Photocalorimetric Spectroscopy of Thin Film Adsorbates.

A new technique for studying surface adsorbed molecules is presented. The technique, photocalorimetric spectroscopy, uses an ultransensitive thermometer to measure the amount of heat released upon relaxation of photoexcited surface absorbate molecules. Photocalorimetric absorption spectra are recorded by measuring this heat as a function of the wavelength of the photoexcitation source. Both the thermometer and sample film are supported on a sapphire crystal wafer substrate which is mounted on a liquid helium cold-finger under ultrahigh vacuum. By precoating the sapphire wafer before dosing with adsorbate molecules, a wide variety of surface-adsorbate systems can be studied. An integral thin film resistive heater facilitates several useful thermodynamic measurements. The technique is applicable over the full range of electronic and vibrational excitation of the adsorbate molecules. The combination of spectroscopic and thermodynamic data can be used to determine the energies, orientations, and dynamics of surface-adsorbate interactions. As a demonstration of the technique, the results of several experiments are presented. These experiments focus on the adsorption of pyridine and carbon monoxide on polycrystalline nickel surfaces. The data include heats of adsorption, thermal desorption spectra, and electronic and vibrational absorption spectra. From the results, it is shown that photocalorimetric spectroscopy reveals coverage dependent changes in the bonding and orientation of molecules in thin films.Ph.D.Physical chemistryUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160445/1/8502955.pd