Closed Cycle Liquid 4He Cryogenic Cooler: Toward Controlling Chemical Reactions

Abstract

In an attempt to control gas phase chemical reactions of CaH and Li by manipulating electron spin states in a 4 Kelvin cryostat, it was observed that the unpolarized free reaction rate coefficient, 10−11 cm3 s −1 , is of the same order as the depolarization rate coefficient of the CaH, 10−11 cm3 s −1 , making it impossible to control the reaction. By cooling our reactants to 1 K in a new closed cycle liquid 4He cooler the depolarization rate coefficient of CaH was reduced to 10−13 cm3 s −1 . This thesis will discuss the design and implementation of a closed cycle liquid 4He cryogenic cooler (LHCC) capable of cooling gas phase samples to 1 Kelvin by means of laser ablation and helium buffer gas cooling, the samples are then observed by laser absorption spectroscopy. This LHCC has been used in the measurement of the depolarization rate of CaH over a range of temperatures from 1 K to 2.2 K and investigating the chemical reaction CaH+Li → Ca+LiH+Energy.