Rotational Excitation Of Molecules Research Articles

Application of Boltzmann's transfer equation to nonlinear propagation of a plane polarized monochromatic EM wave in ionospheric plasma

The ionospheric plasma (mainly D and E layers) has been modeled as a slightly ionized plasma, with highly dominant scattering of electrons by N2 and O2 molecules. Scattering by elastic collisions and inelastic collisions with rotational excitation of molecules has been considered on the basis of the measured scattering parameter δ≠2m/M as a function of electron speed. The assumptions in this paper are valid when the collision frequency ν is much larger than the gyro-frequency ωh; this is true for many situations in the D layer and is of academic interest for the E-layer; the present formulation is valid in all the cases when the magnetic field is along the electric vector of the wave. In this paper, the nonlinear propagation parameters of a plane polarized monochromatic electromagnetic wave in the plasma have been obtained by substituting the appropriate expression for the current density J, as a function of E, the electric field of the wave. J is of course obtained using the solution of the corresponding Boltzmann transfer equation. The analysis is limited by ν≫ωh. There are many books and reviews available on nonlinear propagation in plasma. The applications of such studies start from demodulation, cross-modulation, phase modulation, etc., in the ionosphere.

Rotational excitation of molecules with long sequences of intense femtosecond pulses

We investigate the prospects of creating broad rotational wave packets by means of molecular interaction with long sequences of intense femtosecond pulses. Using state-resolved rotational Raman spectroscopy of oxygen, subject to a sequence of more than 20 laser pulses with peak intensities exceeding $10^{13}$ W/cm$^{2}$ per pulse, we show that the centrifugal distortion is the main obstacle on the way to reaching high rotational states. We demonstrate that the timing of the pulses can be optimized to partially mitigate the centrifugal limit. The cumulative effect of a long pulse sequence results in high degree of rotational coherence, which is shown to cause an efficient spectral broadening of probe light via cascaded Raman transitions.

From Gyroscopic to Thermal Motion: A Crossover in the Dynamics of Molecular Superrotors

Localized heating of a gas by intense laser pulses leads to interesting acoustic, hydrodynamic and optical effects with numerous applications in science and technology, including controlled wave guiding and remote atmosphere sensing. Rotational excitation of molecules can serve as the energy source for raising the gas temperature. Here, we study the dynamics of energy transfer from the molecular rotation to heat. By optically imaging a cloud of molecular superrotors, created with an optical centrifuge, we experimentally identify two separate and qualitatively different stages of its evolution. The first non-equilibrium "gyroscopic" stage is characterized by the modified optical properties of the centrifuged gas - its refractive index and optical birefringence, owing to the ultrafast directional molecular rotation, which survives tens of collisions. The loss of rotational directionality is found to overlap with the release of rotational energy to heat, which triggers the second stage of thermal expansion. The crossover between anisotropic rotational and isotropic thermal regimes is in agreement with recent theoretical predictions and our hydrodynamic calculations.

Communication: Creation of molecular vibrational motions via the rotation-vibration coupling.

Building on recent advances in the rotational excitation of molecules, we show how the effect of rotation-vibration coupling can be switched on in a controlled manner and how this coupling unfolds in real time after a pure rotational excitation. We present the first examination of the vibrational motions which can be induced via the rotation-vibration coupling after a pulsed rotational excitation. A time-dependent quantum wave packet calculation for the HF molecule shows how a slow (compared to the vibrational period) rotational excitation leads to a smooth increase in the average bond length whereas a fast rotational excitation leads to a non-stationary vibrational motion. As a result, under field-free postpulse conditions, either a stretched stationary bond or a vibrating bond can be created due to the coupling between the rotational and vibrational degrees of freedom. The latter corresponds to a laser-induced breakdown of the adiabatic approximation for rotation-vibration coupling.

Nonlinear absorption of intense femtosecond laser radiation in air

A new mechanism of nonlinear absorption of intense femtosecond laser radiation in air in the intensity range I = 10(11)-10(12) W/cm(2) when the ionization is not important yet is experimentally observed and investigated. This absorption is much greater than for nanosecond pulses. A model of the nonlinear absorption based on the rotational excitation of molecules by linearly polarized ultrashort pulses through the interaction of an induced dipole moment with an electric field is developed. The observed nonlinear absorption of intense femtosecond laser radiation can play an important role in the process of propagation of such radiation in the atmosphere.

Adiabatic excitation of rotational ladder by chirped laser pulses

We discuss rotational excitation of molecules by a pair of left and right circularly polarized laser pulses with opposite chirps. The pulses are supposed to be short enough (picosecond or femtosecond) to prevent relaxation, sufficiently intense to induce adiabatic evolution, and far-off-resonant, e.g., infrared. This technique has been demonstrated recently by Villeneuve et al. [Phys. Rev. Lett. 85, 542 (2000)] in rotational dissociation of molecules. We analyze the properties of this technique by using the concepts of level crossing and adiabatic evolution, which allow us to derive analytically the conditions for efficient excitation. We analyze both the cases of intuitive (divergent frequencies) and counterintuitive (convergent frequencies) chirps and examine various initial conditions, including a single J state, coherent and incoherent superpositions of J states. We propose a technique, which can create superrotors by applying a pair of appropriately timed narrow pulses.

Coherent resonance and dipole scattering in rotational excitation of molecules by slow electrons

We present a new approach to rotational-vibrational excitation of molecules by slow electrons, incorporating the shape resonance, the dipole interaction, and the interference between their amplitudes. Excellent agreement is found with the high-resolution differential-cross-section data in CO for vibrational excitation and the simultaneous excitation of individual rotational branches. Accurate state-to-state integral cross sections are given for CO at the resonance energy of 1.8 eV.

Rotational excitation of molecules by slow neutrons. II

The contribution of the spin-orbit interaction to the rotationally inelastic scattering of neutrons from a closed-shell diatomic molecule is studied. For small momentum transfer and $\ensuremath{\Delta}J=\ifmmode\pm\else\textpm\fi{}1$ rotational transitions, the first Born-approximation amplitude depends approximately on the factor $\ensuremath{\Delta}\ensuremath{\omega}D{q}^{\ensuremath{-}1}$ (in atomic units), where $\ensuremath{\Delta}\ensuremath{\omega}$ is the energy transferred, $D$ is the permanent dipole moment of the target, and $q$ is the momentum transferred. The dependence on $\ensuremath{\Delta}\ensuremath{\omega}D$ is the same as that of the coefficient of absorption of long-wavelength radiation, and the dependence on ${q}^{\ensuremath{-}1}$ is characteristic of particle scattering in a dipole potential. Essentially $\ensuremath{\Delta}J=\ifmmode\pm\else\textpm\fi{}1$ cross sections are enhanced because rotational energy transfer can occur over appreciable target-neutron distances. Thus the tiny rotational transition strength $\ensuremath{\Delta}\ensuremath{\omega}D$ is weighted by the large factor ${q}^{\ensuremath{-}1}$. This means that a classical average of the cross section over molecular orientations cannot describe the physical process. The importance of this result to the theory of neutron-optical activity is discussed.

Rotational excitation of molecules by slow neutrons

Elastic and rotationally inelastic differential cross sections are calculated for slow neutrons incident on gaseous para-$\mathrm{H}_{2}^{}{}_{}{}^{+}$. This target is chosen as the simplest open-shell molecular system so that the contribution of electromagnetic forces to the scattering can be studied. It is shown that electromagnetic forces are dominant over nuclear forces in $0\ensuremath{\rightarrow}2$ rotational excitation for small-momentum-transfer collisions (low angles at incident energies about twice the $0\ensuremath{\rightarrow}2$ threshold value). For large-momentum-transfer collisions (wide angles at the higher energies or all angles at energies close to threshold) nuclear forces dominate the scattering. The calculations show that low-angle, small-energy-loss experiments would isolate the weak but long-range electromagnetic forces and thus probe target electronic structure.

Infrared pumping and rotational excitation of molecules in interstellar clouds

view Abstract Citations (92) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared pumping and rotational excitation of molecules in interstellar clouds Carroll, T. J. ; Goldsmith, P. F. Abstract The effects of the rotational excitation of diatomic molecules in interstellar clouds by the absorption of infrared radiation are evaluated. A criterion for infrared pumping to alter significantly populations of the rotational states of ground and first excited vibrational levels is derived, with the effects of collisions neglected, and used to calculate the minimum required filling factor or source temperature for pumping various diatomic molecules. A model infrared source region consisting of a region of warm dust heated by embedded stars or H II regions is then developed which is characterized by an effective temperature, opacity and solid-angle dilution factor as seen by the molecular cloud, and used to derive upper limits to the size of the region affected by the infrared source. Numerical calculations for the case of the CS molecule are presented which indicate that the hot components of sources such as W3-IRS 2/2a and M17a/b emit sufficient infrared flux to perturb significantly the populations of the rotational levels. Such perturbations could lead to erroneous estimates of molecular densities if it is assumed that collisional excitation is the only operative pumping mechanism. Publication: The Astrophysical Journal Pub Date: May 1981 DOI: 10.1086/158865 Bibcode: 1981ApJ...245..891C Keywords: Infrared Radiation; Interstellar Gas; Interstellar Masers; Molecular Clouds; Molecular Excitation; Molecular Rotation; Optical Pumping; Radiative Transfer; Carbon Compounds; Diatomic Molecules; H Ii Regions; Radiation Absorption; Radiation Effects; Sulfides; Astrophysics full text sources ADS |

Behaviour of positrons in molecular gases. I. Rotational excitation of molecules by slow positrons

Cross sections for the rotational excitation of H2, N2 and O2 by the impact of slow positrons are calculated in the distorted wave approximation. The effects of long range and short range interaction are investigated. It is found that the inclusion of the short range interaction affects the cross sections appreciably.

Rotational Excitation of Molecules in Collisions with Slow Electrons

The scattering of slow electrons on dipolar symmetrical-top molecules and on quadrupolar linear molecules is calculated at energies below the threshold for vibrational excitation. The result for symmetrical-top molecules is the same as that obtained previously for linear molecules. The result for quadrupolar linear molecules is independent of the energy of the incident electron.

Contribution of energetic photoelectrons to D -region nonequilibrium electron temperatures

Electron equilibrium energy in the D and lower E region of the ionosphere is calculated from a relationship between electron energy loss and attachment characteristic times. It is shown that solar photodetachment of electrons from O2− ions, which is the major short-term ionization source between 50 and 90 km, is a source of energetic electrons of 2.9-ev mean energy. Thus competition between energy loss by rotational excitation of molecules and dissociative attachment to oxygen will exist as the most probable processes for 3-ev electrons. Approximate conditions are defined that allow calculation of maximum mean electron energy at equilibrium for the undisturbed or slightly disturbed daytime ionosphere. Comparison of these maximum values with a measured electron energy of 0.15 ev at 40 km shows agreement within a factor of 2, which may be accounted for by the disturbed condition of the ionosphere during the measurement. The range of maximum permissible electron energies lies between 12 times ambient at 30 km and 42 times ambient (0.74 ev) at 90 km. A method is outlined whereby one parameter among the set—equilibrium electron energy, energy loss characteristic time, or electron removal characteristic time—can be determined if the other two can be found experimentally and if the mean source energy of the electrons is known. Thus radio measurement techniques such as the Luxembourg effect or rocket probes can be used to relate temperature measurements and electron-density measurements through a reaction model based on estimation of characteristic times of competing processes.