High Rotational Levels Research Articles

Collision-induced excitation of ammonia in warm interstellar and circumstellar environments

ABSTRACT Ammonia (NH3) is the first polyatomic molecule detected in the interstellar medium. Both its spectroscopic and collisional properties have been extensively studied earlier, and NH3 has often been used in laboratory astrophysics studies to compare high-level scattering calculations with state-of-the-art experiments. Nevertheless, some of its important collisional properties remain unresolved. In this paper, we report state-to-state and thermally averaged collisional data for the rotational excitation of NH3 by H2 calculated with the close-coupling quantum theory. Both nuclear spin symmetries (ortho/para) of the colliders are studied. Similar research has been carried out previously, providing rate coefficients up to a temperature of 200 K for rotational states with internal energy up to ∼420 cm−1. Here, we have computed cross sections for collision energies up to 4700 cm−1 and rate coefficients up to 500 K. Most of the rotation-inversion levels of ammonia have been considered below the first vibrational excitation threshold, leading to a total of 33 ortho- and 62 para-NH3 states. We have compared our results with the most accurate data for He and H atoms available in the literature. The propensity rules have also been analysed in the case of high rotational levels of NH3. The rate coefficients obtained by averaging over the thermal H2 relative populations exhibit significantly larger magnitudes than the state-to-state collisional data in the case of NH3 transitions with large internal energy difference, when the rotational energy transfer between the colliders is strong.

Observation of rotationally dependent fine-structure branching ratios near the predissociation threshold N(2D5/2,3/2) + N(2D5/2,3/2) of 14N2.

Photofragment spin-orbit fine-structure branching ratios have long been predicted to depend on the rotational quantum number J' by theory near the dissociation thresholds of several diatomic molecules, while this has rarely been observed in any photodissociation experiments yet. Here, we measured the fine-structure branching ratios N(2D5/2)/N(2D3/2) produced in the N(2D5/2,3/2) + N(2D5/2,3/2) channel at the b'1Σu +(v = 20) state of 14N2 by using our vacuum ultraviolet (VUV)-pump-VUV-probe time-sliced velocity-mapped ion imaging setup. It is found that 14N2 almost exclusively dissociates into the spin-orbit channel N(2D5/2) + N(2D3/2) at low rotational levels and gradually approaches the statistical or diabatic limit by distributing all possible spin-orbit channels at higher rotational levels. The strongly rotationally dependent fine-structure branching ratios should be due to the increasing strength of nonadiabatic Coriolis interaction among various dissociative states in the so-called "recoupling zone" as J' increases. They are supposed to provide unprecedented information on the near threshold photodissociation dynamics of 14N2.

Anisotropy Measurements from the Near-Threshold Photodissociation of the N2-NO Complex.

We have used velocity map ion imaging to measure the angular anisotropy of the NO (A) products from the photodissociation of the N2-NO complex. Our experiment ranged from 108 to 758 cm-1 above the threshold energy to form NO (A) + N2 (X) products, and these measurements reveal, for the first time, a strong angular anisotropy from photodissociation. At 108 cm-1 above the photodissociation threshold, we observed NO (A) photoproducts recoil preferentially perpendicular to the laser polarization axis with an average anisotropy parameter, β = -0.25; however, as the available energy was increased, the anisotropy increased, and at 758 cm-1 above the threshold energy, we found an average β = +0.28. The observed changes in the angular anisotropy of the NO (A) photoproduct are qualitatively similar to those observed for the photodissociation of the Ar-NO complex and likely result from changes in the region of the excited state potential energy surface accessed during the electronic excitation. At the lowest available energy, we also noted a large contribution from hotband excitation; however, this contribution decreased as the available energy increased. The outsized contribution at the lowest available energy may result from hotbands having better Franck-Condon overlap with the excited electronic state near threshold. Finally, we contrast the experimental center of mass translational energy distribution with a statistical energy distribution determined from phase space theory. The experimental and statistical distributions show pronounced disagreement, particularly at low kinetic energies, with the experimental one showing less dissociation resulting in high rotational levels of the fragments.

Isotope effect of rovibrational distribution of hydrogen molecules desorbed from amorphous carbon

When the hydrogen isotope atom is injected into the amorphous carbon with the incident energies E in of 20, 50, and 80 eV, we obtain the following physical quantities of hydrogen isotope atoms/molecules emitted from the amorphous carbon using molecular dynamics and heat conduction hybrid simulation. The physical quantities are the time evolution of the emission rate, the depth distribution of the original location of the hydrogen emitted from the target, the polar angular dependence, and the translational, rotational, and vibrational energy distributions. In addition, the approximate analysis yields the emission distributions at the vibrational (v) and rotational (J) levels. Using these distributions, we evaluate the rotational temperature T rot for v = 0 and small J states. From the above, it is found that molecules with higher rotational levels J tend to be emitted as E in increases or as the mass of hydrogen isotope increases. Moreover, the isotope effect appears in the mass dependence of T rot.

Modelled effects of temperature gradients and waves on the hydroxyl rotational distribution in ground-based airglow measurements

Abstract. Spectroscopy of the hydroxyl (OH) airglow has been a commonly used way to remotely sense temperatures in the mesopause region for many decades. This technique relies on the OH rotational state populations to be thermalized through collisions with the surrounding gas into a Boltzmann distribution characterized by the local temperature. However, deviations of the rotational populations from a Boltzmann distribution characterized by a single temperature have been observed and attributed to an incomplete thermalization of the OH from its initial, non-thermodynamic-equilibrium distribution. Here we address an additional cause for the apparent amount of excess population in the higher rotational levels of the OH airglow brought about by integrating these OH emissions through vertical gradients in the atmospheric temperature. We find that up to 40 % of the apparent excess population, currently attributed to incomplete thermalization, can be due to the vertical temperature gradients created by waves. Additionally, we find that the populations of the different upper vibrational levels are affected differently. These effects need to be taken into account in order to assess the true extent of non-thermodynamic-equilibrium effects on the OH rotational populations.

First Detection of A–X (0,0) Bands of Interstellar C2 and CN

Abstract We report the first detection of C2 (0,0) and CN (0,0) absorption bands in the interstellar medium. The detection was made using the near-infrared (0.91–1.35 μm) high-resolution (R = 20,000 and 68,000) spectra of Cygnus OB2 No. 12 collected with the WINERED spectrograph mounted on the 1.3 m Araki telescope. The A–X (1,0) bands of C2 and CN were detected simultaneously. These near-infrared bands have larger oscillator strengths, compared with the A–X (2,0) bands of C2 and CN in the optical. In the spectrum of the C2 (0,0) band with R = 68,000, three velocity components in the line of sight could be resolved and the lines were detected up to high rotational levels (J″ ∼ 20). By analyzing the rotational distribution of C2, we could estimate the kinetic temperature and gas density of the clouds with high accuracy. Furthermore, we marginally detected weak lines of 12C13C for the first time in the interstellar medium. Assuming that the rotational distribution and the oscillator strengths of the relevant transitions of 12C2 and 12C13C are the same, the carbon isotope ratio was estimated to be 12C/13C = 50–100, which is consistent with the ratio in the local interstellar medium. We also constrained the oscillator strength ratio of the C2 (0,0) and (1,0) bands, for which there exists a discrepancy between theoretical calculations and experimental results. This unique constraint obtained from astronomical observation will contribute to improving the accuracy of the oscillator strength measurement, which will lead to further advancements of the C2 excitation model and allow the physical conditions of clouds to be derived.

Fourier-transform spectroscopy of the A2Πi − X2Σ+ system in CO+ and deperturbation analysis of the A2Πi(v = 0, 1) levels

The A2Πi state of 12C16O+ has been reinvestigated using high-resolution, emission spectra obtained via Fourier-transform spectroscopy of the (0, 1), (0, 2), (1, 0) and (1, 1) bands of the Comet–Tail (A2Πi − X2Σ+) system with accuracy of up to 0.005 cm−1. The high temperature (900 K) of the emission discharge inside the hollow-cathode lamp permitted monitoring of high rotational quantum levels in A2Πi(v = 0, 1) up to J = 48.5. All these bands together with the X2Σ+ − X2Σ+(10, 1) and (10, 2) transitions deriving from a complex, spin-orbit, spin-electronic and L-uncoupling A2Π(v = 0) ∼ X2Σ+(v = 10) interaction as well as the B2Σ+ − X2Σ+(6, 10) frequency transitions from our previous study were taken into consideration. In total, 1849 spectral lines were included in the comprehensive deperturbation analysis. This fit leads to a much improved description in terms of deperturbed molecular constants and interaction parameters compared to the previous studies of the same energy region in CO+. Moreover, the ro-vibronic term values of the A2Πi(v = 0) level and its perturber X2Σ+(v = 10) were determined as well as Λ-doubling of A2Πi(v = 0,1), γ-doubling of X2Σ+(v = 10) and percentage 2Π character of these levels were considered in detail.

Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm.

Roaming dynamics is observed in the photodissociation of formic acid (HCOOH) at 230 nm by using the slice imaging method. In combination with rotational state selective (2 + 1) resonance-enhanced multiphoton ionization of the CO fragments, the speed distributions of the CO fragments exhibit a low recoil velocity at low rotational levels of J = 9 and 20, while the velocity distributions of CO at high rotational levels of J = 30 and 48 show a relatively large recoil velocity. The experimental results indicate that the roaming of OH radical should be related with the formation of CO + H2O channel at the present photolysis energy. Unlike the roaming pathways occurring in H2CO that can be described by loose flat potential, our CO speed distribution analysis suggests the presence of a "tight" flat potential in the roaming dynamics of HCOOH molecules.

Precision spectroscopy and comprehensive analysis of perturbations in the A1∏(v = 0) state of 13C18O

ABSTRACTWe have reinvestigated the A level of 13C18O using new high-resolution spectra obtained via multi-photon laser excitation as well as with synchrotron-based Fourier-transform absorption spectroscopy of the A, e, d, a, and a bands. In addition, Fourier-transform emission spectroscopy in the visible range is performed on the band. Spectra of the B band are measured in order to tie information from the latter emission data to the level structure of A. The high pressures in the absorption cell at the synchrotron and the high temperatures in the emission discharge permitted monitoring of high rotational quantum levels in A up to J=43. All information, in total over 900 spectral lines, was included in an effective Hamiltonian analysis of the Alevels that are directly perturbed by the e, d, a, D, I close-lying levels and the e, d, a remote levels, as well being indirectly influenced by the a state. The influence of 9 further perturber levels and their interactions was investigated and are not significant for reproducing the present experimental data. This analysis leads to a much improved description in terms of molecular constants and interaction parameters, compared to previous studies of the same energy region for other CO isotopologues.

The harpooning mechanism as evidenced in the oxidation reaction of the Al atom.

The harpooning mechanism has long been proposed for elementary reaction dynamics involving metals. It is characterized by an initial electron transfer (ET) process from the metal to the oxidant molecule. For the titled reaction Al + O2, the ET distance can be predicted to be 2.6 Å by simply calculating the energy difference between the ionization energy of the Al atom and the electron affinity of the O2 molecule. Hereby we experimentally derived the maximum impact parameter bmax of 2.5 ± 0.2 Å for the titled reaction, in consistency with the predicted ET distance. This derivation of bmax was achieved by using the crossed molecular beam experiment at a collision energy of 507 cm-1 (i.e. 1.45 kcal mol-1) with a high resolution time-sliced ion velocity imaging detection of the state-selective AlO products based on the (1 + 1) resonance-enhanced multiphoton ionization. The small rotational constant of the AlO(X2Σ+) radical (Be = 0.6413 cm-1) facilitated the formation of the AlO(v = 0) products in high rotational levels up to the energetically limited state, Nmax = 52, with an almost zero velocity mapping. Hence, in this extreme angular momentum disposal case, the collisional orbital angular momentum l was nearly completely channeled into the product rotational angular momentum as a consequence of the conservations of energy and angular momentum, offering a reaction system that breaks the restriction of kinematically favored mass combination in order to obtain information on the impact parameters. The present study yields the first direct derivation of bmax from the maximum rotational level of products under the experimental condition with the recoil energy E'T ≈ 0. This, in turn, provides solid evidence in supporting the harpooning mechanism.

Terahertz pulse shaping through propagation in a gas of symmetric top molecules

Symmetric top molecules of methyl iodide are irradiated with a terahertz pulse generated by a two-color plasma and shaped by a short propagation in air. Free-induction decay is emitted by the excited molecular sample and then propagates in air before detection. The experimental data show that the input terahertz (THz) pulse undergoes strong reshaping through absorption and dispersion. This leads to narrow wave packets at revival times due to the excitation of high rotational energy levels. Typically, a THz burst of duration $\ensuremath{\simeq}15\ensuremath{-}20$ ps is produced periodically, with a central frequency of $\ensuremath{\simeq}1$ THz and a width that can be as narrow as 60--80 GHz. Pulse shaping based on propagation can be useful for quantum control in molecules. We provide a theoretical description of this wave propagation based on the Maxwell-Bloch equation. The observed experimental signal is in good agreement with the numerical simulations.

High-resolution and broadband mid-infrared Zeeman spectrum of gas-phase nitric oxide using a quantum cascade laser around 5.2 μm

A broadly tunable single-mode quantum cascade laser operating around 5.2 μm has been used to study the high-resolution and broadband mid-IR Zeeman spectrum of Gas-phase 14N16O. The Zeeman spectrum signals in the R (from R0.5 to R22.5), Q (from Q0.5 to Q6.5) and P (from P1.5 to P14.5) branches of the fundamental vibrational transition, have been recorded with both large spectral region (over the range of 1824 –1947 cm−1) and high spectral resolution (<0.001 cm−1) for the first time. We simulated the patterns of magnetic Zeeman split lines in the R, Q, and P branches, and calculated the Landé g-factor of the and components for pure Hund’s coupling case (a) and (b), and intermediate coupling (a–b) case. The experimental Zeeman spectrum measured at various values of magnetic field strength (from 0.04 to 0.32 T) at conditions of both and was calibrated by a 101.6 mm long solid germanium etalon with a free spectral range (FSR) of 0.012 cm−1. Good agreement between the calculated Landé g-factors using Radford’s theory and their best fit values inferred from the experimental data was obtained, which suggests that the Landé g-factor in Hund’s coupling case (a–b) is a better approximation, either at higher rotational quantum numbers, or at lower rotational levels, for both and subsystems. Our results provide a more general evidence to determine the coupling case of NO than previously available, because we can measure and simulate the high-resolution and broadband mid-IR Zeeman spectrum for both low and high rotational levels of the two subsystems.

Trajectory and Model Studies of Collisions of Highly Excited Methane with Water Using an ab Initio Potential.

Quasi-classical trajectory studies have been performed for the collision of internally excited methane with water using an accurate methane-water potential based on a full-dimensional, permutationally invariant analytical representation of energies calculated at a high level of theory. The results suggest that most energy transfer takes place at impact parameters smaller than about 8 Bohr; collisions at higher impact parameters are mostly elastic. Overall, energy transfer is fairly facile, with values for ⟨ΔEdown⟩ and ⟨ΔEup⟩ approaching almost 2% of the total excitation energy. A classical model previously developed for the collision of internally excited molecules with atoms (Houston, P. L.; Conte, R.; Bowman, J. M. J. Phys. Chem. A 2015, 119, 4695-4710) has been extended to cover collisions of internally excited molecules with other molecules. For high initial rotational levels, the agreement with the trajectory results is quite good (R(2) ≈ 0.9), whereas for low initial rotational levels it is only fair (R(2) ≈ 0.7). Both the model and the trajectories can be characterized by a four-dimensional joint probability distribution, P(J1,f,ΔE1,J2,f,ΔE2), where J1,f and J2,f are the final rotational levels of molecules 1 and 2 and ΔE1 and ΔE2 are the respective changes in internal energy. A strong anticorrelation between ΔE1 and ΔE2 is observed in both the model and trajectory results and can be explained by the model. There is evidence in the trajectory results for a small amount of V ↔ V energy transfer from the water, which has low internal energy, to the methane, which has substantial internal energy. This observation suggests that V ↔ V energy transfer in the other direction also occurs.

Technical Note: On the possibly missing mechanism of 15 μm emission in the mesosphere–lower thermosphere (MLT)

Abstract. Accurate knowledge of the rate as well as the mechanism of excitation of the bending mode of CO2 is necessary for reliable modeling of the mesosphere–lower thermosphere (MLT) region of the atmosphere. Assuming the excitation mechanism to be thermal collisions with atomic oxygen, the rate coefficient derived from the observed 15 μm emission by space-based experiments (kATM = 6.0 × 10−12 cm3s−1) differs from the laboratory measurements (kLAB =(1.5-2.5) × 10−12 cm3s−1) by a factor of 2–4. The general circulation models (GCMs) of Earth, Venus, and Mars have chosen to use a median value of kGCM = 3.0 × 10−12 cm3s−1 for this rate coefficient. As a first step to resolve the discrepancies between the three rate coefficients, we attempt to find the source of disagreement between the first two. It is pointed out that a large magnitude of the difference between these two rate coefficients (kx ≡ kATM - kLAB) requires that the unknown mechanism involve one or both major species: N2, O. Because of the rapidly decreasing volume mixing ratio (VMR) of CO2 with altitude, the exciting partner must be long lived and transfer energy efficiently. It is shown that thermal collisions with N2, mediated by a near-resonant rotation-to-vibration (RV) energy transfer process, while giving a reasonable rate coefficient kVR for de-excitation of the bending mode of CO2, lead to vibration-to-translation kVT rate coefficients in the terrestrial atmosphere that are 1–2 orders of magnitude larger than those observed in the laboratory. It is pointed out that the efficient near-resonant rotation-to-vibration (RV) energy transfer process has a chance of being the unknown mechanism if very high rotational levels of N2, produced by the reaction of N and NO and other collisional processes, have a super-thermal population and are long lived. Since atomic oxygen plays a critical role in the mechanisms discussed here, it suggested that its density be determined experimentally by ground- and space-based Raman lidars proposed earlier.

Probing the structure and dynamics of molecular clusters using rotational wave packets.

Rotational wave packets of the weakly bound C(2)H(2)-He complex have been created using impulsive alignment. The coherent rotational dynamics were monitored for 600 ps enabling extraction of a frequency spectrum showing multiple rotational energy levels up to J = 4. spectrum has been combined with ab initio calculations to show that the complex has a highly delocalized structure and is bound only by ca. 7 cm(-1). The experiments demonstrate how highly featured rotational spectra can be obtained from an extremely cold environment where only the lowest rotational energy states are initially populated.

Analysis of Anomalous Diffuse Interstellar Bands in the Spectrum of Herschel 36

AbstractThe extraordinary DIBs observed toward Herschel 36 (Dahlstrom et al. 2013) have been analyzed (Oka et al. 2013). The analysis led us to a new way to classify the carriers of DIBs depending on whether the molecules are polar or non-polar. The pronounced Extended Tails toward Red (ETR) observed for DIBs λ5780.5, λ5797.1, and λ6613.6 are explained as due to radiative excitation of high rotational levels of polar carrier molecules in an environment with high radiative temperature ~90 K. Other DIBs (e.g., λ5849.8, λ6196.0, and λ6379.3) which do not show ETR are likely due to non-polar molecules. Model calculations taking into account the interplay of radiative and collisional effects reproduce the observed ETR using realistic molecular parameters if the radiative temperature is sufficiently high (~90 K). The calculation suggests that the carriers of DIBs with ETR are likely medium size molecules with 3 - 6 heavy atoms unless the radiative temperature is much higher.

Infrared Spectroscopy of CO Ro-Vibrational Absorption Lines toward the Obscured AGN IRAS 08572+3915

Abstract We present high-resolution spectroscopy of gaseous CO absorption in the fundamental ro-vibrational band toward the heavily obscured active galactic nucleus (AGN) IRAS 08572$ +$ 3915. We have detected absorption lines up to highly excited rotational levels ($ J$$ \leqq$ 17). The velocity profiles reveal three distinct components, the strongest and broadest ($ \Delta v$$ &amp;gt;$ 200 km s$ ^{-1}$ ) of which is due to blueshifted ($-$ 160 km s$ ^{-1}$ ) gas at a temperature of $ \sim$ 270 K absorbing at velocities as high as $-$ 400 km s$ ^{-1}$ . A much weaker but even warmer ($ \sim$ 700 K) component, which is highly redshifted ($ +$ 100 km s$ ^{-1}$ ), is also detected, in addition to a cold ($ \sim$ 20 K) component centered at the systemic velocity of the galaxy. On the assumption of local thermodynamic equilibrium, the column density of CO in the 270 K component is $ N_{\rm {CO}}$$ \sim$ 4.5 $ \times$ 10$ ^{18}$ cm$ ^{-2}$ , which in fully molecular gas corresponds to an H$ _2$ column density of $ N_{\rm {H_2}}$$ \sim$ 2.5 $ \times$ 10$ ^{22}$ cm$ ^{-2}$ . The thermal excitation of CO up to the observed high rotational levels requires a density greater than $ n_{\rm c}$ (H$ _2$ ) $ &amp;gt;$ 2 $ \times$ 10$ ^{7}$ cm$ ^{-3}$ , implying that the thickness of the warm absorbing layer is extremely small ($ \Delta d$$ &amp;lt;$ 4 $ \times$ 10$ ^{-2}$ pc), even if it is highly clumped. The large column densities and high radial velocities associated with these warm components, as well as their temperatures, indicate that they originate in molecular clouds near the central engine of the AGN.

EXCITATION OF C2IN DIFFUSE INTERSTELLAR CLOUDS

We investigate the effects and the implications of incorporating new collision and radiative rates in modeling the excitation of diatomic carbon molecule. The present results suggest that diffuse and translucent interstellar clouds may present a structure in which regions with different densities and kinetic temperatures overlap along the line of sight, such as core-halo clouds, the nested structure of the molecular gas, and clumpiness. Such conclusion reflects the response of the C{sub 2} rotational ladder to the interplay of thermal and radiative conditions, with low and high rotational levels tracing different regions of the parameter space. To relieve constraints to the formation and excitation of C{sub 2} molecules, we propose a scenario in which the chemistry in diffuse clouds is supplemented by chemistry in many transient and tiny perturbations.

Study of the excitation mechanisms of the second positive system in the negative glow of a N2–Ar discharge

In an Ar–N2 discharge, the high excitation transfer from Ar(3P2,0) to N2 produces an overpopulation of the high rotational levels of the bands of the second positive system (SPS), and so the spectra interpretation is not straightforward.This paper presents a fit function for the SPS bands measured in Ar–N2, which allows us to study the excitation process contributions to the N2(C) level.The procedure was tested in the negative glow of a pulsed Ar–N2 discharge at a pressure of 2.5 Torr, for different mixture concentrations. In this discharge, through the fitting, it was possible to calculate the variation of the N2(C) densities produced by different excitation processes as well as the variation of Ar metastable density.

Perturbation Analysis of the υ = 6 Level in the d3Δ State of CS Based on Its Near-Infrared Absorption Spectrum

The spectrum of CS was recorded in the region of 12,086-12,630 cm(-1) by employing optical heterodyne concentration modulation laser absorption spectroscopy. Nearly 350 transitions were assigned to the (6, 0) band in the d(3)Δ-a(3)Π system of CS. The overtone transitions of the (12, 0) band in the a(3)Π(2)-a(3)Π(0) transition were first observed due to the perturbation interaction between d(3)Δ(1) and a(3)Π(2). The Λ doubling in the a(3)Π(1) state was also resolved at high rotational levels. The molecular constants of the a(3)Π (υ = 0) and d(3)Δ (υ = 6) levels and the perturbation parameters of the d(3)Δ (υ = 6) level were determined through nonlinear least-squares fitting using effective hamiltonians. The calculations of mixing fractions of the perturbed states were performed in order to obtain precise information on the perturbations of the d(3)Δ (υ = 6) levels. The mechanisms for perturbations of d(3)Δ (υ = 6) with the a(3)Π (υ = 12) and A(1)Π (υ = 1) levels, especially for the second-order perturbation, were discussed and explained according to first-order nondegenerate perturbation theory.