High-resolution time resolved spectroscopy of collisionless molecular beams. II. Energy randomization and optical phase relaxation of molecules in crossed laser and molecular beams
This paper outlines new ways for separating the different channels of optical dephasing of molecules in beams. It is shown that both the population loss and the optical phase relaxation rates can be obtained under collisionless conditions (molecular beams). These dephasing rates, which measure directly the homogeneous width of the prepared resonance, can be obtained with an energy resolution of better than one part in 108. Under these conditions the solution of the density matrix equations of motion is given for effusive and nozzle beams which represent a statistically open ensemble. The results, which we applied to our recent measurements of optical T1 and T2, are discussed in different limits of power broadening, beam geometry, detector characteristics, temperature of the oven (or material container), and the transit time the molecules of different velocities spend in the laser beam. Furthermore, we indicate that the treatment of coherent transients in beams utilizing conventional Bloch equations is not valid since there is a loss of optically excited molecules. Finally, we discuss possible differences between small and large molecules when they undergo radiationless (or reactive) processes following the selective laser excitation (~10 kHz–10 MHz).
Additional Information© 1978 American Institute of Physics. Received 17 October 1977. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by ACS, for partial support of this research. This work was also supported in part by the Energy Research and Development Administration and partly by the Research Corporation. One of us (A.H.Z.) would like to thank Professor Don Levy of the University of Chicago for very stimulating discussions regarding the discussions of I2He complex. Arthur Amos Noyes Laboratory of Chemical Physics, Contribution No. 5677.
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