Abstract
The kinetics of rotational inelastic NH3–NH3 collisions are recorded using pump–probe experiments, carried out with a K-band waveguide chirped pulse Fourier transform microwave spectrometer, in which the population of one inversion doublet is altered by the pump pulse. Due to self-collisions, the resulting deviation from equilibrium propagates to other states and, thus, can be interrogated by probe pulses as a function of the pump–probe delay time. A clear hierarchy of the state-to-state collision processes is found and subsequently translated into propensity rules. State-to-state rate coefficients are estimated, first via an analysis of the kinetics, and then more robustly and accurately derived from the pressure-dependent measurements using a global fitting procedure.
Highlights
R eliable data on state-to-state rate coefficients are essential for accurately modeling rarefied gases
The most recent works have focused on collisions at low temperatures with He or H2 due to their astronomical importance.14−17 There is a large body of work on the use of double resonance techniques that, in particlar, employ time-resolved spectroscopy to derive rate coefficients for collisional processes, e.g., for vibrationally excited ammonia
No individual experimental stateto-state rate coefficients for vibrational ground-state ammonia and many other collision systems are available, a deficiency we seek to remedy by the method we propose here
Summary
Though, that T1 is determined by the net effect of all inelastic processes resulting in a population loss from the pumped doublet of the [3,1] state; it is not at all obvious which states are involved in the transfer or to what extent This uncertainty is the motivation for further studying the temporal behavior of the state-resolved signal difference as a Figure 3. The respective rate coefficients, listed, vary over 2 orders of magnitude from 10−11 to 10−9 cm s−1, as expected from the observed hierarchy of processes With this hierarchy in mind, the negative signals recorded for the neighboring (J,K) states can be interpreted.
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