Collision-induced Enhancement Research Articles

Collision-induced amplification of short-wave radiation on transitions to the ground state of atoms

A new version of obtaining short-wave lasing on transition between highly excited states and the ground state of active atoms in the buffer gas atmosphere is studied theoretically. The mechanism of obtaining population inversion on such a transition is associated with the establishment of the local Boltzmann distribution of populations in a group of highly excited levels due to frequent collisions. If the excitation of the upper-lying level is performed by two laser radiation sources with frequencies ω1 and ω2, short-wave lasing can be obtained at a frequency close to the total frequency ω1 + ω2. The conditions for the emergence of population inversion are analyzed and simple analytic formulas are derived. It is shown that collision-induced enhancement of short-wave radiation can occur for pumping intensities on the order of 100 W/cm2. For pumping intensities on the order of 1000 W/cm2, the amplification factor for short-wave radiation may attain values of 3 cm−1 (for an active atom concentration of N ∼ 1015 cm−3), which is sufficient for the development of lasing per path through an active medium (superradiance condition) for a length of the active medium on the order of 10 cm.

Collisionally enhanced isotopic selectivity in multiphoton dissociation of vibrationally excited CF3H

We have studied infrared multiphoton dissociation of CF3H pre-excited to the second C–H stretch overtone under collisional conditions in view of developing a laser isotope separation scheme for carbon-13. This single stage process results in a C2F4 product that has been enriched in carbon-13 to a level as high as 99% starting from a naturally abundant sample, implying an isotopic selectivity in excess of 9000. While most of the selectivity is gained at the pre-excitation step, it can be increased up to a factor of 16 by collisions of the pre-excited CF313H species with room temperature molecules. This collision-induced enhancement in selectivity becomes evident from the dependence of the isotopic enrichment on both the total sample pressure and the time-delay between the two lasers, and we propose two different models that can account for this behavior. Finally, we evaluate the practical relevance of this two-laser scheme for isotope separation.

Collision-induced absorption in the ν2 fundamental band of CH4. II. Dependence on the perturber gas

The integrated intensities of the collision-induced enhancement spectra of the ν2 band of CH4 perturbed by rare gases and linear molecules (N2, H2, and CO2) are calculated theoretically using the quadrupole transition moment obtained from an analysis of CH4–Ar spectra. In addition to the isotropic quadrupole mechanism responsible for the enhancement in CH4-rare gases, there is additional absorption arising from the anisotropic quadrupole mechanism in the case of molecular perturbers. This latter effect involves the matrix element of the anisotropic polarizability for the ν2 transition in CH4 that is available from the analysis of the depolarized Raman intensity measurements. Overall, the theoretical values for the slope of the enhancement spectra with respect to the perturber density are in reasonably good agreement with the experimental results, thus confirming that the collision-induced absorption arises primarily through the quadrupolar induction mechanism.

Collision-induced absorption in the ν2 fundamental band of CH4. I. Determination of the quadrupole transition moment

An experimental value for the quadrupole transition moment of the ν2 fundamental band of CH4 has been determined by fitting the collision-induced enhancement spectrum of CH4 with Ar as the perturber. The observed quadrupole-induced absorption increases linearly with the Ar density, ρAr, and is comparable to the allowed dipole intensity due to Coriolis interaction with the ν4 band at approximately 125 amagats. Ignoring vibration-rotation interaction and Coriolis interaction,, we equate the measured slope of the integrated intensity versus ρAr to the theoretical expression for the quadrupole-induced absorption, and obtain the value |〈0|Q|ν2〉|=0.445 ea02 for the quadrupole transition matrix element. A theoretical value 〈0|Q|ν2〉=0.478 ea02 has been determined by large-scale ab initio calculations and, considering both the theoretical approximations and experimental uncertainties, we regard the agreement as good, thus confirming our interpretation of the enhancement as due to the quadrupole collision-induced mechanism.

Collision-Induced Absorption in the a1Δg(v′=0)←X3Σg−(v=0) Transition in O2–CO2, O2–N2, and O2–H2O Mixtures

When O2 is perturbed by collisions with other molecules, the weak spin-forbidden magnetic dipole transition a1Δg←X3Σg− shows a broad continuum absorption underlying the sharp lines. This collision-induced enhancement absorption plays a role in the Earth's atmosphere and much experimental work has been carried out to measure the binary absorption coefficient with different perturber gases. Recent work on the v′=0←v=0 band in O2–CO2 mixtures yielded a value for the coefficient that was approximately three times that of earlier measurements on O2–N2 mixtures. In the present note, we calculate the absorption theoretically assuming that the long-range quadrupole-induced dipole mechanism is dominant. Using experimental polarizability matrix elements of CO2 and ab initio results in the literature for the quadrupolar transition matrix element for O2, we find good agreement for O2–CO2 mixtures without any adjustable parameters. The agreement for O2–N2 is less good, and because of the much smaller polarizability of N2 than of CO2, we suggest that one has to include a short-range component in addition to the long-range one treated here. We also calculate the binary absorption coefficient for O2–H2O, for which no experimental data are available, and we synthesize the corresponding spectrum for use in atmospheric modeling.

The collisional energy transfer between the D 1 Π and d 3 Π states of the NaK dimer induced by argon buffer gas

Π→X1Σ+ transitions and the continuum spectrum of d3Π→a3Σ+ transitions of the NaK dimer in a heat pipe could be observed in the range of 500–700 nm. The collision-induced enhancement effects of the intensities Id→a and the quenching effect of the intensities ID→X by collisions with argon buffer gas at different pressures were measured experimentally. Based on the stationary collisional model and lifetime measurements, the quenching cross sections and cross sections for collision-induced energy transfer between the D1Π and d3Π states could be estimated.

COLLISION-INDUCED ENHANCEMENT EFFECT OF THE INTENSITIES OF 23∏g—13∑u+ DIFFUSE SPECTRUM FOR K2 DIMER

The 23Πg-13∑u+ diffuse fluorescence spectrum around 5730? for K2 dimer excited by 4415.6? CW Isser line was obtained. The collision-induced enhancement effect of the maximum intensities Idiff of the diffuse spectrum versus the pressure P ot buffer gas Ar was studied experimentally. The energy transfer processes of C1Πu-23Πg was showed by stable collision model. The Idiff-P relation was deduced and a satisfactory fitting for the experimental data was obtained by use of this model. This fitting showed that the energy crossing rate and collision-induced transfer rate could be obtained by this model with some alternations of the experimental setup.

Decomposition of the photoabsorption continuum underlying the Schumann-Runge bands of 16O 2—II. Role of the 1 3Π g state and collision-induced absorption

Measurements are presented of molecular oxygen photoabsorption cross-sections and pressure coefficients taken at selected minima between rotational lines of the Schumann-Runge band system. Both room-temperature and liquid-nitrogen-temperature results are presented from 1760–1980 Å, and corrections are applied for the effect of the wings of the rotational lines. Absorption into the B 3∑ − u and A 3∑ + u + and is found to be insufficient to account for the total observed cross-section, and it is proposed that transitions to the 1 3 Π g valence state account for the remainder. The pressure dependence of the cross-sections is consistent with collision-induced enhancement of the intensities of the forbidden transitions X 3∑ − g → A 3∑ + g and X 3∑ − g → 1 3Π g , while the temperature dependence of the pressure coefficients is not consistent with absorption due to stable (O 2) 2 dimers.

Collision-induced fundamental band of D 2 in D 2He and D 2Ne mixtures at different temperatures

The collision-induced enhancement absorption spectra of deuterium in its fundamental region in D 2He at 77, 195, and 273 K and in D 2Ne at 77, 195, 273, and 298 K have been recorded with an absorption path length of 26 cm for gas densities up to 670 amagat for several base densities of deuterium. At each experimental temperature, binary and ternary absorption coefficients for the fundamental band were derived from the measured integrated intensities. The theory of the “exponential-4” model for the induced dipole moment of the colliding pair of molecules was applied to the binary absorption coefficients. The overlap parts of these coefficients were obtained by subtracting the calculated quadrupolar parts from the experimental values. The magnitude parameter λ, and the range ϱ, of the overlap part of the induced dipole moment, were determined for the colliding molecular pairs D 2He and D 2Ne by obtaining the best fit of the calculated overlap part of the binary absorption coefficients as a function of temperature to the experimental values of the overlap part. Derived parameters λ, ϱ, and the overlap induced dipole moment μ(σ) (at the Lennard-Jones intermolecular diameter σ) for D 2He and D 2Ne are as follows. Mixture λ ϱ μ(σ) D 2He 4.7 × 10 −3 0.24 Å 2.55 × 10 −2 ea 0 D 2Ne 8.5 × 10 −3 0.26 Å 4.56 × 10 −2 ea 0