STARK-MODULATED INFRARED-LASER ENHANCED TRANSIENT NUTATION MEASUREMENT OF $T_{1}$, AND $T_{2}$ FOR AMMONIA
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Date
1975
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Publisher
Ohio State University
Abstract
We have extended the time-resolved infrared-microwave double-resonance transient-nutation method of Levy $et al.,^{1}$ to allow simultaneous measurement: of the phenomenological relaxation times for both the diagonal ($T_{1}$) and off-diagonal ($T_{2}$) components of the density matrix for the 2-level microwave system in the Feynmann-Vernon-Hellwarth representation. The method involves a variable time delay between the ir pulse which perturbs the state populations and the turning off of a Stark field which induces the nutations. The values of the pressure dependence of the relaxation times in the (8, 7) microwave inversion doublet of $^{14}NH_{3}$ are $1/2\Pi T_{1} = [(25.4 \pm 2.0)p + 0,028]$ MHz and $1/2\Pi T_{2} = [{24,5 \pm 2.0)p + 0.087]$ MHZ, where p is the pressure of ammonia in Torr. While the pressure dependence for both relaxation times is the same within experimental error, $T_{1}$, is slightly longer than $T_{2}$ at a given pressure due to the difference in their zero-pressure intercepts. The intercept for $T_{1}$ approximately matches the mean free time between wall collisions (25 kHz); whereas, the intercept for $T_{2}$ approximately matches the mean free time between wall collisions plus the inhomogeneous microwave Doppler width (68 $H_2$) at $300^\circ K$.
Description
$^{1}$ J. M. Levy, J.H.-S. Wang, S. G. Kukolich and J. I. Steinfeld, Phys. Rev. Letters 29, 395 (1972); Chem. Phys. 1, 141 (1973). This work was sponsored by the National Science Foundation.""
Author Institution: Department of Chemistry, Massachusetts Institute of Technology
Author Institution: Department of Chemistry, Massachusetts Institute of Technology