Foreign Gas Collisions Research Articles

Suppression of superradiance from an ensemble of two-level atoms in the presence of foreign gas collisions

I compute the suppression in the superradiant radiation flux from an ensemble of homogeneously-broadened two-level atoms in a slab geometry configuration due to the increase in the polarization transverse decay rate caused by decoherence due to foreign gas collisions. I show that the differences between the total width of the polarization at low excitation with the magnitude of the real part of the eigenvalues and of the dominant and leading modes of the 1-D Lienard-Wiechert kernel are the critical parameters that determine the particular regimes for the superradiant flux dependence on the collision width.

Time dependent measurements of nitrous oxide and carbon dioxide collisional relaxation processes by a frequency down-chirped quantum cascade laser: Rapid passage signals and the time dependence of collisional processes

Intrapulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of the gas sample. This is due to the rapid sweep of the radiation through the absorption features. The intrapulse time domain spectra closely resemble those recorded in coherent optical nutation experiments. In the present paper, the frequency down-chirped technique is employed to investigate the nitrous oxide-foreign gas collisions. We have demonstrated that the measurements may be characterized by the induced polarization dominated and collision dominated measurement limits. The first of these is directly related to the time dependence of the long range collision cross sections. Among the collisional partners considered, carbon dioxide shows a very unusual behavior of rapid polarization damping, resulting in the production of symmetrical line shapes at very low gas buffer pressures. In the collision dominated regime, the pressure broadening parameters, which we have derived, are comparable at slow chirp rates, with those derived from other experimental methods. By comparing the pressure broadening coefficients of Ar, N(2), and CO(2) with those of He, making use of the chirp rate independence of the pressure broadening by helium, we have shown that at higher chirp rates there is clear evidence of the chirp-rate dependence of the pressure broadening parameters of N(2) and CO(2).

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.

Monte Carlo simulation of radiation trapping in Hg-Ar fluorescent discharge lamps

The line spectra of emitted resonance radiation from mercury and the effective decay rates of the Hg 63P1 and 61P1 levels in mercury–argon discharges are simulated by a Monte Carlo method. The hyperfine splitting, the natural isotopic composition, collisional transfer of excitation, foreign gas collisions and quenching are considered to describe in detail the 253.7 nm and 184.9 nm lines. The calculations are performed for Hg vapor densities corresponding to coldest spot temperatures of 5–100°C, and discharge parameters typical for fluorescent lamp operation. The densities of the Hg 63P1 and 61P1 levels are consistently estimated by means of a set of balance equations for the Hg 63P0, 63P1, 63P2, and 61P1 excited states. The resulting uv radiation output of the discharge is then estimated for a tube radius of 18 mm, argon pressure of 400 Pa, discharge current 0.4 A, and wall temperatures of 20–80°C. The results obtained show a good agreement as compared with published experimental data.

Vibrational Energy Transfer in Ozone Excited to the (101) State: Double-Resonance Measurements and Semiclassical Calculations in the 200−300 K Temperature Range

The relaxation of ozone excited into the (101) vibrational state is studied in O3−O2 and O3−N2 gas mixtures over the 200−300 K temperature range using a laser double-resonance (DR) technique. Rate coefficients for V−V transfer processes occurring in O3−O3 collisions are experimentally determined and theoretically calculated. The temperature exponent n obtained by fitting the calculated values with a law of the form k(T) = k(T0)(T0/T)n is in very good agreement with the experimental values. To model the vibrational energy deactivation processes occurring upon O3−foreign gas collisions, the rate coefficients for levels higher than the first dyad are deduced from the rate coefficients for the lowest states using simple scaling. The simulated DR signals obtained by a kinetic model using these values are in good agreement with the corresponding experimental DR signals over the entire temperature range.

Speed dependence of rotational relaxation induced by foreign gas collisions: Studies on CH3F by millimeter wave coherent transients

The dependence of rotational relaxation rates on the speed of absorbing molecules has been studied by millimeter wave coherent transients for the J,K=1,1–2,1 rotational transition of methyl fluoride (CH3F). A new phenomenological model used to describe such a speed dependence has been introduced. It leads to a quite simple analytical expression for time-domain transient signals, the Fourier transform of which corresponds to the frequency-domain line shape (speed-dependent Voigt profile). The investigations were carried out on mixtures of CH3F with He, Ar, Xe, H2, D2, N2, and O2, yielding parameters which characterize the speed dependence of the observed decay rates and its pressure and temperature dependence. Special emphasis was given to the key role of the mass ratio of collision partners which clearly allowed the relation of the observed nonexponential decay behavior to collisional effects. However, the observations cannot be explained exclusively with consideration of speed-dependent rates, but must also be discussed with reference to velocity-changing collisions. The observed temperature dependence of the rates may allow discrimination between these two different collisional effects which lead to departures from Voigt profile line shapes.

Rotational relaxation rates for theJ= 0–1 transition of N2O by self-collisions and foreign gas collisions

Time domain techniques using pulse methods in microwave spectroscopy have been employed to determine the population and coherence decay rates, 1/T 1 and 1/T 2, for the J = 0–1 rotational transition of 15N2O. The pressure dependence of both relaxation rates has been investigated for the pure gas. Pressure broadening of the line (∼ 1/T 2) has also been studied for mixtures with O2, N2, air and the rare gases He, Ne, Ar, Kr and Xe. The observed transient emission signals were analysed by the standard model of constant coherence decay rates and also assuming dependence of the rates on the speed of the absorber molecules. The experimental results are compared with predictions from theoretical treatments of the binary collisions.

Collisional processes and corresponding rate constants in the B 3Π(0 u+).υ′= 15 State of I 2

The selective laser excitation and decay time measurement of the induced fluorescence has been used to study inelastic collisions of 1 2 *(B 0 u +.υ′= 15. J′) with l 2, 4He, 3He, Ne, Ar. H 2 and D 2. For each collision partner, at least six initial rotational levels, ranging from J i′=12 up to J i′=146. have been excited. Total depopulaton, rotational transfer, vibrational transfer and quenching rate constants were determined for the different collision partners. in I 2 *-I 2 collisions, the quenching rate constant is the largest. While in I 2 *-foreign gas collisions, the rotational transfer is dominant.

Theory of resonant light scattering from two level atoms in the presence of collisions

A theory of resonant light scattering from two level atoms in a gas is developed for an arbitrarily strong pump field. The Doppler effect and two body foreign gas collisions are considered. Elastic, inelastic (upward and downward), and the velocity changing collisions are retained in the general formulation. The impact approximation is not made, and the expressions for all (field dependent) collision rates are generated internally. Particular attention is paid to the lower state interaction, and a more complete parameterization of the theory is achieved. The general theory is non-Markovian and the quantum regression theorem is not used; it reduces to existing theories in special cases. It is shown that the coherent scattering is not a general property of the system if the lower state interaction is present; the "coherent" component acquires a small, but nonzero, width in the strong field region. This width could provide information about some new collision parameters.

Measurements of T2 in excited vibrational states of HCN

Polarization relaxation times (T2) have been measured for the J=6, 9, and 10 l-doublet transitions in the (0110) vibrational level of HCN by means of microwave transient emission. Also, T2 values for relaxation induced by He, H2, and CH3F are obtained for the J=6 transition. The dephasing cross sections for foreign gas collisions are consistent with the force laws operative in each case. The HCN–H2 result provides a comparison with the recently measured cross section of the isoelectronic molecular ion system, HCO+–H2, and confirms the prediction of Green that rotational relaxation rates are only moderately faster for molecular ions.

The influence of hyperfine coherence and of elastic collisions on the circular polarisation of emission from Li 2

This paper gives an account of the factors that influence the degree of involvement of nuclear hyperfine interactions in determining the polarisation ratios of low J′ levels of diatomic molecules excited by circularly polarised light. Experimental estimates are obtained for the nuclear hyperfine coupling constant of the 1Σ u + excited state of 7Li 2. The role of foreign gas collisions is also considered and cross sections for the destruction of orientation obtained for J′ = 10.

Influence of Resonant and Foreign Gas Collisions on Line Shapes

In the limit of a dipole-dipole interaction between neutral atoms and the binary-collision approximation, we present a theory of pressure broadening in which the multiplet structure of the atomic levels is considered. The appropriate Schr\"odinger equations for the collision problem are numerically integrated to obtain solutions for both resonant and foreign gas broadening. The presence of radiation fields is neglected in the first half of the paper, where the collisional time rate of change of the density matrix is found. The pertinent collisional decay parameters are extracted from these equations. The ratios of the parameters of the average magnetic dipole to electric quadrupole moment obtained were 1.21\ifmmode\pm\else\textpm\fi{}0.02 for the relaxation of the entire ensemble in resonant broadening, 0.966\ifmmode\pm\else\textpm\fi{}0.012 for the relaxation of the initially excited atoms in resonant broadening, and 1.12\ifmmode\pm\else\textpm\fi{}0.02 for the relaxation of the initially excited atoms in foreign gas broadening. In the second half of the paper, the collisional broadening results are incorporated in a systematic evaluation of (i) spectral profiels (effects arising from the velocity and recoil of the emitting atom are neglected), (ii) Hanle-effect line shapes, and (iii) laser phenomena. For the case of resonant broadening, the radiation trapping process is reviewed and its influence on line shapes discussed. Finally, a detailed comparison of the theory with experimental findings as well as with previous theoretical results is made. We conclude that the dipole-dipole interaction is sufficient to explain most cases of resonant and some cases of foreign gas broadening.