Knowledge Bank

We have extended the time-resolved infrared-microwave double-resonance transient-nutation method of Levy $etal.,1$ to allow simultaneous measurement: of the phenomenological relaxation times for both the diagonal ($T1$) and off-diagonal ($T2$) 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 $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}NH3$ are $1/2\Pi T1=[(25.4\pm 2.0)p+0,028]$ MHz and $1/2\backslash Pi\; T\_\{2\}\; =\; [\{24,5\; \backslash pm\; 2.0)p\; +\; 0.087]$ MHZ, where p is the pressure of ammonia in Torr. While the pressure dependence for both relaxation times is the same within experimental error, $T1$, is slightly longer than $T2$ at a given pressure due to the difference in their zero-pressure intercepts. The intercept for $T1$ approximately matches the mean free time between wall collisions (25 kHz); whereas, the intercept for $T2$ approximately matches the mean free time between wall collisions plus the inhomogeneous microwave Doppler width (68 $H2$) at $300\circ K$.