Dependence of Q-branch intensity of pendular state molecules on electric field and molecular orientation and their applications

A new method to measure the orientation of cold polar linear molecules using the relative intensity of the Q branch spectrum of the pendular state of linear molecules is presented. The relationship between the orientation of HCN-N₂ and Q-branch intensity of pendular state spectrum of the C-H stretch of the ground state of HCN-N₂ is studied, and it is found that the intensity of its Q branch spectrum increases with the increase of the electric field strength, but the intensity of the R- and P-branches decreases with the increase of the electric field strength. The relative intensity of the Q branch spectrum (ratio of spectral intensity of Q branch and total intensity of P, Q, R branches) can be used to represent the electric field intensity. Also, the study shows the orientation of the molecule increases with the increase of electric field strength. So it is proposed that the method which uses the relative intensity of the Q branch spectrum can be applied to measure the orientation of the polar molecule.