State-to-state rotational rate coefficients for ammonia self collisions from pump-probe chirped-pulse experiments

Schlemmer, Stephan

Permalink

https://hdl.handle.net/2142/100727 Copy

Description

Title

State-to-state rotational rate coefficients for ammonia self collisions from pump-probe chirped-pulse experiments

Author(s)

Schlemmer, Stephan

Contributor(s)

• Caselli, Paola
• Endres, Christian

Issue Date

2018-06-19

Keyword(s)

Dynamics and kinetics

Abstract

Rotational state populations of ammonia are inferred from chirped pulse spectra of its tunneling doublets in a room temperature K-band waveguide experiment where many tunneling doublets can be addressed by a single chirped pulse excitation. The thermal distribution of states is altered by a pump pulse where the population of the tunneling doublet of a single rotational state is inverted by a π-pulse within roughly 100 ns. The resulting deviation from equilibrium is then propagating to other states due to collisions and interrogated by a probe pulse from which the state populations of many rotational states are inferred at once. From the free induction decays (FID) of the individual states the relaxation time of the radiation-induced superposition state of the two level tunneling system (T2) is inferred. Also the collisional relaxation time (T1) for the difference in the population of the two-level system is determined. These values exhibit a linear pressure dependence, the slope of which agrees very well with previous measurements a . Analysis of probe FID signals from these pump-probe experiments reveals the well known hierachy of collisional relaxation in ammonia which was first found by Oka fifty years ago through steady state intensity measurements b . Collision-induced transitions within the tunneling doublet (∆J = 0) determined from T1 measurements are faster than ∆J = ±1 transitions. Of those the ∆K = 0 transitions are much faster than those with ∆K 6= 0. Due to this hierachy of inelastic processes and thanks to the fast optical pumping experiments state-to-state rates can be measured. As a result, from the pressure dependence of the measured rates state-to-state rate coefficients are determined. Those rate coefficients agree very well with results of simulations of all coupled states which fit with the temporal behavior of the complete pump probe experiments where many individual (J,K) rotational states can be addressed step by step by separate probe-pump-probe pulse sequences.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/100727

DOI

10.15278/isms.2018.TD01

Copyright and License Information

Copyright 2018 Stephan Schlemmer

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