Absorption line asymmetry: Speed-dependent effects observed from coherent transients on the 86 GHz line of HC15N in collision with polar and diatomic molecules and with rare gases

•Lineshape asymmetries: comparison of hypergeometric and quadratic SD-Voigt profiles.•HCN relaxation induced by polar and diatomic molecules and by rare gases.•Temperature dependence of broadening and frequency-shifting rates and of SD-parameters.•Consequences of e.m. field frequency instabilities on absorption lineshapes.•Lineshape analysis from time domain coherent transients (optical precession).