Cyclotron Resonance in Gases. II. Cross Sections for Dipolar Gases and for CO2

Abstract

The interaction of dipolar molecules with very low-energy electrons arises almost entirely from the monopole—dipole interaction. These cross sections have been measured by analyzing the widths and shapes of the cyclotron-resonance absorption lines of electrons in a gaseous mixture of N2 and dipolar gases. The electron collision cross sections σD for the dipolar molecules could be reasonably well fitted to the relation σD = 180D2(v0/v)2, where σD is in square angstroms, D is the dipole moment in Debye units, v is the electron velocity, and v0 is the electron velocity of 298°K electrons (1.16×107 cm/sec). A Born-approximation calculation yields the same result with the coefficient equal to 130 rather than 180. CO2 has an anomolously large cross section for a nonpolar molecule. The experimental data is compatible with a cross section that is proportional to v−1, but it can also be fitted by a dipolar cross section with D=0.67 which is set equal to zero for v<5×106 cm/sec. This model corresponds to the physical situation in which the faster electrons see the instantaneous dipole moment due to bending motions in CO2, and the slower electrons feel the interaction over several vibrational periods and hence do not see a net dipole moment. However, estimates based on infrared data indicate that the effective dipole moment should only be about D=0.16 in the ground state. The cross section of N2O also shows evidence of being effected by instantaneous dipole moments.