Direct measurements of transition dipole matrix elements using optical nutation

We describe a method of directly determining transition dipole matrix elements which we have used to make the first measurements of this quantity in an asymmetric rotor. A low pressure gas (a few mtorr NH2D) which is subjected to a cw CO2 laser beam is suddenly Stark switched into resonance. An optical nutation signal is detected whose frequency yields -E0/h/, where - is the transition dipole matrix element of interest and E0 is the optical field strength which may be determined from power and beam profile measurements. Thus an observation of the nutation frequency along with the power measurements yields -. This was done for two transitions of NH2D giving transition dipole matrix elements along two axes in the molecule: 〈ν2=0a‖ (∂μa/∂Q) Q‖ν2=1a〉=0.112±0.009 D and 〈0a‖ (∂μc/∂Q) Q‖1s〉=0.158±0.009 D. The extension of this technique to nonpolar molecules and its advantages over conventional steady state laser methods are discussed.