Study of energy transfer from laser excited CO2 to chloromethanes

Collisional energy transfer from CO₂(ν₃) to chloromethanes was studied in the temperature range 300–700 K using the laser induced fluoresce technique. Theoretical calculations using SSH and SB theories were carried out to identify the CO₂(ν₃) deactivation process. The experimental probabilities were found to decrease linearly with increasing number of substituted chlorine atoms.     

Fluorescence behavior of 7-hydroxycoumarine excited by one-photon and two-photon absorption by means of a tunable dye laser

The fluorescence spectrum of 7-hydroxycoumarine in ethanol excited by a pulsed tunable dye laser reveals different features when excitation proceeds via one-photon and two-photon absorption. In the former case the spectrum shows two peaks delayed in time by approximately 2 ns and characterized by different lifetimes. The relative intensity of these peaks is unaffected by significant changes in the dye concentration and therefore the formation of an “exciplex” species seems to be here confirmed. In the spectrum obtained by two-photon excitation the second peak at longer wavelength is absent. Results are presented for both room and liquid nitrogen temperatures as well as for other solvents such as glycerin and EPA. These results are considered to be important for the evaluation of absolute two-photon cross sections where the quantum efficiencies of one- and two-photon processes are assumed to be the same.     

Energy transfer among excited vibrational states of SF6

Studies of the risetime of laser induced 16μ fluorescence in SF₆ show the rapid equilibration (1.5 μsec torr) among the excited vibrational states. The subsequent decay of fluorescence consists of 2 exponentials. The faster of these is due to vibration-translation energy transfer (160μsec torr) and the slower is due to thermal cooling of translationally hot gas.     

Study of near resonant energy transfer from laser excited CO2 to SO2

Collisional energy transfer from CO₂ to SO₂ was studied subsequent to pumping of CO₂ (ν₃) by a Q-switched laser. The measurements were made in the temperature range 300–800 K and in the pressure range 1–30 Torr. The fluorescence from the ν₃ level of CO₂ was monitored with the help of a Ge:Au detector at 77 K with an estimated response time of ≈2 μs. The probability of the energy transfer was found to be increasing with increasing temperature. The probable kinetic models for the V---V relaxation pathways were discussed and the experimentally measured energy transfer rate is related to the cross-over transfer processes. Theoretical calculations using both a simple SSH-breathing sphere model and the Sharma-Brau theory were carried out to evaluate the probabilities of the involved cross-over energy transfer processes and the results were compared with the experimental rates.     

Rotation-vibration-translation energy transfer in laser excited Li2 (B1Πu)

Using the method of laser fluorescence, inelastic collisions with rare gas atoms of electronically excited ⁷Li₂ molecules in the υ = 2 and 4 levels were studied. Vibrational transitions ranging from Δ = +2 to ?4 were observed. The simultaneous rotational transitions were completely resolved, and detailed rate constants k_{Δυ, ΔJ} for specific collision- induced quantum jumps Δυ, ΔJ were determined. The effect of secondary rotational relaxation was eliminated by an extrapolation to zero pressure. By integration over ΔJ, rate constants k_Δυ, were found. They are, within the error limits, independent of the collision partner and on the initial υ (2 or 4) and depend rather weakly on Δυ. These findings are compared with theoretical results from various methods, generally based on a collinear collision model. The apparent disagreement in all respects suggests strongly the importance of rotational degrees of freedom in the collision. Experimental evidence for this is the large amount of V — R transfer observed, which about equals the V — T transfer. The mean cross sections σ(Δυ) for specific vibrational transitions Δυ range between 6 and 15 A², among the largest ever observed.     

On the mechanism of intermode vibration-to-vibration energy transfer in CH3F excited by a tea CO2 laser

The v₃ mode of CH₃F was excited by irradiation with a TEA CO₂ laser pulse, and the time-resolved emission spectra of the v₃ overtone and the 3 μ;m region were observed. The results indicate that the population of the v₄ level behaves kinetically in the same manner as that of 2v₃ or 3v₃. This suggests an efficient energy transfer between these levels.     

Energy transfer dye laser: Confirmation of energy transfer by reabsorption effect

The laser and fluorescence behavior was studied for the mixed dye system containing various concentrations of 3,3′-diethylthiacarbocyanine iodide and rhodamine 6G. A new way of confirming the occurrence of energy transfer is presented on the basis of the reabsorption effect.     

Raman analysis of SF6 molecular beams excited by a cw CO2 laser

In a molecular beam the effects of vibrational pumping of SF₆ (ν₃ = 948 cm^?1) are studied, using a line-tunable cw CO₂ laser. Intracavity spontaneous Raman scattering is used for analysis. For excitation in the collision regime (x_E/D ≤ 1), a thermal redistribution of the ν₃ excitation over all vibrational modes is found, together with an average absorption up to six photons per molecule. The infrared absorption profile shows a red-shift of 6 cm^?1. For excitation in the relatively rare collision regime (x_E/D ? 4), a structured non-thermal ν₁ Raman spectrum is observed, especially in the case of seeded molecular beams (10% in He). The observed hot-band peaks can be explained in terms of single-photon absorptions and collision-induced near-resonant V-V energy transfer, leading to single, double and triple excitations of the ν₃ mode. The value of T_rot in the beam is found to influence sensitively the non-resonant energy-transfer rate [e.g. hν₃(948 cm^?1)+ΔE_rot → h(ν₄ + ν₆)(962 cm^?1) relative to the near-resonant transfer rate (hν₃ + hν₃ → 2hν₃ + 3.5 cm^?1)].     

Photoexcitation spectroscopy of BO2 with a single frequency dye laser

The (0,0,0) ²Π_3/2−(1,0,0) ²Π_3/2 bandhead of ¹¹BO₂ has been studied with a tunable dye laser. Rotational transitions between J = 1.5 and J = 33.5 were assigned. For the measurements a combination of photoexcitation and laser induced fluorescence spectroscopy has been used; the method is extremely useful for the assignment and deconvolution of unresolved complex bands.     

Mode-to-mode vibrational energy transfer in D2CO: Evidence of coriolis-enhanced rotational selectivity

Infrared—ultraviolet double resonance spectroscopy is used to demonstrate rapid collision-induced V-V transfer between the v₆ and v₄ vibrational manifolds of D₂CO. The rate of transfer is at least gas-kinetic and is explained in terms of Coriolis coupling and rotationally specific, quasi-resonant relaxation channels     

A laser induced fluorescence study of vibrational energy transfer in dideuteromethane

V—V and V—T/R rates of ν₁, ν₂, ν₃, ν₆, ν₈, and ν₉ of CH₂D₂ were measured and are reported. The deactivation efficiencies of rare gas collision partners were measured and calculated by SSH theory, the results are reported and discussed. Possible V—V pathways are presented and discussed.     

Model discrimination for rotational energy transfer in the Ar-N2 system

The jackknife test of Rothstein et al. is applied to discrimination between several different models used to compute the rotationally inelastic cross sections for the Ar-N₂ system. The modified exponential models are the best models, except for the case where the energy gap is small, when power laws are best.     

Semi-classical calculations of rotational/vibrational transitions in He-H2

The applicability of the classical trajectory equations in three-dimensional calculations of rot/vib transitions in the He-H₂ system has been investigated. The vibrational relaxation time is calculated and the agreement with experimental data is excellent for temperatures above 250 K. The method has been used to determine the differential cross-sections for vib/rot excitation at 1.09 eV total energy and comparison is made with recent quantum mechanical effective potential calculations.     

A laser induced fluorescence study of vibrational energy transfer processes in cyclopropane

Results of infrared laser induced fluorescence studies on cyclopropane are presented. Molecules were excited from the ground state to the v₁₀ level of cyclopropane using a Q-switched CO₂ laser operating on either the P(14) or P(20) transition of the 9.6 μ branch. Fluorescence was observed from the v₆, v₈, v₁₀ + v₁₁ and v₅ + v₁₀ levels of cyclopropane. The self-deactivation of vibrationally excited cyclopropane through V → T/R processes was found to have a rate of 8.0 ± 1.5 ms^?1 torr^?1. Deactivation by rare gas collisions was also studied with comparison to simple V → T and V → R theories. V → V equilibration processes are discussed involving the v₆, v₈, v₁₀, v₁₁, and v₁₀ + v₁₁ levels.     

Inaccuracy of an approximate dynamical treatment of He-H2 vibrational energy transfer

The recent semi-classical approach of Shin is applied to He-H₂ vibrationally-inelastic collisions using the Gordon-Secrest potential surface. The calculated de-excitation cross sections, when compared to accurate coupled-states results are too large, particularly at low collision energies. As a result the rate constants for vibrational relaxation are artificially enhanced at low temperature, leading to a fortuitous agreement with experiment.     

Rapid sensitization of vibrational energy levels of N2O in collisions with SF6 excited by a CO2 laser

Experimental investigations of mixtures containing predominantly N₂O and small amounts of SF₆ demonstrate that rapid interspecies pooling of vibrational energy can occur to produce a pulse of excess vibrational energy in the ν₃ mode of N₂O following excitation of SF₆ by a Q-switch CO₂ laser. This increased population in the ν₃ mode of N₂O can occur on a time scale shorter than that on which collision-induced VV processes redistribute vibrational energy among the modes of SF₆. The equilibration takes place in three discernible stages: (1) a rapid pooling of energy between a limited number of levels of the SF₆ and N₂O, then (2) a slower collision-dependent VV process that equilibrates all the vibrational modes in the system, with (3) a subsequent VT,R process that returns the system to its initial state. Argon is shown to accelerate selectively process (2) with an efficiency consistent with the previously measured ability of argon to accelerate the VV process in pure SF₆. The experimental evidence indicates that other modes in N₂O do not become involved on the time scale on which direct crossing to ν³ occurs. Additionally, on the time scale preceding the SF₆ VV equilibration, a fast collision-dependent process competes with the transfer of excitation to N₂O. The production of a pulse of excitation in N₂O is eliminated when isotopically substituted N₂O (¹⁴N¹⁵NO) is used instead under the same conditions because the crossing rate to the ν₃ mode of N₂O is decreased sufficiently when ¹⁵N is substituted for ¹⁴N that it no longer can compete with the VV equilibration among the modes in SF₆.     

Anharmonicity in rotational and vibrational excitation of H2 by Li+ collisions

Integral cross sections for pure rotational and vibrational-rotational excitation of H₂(X¹Σ⁺_g) by Li⁺(¹S) impact are computed by close-coupling methods at 0.2, 0.6, and 1.2 eV in the c.m. system using vibrational functions that are numerical solutions of the one-dimensional radial Schrödinger equation for harmonic, Morse, and adiabatically corrected Kolos-Wolniewicz (KW) potential functions. Comparison of results employing KW and Morse functions shows excellent agreement for all transitions studied. Findings using harmonic oscillator functions, however, differ noticeably from KW and Morse values for vibrational (0 → 1) and very large rotational (Δj = 10) transitions, but are satisfactory for lower order (0 → 2, 4, 6, 8) rotational transitions.     

Single-Crystal Growth and Classification of EuAlF5 and Solid Solutions M(II)1−xEuxAlF5 (M=Ca, Sr, Ba) within the Structural Family of Tetragonal M(II)M(III)F5 Compounds

The compound EuAlF₅, as well as the solid solutions Ca_0.19(1)Eu_0.81(1)AlF₅, Sr_0.15(1)Eu_0.85(1)AlF₅, Sr_0.55(1)Eu_0.45(1)AlF₅, Sr_0.77(1)Eu_0.23(1)AlF₅, and Ba_0.62(1)Eu_0.38(1)AlF₅, crystallize in colorless tetragonal columns. These have been prepared by solid state reactions at 900°C, starting from mixtures of the binary fluorides. According to Vegard's rule the solid solution Sr_1−xEu_xAlF₅ shows a linear dependence of the crystal volume on the molar ratio Eu/Sr. All crystal structures have been refined from single-crystal diffractometer data. EuAlF₅ and the M_1−xEu_xAlF₅ (M=Ca, Sr) compounds obtained are isotypic with β-SrAlF₅. They crystallize in a superstructure in space group I4₁/a (no. 88) with 64 formula units and lattice parameters a≈19.9 Å, c≈14.3 Å. The structure is characterized by chains of trans-corner-sharing [AlF_4/2F_2/1] and branched [AlF_5/1F_1/2] octahedra forming a channel structure. Inside the channels isolated ordered dimeric units [AlF_4/1F_2/2]₂ are located. The divalent metal atoms show coordination numbers 8 and 9; they connect the [AlF₆] octahedra into a three-dimensional structure. Ba_0.62(1)Eu_0.38(1)AlF₅ is isotypic with the corresponding Sr compound Ba_0.43(1)Sr_0.57(1)AlF₅, and it crystallizes with 16 formula units and lattice parameters a=14.3860(7) Å, c=7.2778(3) Å in space group I4/m (no. 87). The network structure is identical with that of EuAlF₅. Instead of the dimeric units, infinite chains [AlF_4/1F_2/2]_∞ of trans-corner-sharing [AlF₆] octahedra extending along the c- axis are located inside the channels. The bridging fluorine atoms of this chain show large anisotropic displacement parameters, but no superstructure reflections have been observed for this compound.     

Population transfer to excited vibrational levels of H2 molecule by stimulated hyper-Raman passage with chirped laser pulses

We have theoretically investigated the population transfer from the initial ground rovibrational level v(g)=0, J(g)=0 to the final rovibrational levels v(f)=1,2, J(f)=0 of the ground electronic state X (1)Sigma(g) (+) via the resonant intermediate level v(i)=6, J(i)=0 of the excited electronic state EF (1)Sigma(g) (+) of H(2) molecule by (2+2)-photon stimulated hyper-Raman passage (STIHRP). The density matrix technique has been employed to evaluate the population transfer to the final target levels using linearly chirped pump and Stokes laser pulses with different chirp rates. Both the pulses are considered to have the same temporal shape, pulse width, and linear parallel polarizations. We have studied in detail the dependence of the population transfer on the set of laser parameters for pulse (peak) intensities in the ranges of 1.5 x 10(11)-1.0 x 10(12) and 1.0 x 10(12)-7.0 x 10(12) W/cm(2). The corresponding pulse widths (full width at half maximum) are of the order of 115-200 and 15-30 ps. We have found that the chirp rate parameters can be optimized to achieve almost complete population transfer from the ground (g) to the final (f) target levels. This, to our knowledge, is the first application of a (2+2)-photon STIHRP process with chirpings to a model molecular system (H(2)). The study demonstrates the suitability of the chirped (2+2)-photon STIHRP technique for selective and almost total inversion of vibrational population in a diatomic molecule.