An Experimental Setup for the Characterization of Ices of Astrophysical Interest

Abstract

Determination of densities of ices is of great importance in understanding different astrophysical scenarios. We present here a laser-interferometry experimental setup to determine the density of various mixtures of ice films. It consists of basically a vacuum chamber pumped to a base pressure of about 10⁻⁷mbar (measured by a Pirani-hot cathode) by a turbomolecular and a rotary vane pump. Ice films is deposited on a 6 MHz quartz crystal micro balance (QCMB) which we use as a substrate. Change in the frequency of the QCMB indicates mass deposition or mass removal. The QCMB substrate can be cooled down to 10 K using a closed-cycle helium refridgeration system equipped with a dual stage cold head assembly. In the first stage a cold shirt is cooled down to a temperature of 75 K which acts as a thermal radiation shield from the chamber wall. In the second stage the cold head finger is cooled down to 10 K. The temperature of the substrate is measured using a diode sensor and the temperature can be controlled at any temperature between 10 K and 300 K. Ice film is deposited on the substrate by leaking the desired gas mixture into the chamber through a leak valve. The density of ice film is obtained by knowing the thickness and the total mass of the film deposited. The thickness of the ice film is measured by interferometry and the mass is determined from the change in the crystal frequency. A laser beam is allowed to fall on the ice film and the reflected intensity is measured with a photodiode. The reflected intensity shows interference maxima and minima with ice deposition. A software routine fit is used to determine the thickness.