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Register a new userRotational quenching of H2CO by molecular hydrogen: cross-sections, rates, pressure broadening
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We compute the rotational quenching rates of the first 81 rotational levels of ortho- and para-H2CO in collision with ortho- and para-H2, for a temperature range of 10-300 K. We make use of the quantum close-coupling and coupled-states scattering methods combined with the high accuracy potential energy surface of Troscompt et al. (2009a). Rates are significantly different from the scaled rates of H2CO in collision with He; consequently, critical densities are noticeably lower. We compare a full close- coupling computation of pressure broadening cross sections with experimental data and show that our results are compatible with the low temperature measurements of Mengel & De Lucia (2000), for a spin temperature of H2 around 50 K.
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The relative orientation of colliding molecules plays a key role in determining the rates of chemical processes. Here we examine in detail a prototypical example: rotational quenching of HD in cold collisions with H2. We show that the rotational quenching rate from j=2 -> 0, in the v=1 vibrational level, can be maximized by aligning the HD along the collision axis and can be minimized by aligning the HD at the so called magic angle. This follows from quite general helicity considerations and suggests that quenching rates for other similar systems can also be controlled in this manner.
Theoretical cross sections for the pressure broadening by hydrogen of rotational transitions of water are compared to the latest available measurements in the temperature range 65-220 K. A high accuracy interaction potential is employed in a full close coupling calculation. A good agreement with experiment is observed above ~80 K while the sharp drop observed experimentally at lower temperatures is not predicted by our calculations. Possible explanations for this discrepancy include the failure of the impact approximation and the possible role of ortho-to-para conversion of H2.
Interpretation of solar polarization spectra accounting for partial or complete frequency redistribution requires data on various collisional processes. Data for depolarization and polarization transfer are needed but often missing, while data for collisional broadening are usually more readily available. Recent work by Sahal-Brechot and Bommier concluded that despite underlying similarities in the physics of collisional broadening and depolarization processes, relationships between them are not possible to derive purely analytically. We aim to derive accurate numerical relationships between the collisional broadening rates and the collisional depolarization and polarization transfer rates due to hydrogen atom collisions. Such relationships would enable accurate and efficient estimation of collisional data for solar applications. Using earlier results for broadening and depolarization processes based on general (i.e. not specific to a given atom), semi-classical calculations employing interaction potentials from perturbation theory, genetic programming (GP) has been used to fit the available data and generate analytical functions describing the relationships between them. The predicted relationships from the GP-based model are compared with the original data to estimate the accuracy of the method.
Aims. We seek to understand how the 4.8 GHz formaldehyde absorption line is distributed in the MON R2, S156, DR17/L906, and M17/M18 regions. More specifically, we look for the relationship among the H2CO, 12CO, and 13CO spectral lines. Methods. The four regions of MON R2 (60x90), S156 (50x70), DR17/L906 (40x60), and M17 /M18 (70x80)were observed for H2CO (beam 10), H110a recombination (beam 10), 6 cm continuum (beam 10), 12CO (beam 1), and 13CO (beam 1). We compared the H2CO,12CO,13CO, and continuum distributions, and also the spectra line parameters of H2CO,12CO, and 13CO. Column densities of H2CO,13CO, and H2 were also estimated. Results. We found out that the H2CO distribution is similar to the 12CO and the 13CO distributions on a large scale. The correlation between the 13 CO and the H2CO distributions is better than between the 12CO and H2CO distributions. The H2CO and the 13CO tracers systematically provide consistent views of the dense regions. T heir maps have similar shapes, sizes, peak positions, and molecular spectra and present similar centr al velocities and line widths. Such good agreement indicates that the H2CO and the 13CO arise from similar regions.
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