Figure 1: Physical picture of the site selectivity achieved by X-ray rotational heating.

The rotational heating is caused by the angular momentum transferred to the molecule by the electron ejected from the heavy Cl or the light H atoms. In the ground state, the molecule rotates with the angular velocity ω due to the finite temperature. The hydrogen and chlorine atoms have different distances from the centre of gravity , where M=m_H+m_Cl is the total mass of the molecule. This makes the velocity of the light H atom much larger than the velocity of the heavy Cl atom: . During the rotation, the electron energy will exhibit a Doppler shift , which changes in the interval (−kωR_n,+kωR_n) due to the change in the angle between v_n and the electron momentum k. This variation of the Doppler shift results in a Doppler line broadening which is different for light (n=H) and heavy (n=Cl) atoms. Taking into account that j_n=R_n × k, we can conclude that the Doppler shift k·v_n is nothing else but the rotational Doppler shift j_n·ω, which is larger for the light atom. The X-ray polarization vector e is parallel to the electron momentum k, while the photon propagation axis is perpendicular to the figure plane.