Thesis of Prakash Gyawali

Defense of thesis Prakash Gyawali - laboratory PhLAM

Terahertz spectroscopy of molecules and molecular complexes of atmospheric interest exhibiting large amplitude motions

Abstract :

The atmosphere comprises a diverse array of molecules and species. Among these, water vapor and its complexes have a significant role in the phenomenon of global warming and climate change. Spectroscopic analysis of such complexes is essential for understanding various atmospheric processes. However, there is still a limited knowledge on weakly bounded water complexes in the terahertz wave range due to their complex spectral features and experimental challenges. Spectral complexity often results from the large amplitude motions limited by low potential barriers.
We present the development of pulsed-jet emission spectrometer intended for studies of molecular complexes in the terahertz range. Experimental developments were accompanied by benchmarking large amplitude motions models on the molecules of atmospheric interest. We started with acetyl halides exhibiting a periodic torsional motion of methyl group. We obtained accurate models of the rotational spectra of acetyl chloride and acetyl bromide within experimental accuracy. Subsequently, the rotational spectra of methylamine in its first excited torsional state were studied. Methylamine is characterized by two large amplitude motions: torsion and inversion. The rotational spectrum of methylamine was analyzed using the so-called "hybrid" model that for the first time allowed accurate assignment and modeling of the lowest excited torsional states. Finally, the high-resolution rotational spectra of ammonia-water weakly bounded complex were measured using the newly built spectrometer. For the analysis of ammonia-water which exhibits two large amplitude motions similar to methylamine, we also applied the "hybrid" approach demonstrating thus its advantage in the application to excited states and low barrier cases.

Keywords : Terahertz,Large amplitude motions,Bi-molecular complexes,High-resolution spectroscopy