Collisional Perturbations Research Articles

Ab initio quantum scattering calculations and a new potential energy surface for the HCl(X1Σ+)-O2(X3Σg-) system: Collision-induced line shape parameters for O2-perturbed R(0) 0-0 line in H35Cl.

The remote sensing of abundance and properties of HCl-the main atmospheric reservoir of Cl atoms that directly participate in ozone depletion-is important for monitoring the partitioning of chlorine between "ozone-depleting" and "reservoir" species. Such remote studies require knowledge of the shapes of molecular resonances of HCl, which are perturbed by collisions with the molecules of the surrounding air. In this work, we report the first fully quantum calculations of collisional perturbations of the shape of a pure rotational line in H35Cl perturbed by an air-relevant molecule [as the first model system we choose the R(0) line in HCl perturbed by O2]. The calculations are performed on our new highly accurate HCl(X1Σ+)-O2(X3Σg-) potential energy surface. In addition to pressure broadening and shift, we also determine their speed dependencies and the complex Dicke parameter. This gives important input to the community discussion on the physical meaning of the complex Dicke parameter and its relevance for atmospheric spectra (previously, the complex Dicke parameter for such systems was mainly determined from phenomenological fits to experimental spectra and the physical meaning of its value in that context is questionable). We also calculate the temperature dependence of the line shape parameters and obtain agreement with the available experimental data. We estimate the total combined uncertainties of our calculations at 2% relative root-mean-square error in the simulated line shape at 296K. This result constitutes an important step toward computational population of spectroscopic databases with accurate abinitio line shape parameters for molecular systems of terrestrial atmospheric importance.

Fully quantum calculations of O2-N2 scattering using a new potential energy surface: Collisional perturbations of the oxygen 118GHz fine structure line.

A proper description of the collisional perturbation of the shapes of molecular resonances is important for remote spectroscopic studies of the terrestrial atmosphere. Of particular relevance are the collisions between the O2 and N2 molecules-the two most abundant atmospheric species. In this work, we report a new highly accurate O2(X3Σg -)-N2(X1Σg +) potential energy surface and use it for performing the first quantum scattering calculations addressing line shapes for this system. We use it to model the shape of the 118GHz fine structure line in O2 perturbed by collisions with N2 molecules, a benchmark system for testing our methodology in the case of an active molecule in a spin triplet state. The calculated collisional broadening of the line agrees well with the available experimental data over a wide temperature range relevant for the terrestrial atmosphere. This work constitutes a step toward populating the spectroscopic databases with ab initio line shape parameters for atmospherically relevant systems.

Self-interacting dark matter cusps around massive black holes

We adopt the conduction fluid approximation to model the steady-state distribution of matter around a massive black hole at the center of a weakly collisional cluster of particles. By "`weakly collisional" we mean a cluster in which the mean free time between particle collisions is much longer than the characteristic particle crossing (dynamical) time scale, but shorter than the cluster lifetime. When applied to a star cluster, we reproduce the familiar Bahcall-Wolf power-law cusp solution for the stars bound to the black hole. Here the star density scales with radius as $r^{-7/4}$ and the velocity dispersion as $r^{-1/2}$ throughout most of the gravitational well of the black hole. When applied to a relaxed, self-interacting dark matter (SIDM) halo with a velocity-dependent cross section $\sigma \sim v^{-a}$, the gas again forms a power-law cusp, but now the SIDM density scales as $r^{-\beta}$, where $\beta = (a+3)/4$, while its velocity dispersion again varies as $r^{-1/2}$. Results are obtained first in Newtonian theory and then in full general relativity. Although the conduction fluid model is a simplification, it provides a reasonable first approximation to the matter profiles and is much easier to implement than a full Fokker-Planck treatment or an $N$-body simulation of the Boltzmann equation with collisional perturbations.

Electric field induced anomalies in frequency shift and collisional perturbations in the 5snd 1D2 Rydberg series of strontium in a heatpipe setup with ionization detection

We have examined the effect of weak DC electric field (2–20 V/cm) complimented by foreign gas collisions on the bound J = 2 even-parity 5snd 1 D 2 Rydberg states of neutral strontium . We use resonant two-photon transverse excitation, employing a narrow bandwidth tunable dye laser and an atomic jet in a heatpipe setup with ionization detection. In this paper we report certain anomalies in the observed spectra covering principal quantum number range n = 27–42 indicating a frequency shift reversal with nearly quadratic dependence on the field strength above an anti-resonance region. Furthermore, we have observed the emergence of highly localized doubly-excited 4d 2 states, including a remarkably broad perturber with large angular momentum , uncovering orbital contraction effect. This non-Rydberg excitation, which intrudes upon the two-photon spectrum with large energy overlap is due to single-photon excitations from the 5s5p 1 P 1 resonance level following molecular dissociation of the Sr 2 dimers suitably governed by binary atomic collisions . Our study which involves laser excitation complimented by electric field and collisions using inexpensive home-made setup opens up the possibilities for a new class of experiments, with considerable simplicity in the choice of excitation schemes for both Rydberg and non-Rydberg transitions, to reach states lying at high energies which cannot otherwise be accessed from the ground state due to parity and selection rules, while allowing one to probe localization properties of atomic wave functions.

Collisional perturbation of radio-frequencyE1transitions in an atomic beam of dysprosium

We have studied collisional perturbations of radio-frequency (rf) electric-dipole (E1) transitions between the nearly degenerate opposite-parity levels in atomic dysprosium (Dy) in the presence of 10 to 80 $\mu$Torr of H$_\text{2}$, N$_\text{2}$, He, Ar, Ne, Kr, and Xe. Collisional broadening and shift of the resonance, as well as the attenuation of the signal amplitude are observed to be proportional to the foreign-gas density with the exception of H$_2$ and Ne, for which no shifts were observed. Corresponding rates and cross sections are presented. In addition, rates and cross sections for O$_2$ are extracted from measurements using air as foreign gas. The primary motivation for this study is the need for accurate determination of the shift rates, which are needed in a laboratory search for the temporal variation of the fine-structure constant [A. T. Nguyen, D. Budker, S. K. Lamoreaux, and J. R. Torgerson, Phys. Rev. A \textbf{69}, 22105 (2004)].

Rotational time‐domain CARS in H2: departure from statistically independent collisional dephasing model

AbstractTime‐domain CARS was applied to study dephasing of H2 S0(0) and S0(1) rotational transitions in the density range from 0.004 to several amagat at 296 and 80 K. Dephasing of the S0(0) transition in pure H2 corresponds satisfactorily to the theoretical model of statistically independent collisional perturbations of translational and rotational molecular motions. Experimental dephasing curves of the S0(1) transition in pure H2 in the density region of the Dicke effect manifestation are steeper than the theoretical curves calculated on the basis of the statistically independent dephasing model. This departure is especially noticeable at liquid nitrogen temperature. At the same time, dephasing curves of the S0(1) transition in He as a buffer gas measured at room temperature demonstrate fairly good coincidence with the theoretical model. The departure of the experimental dephasing curves of the S0(1) transition in pure H2 from the statistically independent model can be mainly attributed to resonance collisions. Copyright © 2003 John Wiley & Sons, Ltd.

Dicke effect in hydrogen S0(0) rotational transition observed by time‐domain CARS

AbstractThe Dicke effect in the hydrogen S0(0) rotational transition (354 cm−1) in forward scattering geometry was observed using time‐domain CARS. Dicke effect measurements were performed at room temperature and with cooling by liquid nitrogen. The observed slowest decays of the impulse responses correspond to the narrowest S0(0) transition spectral linewidths of 1.1 × 10−3 cm−1 (296 K) and 0.45 × 10−3 cm−1 (85 K). At 85 K, the impulse responses coincided satisfactorily with the theoretical values calculated for statistically independent collisional perturbations of translational and rotational motions. At 296 K, some discrepancy between experiment and theory was observed. Copyright © 2002 John Wiley & Sons, Ltd.

Trans-neptunian objects

The trans-neptunian objects (TNOs) constitute a new class of solar system object that was discovered only recently to exist beyond the orbit of Neptune. About 400 trans-neptunian objects have been detected over the past nine years and more than ten new objects are being discovered every month. All of the TNOs known to date fit into three dynamical classes: the classical, the resonant and the scattered objects. The total mass of the TNOs currently orbiting the Sun is estimated from the observed luminosity distribution to be of the order of 10–20% of the Earth's mass. However, theoretical investigations of the formation and evolution of the trans-neptunian belt into its currently observed shape suggest that it was much more massive in the past. The physical characterisation of TNOs starts to reveal some of the basic properties of these objects, such as size, shape and rotation and provides a first glance into the diversity of their surfaces. TNOs cover a very diverse range of colours, possibly reflecting different surface compositions. First evidence for the presence of water ice was found in a spectrum of one TNO while others do not show the characteristic absorption bands. The TNOs are now regarded as the likely source of some short-period comets. Owing to giant-planet and collisional perturbations, some TNOs may evolve into Centaurs, i.e. objects orbiting the Sun in the region between Jupiter and Neptune, which are further perturbed to become Jupiter-family short-period comets. Together with smaller debris generated by collisional shattering, the TNOs might represent a belt that has evolved from a more massive circumstellar disc into its present structure.

Van der Waals Broadening of the Electronic Spectra of 1-Azabicyclo[2.2.2]octane: Inversion into Lennard-Jones Pair Potentials

Collisional perturbations of the S0 → S1 and S0 → S2 electronic transitions of 1-azabicyclo[2.2.2]octane (ABCO) by Ar, He, Xe, and SF6 are studied under static-cell conditions at 296 K. Pressures ranged up to ca. 20 atm for Xe and SF6 and ca. 80 atm for Ar and He. For all perturbers except for He, the vibronic features are red-shifted and broadened asymmetrically to the blue. For He a slight blue shift is observed. Linear shift coefficients for the 0−0 bands of these transitions are reported for the perturbers studied. The spectral perturbations caused by these agents are analyzed globally for each transition using an iterative Fourier reconvolution (IFR) method. Because of the distortion of the more weakly allowed S0 → S1 transition by the S0 → S2 transition, the IFR method was applied to the latter system using Lennard-Jones (LJ) pair potentials to model the interactions between the perturber and the S0 and S2 states of ABCO. In all cases good overall fits are achieved (χ2 ca. 10-4). For all perturbers, the LJ R values are larger for the S2-perturber complex than for the ground state. For He and Ar, the excited-state well depth is smaller relative to the ground state; for SF6 and Xe, the opposite is found. Results are compared with fluorescence lifetime and efficiency data of ABCO acquired at perturber pressures below 1 atm. The results of recent studies of ABCO−Ar in a supersonic beam are discussed in the context of the LJ potentials obtained in this study.

Collisional cooling and ablation product dynamics observed by resonant ionization spectroscopy of nascent carbon monoxide from 193 nm laser ablation of polyimide

Rotational and vibrational energy distributions of carbon monoxide, which is liberated during the 193 nm excimer laser ablation of polyimide, have been measured using mass-resolved resonant multiphoton ionization spectroscopy. Using 150 mJ/cm2 fluence (1000 Å removed per pulse), the rotational temperature is seen to decrease monotonically from 1150 to 650 K from beginning to end of the ablation plume. Additionally, the vibrational temperature is measured to be 3400 K and is constant over time. When using 6 mJ/cm2 to ablate (0.1 Å removed per pulse), the rotational and vibrational temperatures are measured to be 700 and 900 K, respectively. The influence of collisional perturbations and high ablation fluences on the measured rotational and vibrational temperatures is discussed.

Infrared studies in the ν 1 of ONCI: CO laser line coincidences and pressure broadening data

46 Spectral quasi-coincidences between CO laser lines and ClNO absorption lines are given. The ONCl ν 1 spectra have been obtained using a Fourier transform spectrometer. In four cases the discrepancies between the frequencies are smaller than the Doppler width. Pressure broadening parameters resulting from collisional perturbations by Ar for two rotational transitions in the ν 1 band of ONCl have been measured using a diode laser spectrometer.

Photoacoustics using radio-frequency laser-optogalvanic detection: a new technique for low-pressure photoacoustic spectroscopy. Magnetic predissociation of the state of I2

A new technique for low-pressure photoacoustic spectroscopy, one in which a low-power radio-frequency discharge is used as a sensitive acoustic-wave sensor, is presented. This contactless technique is a variation of the laser-optogalvanic method and is termed PARFLOG (photoacoustics using radio-frequency laser-optogalvanic detection).The ability to prosecute low-pressure photoacoustic measurement at a reduced level of collisional perturbations enables us to study intramolecular processes such as level crossings, predissociation, field effects, etc. Results for magnetic predissociation of the [Formula: see text] state of iodine are presented.

Rotational inelastic cross sections for OCS–Ar, OCS–He, OCS–H2 collisions: A comparison between theory and experiment

Pressure broadening parameters resulting from collisional perturbations by Ar, He, and H2 for a number of rotational transitions in the ν3 band of OCS have been measured using a diode laser spectrometer. The data are compared with theoretical values obtained using intermolecular potentials previously suggested for these systems and infinite order sudden approximation molecular scattering calculations.

Stochastic diffusion in inverse square fields: General formulation for interplanetary gas

The Fokker-Planck equation for small stochastic changes to particles in Kepler orbits has to be formulated in terms of the integrals of motion. We generalize the modelling of proton and electron collisional perturbations to gas particles on trajectories through the solar system in order to include both spatial and velocity diffusion. The general solution is obtained in terms of a 4-dimensional normal distribution. Treatment of the singularity in the Fokker-Planck operator reduces the dimensionality by one. In addition to extending earlier results for anisotropic collisional heating in the thermal approximation, the present formulation gives the changes in density due to the mean repulsive force and to perturbations of trajectories (spatial diffusion). The net diffusion is almost everywhere towards the sun and the density increase is significant in the downstream hydrogen wake, particularly where destructive depletion is strong and gravitational focussing weak.

Multiphoton absorption of intense HF laser radiation by methanol

Cross sections for absorption of the HF P1 (6) line by methanol were obtained for pressures between 4.2 Pa and 2.75 kPa. For pressures below 200 Pa, cross sections decrease with increasing laser fluence. However for higher pressures cross sections are independent of fluence. Collisional perturbations are likely responsible for this change of behaviour.

Collisional perturbations of time-resolved photon scattering from molecular levels

A theoretical study is presented of the effects of collisional perturbations on the time-resolved resonant and near-resonant photon scattering from a molecular system. The photon counting rate is formulated in terms of tetradic Green's functions in Liouville space. The collisional and radiative damping effects are handled by an effective-Liouvillian formalism. The formal expressions are disentangled by invoking a series of successive approximations, assuming short correlation times of the thermal bath, neglecting relaxation processes in the ground electronic state, considering rotational relaxation as providing the dominant damping mechanism in the excited electronic manifold, specializing to weak electromagnetic fields, neglecting some off-resonance processes and disregarding collision-induced radiative processes. The photon counting rate is expressed as a triple convolution of the photon counting rate for the free molecule with the Doppler Gaussian profile and with a Lorentz profile. The latter incorporates purely collisional effects due to intrastate cross relaxations and proper ${T}_{2}$ processes. The theory accounts well for the recent experimental data of Williams, Rousseau, and Dworetsky on collisionally perturbed ${\mathrm{I}}_{2}$ molecules.

Theory of collision effects on atomic and molecular line shapes

A review of recent developments in the theory of the effects of binary collisions on the spectral profiles associated with atomic and molecular systems is presented. To consistently account for collisional perturbations of both the internal energy levels and the velocity of active (emitting or absorbing) atoms or molecules, one must use a theory in which the center-of-mass motion of the active atoms has been quantized. Following this procedure general equations for absorption or emission line shapes are obtained. The line shapes may exhibit narrowing or broadening with increasing perturber pressure, depending upon the nature of the collision interaction. The physical significance of the collision mechanisms giving rise to such behavior is discussed, as is the experimental evidence in support of the theory. Various applications of the theory are presented.

Fluorescence, Phosphorescence, and Triplet Formation in Biacetyl at Low Pressures

The S1 → T intersystem crossing in biacetyl vapor has been examined by direct observation of emission from both the initial and final states. When biacetyl is excited by absorption of 4358-Å radiation, both the singlet- and the triplet-emission yields are independent of pressure in the range 40–0.1 torr. Since isolated molecule conditions are established for singlet relaxation in the lower limit of this range, triplet formation must occur in biacetyl in the absence of collisional perturbations involving the excited singlet state. Triplet emission continues to be observable at pressures descending to 0.03 torr, but the emission yield decreases due to triplet deactivation at the walls. The results are discussed within the framework of recent theories of radiationless transitions.

Vibrational Dynamics in Liquid Water: A New Interpretation of the Infrared Spectrum of the Liquid

In comparing the infrared spectra of liquid water and simple hydrates, two anomalies are noted which are apparently inexplicable in terms of presently acceptable theories of the liquid. These anomalies are explained in terms of a Maxwellian distribution of collisional interactions between water molecules, which when put in quantitative form reveals (1) that most molecules in the liquid are highly distorted by collisional perturbations, and (2) that there is a broad distribution of distortion among the molecules in the liquid. When translated into vibrational dynamics, these concepts lead to a continuum distribution of ν1 and ν3 modes of varying distorted molecules. It is shown that in its stretching motion, liquid water behaves dynamically as a continuum of OH bond oscillators of different frequencies, demonstrating both weak inter- and intramolecular vibrational interactions. A new interpretation of the infrared spectrum of liquid water is given in terms of this model, and other spectral evidence is offered in support of it.