Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

Abstract

A temperature measurement technique using SF6 molecules as tiny probe thermometers is described, and results are presented, for large (CO2)N van der Waals clusters (with N ≥ 102) in a cluster beam. The SF6 molecules captured by (CO2)N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF6 in the incoming beam. As a result, the nanoparticle temperature in the (CO2)N cluster beam is estimated as Tcl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO2 laser are performed to determine the velocity (kinetic energy) of SF6 molecules sublimating from clusters, and the cluster temperature is found to be Tcl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.