Time domain modeling of spectral collapse in high density molecular gases

Lindblad parameters from high resolution spectroscopy to describe collision-induced rovibrational decoherence in the gas phase-Application to acetylene

Within the framework of the Lindblad master equation, we propose a general methodology to describe the effects of the environment on a system in the dilute gas phase. The phenomenological parameters characterizing the transitions between rovibrational states of the system induced by collisions can be extracted from experimental transition kinetic constants, relying on energy gap fitting laws. As the availability of these kinds of experimental data can be limited, this work relied on experimental line broadening coefficients, however still using energy gap fitting laws. The 3 μm infrared spectral range of acetylene was chosen to illustrate the proposed approach. The method shows fair agreement with available experimental data while being computationally inexpensive. The results are discussed in the context of state laser quantum control.

Molecular dynamics simulations for CO2 spectra. IV. Collisional line-mixing in infrared and Raman bands

Ab initio calculations of the shapes of pure CO2 infrared and Raman bands under (pressure) conditions for which line-mixing effects are important have been performed using requantized classical molecular dynamics simulations. This approach provides the autocorrelation functions of the dipole vector and isotropic polarizability whose Fourier-Laplace transforms yield the corresponding spectra. For that, the classical equations of dynamics are solved for each molecule among several millions treated as linear rigid rotors and interacting through an anisotropic intermolecular potential. Two of the approximations used in the previous studies have been corrected, allowing the consideration of line-mixing effects without use of any adjusted parameters. The comparisons between calculated and experimental spectra under various conditions of pressure and temperature demonstrate the quality of the theoretical model. This opens promising perspectives for first principle ab initio predictions of line-mixing effects in absorption and scattering spectra of various systems involving linear molecules.

Line-mixing in absorption bands of linear molecules diluted in high-density rare gases: measurements and modeling for OCS-He

Absorption coefficients in the bending ν2 and stretching 2ν3 bands of OCS perturbed by He at high pressures up to 300 atm have been measured and analyzed in the impact-approximation region by two theoretical models accounting for the line mixing. In the first approach, the frequency-independent relaxation operator is treated semi-classically [N. N. Filippov and M. V. Tonkov, J. Quant. Spectrosc. Radiat. Transf. 50, 111 (1993)] with adiabatic corrections. To characterize the strength of collisions and the line mixing efficiency a single parameter G is used and its value is estimated from the intermolecular interaction potential. The adiabaticity of collisions and the corresponding correction to the collisional cross sections amplitude are taken into account by two other parameters ν(corr) and σ deduced from fitting to experimental line widths. In the second approach, the symmetrized non-Markovian relaxation operator of Energy-Corrected Sudden type developed for rototranslational Raman spectra of linear rotators [J. Buldyreva and L. Bonamy, Phys. Rev. A 60, 370 (1999)] and satisfying all the basic properties (in particular, the detailed balance relation and the double-sided sum rules) is adapted to absorption spectra for the first time. Its off-diagonal elements are determined via the common adiabaticity factor and the basic transition rates whose parameters are adjusted on experimental values of isolated line widths. Both models provide a very consistent description of OCS-He 2ν3 and ν2 band intensities up to the maximal experimentally studied pressure, with a slightly more realistic picturing of the inter-branch exchange by the second approach.