 1Πg valence state.

Spectroscopy and ion thermometry of C2− using laser-cooling transitions

A prerequisite for laser cooling a molecular anion, which has not been achieved so far, is the precise knowledge of the relevant transition frequencies in the cooling scheme. To determine these frequencies we present a versatile method that uses one pump and one photodetachment light beam. We apply this approach to C$_{2}^{-}$ and study the laser cooling transitions between the electronic ground state and the second electronic excited state in their respective vibrational ground levels, $B ^{2} \Sigma _{u} ^{+}(v=0) \leftarrow X ^{2} \Sigma _{g} ^{+}(v=0) $. Measurements of the R(0), R(2), and P(2) transitions are presented, which determine the transition frequencies with a wavemeter-based accuracy of $0.7\times10^{-3}$ cm$^{-1}$ or 20 MHz. The spin-rotation splitting is resolved, which allows for a more precise determination of the splitting constants to $\gamma ' = 7.15(19)\times10^{-3}$ cm$^{-1}$ and $\gamma '' = 4.10(27)\times10^{-3}$ cm$^{-1}$. These results are used to characterize the ions in the cryogenic 16-pole wire trap employed in this experiment. The translational and rotational temperature of the ions cooled by helium buffer gas are derived from the Doppler widths and the amplitude ratios of the measured transitions. The results support the common observation that the translational temperature is higher than the buffer gas temperature due to collisional heating under micromotion, in particular at low temperatures. Additionally, a rotational temperature significantly lower than the translational is measured, which agrees with the notion that the mass weighted collision temperature of the C$_{2}^{-}$-He system defines the internal rotational state population.

Analysis of nonequilibrium atomic and molecular nitrogen radiation in pure N2 shockwaves

This paper presents an analysis of data previously reported from a test series in the Electric Arc Shock Tube Facility for incident shocks composed of pure nitrogen. The present work focuses on the lowest velocity measured in the test (approximately 7 km/s) where both molecular and atomic radiation are significant. Analysis of the spectral data with the NEQAIR radiative transport code obtains post-shock trends in temperature and number densities of the states of N2 and atomic N. The results suggest that chemical, and possibly thermal, equilibrium has not been obtained within the distance/time measured (about 5 cm/80 µs). In the nonequilibrium region of the shock, differences between rotational temperatures of different species are observed, as well as differences between rotational and vibrational temperatures. Population of rotational states above the dissociation limit is observed to occur over a distance of several cm. Finally, these data suggest that predissociation strongly affects the C3Πu state of N2, leading to a lower measured radiation than predicted. An effective average pre-dissociation rate is shown to produce reasonable agreement under quasi-steady state approximation.

Rotational dephasing of the molecular alignment by centrifugal distortion [Invited

The dephasing of molecular alignment can lead to the deformation of the alignment signal during its periodic revivals. Most studies are concentrated on the first few rotational revival periods of the molecular alignment and neglect the dephasing effect. However, study of the alignment dephasing is still of great significance for both the long-term dynamics of the molecular alignment and the dephasing itself. In this work, we theoretically demonstrate that the dephasing effect is correlated with both the rotational temperature and the rotational revival period of the molecules. The results present that the dephasing is especially significant for those molecules with long rotational revival period at high rotational temperatures. The physics behind it is explored by taking advantage of the coherence of the rotational quantum state population. This work deepens our understanding of rotational dynamics and rotational spectroscopy in molecular alignment.

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.

Photodetachment in cold ion traps

We present a computational investigation of the photodetachment (PD) dynamics of C2H− in its ground, electronic 1Σ state, cooled in an ion trap with He as a buffer gas. Our analysis employs a range of PD rates from quantum dynamics, selective choices of the anion’s initial rotational state populations as the laser is switched on, and different operational trap conditions. Our results show that a variety of outcomes for the initial state-dependent PD process is possible by selecting different initial conditions in the cold trap, the latter generating a dynamical interplay between the anion’s populations of rotational states, induced by the buffer gas, and the chosen laser frequency and laser power. Specific parameter selections during the PD process will be shown to cause different abundances of the rotational states of the anion in the trap.Graphical abstract

Diagnostics of Collisions between Electrons and Water Molecules in Near-ultraviolet and Visible Wavelengths

Abstract We studied the dissociation reactions of electron impact on water vapor for several fragment species at optical and near-ultraviolet wavelengths (200–850 nm). The resulting spectrum is dominated by the hydrogen Balmer series, by the OH (A 2Σ+ − X 2Π) band, and by the emission of ionic H2O+(A 2A1 − X 2B1) and OH+(A 3Π − X 3Σ−) band systems. Emission cross sections and reaction channel thresholds were determined for energies between 5 and 100 eV. We find that the electron impact dissociation of H2O results in an emission spectrum of the OH (A 2Σ+ − X 2Π) band that is distinctly different from the emission spectra from other excitation mechanisms seen in planetary astronomy. We attribute the change to a strongly non-thermal population of rotational states seen in planetary astronomy. This difference can be utilized for remote probing of the contribution of different physical reactions in astrophysical environments.

Modeling state-selective photodetachment in cold ion traps: Rotational state "crowding" in small anions.

Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH-(1Σ), MgH-(1Σ), and C2H-(1Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions.

A Spectral Analysis of the Centimeter Regime of Nearby Galaxies: RRLs, Excited OH, and NH3

Centimeter-wave transitions are important counterparts to the rotational mm-wave transitions usually observed to study gas in star-forming regions. However, given their relative weakness, these transitions have historically been neglected. We present Australia Telescope Compact Array 4cm- and 15mm-band spectral line observations of nine nearby star-forming galaxies in the H75 array configuration. Thirteen different molecular lines are detected across the sample from OH, NH$_3$, H$_2$O, H$_2$CO, and c-C$_3$H$_2$, as well as 18 radio recombination lines (RRLs) in NGC 253. Excited OH $^2\Pi_{3/2}$ absorption is detected towards NGC 253 (J=5/2), NGC 4945 (J=9/2), and Circinus (J=9/2); the latter two represent only the third and fourth extragalactic J=9/2 detections. These lines in Circinus suggest rotation temperatures in excess of 2000 K, and thus it is likely that the populations of OH rotational states are not governed by a Boltzmann distribution. Circinus's OH lines are blueshifted from the systemic velocity by ~35 km s$^{-1}$, while NGC 4945's are redshifted by ~100 km s$^{-1}$. NGC 4945's OH absorption likely indicates infall onto the nucleus. The NH$_3$ (1,1) through (6,6) lines in NGC 4945 display a superposition of emission and absorption similar to that seen in other dense gas tracers. Strong (3,3) emission points towards maser activity. The relative NH$_3$ absorption strengths in NGC 4945 show similar anomalies as in previous studies of Arp 220 (weak (1,1) and strong (5,5) absorption). A trend towards higher LTE electron temperatures with increasing RRL frequency is present in NGC 253, likely indicative of stimulated emission within the nuclear region.

Determination of the normal A2Π state in MgF with application to direct laser cooling of molecules.

We report high resolution electronic spectroscopy of cold magnesium monofluoride (MgF) molecules in the gas phase, which are created by a combination of laser ablation, chemical reaction, and 6 K helium buffer-gas cooling. Thanks to the sufficient population in the low-lying rotational states, the P, Q, and R branches in the electronic transition of the X2Σ+ to A2Π state are able to be measured unambiguously by in-cell absorption spectra. For the first time, we show that the A2Π state of MgF is actually a normal state, not an inverted one. The laser cooling relevant transitions X2Σ+v=0,1,N=1→A2Π1/2(v=0,J'=1/2) are also identified, along with the hyperfine structure of the X2Σ+(v = 0, N = 1) state. This study provides an important step for ongoing laser cooling experiments of MgF molecules.

Towards state selective recombination of H3+ under astrophysically relevant conditions.

We present studies on the thermalisation of H3+ ions in a cold He/Ar/H2 plasma at temperatures 30-70 K. We show that we are able to generate a rotationally thermalised H3+ ensemble with a population of rotational and nuclear spin states corresponding to a particular ion translational temperature. By varying the para-H2 fraction used in the experiment we are able to produce para-H3+ ions with fractional populations higher than those corresponding to thermodynamic values. At 35 K, only the lowest rotational states of para and ortho H3+ are populated. This is the first step towards experimental studies of electron-molecular ion recombination processes with precisely specified quantum states at astrophysically relevant temperatures.

Synthetic dimensions in ultracold polar molecules

Synthetic dimensions alter one of the most fundamental properties in nature, the dimension of space. They allow, for example, a real three-dimensional system to act as effectively four-dimensional. Driven by such possibilities, synthetic dimensions have been engineered in ongoing experiments with ultracold matter. We show that rotational states of ultracold molecules can be used as synthetic dimensions extending to many – potentially hundreds of – synthetic lattice sites. Microwaves coupling rotational states drive fully controllable synthetic inter-site tunnelings, enabling, for example, topological band structures. Interactions leads to even richer behavior: when molecules are frozen in a real space lattice with uniform synthetic tunnelings, dipole interactions cause the molecules to aggregate to a narrow strip in the synthetic direction beyond a critical interaction strength, resulting in a quantum string or a membrane, with an emergent condensate that lives on this string or membrane. All these phases can be detected using local measurements of rotational state populations.

Intensity-independent molecular rotational decoherence lifetimes measured with mean wavelength shifts of femtosecond pulses

We report on an experimental investigation on the dynamic decoherence process of molecular rotational wavepackets during femtosecond laser filamentation based on time-dependent mean wavelength shifts of a weak probe pulse. Details of periodic revival structures of transient alignment can be readily obtained from the measured shifted spectra due to the periodic modulation of the molecular refractive index. Using the method, we measured decoherence lifetimes of molecular rotational wavepackets in N2 and O2 under different experimental conditions. Our results indicate that decoherence lifetimes of molecular rotational wavepackets are primarily determined by the relative population of rotational states in the wave packet and intermolecular collisions, rather than the focusing intensity.