Distribution Of Rotational Levels Research Articles

Rotational Cooling Effect on the Rate Constant in the CH3F + Ca+ Reaction at Low Collision Energies.

The rotational cooling effect on the reaction rate constant of the gas-phase ion-polar-molecule reaction CH3F + Ca+ → CH3 + CaF+ was experimentally studied at low collision energies. Fluoromethane molecules showed higher reactivity as the rotational temperature decreased. The experimental rate constants were compared with the capture rate constants which were obtained by the Perturbed Rotational State (PRS) theory assuming the rotational level distribution corresponding to the experimental conditions. The PRS result shows a strong dependence of the capture rate constants on the rotational level distribution in accordance with the experimental findings. However, the PRS capture rate constants deviate from the measurement values as the average collision energy increases especially when the fluoromethane molecules are rotationally cooled far below room temperature. The present paper suggests that the rotational state distribution significantly affects the rate constants of ion-polar-molecule reactions and is one of the important issues to be considered in the study of molecular synthesis in the interstellar medium, where the thermal equilibrium is not necessarily established.

On the rotational–translational equilibrium in non-thermal argon plasmas at atmospheric pressure

This work examines the rotational–translational equilibrium in non-thermal, argon-based plasmas at atmospheric pressure. In particular, rotational temperatures (T rot) and neutral gas temperatures (T g) are compared along the axis of plasma columns sustained by either radiofrequency (RF) or microwave (MW) electromagnetic fields. Water vapours or N2 admixtures are added to the high-purity argon plasmas to record the rotational temperatures from the emission spectra of either the OH(A2Σ + − X2Π i ) or the N2 +(B2Σ u + − X2Σ g + ) rovibrational systems. T g values are also deduced from the line broadening of selected Ar emission lines using an hyperfine spectrometer. In the MW Ar/H2O plasma, T g decreases from ∼2100 K close to the wave launcher to ∼1600 K near the end of the plasma column, while T rot is mostly constant in the 1500 K range. In presence of N2 admixtures instead of water vapours, T g is higher by about 300 K (from ∼2400 K to ∼1900 K), while T rot decreases from ∼3200 K to ∼2750 K along the plasma column. A discrepancy between T g and T rot is also observed in the much colder RF plasmas with T g ∼ 400 K and T rot ∼ 515 K. Such departure from the rotational–translational equilibrium in both plasmas is ascribed to the influence of electrons competing with neutrals to impose their own temperature on the distribution of rotational levels of both ground and excited states.

Importance of the Parallel Component of the Transition Moments to the 1Π1 (5A') and 3Π1 (3A') Excited States of ICN in the Ã-Band Photodissociation.

ICN is one of the few simple triatomic molecules whose photodissociation mechanisms have been thoroughly investigated. Since it has a linear structure in the electronic ground state, the dissociation follows a photoexcitation at a linear or slightly bent structure. It is generally believed that the Ã-band consists of the dominant excitation to 3Π0+ (4A') with the transition dipole moment (TDM) parallel to the molecular axis ( z), a slightly weaker transition to 1Π1 (5A', 4A″), and a much weaker transition to 3Π1 (3A', 2A″), both of the latter two having perpendicular TDMs. In the present work, we have theoretically studied the geometry dependence of these TDMs and found a pronounced θ (bending angle) dependence in the parallel ( z) component of the TDMs to 1Π1 (5A') and 3Π1 (3A'), both of which should be zero at a linear geometry by symmetry and thus have been previously ignored. We estimated that the z component TDM to 1Π1 (5A') has a contribution of 15-20% to the total absorption cross-section at 249 nm at room temperature. Interestingly, the TDM to 3Π0+ (4A') does not exhibit such θ dependency and thus has only the z component. We compare the TDMs of ICN and CH3I molecules having similar excited states. The fact that all the TDMs to 3A', 4A', and 5A' have nonnegligible z components implies the importance of the coherent excitation contributions to various observables of CN fragment, such as the anisotropy parameter, the orientation parameter, and the rotational level distribution as well as the rotational fine structure level distribution.

Kinetic description of non-Boltzmann OH rotational distribution in nonequilibrium stationary plasmas.

We present a model that describes the departure from equilibrium of the OH(A) rotational level distribution in collisional plasmas. In this model, the OH(A) rotational state densities are governed by a rate equation including: (i) a balanced rotational energy transfer process with the buffer gas species; (ii) unbalanced exothermic reactions, which pump rotationally excited states into the system. Based on the prior assumptions, we formally derive a model function describing the non-Boltzmann distribution. This function depends on five parameters, each of which has a physical meaning. The temperature is given as one of the model function parameters, which can be readily identified with the translational temperature of the buffer gas. The validity of the model was tested by means of the least-squares fitting of data found in literature. Based on this analysis we propose the formation processes of rotationally excited OH(A) in several discharge conditions.

Rovibrational laser control targeting a dark state in acetylene. Simulation in the Ns = 1, Nr = 5 polyad

ABSTRACTOptimal control theory in the Liouville space is used to perform rovibrational control by means of a laser pulse in a polyad of acetylene in order to populate a dark vibrational state. The initial mixed state is a truncated Boltzmann distribution of rotational levels from J=27 to J=31 of the ground vibrational state. The target state is a rotational equidistribution of levels ranging from J=28 to J=32 of the first excited vibrational dark state including quanta of energy in each bending modes, with positive vibrational angular momenta. The simulation is performed by using a manifold of eigenstates of a full-dimensional Hamiltonian calibrated by high precision spectroscopy known as the global acetylene Hamiltonian [B. Amyay et al., J. Chem. Phys. 131, 114301 (2009)]. The control is successful as an Uhlmann's fidelity of 0.98 is reached.

Production of rovibronic-ground-state RbCs molecules via two-photon-cascade decay

We report the production of ultracold RbCs molecules in the rovibronic ground state, i.e., $X^1\Sigma^+ (v=0, J=0)$, by short-range photoassociation to the $2^3\Pi_{0}$ state followed by spontaneous emission. We use narrowband depletion spectroscopy to probe the distribution of rotational levels formed in the $X ^1\Sigma^+(v=0)$ state. We conclude, based on selection rules, that the primary decay route to $X ^1\Sigma^+(v=0)$ is a two-step cascade decay that leads to as much as $33\%$ branching into the $J=0$ rotational level. The experimental simplicity of our scheme opens up the possibility of easier access to the study and manipulation of ultracold heteronuclear molecules in the rovibronic ground state.

Photodissociation dynamics of lithium chloride: Contribution of interferometric predissociation

Continuum photoabsorption of lithium chloride (LiCl) was investigated using a coupled-channel time-dependent wave packet approach. Photodissociation cross sections for the production of ground-state Li and Cl atoms were computed up to temperatures of 1500 K for a thermal distribution of rotational levels. At such temperatures, LiCl is believed to be the primary Li-bearing gas in cool stellar atmospheres. Narrow Rydberg resonances in the total absorption spectrum are found to dominate the thermally averaged cross section due to the large density of Rydberg states in the predissociation gap. Comparison with measured photoabsorption cross sections, where available, is made.

Blackbody thermometry with cold molecular ions and application to ion-based frequency standards

We have used laser spectroscopy to measure the rotational level distribution of trapped molecular $\mathrm{H}{\mathrm{D}}^{+}$ ions at translational temperatures in the millikelvin range. The $\mathrm{H}{\mathrm{D}}^{+}$ ions are loaded into an ion trap by electron-impact ionization, and sympathetically cooled using laser-cooled ${\mathrm{Be}}^{+}$ ions which are already stored in the trap. Under our experimental conditions, the internal (rotational) degrees of freedom turn out to be independent of the translational degrees of freedom, and an effective rotational temperature close to room temperature is found. The near absence of background-gas collisions allows the rotational temperature to be related directly to the temperature of the ambient blackbody radiation (BBR). This feature suggests the use of molecular ions for BBR thermometry, which may help to improve the accuracy of frequency standards based on trapped atomic ions. For the spectroscopic measurement of the rotational populations, we propose a nondestructive technique.

Determination of the neutral gas temperature of nitrogen-containing low-pressure plasmas using a two-temperature model

The neutral gas temperature, which is an important process parameter in low-pressure plasmas, is often determined by evaluating the shape of rotational spectra of appropriate molecules. However, the observed population of the rotational levels is determined by the excitation mechanisms and the rotational level population of the electronic states from which the upper electronic state of the chosen molecule band is populated. We used the first negative system of the nitrogen molecule ion for temperature determination in different low-pressure discharges. We will show that the spectra can be fitted best assuming that they are composed of two contributions representing two populations of the nitrogen molecule ion with distinctly different rotational temperatures. Thus, at least two excitation channels of the upper electronic state have to be considered which, in our opinion, are the electron impact on the ground state neutral nitrogen molecule connected with ionization and excitation (rotational level distribution remains essentially unchanged and reflects the gas temperature) and the impact of nitrogen molecules in high vibrational states on the ground-state nitrogen molecule ion (connected with rotational excitation, rotational temperature much higher than the gas temperature). Considering this, more reliable results are obtained compared to the conventional method where a single Boltzmann distribution of the rotational levels is assumed. Here, often, too high temperatures (up to several 100 K) are determined.

Spatial distribution of the neutral gas temperature in rf and dc magnetron sputter discharges determined by fitting rotational spectra of the N 2+ using a two-temperature fit

The spatial distribution of the neutral gas temperature in a dc and rf magnetron sputter discharge (boron target) with pure nitrogen was estimated by fitting the rotational spectra of the first negative system of the nitrogen molecule ion. The rotational levels of the excited nitrogen molecule ion are populated mainly by two mechanisms. Only one of them leads to a rotational level distribution corresponding to the neutral gas temperature. Therefore, fitting of the spectra assuming a single Boltzmann distribution of the rotational levels often leads to unreliable and too high rotational temperatures and the spectra have to be fitted using a two-temperature model. Beside the neutral gas temperature the value of the second temperature as well as the contribution of the higher rotationally excited molecules to the spectra are studied in more detail. The dependence of the spatial distribution of the neutral gas temperature on pressure and discharge power was investigated. Additionally, the results are compared to rotational temperatures obtained by fitting the spectra assuming a single Boltzmann distribution of the rotational levels. These rotational temperatures often do not reflect the neutral gas temperature and wrong dependences would be concluded.

Production of CH(X2Π) from the multiphoton dissociation of CH2CO at wavelengths of 279.3 and 308 nm

CH(X2Π) has been observed by laser-induced fluorescence (LIF) spectroscopy as the product of the two-photon dissociation of ketene (CH2CO) at 279.3 and 308 nm. The nascent distribution of rotational levels is Gaussian in profile, consistent with a ‘rotational reflection’ principle in the dissociation. Thermodynamic arguments imply a fragmentation pathway to CH + HCO following an initial one-photon absorption to the 1A″ excited state of ketene, a second photon absorption, and dissociation following rearrangement via a formylmethylene isomer. Analysis of Λ doublets in the LIF spectra shows no orbital alignment of CH produced on photolysis at 279.3 nm, but some propensity for Π(A′) symmetry alignment (π orbital of CH parallel to the plane of rotation) for photolysis at 308 nm.

Photoinitiated H- and D-atom reactions with N2O in the gas phase and in N2O–HI and N2O–DI complexes

Reactions of H atoms with N2O have two product channels yielding NH+NO and OH+N2. Both channels were observed via NH A 3Π←X 3∑ and OH A 2∑←X 2Π laser-induced fluorescence spectra. Photoinitiated reactions with N2O–HI complexes yield a much lower [NH]/[OH] ratio than under the corresponding bulk conditions at the same photolysis wavelength. For hot D-atom reactions with N2O, this effect is somewhat more pronounced. These results can be interpreted in terms of entrance channel geometric specificity, namely, biasing hydrogen attack toward the oxygen. Another striking observation is that the OH and OD rotational level distributions (RLD) obtained under bulk conditions differ markedly from those obtained under complexed conditions, while the NH as well as the ND RLD are similar for the two environments. In addition, OH Doppler profiles change considerably in going from bulk to complexed conditions, while such an effect is not observed for NH. The changes observed with the OH RLD are most likely due to OH–halogen interactions and/or entrance channel specificity. Under bulk conditions, the Doppler shift measurements indicate a large amount of N2 internal excitation (i.e., ∼25 000 cm−1) for the OH (v=0) levels monitored. This is consistent with a reaction mechanism involving an HNNO° intermediate. The hot hydrogen atom first attaches to the terminal nitrogen of N2O and forms an excited HNNO° intermediate having a relatively elongated N–N bond compared with N2O. Then the H atom migrates from nitrogen to oxygen and exits to the N2+OH product channel, leaving N2 vibrationally excited. A simple Franck–Condon model can reconcile quantitatively the large amount of N2 vibrational excitation.

The dynamical Renner–Teller effect. II. Rotationally inelastic scattering of NCO(X̃ 2Π,0010)+He

State-resolved integral cross sections for the inelastic scattering of the Hund’s case (a) radical NCO(X̃ 2Π) by He have been measured in a crossed-beam apparatus. Two different types of measurements are reported: (1) state-resolved integral cross sections at fixed collision energies from 0.94–3.05 kcal mol−1 and (2) the collision energy dependence of state-specific product states for both 2Π3/2 and 2Π1/2 spin–orbit states of NCO(X̃ 2Π,0010). As in a preliminary account of this work for E0=3.74 kcal mol−1 [J. Phys. Chem. 91, 9630 (1991)], herein referred to as part I, dramatically different rotational level distributions were found for spin–orbit conserving or changing collisions. It is argued that this behavior and many other observations on this system are generic to the inelastic scattering of Hund’s case (a) 2Π radicals. A conceptual framework to understand these results is elucidated in detail. The direct connection between the experimental observables and the Hund’s case classification is made for the inelastic scattering of radicals. This connection can be viewed as a dynamical consequence of the Renner–Teller effect.

A crossed-beam study of the state-resolved dynamics of CH(X 2Π) + D2. II. The isotopic exchange channel

The state-to-state integral cross sections for the isotopic exchange reaction of CH(X 2Π) with D2 to produce CD(X 2Π) have been measured in a crossed-beam apparatus by the laser-induced fluorescence method. Two types of measurements were performed: (1) the translational energy dependence of an individual quantum state of the product and (2) the state distribution of the products at fixed and well-defined translational energy. To understand some of the finer details of the reaction dynamics similar experiments were also carried out on the reaction CD(X 2Π) with H2 to give CH(X 2Π). For the isotopic exchange channel, the cross section decreased rapidly with increasing translational energy, signifying a complex formation reaction mechanism. The CD(CH) product rotational level distributions are substantially colder than a statistical expectation and are interpreted as the result of a multiple-impact collision between the receding products in the exit channel. A novel frequency-locking mechanism, similar to that proposed to understand the dynamics of inelastic collisions between CH and D2 (the preceding paper), is suggested to explain anomalous peaks in the product rotational level distribution for the isotopic exchange channel. However, the fine-structure state distributions for the reactive process, which are very different from that for the inelastic channel, cannot be rationalized by conventional interpretation. Some basic concepts about fine-structure selectivity in chemical reactions have been developed. With these concepts, it is conjectured that the fine-structure state distributions of open-shell molecules arising from reactive encounters could provide a ‘‘fingerprint’’ of the electronic wave function at the transition state.

State-to-state integral cross sections for the inelastic scattering of CH(X 2Π)+He: Rotational rainbow and orbital alignment

The state-to-state integral cross sections for the inelastic scattering of CH(X 2Π) with He were measured in a newly constructed crossed molecular beam machine. Use of laser-induced fluorescence in an unconventional flux mode of detection provided single fine-structure state specific detection of the products. Two types of measurements were performed to further our understanding of the collision dynamics of open shell systems: (1) the product state distribution at a fixed and well-defined collision energy and (2) the dependence on collision energy of product state-resolved cross sections. A qualitative understanding of the collision dynamics can be obtained by properly factoring out features dependent on the fine-structure states, i.e., effects involving individual Λ-doublet states and features dependent on the rotational level alone, i.e., effects remaining after summing over all four fine-structure states associated with a given rotational quantum number. As for the fine-structure effects, a preferential population of product Λ-doublet states with reflection symmetry Π(A″) was observed. The physical origin of this observed electronic orbital alignment can be attributed to a quantum interference phenomenon, as detailed in the accompanying paper. At the rotational level, the dominance of rotational rainbow scattering is unambiguously identified from both the existence of dynamical thresholds and a strong correlation between rotational level distributions at fixed translational energy and level specific excitation functions. These effects combined with other experimental observations lead us to visualize the CH+He scattering dynamics in a novel fashion. The collision can be regarded as a series of approximately independent sequential events each mediated by different regions of the interaction potential during the course of the whole encounter.

Characterization of OH Radical 306.4 nm Emission in Argon and Helium Reduced-Pressure ICPs

The A → X emission of OH at 306.4 nm has been studied in reduced-pressure inductively coupled argon and helium plasmas. Under a variety of conditions of power and flow rate, a comparison of the A state rotational level distributions shows significant differences in the two plasmas. The rotational level distributions are generally nonlinear but can be quantitatively described as the sum of two separate Boltzmann distributions.

Energy distribution in the nitric oxide fragments from the ν7 vibrational predissociation of NO–C2H4

The rotational level distribution of the NO fragments formed as a result of the predissociation of the vibrationally excited NO–C2H4 (ν7) van der Waals molecule was measured by laser excited fluorescence techniques. The distribution was found to be Boltzmann in character, described by the rotational temperature 75±15 K. An average kinetic energy release of ≊105 cm−1 per fragment, in an isotropic flux distribution, was determined from Doppler profiles of the NO fragments in selected rotational levels.

Disturbance of the Boltzmann population distribution of rotational levels in a free nitrogen jet

Disturbance of the Boltzmann population distribution of rotational levels in a free nitrogen jet

Energy partitioning in the VUV photodissociation of ClNO as a function of excitation energy

The fluorescence of NO Rydberg states resulting from the VUV photodissociation of ClNO with synchrotron radiation between 120 and 165 nm has been studied. The excitation spectra for the production of NO A 2Σ+ v′ = 0,1,2, C 2Π v′ = 0, D 2Σ+ v′ = 0 have been measured and show by comparison with the ClNO absorption spectrum a state-to-state correlation. The internal energy content (vibrational and rotational) of the NO A 2Σ+ fragment has been determined as a function of excitation energy. The rotational level distribution of NO(A) is not of Boltzmann type but follows a Gaussian-like distribution as shown from the detailed analysis of the fluorescence spectrum obtained at 147 nm. The excess energy dependence of the rotational and vibrational energy disposal deviates strongly from statistical predictions and can be explained on the basis of a simple quasidiatomic impulsive model. The time decays of NO(A) have been recorded and the rate coefficients for quenching by ClNO was found to be larger for NO(A) resulting from ClNO photolysis than for the thermalized species. This effect is attributed to the translational excitation of the separating NO fragment.

Analysis of rotational nonequilibrium behavior in hydrogen fluoride/deuterium fluoride systems

A surprisal correlation of the experimental data for the V→R, T processes occurring in hydrogen fluoride/deuterium fluoride (HF/DF) chemical lasers indicates that the resulting product rotational level distributions may be very narrow. This result can be used to greatly simplify the modeling effort because a one-to-one correspondence between reactant and product states can be assumed. A similar analysis has also been used to correlate the published experimental results for R→R, T relaxation of HF/DF molecules. The correlation compares well with the trajectory analysis of Wilkins. Since the available experimental data for R→R, T relaxation relate to ambient conditions, the trajectory analysis temperature dependence was assumed.